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External
Generative Organs
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Mons Pubis
The mons
pubis, or mons veneris, is the fat-filled cushion that lies over the symphysis
pubis. After puberty, the skin of the mons pubis is covered by curly hair that
forms the escutcheon. In women, it is distributed in a triangular area, the
base of which is formed by the upper margin of the symphysis. In men, the
escutcheon is not so well circumscribed.
These
structures vary somewhat in appearance, principally according to the amount of
fat that is contained within the tissues. Embryologically, the labia majora are
homologous with the male scrotum. The round ligaments terminate at the upper
borders. After repeated childbearing, the labia majora are less prominent. They
are 7 to 8 cm in length, 2 to 3 cm in width, and 1 to 1.5 cm in thickness, and
are somewhat tapered at the lower extremities. In children and nulliparous
women, the labia majora usually lie in close apposition, whereas in multiparous
women, they may gape widely. They are continuous directly with the mons pubis
above and merge into the perineum posteriorly at a site where they are joined
medially to form the posterior commissure.
Before
puberty, the outer surface of the labia is similar to that of the adjacent
skin, but after puberty the labia are covered with hair. In nulliparous women,
the inner surface is moist and resembles a mucous membrane, whereas in
multiparous women, the inner surface becomes more skinlike. The labia majora
are richly supplied with sebaceous glands. Beneath the skin, there is a layer
of dense connective tissue that is rich in elastic fibers and adipose tissue
but is nearly void of muscular elements. Unlike the squamous epithelium of the
vagina and cervix, there are epithelial appendages in parts of the vulvar skin.
A mass of fat beneath the skin provides the bulk of the volume of the labium,
and this tissue is supplied with a rich plexus of veins.
The labia minora
vary greatly in size and shape. In nulliparous women, they usually are not
visible behind the nonseparated labia majora. In multiparas, it is common for
the labia minora to project beyond the labia majora.
Each labium
minus is a thin fold of tissue that is moist and reddish, similar in appearance
to a mucous membrane. The labia minora are covered by stratified squamous
epithelium. Although there are no hair follicles in the labia minora, there are
many sebaceous follicles and, occasionally, a few sweat glands. The interior of
the labial folds is composed of connective tissue with many vessels and some
smooth muscular fibers. They are supplied with a variety of nerve endings and
are extremely sensitive. The tissues of the labia minora converge superiorly,
where each is divided into two lamellae; the lower pair fuse to form the frenulum
of the clitoris, and the upper pair merge to form the prepuce.
Inferiorly, the labia minora extend to approach the midline as low ridges of
tissue that fuse to form the fourchette.
The clitoris
is the principal female erogenous organ. It is the homologue of the penis and
is located near the superior extremity of the vulva. This erectile organ
projects downward between the branched extremities of the labia minora. The
clitoris is composed of a glans, a corpus, and two crura. The glans is made up
of spindle-shaped cells, and in the body there are two corpora cavernosa, in
the walls of which are smooth muscle fibers. The long, narrow crura arise from
the inferior surface of the ischiopubic rami and fuse just below the middle of
the pubic arch to form the corpus.
The clitoris
rarely exceeds 2 cm in length. Its free end is pointed downward and inward
toward the vaginal opening. The glans is usually less than 0.5 cm in diameter
and is covered by stratified squamous epithelium that is richly supplied with
nerve endings. The vessels of the erectile clitoris are connected with the
vestibular bulbs.
There is a
delicate network of free nerve endings in the labia majora, labia minora, and
clitoris (Krantz, 1958). Tactile discs are found in abundance in these areas.
Genital corpuscles, which are mediators of erotic sensation, vary considerably
in number. These structures are abundant in the labia minora and in the skin
that overlies the glans clitoris.
The
vestibule is an almond-shaped area that is enclosed by the labia minora
laterally and extends from the clitoris to the fourchette. The vestibule is the
functionally mature female structure of the urogenital sinus of the embryo. In
the mature state, the vestibule usually is perforated by six openings: the
urethra, the vagina, the two ducts of the Bartholin glands, and, at times, the
two ducts of the paraurethral glands, also called the Skene ducts and glands.
The posterior portion of the vestibule between the fourchette and the vaginal
opening is called the fossa navicularis, and it is usually observed only
in nulliparous women
The pair of Bartholin glands
are about 0.5 to 1 cm in diameter, and each is situated beneath the vestibule
on either side of the vaginal opening. They are the major vestibular glands,
and the ducts are 1.5 to 2 cm long and open on the sides of the vestibule just
outside the lateral margin of the vaginal orifice. At times of sexual arousal,
they secrete mucoid material. These glands may harbor Neisseria gonorrhoeae
or other bacteria, which in turn may cause infection and a Bartholin gland
abscess.
The lower
two thirds of the urethra lies immediately above the anterior vaginal wall. The
urethral opening or meatus is in the midline of the vestibule, 1 to 1.5 cm
below the pubic arch, and a short distance above the vaginal opening.
Ordinarily, the paraurethral ducts, also known as the Skene ducts,
open onto the vestibule on either side of the urethra. The ducts occasionally
open on the posterior wall of the urethra just inside the meatus.
Embryologically,
the vestibular bulbs correspond to the anlage of the corpus spongiosum of the
penis. These are almond-shaped aggregations of veins, 3 to 4 cm long, 1 to 2 cm
wide, and 0.5 to 1 cm thick, that lie beneath the mucous membrane on either
side of the vestibule. They are in close apposition to the ischiopubic rami and
are partially covered by the ischiocavernosus and constrictor vaginae muscles.
The vestibular bulbs terminate interiorly at about the middle of the vaginal
opening and extend upward toward the clitoris. During childbirth, they may be
injured and may even rupture to form a vulvar hematoma.
In most
virginal women, the vaginal opening most often is hidden by the overlapping
labia minora. There are marked differences in shape and consistency of the
hymen, which is composed mainly of elastic and collagenous connective tissue.
Both the outer and inner surfaces are covered by stratified squamous
epithelium. The hymen has no glandular or muscular elements, and it is not
richly supplied with nerve fibers.
In the newborn,
the hymen is very vascular and redundant. In pregnant women, its epithelium is
thick, and the tissue is rich in glycogen. After menopause, the epithelium is
thin, and focal cornification may develop. In adult women, the hymen is a
membrane of various thickness that surrounds the vaginal opening more or less
completely. Its aperture varies in diameter from pinpoint size to one that
admits the tip of one or even two fingers.
The
appearance of the hymen cannot be used to determine whether a woman has begun
sexual activity. A fimbriated type of hymen in virginal women may be
indistinguishable from one that has been penetrated during intercourse. As a
rule, however, it is torn at several sites during first coitus, usually in the
posterior portion. Identical tears may occur by other penetration, for example,
tampons used during menstruation. The edges of the torn tissue soon cicatrize,
and the hymen becomes divided permanently into two or more portions that are
separated by narrow sulci. Occasionally with hymenal rupture, there may be
profuse bleeding.
Changes
produced in the hymen by childbirth are usually readily recognizable. Over
time, the hymen consists of several cicatrized nodules of various sizes. Imperforate
hymen is a rare lesion in which the vaginal orifice is occluded completely,
causing retention of menstrual blood
This
musculomembranous structure extends from the vulva to the uterus and is
interposed anteriorly and posteriorly between the urinary bladder and the
rectum . The upper portion of the vagina arises from the müllerian ducts, and
the lower portion is formed from the urogenital sinus. Anteriorly, the vagina
is separated from the bladder and urethra by connective tissue, often referred
to as the vesicovaginal septum. Posteriorly, between the lower portion
of the vagina and the rectum, there are similar tissues that together form the rectovaginal
septum. The upper fourth of the vagina is separated from the rectum by the rectouterine
pouch, also called the cul-de-sac of Douglas. Normally, the anterior
and posterior vaginal walls lie in contact, with only a slight space
intervening between the lateral margins. Vaginal length varies considerably,
but commonly, the anterior and posterior vaginal walls are, respectively, 6 to
8 cm and 7 to 10 cm in length. The upper end of the vaginal vault is subdivided
into the anterior, posterior, and two lateral fornices by the uterine cervix.
These are of considerable clinical importance because the internal pelvic
organs usually can be palpated through their thin walls. Moreover, the
posterior fornix provides surgical access to the peritoneal cavity.
Prominent midline longitudinal
ridges project into the vaginal lumen from the anterior and posterior walls. In
nulliparous women, numerous transverse ridges, or rugae, extend outward
from and almost at right angles to the longitudinal ridges. In postmenopausal
multiparous women, the vaginal walls often are smooth.
The vaginal
mucosa is composed of noncornified stratified squamous epithelium. Beneath the
epithelium is a thin fibromuscular coat, usually consisting of an inner
circular layer and an outer longitudinal layer of smooth muscle. A thin layer
of connective tissue beneath the mucosa and the muscularis is rich in blood
vessels. It is controversial whether this connective tissue—often referred to
as perivaginal endopelvic fascia—is a definite fascial plane in the
strict anatomical sense.
There are no
vaginal glands. After giving birth, fragments of stratified epithelium
occasionally are embedded in the vaginal connective tissue. They may form vaginal
inclusion cysts, which are not true glands. In the absence of glands, the
vagina is kept moist by a small amount of secretion from the cervix. During
pregnancy, there is copious, acidic vaginal secretion, which normally consists
of a curdlike product of exfoliated epithelium and bacteria. Lactobacillus
species are also recovered in higher concentrations than in nonpregnant women
(Larsen and Galask, 1980; McGregor and French, 2000).
The vagina
has an abundant vascular supply. The upper third is supplied by the
cervicovaginal branches of the uterine arteries, the middle third by the
inferior vesical arteries, and the lower third by the middle rectal and
internal pudendal arteries. The vaginal artery may branch directly from the
internal iliac artery. An extensive venous plexus immediately surrounds the
vagina and follows the course of the arteries. Lymphatics from the lower third
of the vagina, along with those of the vulva, drain primarily into the inguinal
lymph nodes. Those from the middle third drain into the internal iliac nodes,
and those from the upper third drain into the iliac nodes.
The many
structures that make up the perineum. Most of the support of the perineum is provided
by the pelvic and urogenital diaphragms. The pelvic diaphragm consists
of the levator ani muscles plus the coccygeus muscles posteriorly. The levator
ani muscles form a broad muscular sling that originates from the posterior
surface of the superior pubic rami, from the inner surface of the ischial
spine, and between these two sites, from the obturator fascia. Some of these
muscle fibers are inserted around the vagina and rectum to form efficient
functional sphincters. In a recent study utilizing magnetic resonance imaging,
Tunn (2003) and Hoyte (2004) and their colleagues used magnetic-resonance
imaging and reported significant variation in the levator ani muscle,
endopelvic fascia, and urethral support in nulliparous women. The urogenital
diaphragm is external to the pelvic diaphragm and includes the triangular
area between the ischial tuberosities and the symphysis. The urogenital
diaphragm is made up of the deep transverse perineal muscles, the constrictor
of the urethra, and the internal and external fascial coverings.
Size and Shape
The uterus
resembles a flattened pear in shape. It consists of two major but unequal
parts: an upper triangular portion, the body, or corpus; and a lower,
cylindrical, or fusiform portion, the cervix, which projects into the
vagina. The isthmus is that portion of the uterus between the internal
cervical os and the endometrial cavity. It is of special obstetrical
significance because it forms the lower uterine segment during pregnancy. The
oviducts, or fallopian tubes, emerge from the cornua of the uterus at
the junction of the superior and lateral margins. The convex upper segment
between the points of insertion of the fallopian tubes is called the fundus.
The round ligaments insert below the tubes on the anterior side. They are
covered by a fold of peritoneum that extends to the pelvic sidewall. These
folds are called the broad ligaments, however, they do not constitute
the anatomical definition of a ligament.
The
prepubertal uterus varies in length from 2.5 to 3.5 cm (Orsini and colleagues,
1984). The uterus of adult nulliparous women is from 6 to 8 cm in length as
compared with 9 to 10 cm in multiparous women. Uteri of nonparous women average
50 to 70 g, and those of parous women average 80 g or more (Langlois, 1970). In
the premenarchal girl, the body of the uterus is only half as long as the
cervix. In nulliparous women, the two are about equal in length. In multiparous
women, the cervix is only a little more than a third of the total length of the
organ. After menopause, uterine size decreases as a consequence of atrophy of
both myometrium and endometrium.
The bulk of
the body of the uterus, but not the cervix, is composed of muscle. The inner
surfaces of the anterior and posterior walls lie almost in contact, and the
cavity between these walls forms a mere slit. The cervical canal is fusiform
and is open at each end by small apertures, the internal os and the external
os.
Pregnancy
stimulates remarkable uterine growth due to hypertrophy of muscle fibers. The
weight of the uterus increases from 70 g to about 1100 g at term. Its total
volume averages about 5 L. The uterine fundus, a previously flattened convexity
between tubal insertions, now becomes dome shaped. The round ligaments now
appear to insert at the junction of the middle and upper thirds of the organ.
The fallopian tubes elongate, but the ovaries grossly appear unchanged.
Cervix
Anteriorly,
the upper boundary of the cervix is the internal os, which corresponds to the
level at which the peritoneum is reflected upon the bladder. The supravaginal
segment is covered by peritoneum on its posterior surface. This segment is
attached to the cardinal ligaments anteriorly, and it is separated from the
overlying bladder by loose connective tissue. The other segment is the lower
vaginal portion of the cervix, also called the portio vaginalis.
Before
childbirth, the external cervical os is a small, regular, oval opening. After
childbirth, the orifice is converted into a transverse slit that is divided
such that there are the so-called anterior and posterior lips of the cervix. If
torn deeply during delivery, it might heal in such a manner that it appears to
be irregular, nodular, or stellate. These changes are sufficiently
characteristic to permit an examiner to ascertain with some certainty whether a
given woman has borne children by vaginal delivery.
The mucosa of the cervical
canal is composed of a single layer of very high ciliated columnar epithelium
that rests on a thin basement membrane. Numerous cervical glands extend from the
surface of the endocervical mucosa directly into the subjacent connective
tissue. These glands furnish the thick, tenacious cervical secretions. If the
ducts of the cervical glands are occluded, retention cysts, known as nabothian
cysts, are formed.
The wall of
the body of the uterus is composed of serosal, muscular, and mucosal layers.
The serosal layer is formed by the peritoneum that covers the uterus. It is
firmly adherent except at sites just above the bladder and at the lateral margins,
where the peritoneum is deflected to form the broad ligaments.
This mucosal
layer lines the uterine cavity in nonpregnant women. It is a thin, pink,
velvet-like membrane that on close examination is found to be perforated by a
large number of minute ostia of the uterine glands. The endometrium normally
varies greatly in thickness, and measures from 0.5 mm to as much as 5 mm. It is
composed of surface epithelium, glands, and interglandular mesenchymal tissue
in which there are numerous blood vessels.
The
epithelium of the endometrial surface is made up of a single layer of closely
packed, high columnar, ciliated cells. The tubular uterine glands are
invaginations of the epithelium. The glands extend through the entire thickness
of the endometrium to the myometrium, which is occasionally penetrated for a
short distance. Histologically, the inner glands resemble the epithelium of the
surface and are lined by a single layer of columnar, partially ciliated
epithelium that rests on a thin basement membrane. The glands secrete a thin,
alkaline fluid. The connective tissue of the endometrium, between the surface
epithelium and the myometrium, is a mesenchymal stroma. Histologically, the
stroma varies remarkably throughout the ovarian cycle.
After menopause,
the endometrium is atrophic and the epithelium flattens. The glands gradually
disappear, and the interglandular tissue becomes more fibrous.
The vascular
architecture of the uterus and the endometrium is of signal importance in
pregnancy. The uterine and ovarian arteries branch and penetrate the uterine
wall obliquely inward and reach its middle third. They then ramify in a plane
that is parallel to the surface and are therefore named the arcuate arteries
(DuBose and colleagues, 1985). Radial branches extend from the arcuate arteries
at right angles and enter the endometrium to become coiled or spiral
arteries. Also from the radial arteries, basal arteries branch at a
sharp angle. The coiled arteries supply most of the midportion and all of the
superficial third of the endometrium. The walls of these vessels are responsive
(sensitive) to the action of a number of hormones, especially by
vasoconstriction, and thus probably serve an important role in the mechanism(s)
of menstruation. The straight basal endometrial arteries extend only into the
basal layer of the endometrium and are not responsive to hormonal action.
Myometrium
The
myometrium makes up the major portion of the uterus. It is composed of bundles
of smooth muscle united by connective tissue in which there are many elastic
fibers. According to Schwalm and Dubrauszky (1966), the number of muscle fibers
of the uterus progressively diminishes caudally such that, in the cervix,
muscle comprises only 10 percent of the tissue mass. In the inner wall of the
body of the uterus, there is relatively more muscle than in the outer layers;
and in the anterior and posterior walls, there is more muscle than in the lateral
walls. During pregnancy, the upper myometrium undergoes marked hypertrophy, but
there is no significant change in cervical muscle content.
The broad
ligaments are made up of two winglike structures that extend from the
lateral margins of the uterus to the pelvic walls. They divide the pelvic
cavity into anterior and posterior compartments. Each broad ligament consists
of a fold of peritoneum. The inner two thirds of the superior margin form the mesosalpinx,
to which the fallopian tubes are attached. The outer third of the superior
margin, which extends from the fimbriated end of the oviduct to the pelvic
wall, forms the infundibulopelvic ligament or suspensory ligament of
the ovary, through which the ovarian vessels traverse.
At the
lateral margin of each broad ligament, the peritoneum is reflected onto the
side of the pelvis. The thick base of the broad ligament is continuous with the
connective tissue of the pelvic floor. The densest portion is usually referred
to as the cardinal ligament—also called the transverse cervical
ligament or the Mackenrodt ligament—and is composed of connective
tissue that medially is united firmly to the supravaginal portion of the
cervix.
A vertical
section through the uterine end of the broad ligament is triangular, and the
uterine vessels and ureter are found within its broad base. In its lower part,
it is widely attached to the connective tissues that are adjacent to the
cervix, that is, the parametrium. The upper part is made up of three
folds that nearly cover the oviduct, the utero-ovarian ligament, and the round
ligament.
The round ligaments
extend from the lateral portion of the uterus, arising somewhat below and
anterior to the origin of the oviducts. Each round ligament is located in a
fold of peritoneum that is continuous with the broad ligament and extends
outward and downward to the inguinal canal, through which it passes to
terminate in the upper portion of the labium majus. In nonpregnant women, the
round ligament varies from 3 to 5 mm in diameter, and is composed of smooth
muscle cells. The round ligament corresponds embryologically to the
gubernaculum testis of men. During pregnancy, the round ligaments undergo
considerable hypertrophy and increase appreciably in both length and diameter.
Each uterosacral
ligament extends from an attachment posterolaterally to the supravaginal
portion of the cervix to encircle the rectum and inserts into the fascia over
the sacrum. Umek and colleagues (2004) used MR-imaging to describe anatomical
variations of these ligaments. The ligaments are composed of connective tissue
and some smooth muscle and are covered by peritoneum. They form the lateral
boundaries of the pouch of Douglas.
The vascular
supply of the uterus is derived principally from the uterine and ovarian
arteries. The uterine artery, a main branch of the internal iliac
artery—referred to in the past as the hypogastric artery—enters the base of the
broad ligament and makes its way medially to the side of the uterus.
Immediately adjacent to the supravaginal portion of the cervix, the uterine
artery divides. The smaller cervicovaginal artery supplies blood to the lower
cervix and upper vagina. The main branch turns abruptly upward and extends as a
highly convoluted vessel that traverses along the margin of the uterus. A
branch of considerable size extends to the upper portion of the cervix, and
numerous other branches penetrate the body of the uterus. Just before the main
branch of the uterine artery reaches the oviduct, it divides into three terminal
branches. The ovarian branch of the uterine artery anastomoses with the
terminal branch of the ovarian artery; the tubal branch makes its way through
the mesosalpinx and supplies part of the oviduct; and the fundal branch is
distributed to the uppermost uterus.
About 2 cm
lateral to the cervix, the uterine artery crosses over the ureter . The
proximity of the uterine artery and vein to the ureter at this point is of
great surgical significance. Because of their close proximity, the ureter may
be injured or ligated during a hysterectomy when the vessels are clamped and
ligated.
The ovarian artery is a
direct branch of the aorta. It enters the broad ligament through the
infundibulopelvic ligament. At the ovarian hilum, it divides into a number of
smaller branches that enter the ovary. Its main stem, however, traverses the
entire length of the broad ligament very near the mesosalpinx and makes its way
to the upper lateral portion of the uterus. Here it anastomoses with the
ovarian branch of the uterine artery. There are numerous additional
communications among the arteries on both sides of the uterus.
When the
uterus is in a contracted state, its numerous venous lumens are collapsed,
however, in injected specimens the greater part of the uterine wall appears to
be occupied by dilated venous sinuses. On either side, the arcuate veins unite
to form the uterine vein, which empties into the internal iliac vein and
thence into the common iliac vein. Some of the blood from the upper uterus, the
ovary, and the upper part of the broad ligament is collected by several veins.
Within the broad ligament, these veins form the large pampiniform plexus
that terminates in the ovarian vein. The right ovarian vein empties into the
vena cava, whereas the left ovarian vein empties into the left renal vein.
During pregnancy, there is marked hypertrophy of the blood supply to the
uterus.
The
endometrium is abundantly supplied with true lymphatic vessels that are
confined largely to the basal layer. The lymphatics of the underlying
myometrium are increased in number toward the serosal surface and form an
abundant lymphatic plexus just beneath it. Lymphatics from the cervix terminate
mainly in the hypogastric nodes, which are situated near the bifurcation of the
common iliac vessels. The lymphatics from the body of the uterus are
distributed to two groups of nodes. One set of vessels drains into the internal
iliac nodes. The other set, after joining certain lymphatics from the ovarian
region, terminates in the periaortic lymph nodes.
The nerve
supply to the pelvic area is derived principally from the sympathetic nervous
system, but also partly from the cerebrospinal and parasympathetic systems. The
parasympathetic system is represented on either side by the pelvic nerve, which
is made up of a few fibers that are derived from the second, third, and fourth
sacral nerves. It loses its identity in the cervical ganglion of
Frankenhäuser. The sympathetic system enters the pelvis by way of the
internal iliac plexus that arises from the aortic plexus just below the
promontory of the sacrum. After descending on either side, it also enters the
uterovaginal plexus of Frankenhäuser, which is made up of ganglia of various
sizes, but particularly of a large ganglionic plate that is situated on either
side of the cervix and just above the posterior fornix in front of the rectum.
Branches
from these plexuses supply the uterus, bladder, and upper vagina. In the 11th
and 12th thoracic nerve roots, there are sensory fibers from the uterus that
transmit the painful stimuli of contractions to the central nervous system. The
sensory nerves from the cervix and upper part of the birth canal pass through
the pelvic nerves to the second, third, and fourth sacral nerves, whereas those
from the lower portion of the birth canal pass primarily through the pudendal
nerve. Knowledge of the innervation of dermatomes and its clinical application
to providing epidural or spinal analgesia for labor and vaginal or cesarean
delivery.
More
commonly called the fallopian tubes, the oviducts vary in length from 8
to 14 cm. They are covered by peritoneum, and their lumen is lined by mucous
membrane. Each tube is divided into an interstitial portion, isthmus,
ampulla, and infundibulum. The interstitial portion is embodied
within the muscular wall of the uterus. The isthmus, or the narrow portion of
the tube that adjoins the uterus, passes gradually into the wider, lateral
portion, or ampulla. The infundibulum, or fimbriated extremity,
is the funnel-shaped opening of the distal end of the fallopian tube. The
oviduct varies considerably in thickness; the narrowest portion of the isthmus
measures from 2 to 3 mm in diameter, and the widest portion of the ampulla
measures from 5 to 8 mm. The fimbriated end of the infundibulum opens into the
abdominal cavity. One projection, the fimbria ovarica, which is
considerably longer than the other fimbriae, forms a shallow gutter that
approaches or reaches the ovary.
The musculature of the
fallopian tube is arranged in an inner circular and an outer longitudinal
layer. In the distal portion, the two layers are less distinct and, near the
fimbriated extremity, are replaced by an interlacing network of muscular
fibers. The tubal musculature undergoes rhythmic contractions constantly, the
rate of which varies with the hormonal changes of the ovarian cycle. The
greatest frequency and intensity of contractions is reached during transport of
ova.
The oviducts
are lined by a single layer of columnar cells, some of them ciliated and others
secretory. The ciliated cells are most abundant at the fimbriated extremity,
elsewhere, they are found in discrete patches. There are differences in the
proportions of these two types of cells in different phases of the ovarian
cycle. Because there is no submucosa, the epithelium is in close contact with
the underlying muscle. In the tubal mucosa, there are cyclical histological
changes similar to those of the endometrium, but much less striking. The mucosa
is arranged in longitudinal folds that are more complex toward the fimbriated
end. On cross sections through the uterine portion, four simple folds are found
that form a figure that resembles a Maltese cross. The isthmus has a more
complex pattern. In the ampulla, the lumen is occupied almost completely by the
arborescent mucosa, which consists of very complicated folds. The current
produced by the tubal cilia is such that the direction of flow is toward the
uterine cavity. Tubal peristalsis is believed to be an extraordinarily
important factor in transport of the ovum.
The tubes
are supplied richly with elastic tissue, blood vessels, and lymphatics.
Sympathetic innervation of the tubes is extensive, in contrast to their
parasympathetic innervation.
Diverticula
may extend occasionally from the lumen of the tube for a variable distance into
the muscular wall and reach almost to the serosa.
The uterus
and tubes arise from the müllerian ducts, which first appear near the upper
pole of the urogenital ridge in the fifth week of embryonic development. This
ridge is composed of the mesonephros, gonad, and associated ducts. The first
indication of the development of the müllerian duct is a thickening of the
coelomic epithelium at about the level of the fourth thoracic segment. This
thickening becomes the fimbriated extremity of the fallopian tube, which
invaginates and grows caudally to form a slender tube at the lateral edge of
the urogenital ridge. In the sixth week of embryonic life, the growing tips of
the two müllerian ducts approach each other in the midline; they reach the
urogenital sinus 1 week later. At that time, a fusion of the two müllerian
ducts to form a single canal is begun at the level of the inguinal crest, that
is, the gubernaculum (primordium of the round ligament). Thus, the upper ends
of the müllerian ducts produce the oviducts and the fused parts give rise to
the uterus. The vaginal canal is not patent throughout its entire length until
the sixth month of fetal life (Koff, 1933). Because of the clinical importance
of anomalies that arise from abnormal fusion and dysgenesis of these
structures.
Ovaries
Compared
with each other, as well as between women, the ovaries vary considerably in
size. During childbearing years, they are from 2.5 to 5 cm in length, 1.5 to 3
cm in breadth, and 0.6 to 1.5 cm in thickness. After menopause, ovarian size
diminishes remarkably. The position of the ovaries also varies, but they
usually are situated in the upper part of the pelvic cavity and rest in a slight
depression on the lateral wall of the pelvis between the divergent external and
internal iliac vessels—the ovarian fossa of Waldeyer. The ovary is
attached to the broad ligament by the mesovarium. The utero-ovarian
ligament extends from the lateral and posterior portion of the uterus, just
beneath the tubal insertion, to the uterine pole of the ovary. Usually, it is
several centimeters long and 3 to 4 mm in diameter. It is covered by peritoneum
and is made up of muscle and connective tissue fibers. The infundibulopelvic
or suspensory ligament of the ovary extends from the upper or tubal pole
to the pelvic wall; through it course the ovarian vessels and nerves.
In young
women, the exterior surface of the ovary is smooth, with a dull white surface
through which glisten several small, clear follicles. As the woman ages, the
ovaries become more corrugated, and in elderly women, the exterior surfaces may
be convoluted markedly.
The ovary
consists of two portions, the cortex and medulla. The cortex is the outer
layer, which varies in thickness with age and becomes thinner with advancing
years. It is in this layer that the ova and graafian follicles are located. The
cortex is composed of spindle-shaped connective tissue cells and fibers, among
which are scattered primordial and graafian follicles in various stages of
development. As the woman ages, the follicles become less numerous. The
outermost portion of the cortex, which is dull and whitish, is designated the tunica
albuginea. On its surface, there is a single layer of cuboidal epithelium,
the germinal epithelium of Waldeyer.
The medulla
is the central portion, which is composed of loose connective tissue that is
continuous with that of the mesovarium. There are a large number of arteries
and veins in the medulla and a small number of smooth muscle fibers that are
continuous with those in the suspensory ligament.
The ovaries
are supplied with both sympathetic and parasympathetic nerves. The sympathetic
nerves are derived primarily from the ovarian plexus that accompanies the
ovarian vessels. Others are derived from the plexus that surrounds the ovarian
branch of the uterine artery. The ovary is richly supplied with nonmyelinated
nerve fibers, which for the most part accompany the blood vessels. These are
merely vascular nerves, whereas others form wreaths around normal and atretic
follicles, and these give off many minute branches that have been traced up to,
but not through, the membrana granulosa.
The earliest
sign of a gonad is one that appears on the ventral surface of the embryonic
kidney at a site between the eighth thoracic and fourth lumbar segments at
about 4 weeks. The coelomic epithelium is thickened, and clumps of cells are
seen to bud off into the underlying mesenchyme. This circumscribed area of the
coelomic epithelium is called the germinal epithelium. By the fourth to
sixth week, however, there are many large ameboid cells in this region that
have migrated into the body of the embryo from the yolk sac. These primordial
germ cells are distinguishable by their large size and certain
morphological and cytochemical features.
When the
primordial germ cells reach the genital area, some enter the germinal
epithelium and others mingle with the groups of cells that proliferate from it
or lie in the mesenchyme. By the end of the fifth week, rapid division of all
these types of cells results in development of a prominent genital ridge.
The ridge projects into the body cavity medially to a fold in which there are
the mesonephric (wolffian) and the müllerian ducts. By the seventh week, it is
separated from the mesonephros except at the narrow central zone, the future
hilum, where the blood vessels enter. At this time, the sexes can be
distinguished, because the testes can be recognized by well-defined radiating
strands of cells (sex cords). These cords are separated from the germinal
epithelium by mesenchyme that is to become the tunica albuginea. The sex cords,
which consist of large germ cells and smaller epithelioid cells derived from
the germinal epithelium, develop into the seminiferous tubules and tubuli rete.
The latter establishes connection with the mesonephric tubules that develop
into the epididymis. The mesonephric ducts become the vas deferens.
In the female embryo, the
germinal epithelium continues to proliferate for a much longer time. The groups
of cells thus formed lie at first in the region of the hilum. As connective
tissue develops between them, these appear as sex cords. These cords give rise
to the medullary cords and persist for variable times (Forbes, 1942). By the
third month, medulla and cortex are defined (see Fig. 2–15). The bulk of the
organ is made up of cortex, a mass of crowded germ and epithelioid cells that
show some signs of grouping, but there are no distinct cords as in the testis.
Strands of cells extend from the germinal epithelium into the cortical mass,
and mitoses are numerous. The rapid succession of mitoses soon reduces the size
of the germ cells to the extent that these no longer are differentiated clearly
from the neighboring cells. These germ cells are now called oogonia.
By the
fourth month, some germ cells in the medullary region begin to enlarge. These
are called primary oocytes at the beginning of the phase of growth that
continues until maturity is reached. During this period of cell growth, many
oocytes undergo degeneration, both before and after birth. A single layer of
flattened follicular cells that were derived originally from the germinal
epithelium soon surrounds the primary oocytes. These structures are now called primordial
follicles and are seen first in the medulla and later in the cortex. Some
follicles begin to grow even before birth, and some are believed to persist in
the cortex almost unchanged until menopause.
By 8 months,
the ovary has become a long, narrow, lobulated structure that is attached to
the body wall along the line of the hilum by the mesovarium, in which
lies the epoöphoron. The germinal epithelium has been separated for the
most part from the cortex by a band of connective tissue—tunica albuginea—which
is absent in many small areas where strands of cells, usually referred to as cords
of Pflüger, are in contact with the germinal epithelium. Among these cords
are cells believed by many investigators to be oogonia that have come to resemble
the other epithelial cells as a result of repeated mitoses. In the underlying
cortex, there are two distinct zones. Superficially, there are nests of germ
cells in synapsis, interspersed with Pflüger cords and strands of connective
tissue. In the deeper zone, there are many groups of germ cells in synapsis, as
well as primary oocytes, prospective follicular cells, and a few primordial
follicles.
From the
first stages of its development until after the menopause, the ovary undergoes
constant change. The number of oocytes at the onset of puberty has been
estimated variously at 200,000 to 400,000. Because only one ovum ordinarily is
cast off during each ovarian cycle, it is evident that a few hundred ova
suffice for purposes of reproduction.
The
glandular elements of ovaries of adult women include interstitial, thecal, and
luteal cells. The interstitial glandular elements are formed from cells of the
theca interna of degenerating or atretic follicles; the thecal glandular cells
are formed from the theca interna of ripening follicles; and the true luteal
cells are derived from the granulosa cells of ovulated follicles and from the
undifferentiated stroma that surround them.
The huge
store of primordial follicles at birth is exhausted gradually after sexual
maturation and through the reproductive span. Block (1952) found that there is
a gradual decline from a mean of 439,000 oocytes in girls younger than 15 years
to a mean of 34,000 in women older than 36 years. In young girls, the greater
portion of the ovary is composed of the cortex, which is filled with large
numbers of closely packed primordial follicles. In young women, the cortex is
relatively thinner but still contains a large number of primordial follicles.
Each primordial follicle is made up of an oocyte and its surrounding single
layer of epithelial cells, which are small and flattened, spindle-shaped, and
somewhat sharply differentiated from the still smaller and spindly cells of the
surrounding stroma.
The oocyte
is a large, spherical cell in which there is clear cytoplasm and a relatively
large nucleus located near the center of the ovum. In the nucleus, there are
one large and several smaller nucleoli, and numerous masses of chromatin.
There are a
number of vestigial wolffian structures that are identified after embryogenesis
of the female reproductive system. Some of these occasionally cause clinical
concerns. The parovarium can be found in the scant loose connective
tissue within the broad ligament in the vicinity of the mesosalpinx. It
comprises a number of narrow vertical tubules that are lined by ciliated
epithelium. These tubules connect at the upper ends with a longitudinal duct
that extends just below the oviduct to the lateral margin of the uterus, where
it ends blindly near the internal os. This canal is the remnant of the wolffian
(mesonephric) duct in women and is called the Gartner duct. The
parovarium, also a remnant of the wolffian duct, is homologous embryologically
with the caput epididymis in men. The cranial portion of the parovarium is the epoöphoron,
or organ of Rosenmüller; the caudal portion, or paroöphoron, is a
group of vestigial mesonephric tubules that lie in or around the broad
ligament. It is homologous embryologically with the paradidymis of men. The
paroöphoron in adult women usually disappears.
The false
pelvis lies above the linea terminalis and the true pelvis below this anatomical
boundary.The false pelvis is bounded posteriorly by the lumbar vertebra and
laterally by the iliac fossa. In front, the boundary is formed by the lower
portion of the anterior abdominal wall.
The true
pelvis is the portion important in childbearing. It is bounded above by the
promontory and alae of the sacrum, the linea terminalis, and the upper margins
of the pubic bones, and below by the pelvic outlet. The cavity of the true
pelvis can be described as an obliquely truncated, bent cylinder with its greatest
height posteriorly. Its anterior wall at the symphysis pubis measures about 5
cm, and its posterior wall, about 10 cm.
The walls of the true pelvis
are partly bony and partly ligamentous. The posterior boundary is the anterior
surface of the sacrum, and the lateral limits are formed by the inner surface
of the ischial bones and the sacrosciatic notches and ligaments. In front, the
true pelvis is bounded by the pubic bones, the ascending superior rami of the
ischial bones, and the obturator foramen.
The sidewalls
of the true pelvis of an adult woman converge somewhat. Extending from the
middle of the posterior margin of each ischium are the ischial spines. These
are of great obstetrical importance because the distance between them usually
represents the shortest diameter of the pelvic cavity. They also serve as
valuable landmarks in assessing the level to which the presenting part of the
fetus has descended into the true pelvis. The sacrum forms the posterior wall
of the pelvic cavity. Its upper anterior margin corresponds to the promontory
that may be felt during bimanual pelvic examination in women with a small
pelvis. It can provide a landmark for clinical pelvimetry. Normally the sacrum
has a marked vertical and a less pronounced horizontal concavity, which in
abnormal pelves may undergo important variations. A straight line drawn from
the promontory to the tip of the sacrum usually measures 10 cm, whereas the
distance along the concavity averages 12 cm.
The
descending inferior rami of the pubic bones unite at an angle of 90 to 100
degrees to form a rounded arch under which the fetal head must pass.
Anteriorly,
the pelvic bones are joined together by the symphysis pubis. This structure
consists of fibrocartilage and the superior and inferior pubic ligaments; the
latter are frequently designated the arcuate ligament of the pubis.
Sacroiliac Joints
Posteriorly,
the pelvic bones are joined by the articulations between the sacrum and the
iliac portion of the innominate bones to form the sacroiliac joints. These
joints also have a certain degree of mobility.
During
pregnancy, relaxation of these joints likely results from hormonal changes.
Abramson and co-workers (1934) observed that relaxation of the symphysis pubis
commenced in women in the first half of pregnancy and increased during the last
3 months. They also observed that this laxity began to regress immediately
after parturition and that regression was completed within 3 to 5 months. The
symphysis pubis also increases in width during pregnancy—more so in multiparas
than in primigravidas—and returns to normal soon after delivery.
There are
important changes in sacroiliac joint mobility. Borell and Fernstrom (1957)
demonstrated that the rather marked mobility of the pelvis at term was caused
by an upward gliding movement of the sacroiliac joint. The displacement, which
is greatest in the dorsal lithotomy position, may increase the diameter of the
outlet by 1.5 to 2.0 cm. This is the main justification for placing a woman
in this position for a vaginal delivery. The increase in the diameter of
the pelvic outlet, however, occurs only if the sacrum is allowed to rotate
posteriorly, that is, only if the sacrum is not forced anteriorly by the weight
of the maternal pelvis against the delivery table or bed (Russell, 1969, 1982).
Sacroiliac joint mobility is also the likely reason that the McRoberts maneuver
often is successful in releasing an obstructed shoulder in a case of shoulder
dystocia (see Chap. 20, Shoulder Dystocia). These changes have also been
attributed to the success of the modified squatting position to hasten
second-stage labor (Gardosi and co-workers, 1989). The squatting position may
increase the interspinous diameter and the diameter of the pelvic outlet
(Russell, 1969, 1982). These latter observations are unconfirmed, but this
position is assumed for birth in many primitive societies.
The pelvic
inlet (superior strait) is bounded posteriorly by the promontory and alae of
the sacrum, laterally by the linea terminalis, and anteriorly by the horizontal
pubic rami and the symphysis pubis. The inlet of the female pelvis typically is
more nearly round than ovoid. Caldwell and co-workers (1934) identified
radiographically a nearly round or gynecoid pelvic inlet in
approximately 50 percent of white women.
Four diameters of the pelvic
inlet are usually described: anteroposterior, transverse, and two obliques. The
obstetrically important anteroposterior diameter is the shortest distance
between the promontory of the sacrum and the symphysis pubis, and is designated
the obstetrical conjugate. Normally, this measures 10 cm or more.
The
transverse diameter is constructed at right angles to the obstetrical conjugate
and represents the greatest distance between the linea terminalis on either
side. It usually intersects the obstetrical conjugate at a point about 4 cm in
front of the promontory. The segment of the obstetrical conjugate from the
intersection of these two lines to the promontory is designated the posterior
sagittal diameter of the inlet.
Each of the
two oblique diameters extends from one of the sacroiliac synchondroses to the
iliopectineal eminence on the opposite side. They average less than 13 cm.
The
anteroposterior diameter of the pelvic inlet that has been identified as the true
conjugate does not represent the shortest distance between the promontory
of the sacrum and the symphysis pubis. The shortest distance is the obstetrical
conjugate, which is the shortest anteroposterior diameter through which the
head must pass in descending through the pelvic inlet. Obstetrical conjugate
cannot be measured directly with the examining fingers. For clinical purposes,
the obstetrical conjugate is estimated indirectly by subtracting 1.5 to 2 cm
from the diagonal conjugate. The latter is determined by measuring the distance
from the lower margin of the symphysis to the promontory of the sacrum.
The
midpelvis is measured at the level of the ischial spines—the midplane, or plane
of least pelvic dimensions. It is of particular importance following engagement
of the fetal head in obstructed labor. The interspinous diameter, 10 cm or
somewhat more, is usually the smallest diameter of the pelvis. The
anteroposterior diameter through the level of the ischial spines normally
measures at least 11.5 cm. Its posterior component (posterior sagittal
diameter), between the sacrum and the line created by the interspinous
diameter, is usually at least 4.5 cm.
The pelvic
outlet consists of two approximately triangular areas that are not in the same
plane. They have a common base, which is a line drawn between the two ischial
tuberosities . The apex of the posterior triangle is at the tip of the sacrum,
and the lateral boundaries are the sacrosciatic ligaments and the ischial
tuberosities. The anterior triangle is formed by the area under the pubic arch.
Three diameters of the pelvic outlet usually are described: the anteroposterior,
transverse, and posterior sagittal.
Pelvic Shapes
In the past,
x-ray pelvimetry was used frequently in women with suspected cephalopelvic
disproportion or fetal malpresentation. Caldwell and Moloy (1933, 1934)
developed a classification of the pelvis that is still used. The classification
is based on the shape of the pelvis, and its familiarity helps the clinician
understand better the mechanisms of labor.
The Caldwell–Moloy
classification is based on measurement of the greatest transverse diameter
of the inlet and its division into anterior and posterior segments. The shapes
of these are used to classify the pelvis as gynecoid, anthropoid, android, or
platypelloid. The character of the posterior segment determines the type of
pelvis, and the character of the anterior segment determines the tendency.
These are both determined because many pelves are not pure but are mixed types;
for example, a gynecoid pelvis with an android tendency means that the
posterior pelvis is gynecoid and the anterior pelvis is android in shape.
From viewing the four basic
types, the configuration of the gynecoid pelvis would intuitively seem suited
for delivery of most fetuses. Indeed, Caldwell and co-workers (1939) reported
that the gynecoid pelvis was found in almost 50 percent of women. In contrast,
in the android pelvis, the posterior sagittal diameter at the inlet is much
shorter than the anterior sagittal diameter, limiting the use of the posterior
space by the fetal head. Moreover, the anterior portion is narrow and triangular.
The extreme android pelvis presages a poor prognosis for vaginal delivery. In
the anthropoid pelvis, the anteroposterior diameter of the inlet is greater
than the transverse. This results in an oval anteroposteriorly, with the
anterior segment somewhat narrow and pointed. Variations of anthropoid-type
pelves are found in about one third of women. The platypelloid pelvis
has a flattened gynecoid shape with short anteroposterior and wide transverse
diameters. Pure varieties are found in fewer than 3 percent of women.
In many
abnormal pelves, the anteroposterior diameter of the pelvic inlet—the
obstetrical conjugate—is considerably shortened. The diagonal conjugate is
clinically estimated by measuring the distance from the sacral promontory to
the lower margin of the symphysis pubis. Two fingers of the dominant hand are
introduced into the vagina. The mobility of the coccyx is first evaluated. The
anterior surface of the sacrum is next palpated from below upward and its
vertical and lateral curvatures noted. In normal pelves, only the last three
sacral vertebrae can be felt without indenting the perineum. Conversely, in
markedly contracted pelves, the entire anterior surface of the sacrum usually
is readily palpable. Next, in order to reach the sacral promontory, the
examiner's elbow must be flexed and the perineum forcibly indented by the
knuckles of the third and fourth fingers. The index and the second fingers are
carried up and over the anterior surface of the sacrum. By deeply inserting the
wrist, the promontory may be felt by the tip of the second finger as a
projecting bony margin. With the finger closely applied to the most prominent
portion of the upper sacrum, the vaginal hand is elevated until it contacts the
pubic arch. The immediately adjacent point on the index finger is marked. The
distance between the mark and the tip of the second finger is the diagonal
conjugate. The obstetrical conjugate is computed by subtracting 1.5 to 2.0 cm,
depending on the height and inclination of the symphysis pubis. If the diagonal
conjugate is greater than 11.5 cm, it is justifiable to assume that the pelvic
inlet is of adequate size for vaginal delivery of a normal-sized fetus.
Engagement
Descent of
the biparietal plane of the fetal head to a level below that of the pelvic
inlet is termed engagement. When the biparietal—the largest—diameter of
the normally flexed fetal head has passed through the inlet, the head is
engaged. Although engagement usually is regarded as a phenomenon of labor, in
nulliparas it may occur during the last few weeks of pregnancy. When it does
so, it is confirmatory evidence that the pelvic inlet is adequate for that
fetal head. With engagement, the fetal head serves as an internal pelvimeter
to demonstrate that the pelvic inlet is ample for that fetus.
Engagement is ascertained by
vaginal examination or by abdominal palpation. With vaginal examination, the
station of the lowermost part of the fetal head in relation to the level of the
ischial spines is determined. If the lowest part of the occiput is at or below
the level of the spines, the head usually, but not always, is engaged. The
distance from the plane of the pelvic inlet to the level of the ischial spines
is approximately 5 cm in most pelves. Although the distance from the biparietal
plane of the unmolded fetal head to the vertex is usually only 3 to 4 cm,
accurate determination of engagement may be difficult if there is considerable
elongation of the fetal head from molding or formation of a caput
succedaneum.
Abdominal
examination is a less satisfactory method to determine engagement. If the
biparietal plane of a term-sized infant has descended through the inlet, the
examining fingers cannot reach the lowermost part of the head. Thus, when
pushed downward over the lower abdomen, the examining fingers will slide over
that portion of the head proximal to the biparietal plane (nape of the neck)
and diverge. Conversely, if the head is not engaged, the examining fingers can
easily palpate the lower part of the head and will converge.
Fixation
of the fetal head occurs when descent proceeds to a depth that prevents its
free movement when pushed right and then left by both hands placed over the
lower abdomen. Fixation is not necessarily synonymous with engagement. Although
a head that is freely movable on abdominal examination cannot be engaged,
fixation of the head is sometimes seen when the biparietal plane is still 1 cm
or more above the pelvic inlet, especially if the head is molded appreciably.
An important
dimension of the pelvic outlet that is accessible for clinical measurement is
the diameter between the ischial tuberosities, variously called the biischial
diameter, intertuberous diameter, and transverse diameter of the outlet.
A measurement of more than 8 cm is considered normal. The measurement of the
transverse diameter of the outlet can be estimated by placing a closed fist
against the perineum between the ischial tuberosities. Usually the closed fist
is wider than 8 cm. The shape of the subpubic arch also can be evaluated at the
same time by palpating the pubic rami from the subpubic region toward the
ischial tuberosities.
Clinical
estimation of midpelvic capacity by any direct form of measurement is not
possible. If the ischial spines are quite prominent, the sidewalls are felt to
converge, and the concavity of the sacrum is very shallow, then suspicion of a
contraction is aroused.
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