The Ovary
Form and Position
The ovary lies on the lateral wall of
the true pelvis. It is held in place by the peritoneum of the
broad ligament, by its attachment to
the body of the uterus, the ovarian ligament, and to some
extent by connective tissue which runs to the lateral wall of the pelvis, the suspensory ligament of
the ovary. The peritoneum of the broad ligament stops at the mesovarium and does not cover the
ovary itself. The ovary is covered by a layer of germinal epithelium (which does not produce
ova), below which lies a tough fibrous coat, the tunica albuginea. In young women the ovary is
about 4cm long and 2cm in diameter. In post-menopausal women the ovary becomes flattened
and shrunk.
Blood Supply
The blood supply to the ovary
arises as for the testes from the abdominal aorta at the level of the
renal arteries. The ovarian artery passes laterally, anterior to the ureter, and inferiorly into the
pelvis. The ovarian artery is an extremely small vessel but supplies the ovary and anastomoses
along the fallopian tube with the uterine artery. The venous drainage of the ovary however often
involves more than one vein, draining more blood than the artery supplies. On the right side the
ovarian vein drains into the inferior vena cava, and on the left side into the left renal vein. The
veins follow the route taken by the arteries.
Lymphatic drainage
The lymphatic drainage of the ovary reflects its origin. The lymphatics drain upwards along the
ovarian vessels to aortic lymph nodes.
Innervation
The innervation of the ovary is entirely by the autonomic nervous system through the aortic
plexus related to the T10 spinal cord segment. The blood supply, nerve supply and lymphatics all
reach the ovary along the suspensory ligament.
Histology
General Organization
The ovary can be divided into an
outer cortex and inner medulla. The cortex is formed of
connective tissue in which are embedded the follicles. The medulla consists of loose connective
tissue and extensive blood vessels. The ovary is covered by a layer of squamous or cuboidal
epithelium which sits on a dense fibrous layer, the tunica albuginea. During development the
gamete forming cells migrate from the wall of the yolk sac to the presumptive ovary. These germ
cells divide to form all of the ova before birth. At birth the ova are contained within primordial
follicles. The egg is surrounded by a layer of squamous epithelial cells. The egg is suspended in
prophase of the first meiotic division. Of the two million ova generated before birth, most
degenerate before puberty leaving about 40,000. Only about 400 will pass through the complete
process of maturation to ovulation. Each month between puberty and menopause an ovum
completes development and is released at ovulation.
Follicular Development
Development from the primordial follicle stage occurs under hormonal
control. Many primary follicles
begin the process of development but most degenerate and are termed atretic. As development
proceeds, the cuboidal follicular cells divide and the follicle becomes
multilayered. A basement
membrane separates the growing follicular cells from the connective tissue of the cortex. As the
follicle grows the connective tissue outside the basement membrane becomes organized around
the follicle, forming an inner vascular theca interna and an outer theca externa. The cells of the
theca interna elaborate and release estrogen. Between the ovum and the inner layer of follicular
cells a glycoprotein layer develops, the zona pellucida. The
primary follicle becomes a secondary
follicle when the follicular cells secrete a fluid which collects
in a space around the ovum, the
follicular antrum. As the follicle grows the antrum becomes larger. The ovum becomes attached
to one wall of the chamber by a stalk of cells, the cumulus oophorus. The ovum is
still encircled
by the zona pellucida and several layers of follicular cells. It takes about fourteen days for the
follicle to mature fully. By maturity the follicle occupies the full thickness of the ovarian cortex
and bulges out from the surface. Just before ovulation the ovum completes the first meiotic
division with the production of the first polar body. At
ovulation the mature or Graafian
follicle
ruptures and the ovum is released, surrounded by several layers of follicular cells, the corona
radiata. At ovulation the division of the ovum is again arrested, this time at metaphase. Division
proceeds only if the ovum is fertilized. All of these stages may be present in the ovary of an
ovulating woman. While it is usual that only one Graafian follicle is produced each month it is
possible for several ova to mature and be released at ovulation simultaneously. Following
ovulation the granulosa cells of the follicle are transformed into the secretory cells of the corpus
luteum. The corpus luteum
produces progesterone. If the ovum is not fertilized the corpus
luteum degenerates after about fourteen days. The degenerating corpus luteum eventually sinks
into the stroma of the ovary as a white scar or corpus albicans. If the ovum is
fertilized the
corpus luteum is maintained and becomes a corpus luteum of pregnancy. The scar which results
from the eventual disintegration of a corpus luteum of pregnancy is larger and persists longer than
the scar resulting from a corpus luteum of menstruation.
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