Structure

The ovaries are covered on the outside by a layer of simple cuboidal epithelium called germinal (ovarian) epithelium. This is actually the visceral peritoneum that envelops the ovaries. Underneath this layer is a dense connective tissue capsule, the tunica albuginea. The substance of the ovaries is distinctly divided into an outer cortex and an inner medulla. The cortex appears more dense and granular due to the presence of numerous ovarian follicles in various stages of development. Each of the follicles contains an oocyte, a female germ cell. The medulla is a loose connective tissue with abundant blood vessels, lymphatic vessels, and nerve fibers.

Oogenesis

Female sex cells, or gametes, develop in the ovaries by a form of meiosis called oogenesis. The sequence of events in oogenesis is similar to the sequence in spermatogenesis, but the timing and final result are different. Early in fetal development, primitive germ cells in the ovaries differentiate into oogonia. These divide rapidly to form thousands of cells, still called oogonia, which have a full complement of 46 (23 pairs) chromosomes. Oogonia then enter a growth phase, enlarge, and become primary oocytes. The diploid (46 chromosomes) primary oocytes replicate their DNA and begin the first meiotic division, but the process stops in prophase and the cells remain in this suspended state until puberty.

Many of the primary oocytes degenerate before birth, but even with this decline, the two ovaries together contain approximately 700,000 oocytes at birth. This is the lifetime supply, and no more will develop. This is quite different than the male in which spermatogonia and primary spermatocytes continue to be produced throughout the reproductive lifetime. By puberty the number of primary oocytes has further declined to about 400,000.

Beginning at puberty, under the influence of follicle-stimulating hormone, several primary oocytes start to grow again each month. One of the primary oocytes seems to outgrow the others and it resumes meiosis I. The other cells degenerate. The large cell undergoes an unequal division so that nearly all the cytoplasm, organelles, and half the chromosomes go to one cell, which becomes a secondary oocyte. The remaining half of the chromosomes go to a smaller cell called the first polar body. The secondary oocyte begins the second meiotic division, but the process stops in metaphase. At this point ovulation occurs. If fertilization occurs, meiosis II continues. Again this is an unequal division with all of the cytoplasm going to the ovum, which has 23 single-stranded chromosome. The smaller cell from this division is a second polar body.

The first polar body also usually divides in meiosis I to produce two even smaller polar bodies. If fertilization does not occur, the second meiotic division is never completed and the secondary oocyte degenerates. Here again there are obvious differences between the male and female. In spermatogenesis, four functional sperm develop from each primary spermatocyte. In oogenesis, only one functional fertilizable cell develops from a primary oocyte. The other three cells are polar bodies and they degenerate.

Ovarian Follicle Development

An ovarian follicle consists of a developing oocyte surrounded by one or more layers of cells called follicular cells. At the same time that the oocyte is progressing through meiosis, corresponding changes are taking place in the follicular cells. Primordial follicles, which consist of a primary oocyte surrounded by a single layer of flattened cells, develop in the fetus and are the stage that is present in the ovaries at birth and throughout childhood.

Beginning at puberty, follicle-stimulating hormone stimulates changes in the primordial follicles. The follicular cells become cuboidal, the primary oocyte enlarges, and it is now a primary follicle. The follicles continue to grow under the influence of follicle-stimulating hormone, and the follicular cells proliferate to form several layers of granulose cells around the primary oocyte. Most of these primary follicles degenerate along with the primary oocytes within them, but usually one continues to develop each month. The granulosa cells start secreting estrogen and a cavity, or antrum, forms within the follicle. When the antrum starts to develop, the follicle becomes a secondary follicle. The granulose cells also secrete a glycoprotein substance that forms a clear membrane, the zona pellucida, around the oocyte. After about 10 days of growth the follicle is a mature vesicular (graafian) follicle, which forms a “blister” on the surface of the ovary and contains a secondary oocyte ready for ovulation.

Ovulation

Ovulation, prompted by luteinizing hormone from the anterior pituitary, occurs when the mature follicle at the surface of the ovary ruptures and releases the secondary oocyte into the peritoneal cavity. The ovulated secondary oocyte, ready for fertilization is still surrounded by the zona pellucida and a few layers of cells called the corona radiata. If it is not fertilized, the secondary oocyte degenerates in a couple of days. If a sperm passes through the corona radiata and zona pellucida and enters the cytoplasm of the secondary oocyte, the second meiotic division resumes to form a polar body and a mature ovum

After ovulation and in response to luteinizing hormone, the portion of the follicle that remains in the ovary enlarges and is transformed into a corpus luteum. The corpus luteum is a glandular structure that secretes progesterone and some estrogen. Its fate depends on whether fertilization occurs. If fertilization does not take place, the corpus luteum remains functional for about 10 days; then it begins to degenerate into a corpus albicans, which is primarily scar tissue, and its hormone output ceases. If fertilization occurs, the corpus luteum persists and continues its hormone functions until the placenta develops sufficiently to secrete the necessary hormones. Again, the corpus luteum ultimately degenerates into corpus albicans, but it remains functional for a longer period of time.

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Anatomical Structure

The uterus is a thick-walled muscular organ capable of expansion to accommodate a growing fetus. It is connected distally to the vagina, and laterally to the uterine tubes.

The uterus has three parts;

• Fundus – top of the uterus, above the entry point of the uterine tubes.
• Body – usual site for implantation of the blastocyst.
• Cervix – lower part of uterus linking it with the vagina. This part is structurally and functionally different to the rest of the uterus.

The Cervix

The cervix is the lower portion of the uterus, an organ of the female reproductive tract. It connects the vagina with the main body of the uterus, acting as a gateway between them.

The cervix is composed of two regions; the ectocervix and the endocervical canal. The ectocervix is the portion of the cervix that projects into the vagina. It is lined by stratified squamous non-keratinized epithelium. The opening in the ectocervix, the external os, marks the transition from the ectocervix to the endocervical canal.

The endocervical canal (or endocervix) is the more proximal, and ‘inner’ part of the cervix. It is lined by a mucus-secreting simple columnar epithelium. The endocervical canal ends, and the uterine cavity begins, at a narrowing called the internal os.

Functions of the cervix

The cervix performs two main functions:

• It facilitates the passage of sperm into the uterine cavity. This is achieved via dilation of the external and internal os.
• Maintains sterility of the upper female reproductive tract. The cervix, and all structures superior to it, are sterile. This ultimately protects the uterine cavity and the upper genital tract by preventing bacterial invasion. This environment is maintained by the frequent shedding of the endometrium, thick cervical mucus and a narrow external os.

Histological Structure

The fundus and body of the uterus are composed of three tissue layers;

• Peritoneum – a double layered membrane, continuous with the abdominal peritoneum. Also known as the perimetrium.
• Myometrium – thick smooth muscle layer. Cells of this layer undergo hypertrophy and hyperplasia during pregnancy in preparation to expel the fetus at birth.
• Endometrium – inner mucous membrane lining the uterus. It can be further subdivided into 2 parts:
  • Deep stratum basalis: Changes little throughout the menstrual cycle and is not shed at menstruation.
  • Superficial stratum functionalis: Proliferates in response to oestrogens, and becomes secretory in response to progesterone. It is shed during menstruation and regenerates from cells in the stratum basalis layer.

Ligaments

The tone of the pelvic floor provides the primary support for the uterus. Some ligaments provide further support, securing the uterus in place.

They are:

• Broad Ligament: This is a double layer of peritoneum attaching the sides of the uterus to the pelvis. It acts as a mesentery for the uterus and contributes to maintaining it in position.
• Round Ligament: A remnant of the gubernaculum extending from the uterine horns to the labia majora via the inguinal canal. It functions to maintain the anteverted position of the uterus.
• Ovarian Ligament: Joins the ovaries to the uterus.
• Cardinal Ligament: Located at the base of the broad ligament, the cardinal ligament extends from the cervix to the lateral pelvic walls. It contains the uterine artery and vein in addition to providing support to the uterus.
• Uterosacral Ligament: Extends from the cervix to the sacrum. It provides support to the uterus.

Vascular Supply and Lymphatics

The blood supply to the uterus is via the uterine artery. Venous drainage is via a plexus in the broad ligament that drains into the uterine veins.

Lymphatic drainage of the uterus is via the iliac, sacral, aortic and inguinal lymph nodes.

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