Female Reproductive System
Consist of two ovaries, two oviducts, uterus, cervix, vagina and the external genetalia.The structure of the human female reproductive system changes considerably from childhood into reproductive maturity and then menopause and it is under direct control of the trophic hormones.
Inaddition to the changes that occur by age the female reproductive system undergo structural and functional modifications at different stages in the monthly menstrual cycle.
Ovaries:
Are almond shaped bodies approximately 3 cm. long, 1.5 cm. width and one cm. thick. They have two major functions:
they are the source of mature ova.
they are endocrine glands producing steroid hormones that prepare the endometrium for conception.
The surface of the ovary is covered by a single layer of cuboidal or low columnar epithelium. These cells commonly flatten with increasing age and when the ovary is enlarged. This epithelium is called Germinal Epithelium, however it is a misleading name because those cells are not the source of the mature gametes. The germinal epithelial cells have prominent microvilli, and a substantial layer of dense connective tissue called Tunica Albugenia separate the surface cells from the underlying ovarian tissues in the mature ovary and it is responsible for the whitish colour of the ovary.
The ovary can be divided into three components:
Ovarian hilum: where blood vessels, lymphatics and nerves enter and leave the ovary and it is continuous with the ovarian medulla.
Ovarian medulla: often contain clusters of stromal cells identical to those occupying the the bulk of the cortex and it contain rich vascular bed embedded in loose connective tissue.there are no sharp limits between cortex and medulla.
Ovarian cortex:has two components:
Supporting stroma
Ovarian follicles containing the oocytes.
The proportion of these two components in the ovary depends on both age and hormonal stimulation, however during the reproductive age the ovarian follicles will occupy the major part of the cortex.
Cortical stroma:
Composed of closely packed spindle shape fibroblast like cells, these cells are arranged haphazardly in the ovary or in a variable whorled pattern. The cytoplasm of these cells is rich in ribosomes & mitochondria with many pinocytotic vesicles and small number of lipid droplets.
Reticulin and collagen fibers are present between the stromal cells & with increasing age, increase in the amount of collagen fibers occur and become more marked with time.
The number of stromal cells together with the amount of lipid droplets in their cytoplasm is both dependant on hormonal stimulation. The increase in lipid accumulation in the stromal cells is called leutinization of the stromal cells and is particularly prominent during pregnancy.
The ovarian stroma has three main functions:
Providing the structural support for the developing ova
Giving rise to theca interna and theca externa cells around the developing follicle.
Secreting steroid hormones. All the stromal cells have the ability to produce steroid hormones at the appropriate circumstances.
Three types of steroid secreting stromal cells are recognized:
Cells that surround the follicles (theca interna and theca externa cells)
Scattered lipid rich leutinized stromal cells.
A population of stromal cells called Enzymatically Active Stromal Cells (EASC) which have marked oxidative and enzymatic activities, thses cells are particularly prominent in postmenopausal women and they appear to secrete testosterone and other androgenic steroids.
Early development of the ovary:
Embryologically: the primordial germ cells (oogonia) appear around the 1st month of embryonic life in the wall of the yolk sac, from here they will migrate to the ovary, during their migration and after arrival they undergo number of mitotic divisions to increase their number & so by the 7th month their number will reach up to 7 million oogonia.
From the 7th month on word these oogonia increase on size & differentiate into primary oocytes which will immediately enter the prophase of the 1st meiotic division and each one of them is surrounded by a single layer of flat cells calledc follicular cells.
By the 7th month the majority of the oogonia have been transformed into primary oocytes however most of these oocytes are lost during a process of degeneration called atrasia. So at birth each ovary contain only about a quarter a million of primay oocytes.
These primary oocytes together with its surrounding follicular cells are called primordial follicles & the ovarian cortex at birth is filled with a large number of primordial follicles embedded in the cortical stroma.
Ovarian follicles:
Each ovarian follicle consists of an oocyte surrounded by one or more layers of follicular or granulose cells.
Primordial follicles: consist of a primary oocyte surrounded by a single layer of flat follicular cells. These primordial follicles are found in the superficial layers of the ovarian cortex. A basal lamina lamina underlies the follicular cells and marks the boundaries between the follicle and the surrounding stroma.
The oocyte in the primordial follicle is:
Spherical cell
About 25 m. in diameter.
Are in the prophase of their 1st meiotic division
Their chromosomes are uncoiled and dont stain intensely.
At puberty a small group of primordial follicles begins a daily process called follicular growth which involves:
modification of the oocytes.
modification of the granulosa cells.
modification of the stroma cells surrounding the follicle
Follicular growth:
The cyclical secretion of FSH from the pituitary gland will stimulate further development of small number (30-40) primordial follicles each month. During this process the oocyte will grow and reach its maximum diameter which is about 120m. in diameter, the nucleus will enlarge, mitochondria increase in number & endoplasmic reticulum will hypertrophies.The primordial follicle will enlarge in size & grow until finally reach the stage of mature follicle. During their growth the follicles will pass through many stages:
Primary follicle:The flat follicular cells surrounding the primordial follicle divide by mitosis and increase in number to form a single layer of cuboidal or columnar follicular cells.
Secondary follicle (Preantral follicle):Continued FSH secretion will induce more proliferation of the follicle so the single layer of follicular cells will proliferate and form a stratified follicular epithelium or called granulosa cells.During this stage both the granulosa cells and the oocyte will share in the secretion of a glycoprotein material eosinophilic in nature formed between the oocytes and the granulosa cells called Zona Pellucida.This membrane is traversed by microvilli protruding out word from the oocyte and also by thin cytoplasmic processes from the inner layer of granulosa cells.Meanwhile the ovarian stromal cells come to lie in a concentric layers around the enlasrging follicle to form a capsule like arrangement and they are called Theca cell layer. When reaching this stage most follicles start to degenerate by a process called atrasia and just a small number will continue to develop.
Tertiary follicle (Vesicular follicle):As the follicle grow (mainly due to increase in number of granulosa cells) it will move to a deeper area of the cortical stroma. A fluid called Liquor folliculi begins to accumulate between the granulosa cells. This fluid contain components of plasma and a substances that are secreted from the granulosa cells with a high concentration of steroid (estrogen, progesterone & androgen).
At this stage the outer capsule of theca cells begins to differentiate into 2 layers:
Inner layer of stromal cells (theca interna):
are larger in size with prominent smooth endoplasmic reticulum and mitochondria with numerous lipid droplets (features characteristic of cells producing steroid) these cells start to secret a steroid hormone called androsteindion that is transported to the granulosa cell layer and under the effect of FSH and aromatase enzyme it will be converted to estrogen. Estrogen then return to the stroma surrounding the follicle to enter its blood vessels and then distributed throughout the body.
Outer layer of stromal cells (theca externa): remain small and compact and has no known secretory function.
The boundaries between the two thecas are not clear and also the boundaries between the theca externa and the rest of the ovarian cortex is not clear. On the other hand the boundaries between the theca interna and the granulosa cells are well defined since their cells are morphologically different and there is a thick basement membrane between them.
Mature Follicle (Preovulatory, Graffian follicle):
The fluid filled spaces between the granulosa cells will coalase with each others forming single large cavity called Antrum, and as a result of fluid accumulation the follicular cavity increase in size, the granulosa cell layer will become thinner, the oocyte will be pushed to one side of the follicle and adhere to its wall. The oocyte will be surrounded by a coat of granulosa cells called Cumulous oophorous and a group of these cells that immediately surround the oocyte called corona radiata. These cells (cumulous oophorus and corona radiata cells) will accompany the oocyte when it leaves the ovary.
During each menstrual cycle only one follicle will grow more than the others to become a dominant graffian follicle, this follicle is about 2.5 cm. in diameter and can be detected by ultrasound.
The whole process from the primordial to mature follicle lasts about 90 days. Only one follicle reach maturity the others undergo atrasia.
Female Reproductive System Dr. Muna Zuhair
Lecture: 1