Read this article to get information about the Compact and Saccular types of Ovaries!
Oogenesis, formation of female gametes i.e. ova or eggs, occurs in the female gonads, the ovaries.
Except birds all vertebrates have paired ovaries (bird have single ovary to reduce body weight—a flight adaptation).
The shape and size of ovaries are most variable in different vertebrate species. In teleost and ganoid fishes they are elongated structures extending over a considerable area of the body.
In mammals, they are flattened and ovoid structures in the resting condition, but during the reproductive phase or breeding phase, they may become rounded in appearance containing mound like protrusions. In amphibians, they are composed of a series of lobes, each of which is a mass of eggs during the breeding season. In reptiles and birds, they have the general form of bunch of grapes.
In fishes the size of the ovaries is enormous compared to the body of the female, which in mammals they are small structures in comparison to the adult body. In fishes during breeding season, each ovary may contain millions of mature eggs, whereas, in some mammals only a single egg commonly matures at a time as in man.
Morphologically, the ovaries of vertebrates may be divided into the following two types:
Compact type ovaries:
This type of ovary is found in cyclostomes, elasmobranchs, teleosts, turtles, crocodiles, birds and mammals. The compact mammalian ovary has the following regions:
1. Medulla:
An inner part containing relatively large blood and lymph vessels and growing follicles.
2. Cortex:
An area outside of and surrounding the medulla, containing early or young and ripe follicles i.e. ova in various stages of development.
3. Tunica albuginea:
Connective tissue layer surrounding the cortex.
4. Germinal epithelium:
Covering epithelium of the ovary providing continuously the primordial germ cells for oogenesis.
The germinal cells divide quickly forming a mass of cells, the egg nest. One cell outgrows the other to become oogonium. Cells surrounding it become the follicle cells that will nourish the oogonium. The mass of the cells thus converted into a primary or young follicle moves deeper into the substance of the ovary.
As the primary oocyte further increases in size, the follicle cells around the oocyte become more numerous and constitute a sort of capsule or theca. These follicles are believed to be important intermediates in the transfer of materials from blood and the surrounding tissues to the growing oocyte.
The outer layer of the theca is of somewhat spindle shaped cells called theca externa, and an inner layer of more rounded cells called theca interna. The oocyte by this time has a thin vitelline membrane surrounded by a thick and non-cellular membrane, the zona-pellucida. Follicular cells lying immediately outside the zona pellucida are large and columnar, and constitute the corona radiata.
Shortly before the growing follicle is due to ‘ripen’, it begins to develop a cavity by rearrangement of the follicle cells. This cavity, the antrum, becomes filled with a semi viscous fluid called the liquor follicule. It is in this stage that it is most frequently designated by the old term Graffian follicle.
The antrum occupies the eccentric position and the follicular cells in several layers around the antrum become granular forming the stratum granulosum. Distinguished from the theca interna large by a more compact arrangement of cells with large nuclei.
At this point where the ovum lies among the follicle cells, they from a hillock projection from the stratum granulosum into the antrum. This hillock is known as the cumulus oopharus or discus proligerous. When first formed, it is broad and low. As the follicle approaches maturity, it becomes more elevated and somewhat undercut.
Finally, the ovum is carried on a slender stalk of cells which readily releases it and allows it to escape in the follicular fluid set free when the follicle ruptures. The free oocyte is soon picked up by the oviduct funnel and pushed down the oviduct by peristalitic waves in its walls.
In some mammals, the antrum soon after becomes filled with blood coming from the ruptured capillaries of the follicle; such a body is called corpus haemorrhagicum. In others such structure is not formed, yet a blood clot appears. In either case the cells of stratum granulosum and the theca interna become endocrine. These are now called lutein cells.
The blood clot along with lutein cells form a characteristic structure known as corpus luteum. Corpus lutea normally develops from all ruptured follicles, but if the liberated ova are not fertilized, the corpora lutea soon degenerate.
If fertilization takes place, it persists as corpus luteum of pregnancy, till the fully developed foetus is delivered at the end of gestation period. If fertilisation does not take place, it is called the corpus luteum of ovulation, which finally shrinks so that only a white scar, called corpus albicans is left in the end (Fig.10).
Saccular type ovaries:
The saccular type ovaries are found only in amphibians among the vertebrates. Such ovary has a cortex and germinal epithelium, somewhat similar to the compact ovary but the area which forms the medulla here is represented by a large lymph space.
The germinal layer gives rise to ovarian follicles each of which enclosing a developing oocyte. Mature follicles rise to the surface of the ovary and liberate the oocyte by a sudden rupture of the follicular wall. Not all follicles reach maturity, some degenerate or become atretic.
