Types of follicle in ovary
There are three types of follicles present at birth and throughout the life of animals in ovary. these are Primordial follicle, Growing follicle and Graafian follicle.
1) Primordial follicle or primary follicle
Consists of an oocyte surrounded by a single layer of epithelial flattened granulosa cells with an irregularly shaped nuclei. This stage is reached at birth in the newborn heifer and the ovary may contain as many as 150,000 of these follicles. The number of primordial follicles decrease to a few as 1,000 in a cow by 15 to 20 years of age.
The number of primordial follicles that undergo folliculogenesis to reach mature, graafian stage from puberty is only a small fraction of the number of follicles available in the pool of primordial follicles. Most follicles will either undergo a process of regression called atresia at some stage of folliculogenesis or remain as primordial follicles with no signs of growth.
2) Growing follicle
Follicles that have left the resting stage as primordial follicles have begun growth but have not developed a theca layer or antrum (cavity).
This follicle has 2 or more layer of granulosa cells surrounding the oocyte; with continued growth, additional layers of granulosa cells appear to surround the oocyte.
A zona pellucida surrounding the oocytes may be seen at this stage.
The number of growing follicle in an ovary at a given time is relatively small in the domestic species and varies with the stage of the estrous cycle.
3) Graafian follicle
These are follicle in which an antrum (cavity) is clearly visible. The Graafian follicle protrude from the surface of the ovary. As the antrum enlarges, the granulosa cell layer is evened out except at the cumulus oophorus where the oocyte rests in a nest of granulosa cells.
The diameter of the oocyte of the cow is 80-120 µm in this stage and is surrounded by the zona pellucida.
Two layer of theca cells, theca interna and theca externa together with the granulosa cells form the wall of the follicle. Theca externa formed by muscle type cells has actin and myosin contractile filaments for follicular contraction.
The granulosa cells maintain contact with the oocyte during folliculogenesis and in the preantral and antral stages from the cumulus oophorus. The cumulus cells maintain contact with the oocyte even as the follicular fluid fills the antrum and eccentrically displace the oocyte. As the follicle matures, the granulosa cells also undergo morphologic differentiation (increased cytoplasmic organelle) for increased steroidogenic function.
Gap junctions are seen between the granulosa cells, theca interna, cells of cumulus oophorus and the oocyte. These gap junctions serve as a important role in movement of small molecules, icons and nutrients from the basement towards the antrum and as channels for hormonal communication between the peripheral cells of the follicle and oocyte.
Primordial follicles lack an independent vasculature. The capillary bed confined to the thecal layer develops as the thecal cells are formed around the follicle these thecal capillaries increase in size and concentrate in the theca interna in close proximity to the basement membrane. Blood flow through these capillaries also ceases as the follicle matures.
The permeability of the follicular wall increases in the pre-ovulating period in some species. So the composition of the follicular fluid is similar to that of the plasma. However due to the metabolic and secretary activities of the follicular cells, the follicular fluid has a lower concentration of gonadotropins and higher concentration of steroid hormones, than in the peripheral blood.
Embryonic development of ova and follicle
Early in the first weeks of embryonic development, the primitive large germ cells called primordial germ cells are present in the yolk sac. These primordial germ cells migrate by ameboid movement from the yolk sac to the genital ridges. This migration occurs in 35 days of gestation in bovine.
After a few days, the gonadal sex can be distinguished by the formation of superficial tunica albuginea and the peripheral location of oogonia in the primitive ovary. This occurs in the 45 days of gestation in bovines.
These oogonia multiply by mitosis after sexual differentiation and enter the prophase of the first meiotic divisions, when they are called the oocytes. The number of oocytes produced may be determined by the level of FSH in the embryo. Oogenesis is the transformation of oogonia into oocytes. Oogenesis is completed before or shortly after birth in all domestic mammals.
The period of oogonial mitosis signifies the end of the major portion of the period of oogenesis and this occurs at 45-100 days of gestation in cattle.
At the end of this period of oogenesis the oocytes enter the first meiotic prophase. In this stage the ova are surrounded by a single layer of follicular epithelium and these are called primordial follicles.
Thus at birth all female animals are born with their full complement of oocytes in primordial or older follicles and these are never replenished but only decrease in number during the life of the animal. Oocytes that fail to get incorporated into the follicles self destruct by apoptosis.