15. Development and maturation of follicles

In oogenesis, the production of the female reproductive cells(ova), two stages can be distinguished, a growth stage and a maturation stage. The maturation stage is linked with changes in the surrounding cells, resulting in the development and maturation of ovarian follicles. Different stages can also be distinguished in that procces.

15.1 Primordial follicle

Primorsial germ cells (oogonia) migrate from the yolk sac into the ovarian cortex during early embryonal development. There they proliferate by mitosis until they reach six million in number in the fourth month of foetal development. Most of them perish during foetal life. Only about one million of them grow and develop into primary oocytes. The primary oocytes,with a diameter of about 30um, has a large vesicular nucleus with a prominent nucleolus, and little eosinophilic cytoplasm. By the seventh month of the foetal life, each primary oocyte reaches the first stsge (prophase) of meiosis and rests there. Primary oocytes become encapsulated by a single layer of squamous folicular cells,which are attached to each othe by desmosomes and separated from the surrounding connective tissue by a basal lamina. The primordial follicle consists of a primary oocyte and a layer of flat follicle .From the beginning of puberty until the climacteric in the non-pregnant femal, 15-20 prmordial follicles continue their maturation with each menstrual cycle. Usually only one of those follicles reaches full maturity and getes to ovulation: the others degenerate and become atretic . So, especially in ovaries of younger women, normally a few mature follicles can be found, which may remain in this stage up to several months (second resting stage).

15.2 Primary follicle

Stimulated by the pituitary follicle-stimulating hormone(FSH),15-20 primordial follicles start to grow during each menstrual cycle ,and their follicle cells, still arranged in one layer, become cuboidal. Between the primary oocyte and follicular cells, a homogeneous eosinophilic membrane of glycoprotein, the zona pellucida, develops(Fig2-1). With further development, follicle cells proliferate and form a stratum of more layers,then termed the zona granulosa. The surrounding connective tissue is arranged in concentric layers (theca folliculi) and then differentiates into two layers,the theca interna and the theca externa. At this stage, the follicle is termed the primary follicle.

15.3 Secondary follicle

Between the follicular cells small spaces appear, which later communicate and form a cavity or hole, termed the antrum folliculi. The space and antrum are filled by the follicular fluid, the liquor folliculi. The antrum is lined by cells of the zona granulosa (granrlosa cells), which form a little mound. Within this mass of cells, termed the cumulus oophorus, is located the oocyte. The granulosa cells surrounding the oocyte are arranged radially to form the corona radiata. The cells of the xorona radiata form processes which cross the zona pellucida and make contact with the surface of the oocyte by cell junctions (gap junctions).The oocyte on its part extends microvilli into the zona pellucida. The theca folliculi now differentiates into an inner theca interna and an outer theca externa. The epitheloid cell acquire the fine structural characteristics of steroid secretory cells .The thcea interna is richly vascularised and secretes oestrogens. The theca externa, on the other hand , remains of a connective tissue nature. At this stage the follicle becomes known as the secondary follicle.

15.4 Tertiary follicle ( mature follicle, Graafian follicle)

In the human female the follicles require 10-14 days to reach the stage of a mature follicle. At this stage the follicle continues to grow , and the antrum also enlarges because of an accumlation of the liquor folliculi. Thus , the zona granulosa also becomes progressively thinner. When the follicle has reached its full size of 1-1.5cm, it occupies the thickness of the cortex and forms a bulge on the surface of the ovary (stigma). As follicular maturity is attained, small irregular spaces, filled with fluid ,appear between the cells of the corona radiata,thus weakening the connection of the ovum with the stratum granulosum. The ovum is surrounded by a thick zzona pellucida and a conspicuous corona radiata .The theca folliculi has attained its greatest development .shortly before ovulation, the first meiotic division is completed and the second meiotic division begins. From then on, the oocyte is termed a secondary oocyte.