Vesicular ovarian follicles

Vesicular Ovarian Follicles (Graafian follicles).—Upon making a section of an ovary, numerous round transparent vesicles of various sizes are to be seen; they are the follicles, or ovisacs containing the ova. Immediately beneath the superficial covering is a layer of stroma, in which are a large number of minute vesicles, of uniform size, about 0.25 mm. in diameter. These are the follicles in their earliest condition, and the layer where they are found has been termed the cortical layer. 

They are especially numerous in the ovary of the young child. After puberty, and during the whole of the child-bearing period, large and mature, or almost mature follicles are also found in the cortical layer in small numbers, and also “corpora lutea,” the remains of follicles which have burst and are undergoing atrophy and absorption.

Beneath this superficial stratum, other large and more or less mature follicles are found imbedded in the ovarian stroma. These increase in size as they recede from the surface toward a highly vascular stroma in the center of the organ, termed the medullary substance (zona vasculosa of Waldeyer). This stroma forms the tissue of the hilum by which the ovary is attached, and through which the bloodvessels enter: it does not contain any follicles.

The larger follicles consist of an external fibrovascular coat, connected with the surrounding stroma of the ovary by a net-work of bloodvessels; and an internal coat, which consists of several layers of nucleated cells, called the membrana granulosa. At one part of the mature follicle the cells of the membrana granulosa are collected into a mass which projects into the cavity of the follicle. This is termed the discus proligerus, and in it the ovum is imbedded. The follicle contains a transparent albuminous fluid.

The development and maturation of the follicles and ova continue uninterruptedly from puberty to the end of the fruitful period of woman’s life, while their formation commences before birth. Before puberty the ovaries are small and the follicles contained in them are disposed in a comparatively thick layer in the cortical substance; here they present the appearance of a large number of minute closed vesicles, constituting the early condition of the follicles; many, however, never attain full development, but shrink and disappear. At puberty the ovaries enlarge and become more vascular, the follicles are developed in greater abundance, and their ova are capable of fecundation.

Discharge of the Ovum.—The follicles, after attaining a certain stage of development, gradually approach the surface of the ovary and burst; the ovum and fluid contents of the follicle are liberated on the exterior of the ovary, and carried into the uterine tube by currents set up by the movements of the cilia covering the mucous membrane of the fimbriæ.

Corpus Luteum.—After the discharge of the ovum the lining of the follicle is thrown into folds, and vascular processes grow inward from the surrounding tissue. In this way the space is filled up and the corpus luteum formed. It consists at first of a radial arrangement of yellow cells with bloodvessels and lymphatic spaces, and later it merges with the surrounding stroma.