|Oogenesis occurs in specialized structures in the cortex of the ovary called ovarian follicles.|
are composed of:
The two images at the left show the typical appearance of follicles in the cortex of the ovary.
Click the images to see them larger and labeled.
|There are three stages in the
development of follicles:
On this page we will consider pre-ovulation only.
The pars distalis of the pituitary gland (hypophysis) controls the process of follicular development and maturation by the secretion of:
The hormonal interactions will be explained in the discussion of the ovarian cycle.
Pre-ovulation: Development of Follicles in the Ovary.
Click each image below to view the histological image.
|The primordial (quiescent) follicle consists of a primary oocyte and a single layer of flattened follicular cells. As the follicle develops, alterations occur in the primary oocyte and the surrounding follicular cells. The primary oocyte produces yolk granules and the follicular cells change from flattened to cuboidal or columnar.|
|The primary follicle consists of a primary oocyte with a single layer of cuboidal/columnar follicular cells. As development proceeds, the number of follicular cells increases by mitosis forming several layers around the primary oocyte. As these cells enlarge they release steroid hormones called estrogens of which estradiol is the dominant one prior to ovulation. During each cycle, a few primary follicles will continue to develop into secondary follicles.|
|The secondary follicle consists of several layers of cuboidal/columnar follicular cells, now collectively called the membrana granulosa which begin to secrete follicular fluid. A thick, amorphous layer, the zona pellucida, forms between the primary oocyte and the membrana granulosa. Previously undifferentiated stromal cells now develop into two distinct layers around the developing follicle: the theca interna and the theca externa (see below). Cells in the theca interna are large, rounded and epithelial-like; cells in the theca externa are smaller, fibroblasts. Both layers of theca cells are separated from the membrana granulosa cells of the follicle by a basement membrane. As the follicular fluid secreted by the membrana graulosa cells accumulates, small pockets of fluid between granulosa cells begin to appear. Usually in human females only one secondary follicle will continue to develop.|
|The secondary-vesicular follicle (below) is characterized by the presence of pockets of follicular fluid within the membrana granulosa. As the follicle continues to develop, the separate pockets fuse to form one large pocket of fluid called the follicular antrum.||During this development of the follicular antrum, the oocyte is still a primary oocyte, arrested in prophase of Meiosis I. It is still surrounded by granulosa cells which are contiguous with the membrana granulosa present around the periphery of the growing follicle.|
|Two regions of cells can be identified in
the layer of granulosa cells surrounding the oocyte:
The other granulosa cells form a layer around the periphery of the follicle and are separated from the theca interna cells by a distinct basement membrane.
|The mature follicle (left), sometimes called the pre-ovulatory follicle, has all of the components of the secondary-vesicular follicle but is much larger and contains one single large antrum of follicular fluid. These follicles are very large and usually extend from the deepest parts of the cortex and protrude from the surface of the ovary. In some species just before ovulation, the primary oocyte in the mature follicle completes meiosis I producing a secondary oocyte and a polar body.|
Pre-ovulation: Development of Theca Cells in the Ovary.
As the oocyte and follicular (granulosa) cells are growing and developing in the ovary, the stromal cells differentiate and develop into the theca interna and theca externa cells. As a follicle goes from a primary to a secondary follicle, the stromal cells immediately surrounding the follicle differentiate into the theca folliculi. The cells closest to the follicle become the theca interna cells, round, foamy cells that secrete androgens, including testosterone. These two male hormones are converted by the granulosa cells to estrogens. The stomal cells farther away from the developing follicle become the theca externa cells, fibroblast-like cells arranged around the follicle outside the theca interna cells.
Click the images on the right to see them enlarged and labeled.
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