Ovarian follicles

Introduction The Ovary : The human ovary consists of an inner medulla and outer cortex with indistinct boundaries. The medulla contains the blood vessels and nerves, while the cortex is occupied by developing follicles. A cross-section of an ovary will reveal follicles in various stages of development. The next sections will describe the histological features of each stage of follicular development and the major functional changes in the cells compose follicles.

Primordial Follicle These follicles consist of oocytes surrounded by a single

layer of squamous follicular cells which have a basal lamina and joined by desmosomes.

Primary oocyte: - sphirecal in shape , about 30 µm in diameter. - it has large vesicular rounded and eccentric nucleus with large nucleolus. - it’s cytoplasm is pale acidophilic and the organelles tend to gather close to the nucleus.

Primary follicle

They are similar to the primordial follicle but: the primary oocyte are larger in size follicular cells become cubical then columnar

A- primordial follicleB- primary follicle

1-oocyte2-follicular cells

Growing follicles Primary oocyte: - it doubles its size - become surrounded by acidophilic membrane called zona pellucida which is glycoprotein in nature produced by both the oocyte and follicular cells. - by EM its cell membrane produce microvilli into the zona pellucida.

Follicular cells: - FSH → the follicular cells proliferate to form many layers - LH → they begin to secrete follicular fluid which accumulate in multiple spaces - these spaces join to form a single cavity called antrum which divides the follicular cells into : a- cumulus oophorus: surround the oocyte, and connect it to one side of the follicle. b- granulosa cells: they line the cavity, polygonal in shape with pale acidophilic cytoplasm and central rounded nuclei. They secrete estrogene hormone.

Theca folliculi: - a capsule result from condensation of stroma around the follicles - later the differentiate into theca interna and theca externa

12345678

Oocyte Pellucid zoneStratum granulosumTheca internaTheca externaAntral follicleCumulus oophorusBasal lamina between theca and stratum granulosum

Grafiaan follicle The Graafian follicle is the stage after the first meiotic

division has completed but before ovulation. The 2ry oocyte, having undergone the first meiotic

division, is located eccentrically, 120 µm, contain 23d-chromosomes.

It is surrounded by the zona pellucida and a layer of several columnar cells known as the corona radiata which send processes into zona pellucida giving nutrition to the oocyte.

The theca folliculi differentiated into: - theca interna: cellular, vascular and secrete estrogene - theca externa: fibrous and less vascular.

Ovulation When the follicular fluid is markedly increased it

ruptures and realese the 2ry oocyte on the surface of the ovary.

the ovum will consist of three structures: oocyte, zona pellucida and corona radiata.

Corpus luteum After release of the ovum, the remaining cells of the

granulosa and theca interna form the corpus luteum. The center contains the remains of the blood clot that formed after ovulation.

Surrounding the clot are glanulosa lutein cells have an appearance characteristic of steroid-producing cells, with pale cytoplasm.

on the outside theca lutein cells which are smaller and more deeply stained. These cells produce progesterone and to a lesser extent cholesterol.

Corpus Albicans If fertilization does not occur, the cells of the

corpus luteum remain active for roughly 14 days until the levels of LH fall and the corpus luteum involutes to form the corpus albicans.

The secretory cells of the corpus luteum degenerate, are phagocytosed by macrophages and replaced by fibrous material.

Atretic Follicle

Each menstrual cycle, several primordial follicles are stimulated to continue development but only one follicles completes development to release an ovum.

The other follicles degenerate through a process called atresia which can occur at any stage of development.

During atresia, granulosa cells undergo apoptosis and are replaced by fibrous material.

The oocyte degenerates and the basement that separated the oocyte from granulosa cells thickens to become the glassy membrane.