In gynecology, the ovarian follicle is a unit consisting of female oocytes, epithal granulosa cells, and the two surrounding connective tissue fringes, theca interna and theca externa, which are localized to the ovarian cortex at advanced stages of follicular maturation. The ovarian follicle and in particular its anatomical auxiliary cells themselves perform important tasks in follicular maturation and sexuality, but at the same time are subject to processes of maturation which, under hormonal control, trigger ovulation in the late stage and flush the oocyte into the tubal funnel in the form of the ovarian follicle. One of the main complaints associated with the ovarian follicle is benign follicular cysts, which can form at any stage of follicular maturation and cause the follicle to grow to a follicle size of more than four centimeters.
What are ovarian follicles?
The egg cell forms a unit in the ovary of the female body during follicle maturation with auxiliary cells, i.e., the surrounding follicular epithelial cells, and the two surrounding connective tissue layers. This unit is known to physicians as the overial follicle, which is sometimes also referred to as the follicle. The follicles undergo a maturation process under hormonal control, during which the two connective tissue layers and the auxiliary cells of the ovarian follicle first form. The resulting ovarian follicles trigger ovulation after they have fully matured. All follicular stages of the ovary are thus anatomically functional structures that contain estrogen-producing cells in particular. Only these cells, in turn, enable the maturation and development of the oocytes.
Anatomy and structure
Ovarian follicles consist of the oocyte, so-called granulosa cells or follicular epithelial cells, and the two connective tissue layers, theca interna and theca externa. The granulosa cells are located in the multilayer granular layer of the ovarian follicle and develop during follicle maturation from epithelial cells of the first maturation stage, the so-called primary follicle. The theca interna, on the other hand, is a differentiated fringe of connective tissue located at the ovarian cortex, where it forms the inner cell layer of the ovarian follicle. This connective tissue layer develops into the secondary follicle as the primary follicle matures. The theca externa is also a differentiated fringe of connective tissue that develops as the follicle progressively matures and eventually lies at the ovarian cortex of the ovarian follicle.
Function and roles
Ovarian follicles are subject to maturation and trigger maturation processes themselves. Even before birth, a woman is endowed with the disposition for primordial follicles, which comprise a single-layer follicular epithelium in addition to the oocyte. These primordial follicles later increased continuously in size and develop into primary follicles in the course of this growth. The primary follicles are thus formed, so to speak, in the first stage of maturation of the follicle and are equipped with single-layer prismatic follicular epithelia. Finally, secondary follicles develop from these primary follicles in the subsequent stage. In the secondary follicles, the oocyte envelops itself in glycoproteins. The follicular epithelium develops into several layers and aligns itself in ray form. The subsequent stage of follicle maturation is that of the tertiary follicle. At this stage, a follicular cavity appears in which cerebrospinal fluid (CSF) collects. This fluid is produced during maturation by the granulosa cells of the ovarian follicle, which have developed from the epithelial cells of the primary follicle during previous maturation. As soon as the oocyte has entered a cell cluster, i.e. the so-called egg mound and thus a docking site, the connective tissue around the cell develops into the two tissue layers theca interna and theca externa. Meanwhile, the follicle continues to increase in size and becomes the so-called Graafian follicle, which is already ready to burst. These growth processes are controlled by the hormone FSH, which has a follicle-stimulating effect and originates from the pituitary gland. As soon as the hormone FSH and the luteinizing hormone LH are present in a specific concentration, ovulation occurs. With the cooperation of contractile cells of the theca externa, it then flushes the egg into the fallopian tube. The majority of the granulosa cells form a protective layer around the oocyte. The connective tissue layer of the theca interna, which is rich in vessels and cells, produces androgens during this process.These androgens reach the helper cell-containing layer of the ovarian follicle by diffusion processes and are converted to estrogens there in aromatization processes. After each ovulation, a retained part of granulosa cells forms the so-called corpus luteum from stored lipids, which is responsible for the production of the hormone progesterone and controls the sexual cycle. Thus, ovarian follicles are involved in several processes of hormone production and thus assume control functions in the field of reproduction and sexuality.
Diseases
One of the best known follicular diseases is follicular cysts. They can arise from the structures of all follicular stages in the ovary, that is, from any form of ovarian follicle in the sense of the maturing egg along with its sheath. As soon as the follicle of any maturation stage exceeds a size of four centimeters, the gynecologist already speaks of a follicular cyst. If this process occurs only after ovulation, the blood-filled, cystic dilatation of the corpus luteum that has just formed is also called a corpus luteum cyst. Cystic processes are benign tumors that can usually be detected by mere palpation during a routine examination by a gynecologist. Follicular cysts often remain largely asymptomatic. The same is true for benign tumors of the ovarian sheath, which develop from superficial cells and often present large space-occupying lesions.