Brockhaus and Efron Encyclopedic Dictionary b17 466-0

An antral follicle, also known as Graafian follicle and tertiary follicle, is an

ovarian follicle An ovarian follicle is a roughly spheroid cellular aggregation set found in the ovaries. It secretes hormones that influence stages of the menstrual cycle. At the time of puberty, women have approximately 200,000 to 300,000 follicles, each with t ...

during a certain latter stage of

folliculogenesis :''Although the process is similar in many animals, this article will deal exclusively with human folliculogenesis.'' In biology, folliculogenesis is the maturation of the ovarian follicle, a densely packed shell of somatic cells that contains a ...

. Definitions differ in where the shift into an antral follicle occurs in the staging of folliculogenesis, with some stating that it occurs when entering the ''secondary'' stage,Page 769
section "formation of the antrum" in: and others stating that it occurs when entering the ''tertiary'' stage.Page 76 in:

Appearance

The antral follicle is marked by the formation of a fluid-filled cavity adjacent to the oocyte called the

antrum ''This is a disambiguation page for the biological term. For the 2018 horror movie, see Antrum (film)'' In biology, antrum is a general term for a cavity or chamber, which may have specific meaning in reference to certain organs or sites in the bod ...

. The basic structure of the mature follicle has formed and no novel cells are detectable. Granulosa and theca cells continue to undergo mitosis concomitant with an increase in antrum volume. The Graaf follicle reaches its maximum diameter (20–22 mm) during ovulation. Antral follicles can attain a tremendous size that is hampered only by the availability of follicle stimulating hormone (FSH), which it is dependent on in this stage of folliculogenesis. By command of an oocyte-secreted morphogenic gradient, the antral follicle's granulosa cells begin to differentiate themselves into four distinct subtypes: ''

corona radiata In neuroanatomy, the corona radiata is a white matter sheet that continues inferiorly as the internal capsule and superiorly as the centrum semiovale. This sheet of both ascending and descending axons carries most of the neural traffic from and ...

'' that surrounds the

zona pellucida The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a specialized extracellular matrix that surrounds the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first ap ...

, ''membrana'' that is interior to the basal lamina, ''periantral'' that is adjacent to the antrum, and ''

cumulus oophorus The cumulus oophorus, (discus proligerus), is a cluster of cells that surround the oocyte both in the ovarian follicle and after ovulation. In the antral follicle, it may be regarded as an extension of the membrana granulosa. The innermost layer ...

'' that connects the membrana and corona radiata granulosa cells together. Each type of cell behaves differently in response to FSH.

Endocrine properties

Theca cells express receptors for

luteinizing hormone Luteinizing hormone (LH, also known as luteinising hormone, lutropin and sometimes lutrophin) is a hormone produced by gonadotropic cells in the anterior pituitary gland. The production of LH is regulated by gonadotropin-releasing hormone (GnRH) ...

(LH). LH kicks off the production of

androgen An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This in ...

s by the theca cells, most notably androstenedione, which are aromatized by granulosa cells to produce

estrogens Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal acti ...

, primarily

estradiol Estradiol (E2), also spelled oestradiol, is an estrogen steroid hormone and the major female sex hormone. It is involved in the regulation of the estrous and menstrual female reproductive cycles. Estradiol is responsible for the development o ...

. Consequently, estrogen levels begin to rise.

Antral follicle count

The ''antral follicle count'' (AFC) is the number of antral follicles, generally in both ovaries taken together if not specified otherwise. It can be determined by

transvaginal ultrasonography Vaginal ultrasonography is a medical ultrasonography that applies an ultrasound transducer (or "probe") in the vagina to visualize organs within the pelvic cavity. It is also called transvaginal ultrasonography because the ultrasound waves go '' ...

. A low AFC is a major factor in the diagnosis of poor ovarian reserve, that is, low fertility characterized by low numbers of remaining

oocytes An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female g ...

in the ovaries, usually accompanied by high follicle stimulating hormone (FSH) levels. Several studies show that an AFC test is more accurate than basal FSH testing for older women (< 44 years of age) in predicting IVF outcome. However, it does not appear to add any predictive information about success rates of an already established pregnancy after IVF. It is also a major determinant of the success of ovarian hyperstimulation. It has been suggested that counting the antral follicles measuring 2–5 or 4–6 mm in diameter is preferable. On the other hand, the number of smaller antral follicles (2–5 mm) is highly correlated with the total number of antral follicles (2–10 mm), and therefore it is suggested that counting all identifiable antral follicles of 2–10 mm in diameter would provide the most practical method for assessment of AFC in clinical practice. Three-dimensional (3D) automated follicular tracking is a developing technique that can substantially decrease both intra- and inter-observer variability in estimating the AFC.

References

{{reflist Reproduction in mammals