ovarian follicle - Development of oocytes in ovarian follicles, Additional images

A structure within the mammalian ovary, part of which may develop into a mature ovum to be released at ovulation; also known as a Graafian follicle [grahfian], after the Dutch physiologist, Reinier de Graaf (1641–73). Each follicle consists of a primordial germ cell (ovum), surrounded by epithelial cells. At birth the human female may have as many as 400 000 follicles contained within the ovaries (of which only 300–400 come to maturity); at this stage they are known as primary follicles. During the early years of life, the primary follicles remain quiescent but many degenerate during this period. With the onset of puberty and menstruation several follicles each month begin to develop further, resulting in usually just one follicle discharging its ovum from the ovary into the abdominal cavity (ovulation). The stimulus to further development of the follicles is due to the ovaries coming into full function in response to the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

During the first half of the menstrual cycle, stimulation of the ovary by FSH causes the development and maturation of the follicle and increased oestrogen secretion. In many mammals, secretion of oestrogen at this time causes the female to be ‘on heat’ (in oestrus), this being the only time she is receptive to the male. In humans, ovulation is about halfway through the menstrual cycle. When oestrogen secretion reaches its peak in the human female, secretion of LH causes the follicle to enlarge quickly and rupture into the abdominal cavity, where the maturing ovum is collected by the uterine (Fallopian) tube. If fertilization occurs, it does so inside the uterine tube, usually within 24 hours of release of the ovum. LH secretion also causes the remains of the follicle to develop into the corpus luteum, which secretes progesterone and some oestrogen. Unless pregnancy occurs, the corpus luteum does not last for more than 12 days (progesterone is therefore not continuously secreted in the non-pregnant female). The secretion of progesterone prepares the lining of the uterus (endometrium) for implantation. If implantation does not result, the cessation of progesterone secretion (which occurs with regression of the corpus luteum) causes the endometrium to disintegrate and bleed (menstruation).

Ovarian follicles are the basic unit of female reproductive biology, they are roughly spherical aggregations of cells found in the ovary. Oocytes are swaddled in a layer of granulosa cells that are together enclosed in a thin layer of Extracellular matrix – the follicular basement membrane or basal lamina - to constitute the ovarian follicle.

As ovarian follicles grow larger they become discernible to the naked eye, these large ovarian follicles are sometimes called Graafian follicles (after Regnier de Graaf).

In humans, oocytes are established in the ovary before birth, and may lay dormant awaiting initiation for up to 50 years (McGee and Hsueh, 2000).

Development of oocytes in ovarian follicles

Gametes (sperm and oocytes) are produced by a special cellular division process called meiosis. These primordial germ cells (PGCs) multiply by mitosis and once they have reached the gonadal ridge they are called oogonia (diploid stem cells of the ovary). Development proceeds and the oogonia become fully surrounded by a layer of cells (pre-granulosa cells). After beginning the meiotic process, the oogonia (now called primary oocytes) can no longer replicate. Once the primary oocytes stop dividing the cells enter a prolonged ‘resting phase’.

For each primary oocyte that undergoes meiosis, only one functional oocyte is produced.

A mature ovarian follicle actually contains a secondary oocyte.

Upon fertilization by sperm, the secondary oocyte continues the second part of meiosis and becomes a zygote.

Additional images

Histology of the preovulatory follicle