HORMONAL CONTROL OF SPERMATOGENESIS

Overview of Control

Spermatogenesis is controlled by the hypothalamus and the anterior pituitary gland working together. The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which travels to the anterior pituitary gland, stimulating it to secrete two gonadotropins (FSH and LH).

Roles of Gonadotropins and Testosterone

• FSH (Follicle Stimulating Hormone): Stimulates Sertoli cells to complete the development of spermatozoa from spermatids.

• LH (Luteinizing Hormone) / ICSH (Interstitial Cell Stimulating Hormone): Stimulates the synthesis of the steroid hormone Testosterone by the Leydig cells (interstitial cells) of the testis.

• Testosterone: Stimulates the growth and development of the germinal epithelial cells (spermatogonia) and works with FSH on the Sertoli cells.

Negative Feedback

A negative feedback mechanism operates to regulate these hormones:

• An increase in the level of Testosterone results in a decrease in secretion of GnRH from the hypothalamus (and possibly reduces LH secretion directly from the pituitary).

• Inhibin, a glycoprotein hormone produced by the Sertoli cells, forms a second feedback inhibition system. If spermatogenesis proceeds too rapidly, Inhibin is released to reduce the secretion of FSH by the anterior pituitary gland.

Role of Cyclic AMP

Both FSH and LH act by causing the release of cyclic AMP (adenosine monophosphate) within the cells they stimulate. Cyclic AMP is known as the ‘second messenger’ system. It stimulates the synthesis of enzymes (e.g., enzymes for testosterone synthesis) inside the nucleus.

Secondary Sexual Characteristics

Testosterone is the main male sex hormone and controls the development and maintenance of secondary sexual characteristics that develop at puberty. These include:

• Development and enlargement of the testes, penis, and reproductive tract glands.

• Increased muscle development.

• Enlargement of the larynx, causing deepening of the voice.

• Growth of pubic, facial, armpit, and chest hair.

• Changes in behavior associated with courtship and mating.

OOGENESIS

Development of Eggs

The production of eggs begins before birth (unlike sperm production).

• During fetal development, oogonia undergo mitosis to form primary oocytes.

• These primary oocytes halt development at prophase of meiosis I throughout childhood.

• They are surrounded by a single layer of granulosa cells (or follicle cells), forming primordial follicles.

• About two million primordial follicles exist just before birth, but only about 450 ever develop into the secondary oocytes that are released.

Follicle Development and Maturation

During a woman’s fertile years, one primordial follicle per month develops into a mature Graafian follicle in response to the hormone FSH.

• The primordial follicle becomes a primary follicle as the granulosa cells multiply.

• Cells from the ovary stroma form outer layers called the theca. The inner theca and granulosa cells secrete female sex hormones.

• A fluid containing oestrogen is secreted by the granulosa cells, creating a fluid-filled space called the antrum (now a secondary follicle).

• The follicle matures into a Graafian follicle (about 1 cm in diameter).

• The primary oocyte divides by meiosis I, producing a secondary oocyte (haploid) and the first polar body.

• The secondary oocyte arrests at metaphase of meiosis II until fertilization.

• At fertilization, the secondary oocyte completes meiosis II, producing the large mature ovum and a second polar body. All polar bodies eventually degenerate.

Hormonal Control of Oogenesis

Control is centered in the hypothalamus and the pituitary gland.

• The hypothalamus secretes GnRH, which stimulates the release of FSH and LH from the pituitary gland.

• In the female, these hormones are secreted in cycles (menstrual cycle, lasting about 28 days), not constantly.

• FSH stimulates the development of the primordial follicle each month.

• Normally, only one egg is produced per cycle. The cycle begins on the first day of menstruation.