Reproductive Hormones and Processes Flashcards

Hormonal Control of Reproduction

Brain-Testicular Axis (Male)

• The production of testosterone and spermatogenesis are controlled by the hypothalamus and anterior pituitary.

• This control system is referred to as the brain-testicular axis.

• Hypothalamus:

  • The hypothalamus produces gonadotropin-releasing hormone (GnRH) in a sustained fashion starting at puberty.

  • GnRH stimulates the anterior pituitary.

• Anterior Pituitary:

  • GnRH stimulates some cells to release follicle-stimulating hormone (FSH).

  • GnRH stimulates other cells to release luteinizing hormone (LH).

• Testes:

  • FSH and LH target cells in the testes (Sertoli cells and Leydig cells).

  • This leads to sperm maturation and testosterone production.

Hormonal Regulation in Males

• The main trigger for GnRH release is a drop in testosterone levels.

• Sustained release of hormones maintains constant testosterone levels and sperm production.

• Luteinizing Hormone (LH):

  • LH binds to Leydig cells, which are located in the spaces between the seminiferous tubules.

  • Leydig cells produce and release testosterone in response to LH.

• Follicle-Stimulating Hormone (FSH):

  • FSH binds to Sertoli cells, which guide sperm maturation.

  • FSH triggers Sertoli cell activity, contributing to sperm maturation.

  • Spermatogenesis requires Sertoli cell activity and testosterone.

Effects of Testosterone

• Testosterone triggers non-gender-specific structures in the developing embryo to develop along male pathways.

• It leads to the development of internal and external genitalia.

• Testosterone is required for sperm development within seminiferous tubules.

• Testosterone is responsible for the development of secondary sexual characteristics, which become noticeable at puberty.

• Secondary Sexual Characteristics:

  • These are dimorphic differences between males and females.

  • Examples include:

    • Increased bone density and muscle mass due to the anabolic effect of testosterone (more protein). The bone's denser because it has more protein. The muscle mass is because of the increased protein.

    • Male and female patterns of hair growth and balding.

    • Differences in adipose tissue deposition (males tend to deposit weight in the torso, while females tend to deposit it in the abdomen and hips).

    • Behavioral differences: Studies on rat fetuses demonstrate that administering testosterone leads to more male patterns of behavior.

Hormonal Control in Females

• The hypothalamic-pituitary axis regulates female reproduction via the same hormones (GnRH, FSH, LH).

• These hormones exert their effects on the ovaries.

• GnRH from the hypothalamus triggers the release of FSH and LH, which bind to cells within the ovaries.

• This leads to egg maturation and the production/release of estrogens and progesterone.

Ovaries and Follicular Development

• Females are born with all the eggs they will ever have (approximately half a million), each surrounded by follicular cells.

• As the egg develops, so do the follicular cells.

• Follicle-Stimulating Hormone (FSH):

  • FSH binds to follicular cells.

  • It initiates their maturation and stimulates them to produce estrogens.

  • Estrogen promotes egg maturation.

Luteinizing Hormone (LH) and Ovulation

• LH is released mid-cycle and triggers the release of one mature follicle (ovulation).

• Ovulation: A mature egg and some surrounding follicular cells rupture from the surface of the ovary.

• After ovulation:

  • LH helps initiate changes in the follicle that has lost its egg.

  • The remaining follicular cells develop into the corpus luteum.

  • The corpus luteum produces estrogen and progesterone.

Functions of Estrogens

• Estrogens are produced by follicular cells and the corpus luteum.

• Functions:

  • Oogenesis: Estrogen triggers and maintains egg development.

  • Development and maintenance of secondary sexual characteristics in females.

  • Regulation of the female reproductive cycle (menstrual cycle).

  • Biological and biochemical effects:

    • Protein metabolism

    • Lipid metabolism (cardiovascular disease in females may arise after menopause due to low estrogen levels).

    • Bone density (lack of estrogen after menopause can lead to osteoporosis).

Progesterone

• Progesterone is present only in the second half of the menstrual cycle (after ovulation).

• It is secreted by the corpus luteum.

• Functions:

  • Maintaining the corpus luteum.

  • Rebuilding the endometrial wall in the uterus.

  • Initiating the development of mammary glands in case of pregnancy.