Gonads and Sex Hormones

Gonads and Their Functions

• The gonads, including the ovaries and testes, serve two major functions
• They produce germ cells like ova in ovaries and spermatozoa in testes
• They synthesize steroid sex hormones, which support the development and function of reproductive organs in both sexes
• Steroid sex hormones support the development of secondary sex characteristics, such as body hair distribution
• They support pregnancy, childbirth, and lactation
• Steroid sex hormones also influence metabolism, cardiovascular function, and bone growth

Estrogens

• Estrogen is primarily produced in the ovaries, but also in the corpus luteum, placenta, and adrenal glands
• In men and postmenopausal women, estrogens are mainly from the conversion of testicular, adrenal, and ovarian androgens in peripheral tissues like adipose tissue and skin
• Estrogens coordinate the development and function of female genitalia and breasts
• During puberty, estrogens regulate the growth of the uterus, breasts, and vagina
• They determine fat deposition patterns for a typical female shape
• Estrogens regulate the pubertal growth spurt and cessation of growth
• In adult women, estrogens regulate the menstrual cycle
• Estrogens contribute to the hormonal regulation of pregnancy and lactation
• Estrogens promote pubertal growth spurt in both boys and girls through an anabolic effect that is weaker than that of testosterone
• Estrogens retard bone resorption and promote epiphyseal fusion with less potency than testosterone
• Estrogens cause salt and water retention, which may contribute to premenstrual symptoms
• Estrogens lower plasma cholesterol which can reduce the risk of atherosclerosis and cardiovascular diseases
• Estrogen production declines post-menopause which can trigger hot flashes, sweating, palpitations, irritability, anxiety, depression, and osteoporosis
• Hormone replacement therapy can alleviate theses symptoms and reduce risks of osteoporosis and coronary heart disease post-menopause
• Hormone replacement therapy can increase the risk of certain types of cancer

Progestogens

• Progestogens are produced in the ovaries during the menstrual cycle and in the placenta during pregnancy
• Progestogens prepare the uterine lining for pregnancy and together with estrogens, stimulate development of mammary glands for lactation

Androgens

• Testosterone, an androgen, is mainly secreted by the testes, with smaller amounts from the adrenal glands and ovaries
• Androgens stimulate the development and growth of the male genital tract
• Testosterone has strong protein anabolic effects that increase muscle mass by promoting protein synthesis
• In the fetus, androgens promote the development of internal and external male genitalia
• During puberty, androgens promote the growth of male sex organs and male secondary sex characteristics.
• Male secondary sex characteristics include pubertal growth spurts, cessation of growth at adult height, deepening of the voice, growth of hair, and increased muscularity and strength
• In adult males, androgens maintain masculinity and regulate sperm production
• Androgens can double muscle mass during puberty by a potent protein anabolic effect
• They can increase bone thickness and length and pubertal growth spurts, ending with the closure of the epiphysis
• Androgens increase basal metabolic rate (BMR) due to its protein anabolic effect
• Androgens increase the number of RBCs, resulting in higher counts in males
• Androgens have weak mineralocorticoid-like activity, causing salt and water retention
• Testosterone levels decrease slightly with age, though this is not as drastic as the estrogen reduction during menopause

Other Endocrine Organs

• Many organs and tissues have secondary endocrine functions including the kidneys, heart, and skin

Kidneys

• Erythropoietin regulates the number of circulating RBCs
• Thrombopoietin regulates the number of circulating platelets
• Calcitriol stimulates calcium reabsorption and inhibits parathyroid hormone production for bone mineralization
• The kidneys produce the enzyme renin, which is important for blood pressure regulation

Heart

• The heart regulates blood sodium, extracellular volume, and blood pressure
• The heart secretes atrial natriuretic peptide which promotes natriuresis, by increasing sodium excretion in the urine, and reducing blood volume and blood pressure

Skin

• The skin regulates activation, inactivation, and removal of hormones, as well as producing hormones
• The skin produces insulin-like growth factor-1 (IGF-1) which is secreted in response to growth hormone and promotes tissue maturation
• IGF-1 is produced in the dermis by dermal fibroblasts
• The skin produces steroid hormones like androgens and estrogens
• Melanocyte-stimulating hormone (MSH), is produced in the skin stimulates melanocytes responsible for skin pigmentation and protection against UV light
• MSH is secreted by the pituitary gland and by the skin
• Vitamin D3 (cholecalciferol) is synthesized by the skin through UV light exposure transported to the liver and kidneys for activation to calcitriol