Other Endocrine Organs + Hormones

Thymus and Its Hormones

irregularly shaped organ in the mediastinum

site of T lymphocyte (involved in the immune response) maturation

secretes Thymosin and Thymopoietin, which act as paracrine signals that assist T lymphocyte maturation

larger and more active in infants and children

shrinks (atrophies) in adults and is eventually replaced mostly by adipose and other connective tissues

Gonads

Gonads include the Testes in males and Ovaries in females

Responsible for production of Gametes, or Sperm Cells in males and Ova in females as well as the sex steroid hormones

Testosterone

Produced mainly by the testes

GnRH from the hypothalamus triggers secretion of LH and FSH from the anterior pituitary

LH stimulates testosterone synthesis and FSH stimulates production of a protein that binds testosterone and concentrates it in the testes

Controlled by negative feedback

Hydrophobic, so easily crosses the plasma membrane and binds to intracellular receptors to influence expression of various genes;

Two basic effects on target cells:

– Anabolic Effects that include those that stimulate bone growth and increase muscle mass

– Androgenic Effects that involve development of male secondary sex characteristics, such as a deeper voice, more prominent facial hair, and increased bone and muscle mass

Estrogens and Progesterone

Produced by the ovaries and controlled by negative feedback involving GnRH, LH, and FSH (same as the other)

estrogens stimulate development of female secondary sex characteristics and the menstrual cycle

affect bone, blood, kidneys, and adipose tissue

Progesterone helps prepare the body for pregnancy and supports fetal development

affects smooth muscle, body temperature, blood clotting, bone tissue, and metabolism

Pineal Gland

part of the epithalamus of the diencephalon

secretes the neurohormone Melatonin

Trigger appears to be related to light and dark cycles

Begins secreting melatonin when ambient light decreases in the evening, and reaches its peak secretion over night

Main target tissues are the sleep-regulating centers in the reticular formation of the brainstem

Appears to adjust the sleep phase of the sleep/wake cycle in some people

Synthetic melatonin is sold as a supplement to promote sleep and prevent jet lag

Effectiveness is questionable

Leptin

protein hormone produced by adipocytes

Can cross the blood brain barrier to interact with neurons in the hypothalamus that control feeding

Acts on these neurons to induce satiety, a feeling of fullness, that prevents overeating

Amount of leptin is related to the amount of adipose tissue

This should prevent humans with excess adipose tissue from overeating, but mechanisms that regulate feeding are complex and involve multiple neural and hormonal factors

synthetic leptin was not effective in reducing appetite for miracle weight loss

Atrial Natriuretic Peptide (ANP)

produced by cardiac muscle cells in response to stretch-sensitive ion channels

ANP targets smooth muscle cells lining blood vessels and causes Vasodilation, which increases vessel diameter

ANP also targets kidney tubules and enhances excretion of sodium ions called Natriuresis

This enhances water excretion because it makes the fluid in the tubules more concentrated and water moves into the tubule by osmosis

This lowers blood volume

Vasodilation and lower blood volume both decrease blood pressure

Endocrine Kidney

Erythropoietin (EPO) Production – Secreted mainly by cells of the kidney in response to a decreased level of oxygen in the blood

Acts on red bone marrow to stimulate development of erythrocytes

Renin Secretion – Renin is an enzyme in blood that converts the plasma protein angiotensinogen to angiotensin-1

Important part of the renin-angiotensin-aldosterone system which maintains blood pressure

Conversion of vitamin D to its active form – Process begins with the skin, but the kidneys convert it to Calcitriol

Stimulated by parathyroid hormone