Hormone Regulation and Function

Study Notes

Serotonin controls sleep, pain control, sexual behavior, and hormone secretion in seasonally breeding mammals.

Lipid-soluble hormones (thyroid and steroid hormones) have intracellular receptors, alter gene expression, and turn specific genes on or off.
Water-soluble hormones (eicosanoids, polypeptide, and amine) have receptors as integral transmembrane proteins on the plasma membrane, using signal transduction.

Gonads develop from mesenchyma, coelomic epithelium, and Primordial Germ Cells.
Genital Ridge formation: genital ridges are formed at the 5th week of development by the proliferation of mesenchyme covered by coelomic epithelium.

Primordial Germ Cells (PGCs) gather at the allantois, then pass into the yolk duct, the intestine, the dorsal mesentery, and finally reach the genital ridges.

Sex cords (vascularized) are formed by a second group of cells deriving from the coelomic epithelium, which moves and surrounds the PGCs.

Secretory process depends on the nature of secretion, blood supply of the secreting tissue, and solubility of the hormone (water-soluble or lipid-soluble).
Endocrine: mediators are secreted into the interstitial fluid, then diffuse into blood capillaries, and circulate until they reach the target cells.

Steroids and Eicosanoids, often carried on transporters.
Steroids:
- Derived from cholesterol, which is synthesized from acetate.
- Cholesterol is converted to pregnenolone in the inner mitochondrial membrane.
- Pregnenolone is converted to sex steroids in the smooth ER.
- Four main families of steroids: progestagens, androgens, oestrogens, and corticosteroids.
- Biosynthetic unit enzymes interconvert them.
Eicosanoids:
- Water-soluble, needing transporters to cross the plasma membrane.
- Comprise Prostaglandins and Leukotrines, with PGs having a greater role in reproductive processes.
- Arachidonic acid is a main precursor for PGs.

The hypothalamo-hypophyseal axis regulates the reproductive system.
GnRH released by the hypothalamus forms a blood vascularization with capillaries of the portal system, allowing rapid hormone transfer.
GnRH controls the expression of LH and FSH, which have a pulsatile mode of release.

LH: acts on Leydig cells, stimulating production of testosterone from cholesterol.
Testosterone: binds to intracellular receptors on Sertoli cells, stimulating them to secrete ABP.
FSH: binds to extracellular receptors on Sertoli cells, stimulating RNA synthesis, testicular fluid production, and protein synthesis (including inhibin and ABP).

Testosterone has a negative feedback on GnRH, inhibiting the secretion of LH and FSH.
Inhibin: produced by Sertoli cells, inhibits the secretion of FSH by negative feedback.