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chapter

34

Endocrine Metabolism V: Reproductive System

(a) A4 P a th w a y

CH3

C =0

P ro g estero n e

2

?H3

c—o

17 a -H ydroxypregnenolone

T e sto ste r o n e

A5-A n drosten ed iol

F IG U R E 34-1

Testosterone biosynthetic pathways, (a) A4 Pathway (preferred pathway in human testes), (b) A5 Pathway (all enzymes

located in the endoplasmic reticulum). Enzymes: 1,3/i-hydroxysteroid dehydrogenase and A5,A4-isomerase; 2,

17«-hydroxyIase (CYP 17); 3, Ci

7

-

2

o-lyase; 4, 17/3-hydroxysteroid dehydrogenase.

achieved in part by a polypeptide produced by Ser-

toli cells called

inhibin,

which specifically inhibits FSH

release. Presumably, the output of inhibin is propor-

tional to the intensity of spermatogenesis and main-

tains spermatogenesis at a relatively constant rate. FSH

release also is modulated by estrogens. Because the

anterior pituitary lacks aromatase and therefore can-

not convert testosterone to estradiol, it is possible that

testicular estrogen exerts a negative feedback on FSH

release.

Regulation of Testicular Steroidogenesis:

Leydig-Neuroendocrine Axis

Leydig cells (interstitial cells) are situated in the vascu-

larized compartments outside the seminiferous tubules