Ganong's Review of Medical Physiology, 23rd Edition

402
SECTION IV
Endocrine & Reproductive Physiology

L
-Dopa decreases prolactin secretion by increasing the forma-
tion of dopamine; bromocriptine and other dopamine agonists
inhibit secretion because they stimulate dopamine receptors.
Chlorpromazine and related drugs that block dopamine recep-
tors increase prolactin secretion. Thyrotropin-releasing hormone
(TRH) stimulates the secretion of prolactin in addition to
thyroid-stimulating hormone (TSH), and additional polypep-
tides with prolactin-releasing activity are present in hypotha-
lamic tissue. Estrogens produce a slowly developing increase in
prolactin secretion as a result of a direct action on the lactotropes.
It has now been established that prolactin facilitates the
secretion of dopamine in the median eminence. Thus, prolac-
tin acts in the hypothalamus in a negative feedback fashion to
inhibit its own secretion (see Clinical Box 25–3).

THE MALE REPRODUCTIVE

SYSTEM

STRUCTURE

The testes are made up of loops of convoluted
seminiferous
tubules,
in the walls of which the spermatozoa are formed
from the primitive germ cells
(spermatogenesis).
Both ends
of each loop drain into a network of ducts in the head of the
epididymis.
From there, spermatozoa pass through the tail of
the epididymis into the
vas deferens.
They enter through the
ejaculatory ducts
into the urethra in the body of the
prostate
at the time of ejaculation (Figure 25–11). Between the tubules
in the testes are nests of cells containing lipid granules, the in-
terstitial cells of Leydig (Figures 25–12 and 25–13), which se-
crete testosterone into the bloodstream. The spermatic
arteries to the testes are tortuous, and blood in them runs par-
allel but in the opposite direction to blood in the pampiniform
plexus of spermatic veins. This anatomic arrangement may
permit countercurrent exchange of heat and testosterone. The
principles of countercurrent exchange are considered in detail
in relation to the kidney in Chapter 38.

GAMETOGENESIS & EJACULATION

Blood–Testis Barrier

The walls of the seminiferous tubules are lined by primitive

germ cells and Sertoli cells, large, complex glycogen-contain-

ing cells that stretch from the basal lamina of the tubule to the

lumen (Figure 25–13). Germ cells must stay in contact with

Sertoli cells to survive, and this contact is maintained by

CLINICAL BOX 25–3

Hyperprolactinemia

Up to 70% of the patients with chromophobe adenomas of

the anterior pituitary have elevated plasma prolactin levels.

In some instances, the elevation may be due to damage to

the pituitary stalk, but in most cases, the tumor cells are ac-

tually secreting the hormone. The hyperprolactinemia may

cause galactorrhea, but in many individuals no demonstra-

ble endocrine abnormalities are present. Conversely, most

women with galactorrhea have normal prolactin levels;

definite elevations are found in less than a third of patients

with this condition.

Another interesting observation is that 15–20% of

women with secondary amenorrhea have elevated prolac-

tin levels, and when prolactin secretion is reduced, normal

menstrual cycles and fertility return. It appears that the pro-

lactin may produce amenorrhea by blocking the action of

gonadotropins on the ovaries, but definitive proof of this

hypothesis must await further research. The hypogonadism

produced by prolactinomas is associated with osteoporosis

due to estrogen deficiency.

As noted previously, hyperprolactinemia in men is asso-

ciated with impotence and hypogonadism that disappear

when prolactin secretion is reduced.

FIGURE 25–11 Anatomical features of the male reproductive system. Left: Male reproductive system. Right: Duct system of the testis.

Bladder Ureter

Vas deferens

Vas deferens

Head of epididymis

Tail of epididymis

Seminiferous tubules

Tunica albuginea

Symphysis Septa Rete testis

Urethra

Epididymis Testis

Scrotum

Cowper’s

(bulbourethral) gland

Ejaculatory duct

Prostate

Seminal vesicle