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Endocrine system (systema endocrinum) 165

stimulates protein synthesis, promotes the breakdown of In males, FSH induces spermatogenesis at the onset
VetBooks.ir fat and reduces glucose uptake. In addition, growth hor- of sexual maturity. Additionally, FSH promotes the pro-
mone induces the release of somatomedin, particularly duction of androgen-binding protein by Sertoli cells. LH
by the liver, which stimulates the production of cartilage stimulates the activity of interstitial cells in the testis and
and bone and thereby contributes to body growth. The has thus also been referred to as interstitial cell-stimulating
activity of somatotropes is controlled by the hypothalamic hormone (ICSH).
regulatory hormones GHRH (growth hormone-releas- The activity of gonadotropic cells is regulated by cir-
ing hormone) and GHIH (growth hormone-inhibiting culating levels of gonadal steroid hormones. An increase
hormone = somatostatin). Within the acidophilic cell in the level of sex steroids inhibits the activity of gonado-
population, the somatotropes appear consistently as tropes (negative feedback). Accordingly, gonadotropic cells
relatively small, round or oval cells lacking cytoplasmic hypertrophy following castration.
processes. Thyrotropes (TSH cells, endocrinocytus thyrotropi-
Lactotropes (endocrinocytus luteotropicus) syn- cus) synthesise thyroid-stimulating hormone (TSH). Of
thesise and store prolactin (PRL) in large, variably sized the cells of the pars distalis, these have the smallest gran-
granules (diameter 500–900 nm). During gestation, pro- ules (diameter around 150 nm).
lactin stimulates development of the mammary gland. Corticotropes (ACTH cells, endocrinocytus cortico-
Lactation is inhibited prior to parturition by high levels tropicus) secrete adrenocorticotropic hormone (ACTH)
of oestrogen and progesterone. As the levels of these and β-lipotropin (β-LPH). ACTH regulates the synthesis
hormones fall, lactation is initiated. Lactotropes often of mineralocorticoids, glucocorticoids and androgens in
occur as single cells, are irregular in shape and appear as the cortex of the adrenal gland.
though they have been squeezed between other cell types.
Secretion of prolactin is stimulated by oxytocin (released PARS TUBERALIS
from the neurohypophysis). During gestation, the adeno- The pars tuberalis is a projection of the adenohypophy-
hypophysis almost doubles in size due to hypertrophy and sis that incompletely surrounds the anterior portion of
hyperplasia of prolactin cells. At the end of the lactation the infundibulum. This component contains dense capil-
period, the volume of the hypertrophied cells is reduced lary networks and is traversed by the portal veins of the
through autophagocytosis. hypothalamo-hypophyseal portal system. The cells of the
Basophilic cells (endocrinocytus basophilicus) are char- pars tuberalis are arranged in clusters. They are largely
acterised by an affinity for basic dyes, particularly aniline chromophobic cells that may include small numbers of
blue in various trichrome stains. The tropic hormones gonadotropes and thyrotropes.
produced by basophilic cells are heavily glycosylated (gly-
coproteins) and their secretory granules are therefore PARS INTERMEDIA
PAS-positive. Basophilic cells are larger than acidophils, The pars intermedia lies between the pars distalis and the
though their secretory granules are typically smaller. neurohypophysis (Figure 9.2). Its parenchyma comprises
Three types of basophilic cells are recognised: basophils and chromophobes arranged in irregular clus-
ters and follicles. The cells of the pars intermedia produce
· gonadotropes, melanocyte-stimulating hormone (MSH), which regu-
· thyrotropes (TSH cells) and lates the distribution of melanin granules in amphibians
· corticotropes (ACTH cells). and reptiles (resulting in darkening of the skin) and stimu-
lates production of melanin by melanocytes in mammals.
Gonadotropes (endocrinocytus gonadotropicus) are The large prohormone pro-opiomelanocortin, synthe-
relatively small, rounded cells containing PAS-positive sised by cells of the pars intermedia, is cleaved to form
granules (diameter 200–400 nm). Gonadotropes produce β-endorphin and two types of MSH, α-MSH and β-MSH.
two types of gonadotropic hormone: Neurosecretory and aminergic axons extend from the
neurohypophysis into the pars intermedia, and basophilic
· follicle-stimulating hormone (FSH) and cells of the pars intermedia often migrate into the neuro-
· luteinising hormone (LH). hypophysis (basophil invasion).

It has not yet been established definitively whether all Neurohypophysis
gonadotropes secrete both hormones, or if particular gon- The neurohypophysis is connected to the supraoptic and
adotrope cell populations secrete only one or the other. paraventricular nuclei of the hypothalamus by a bundle
In females, FSH stimulates cyclic maturation of ovarian of non-myelinated axons (hypothalamo-hypophyseal
follicles. Luteinising hormone directs ovulation and forma- tract). This connection is both structural and functional.
tion of the corpus luteum. The hormones of the neurohypophysis are synthesised in

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