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Endocrine system (systema endocrinum) 163

Hypophysis cerebri (pituitary gland) hormones that influence the activity of almost all other
VetBooks.ir The hypophysis consists of two functionally and embry- endocrine glands. In addition, effector hormones pro-
ologically distinct components, each of which is further duced by the adenohypophysis exert a direct influence on
peripheral tissues.
subdivided:
· adenohypophysis, an ectodermal evagination PARS DISTALIS
from the roof of the stomodeum (Rathke’s pouch), The pars distalis is composed of cords or clusters of endo-
comprising the crine cells situated in close proximity to a dense network
− pars distalis, of sinusoidal capillaries with wide lumina. The endocrine
− pars tuberalis and cells are surrounded by reticular connective tissue contain-
− pars intermedia; ing occasional macrophages (cells of the mononuclear
· neurohypophysis, an extension of the floor of the phagocyte system). Isolated small follicles featuring an
diencephalon, consisting of the outer, single-celled layer of endocrine cells may be found
− infundibulum and throughout the adenohypophysis.
− pars nervosa (neural lobe). The use of routine histological staining methods
(haematoxylin and eosin, trichrome stains) allows endo-
Both parts of the hypophysis (adeno- and neurohypophy- crine cells to be divided into two categories, based on the
sis) are enclosed in a common connective tissue capsule staining properties of their secretory vesicles:
which is continuous with the dura mater. In carnivores,
ruminants and the pig, part of the lumen of Rathke’s · chromophobic cells and
pouch remains as the hypophyseal cavity between the · chromophilic cells.
pars distalis and pars intermedia.
A particular feature of the hypophysis is its character- This classification is not indicative of the functional prop-
istic vasculature (Figure 9.2). The inferior hypophyseal erties of the cells. Functional cell types can be identified
artery primarily supplies the neurohypophysis. The supe- using immunohistochemical techniques.
rior hypophyseal artery provides blood to the capillary
bed in the pars tuberalis of the adenohypophysis, from chRomoPhobic cells (endocRinocytus chRomoPhobus)
which capillary loops extend into the median eminence Chromophobic cells are those in which commonly used
and infundibulum. This primary capillary bed receives the histological stains do not reveal the presence of secre-
secretory products of the neurons of the ventromedial and tory granules (Figure 9.2). They have relatively little,
infundibular nuclei (RH, IH). light-coloured homogeneous cytoplasm and a compara-
Vessels draining the primary capillary plexus (por- tively large nucleus. The electron microscope reveals
tal veins) course through the pars tuberalis to the that most of these cells do in fact contain endocrine
pars distalis where they expand into a secondary capil- granules.
lary plexus (Figure 9.2), downstream from the primary Chromophobic cells are considered to be undifferenti-
plexus. This enables the delivery of the hypothalamic ated, non-secretory stem cells that transform into specific
regulatory hormones to the cells of the pars distalis. The chromophilic cells when required. Conversely, chromo-
vascular bed formed by the consecutive capillary beds and philic cells can degranulate and become chromophobic.
connecting portal veins are referred to as the hypothalamo- This proposition is supported by the observation that,
hypophyseal portal system. with the light microscope, many cells (around 50%) of the
The neurohypophyseal vascular bed supplied by the adenohypophysis appear chromophobic, while the elec-
inferior hypophyseal artery is connected to that of the tron microscope reveals that hardly any cells lack secretory
adenohypophysis by small vessels. The effector hormones granules. On this basis it has been suggested that chromo-
vasopressin and oxytocin, originating from the supraoptic philic cells exhibit cyclic secretory activity incorporating
and paraventricular nuclei, are released into this vas- stages of storage and synthesis.
cular network. Hypophyseal veins drain into the sinus
cavernosus. chRomoPhilic cells (endocRinocytus chRomoPhilus)
According to the affinity of their secretory granules for
Adenohypophysis acidic or basic dyes, chromophilic cells (Figures 9.2 to 9.4)
The adenohypophysis is larger than the neurohypophysis are classified as:
in the horse, ruminant and dog. In the cat and pig, the neu-
rohypophysis is slightly bigger than the adenohypophysis. · acidophilic cells and
The adenohypophysis serves as the overarching endo- · basophilic cells.
crine regulatory centre of the body, producing tropic

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