Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

I. Hypothalamus-Pituitary System
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organization of cytoplasmic organelles for peptide synthesis
and release. Characteristics such as the form, size, and
location of the various organelles and the size and shape
of the cells and their nuclei allow identification of adenohypophyseal
cells at the ultrastructural level ( Batten and
Ingleton, 1987 ; Mikami, 1986 ).
The distribution of the various secretory cells of the AL
is not random, different hormone-secreting cells preferentially
accumulate at different sites in the AL. This topological
organization of the AL is well known in the human
and may also be true for domestic animals. The gland comprises
a central “ mucoid ” wedge containing thyrotropes
and corticotropes and lateral wings containing somatotropes
and lactotropes. The gonadotropes are distributed
diffusely throughout the gland. Also, the numerical contribution
of each AL cell type is not the same. In the pituitary
anterior lobe of humans the distribution is corticotrope
(10% to 15%), thyrotrope (5%), somatotrope (50%), lactotrope
(10% to 30%), and gonadotrope (15% to 20%).
The concept of one pituitary cell making one hormone
is an oversimplification. Immunoelectron microscopy has
shown that one hormone can be produced by the same or different
cell populations. In some instances, multiple hormones
are produced by the same cell population and have even been
visualized within the same secretory granule. The majority
of gonadotropes and somatomammotropes is multihormonal
cells that contain LH and FSH or GH and PRL, respectively.
Also, other combinations such as gonadotroph hormones and
ACTH or TSH and ACTH have been described.
In addition to the functionally distinctive AL cell types
noted above, cells termed “ stellate ” or “ folliculostellate ”
also occur within the normal pituitary. Folliculostellate
cells lack secretory granules as well as hormone secretion
and are characterized by processes that insinuate themselves
between the AL cells. The physiological role of
folliculostellate cells is uncertain. They are thought to be
supportive in nature and synthesize a variety of chemical
messengers, intrapituitary growth factors, and cytokines
that exert local paracrine effects on cell function and proliferation
( Denef, 1994 ).
6 . Cells of the Intermediate Lobe (IL)
The predominant IL cell in mammals is the melanotrope, a
cell with immunoreactivity for α -MSH, which is sparse in
the AL ( Halmi and Krieger, 1983 ). In some species, including
the dog ( Halmi et al. , 1981 ) and the horse ( Amann
et al. , 1987 ), the IL is cytologically heterogeneous. ACTHcontaining
cells (B cells) have been found to be dispersed
among the predominant melanotropes (A cells). In the dog,
the immunoreactive ACTH content of the IL even exceeds
that in the AL ( Halmi et al. , 1981 ). In ferrets, as in dogs
and cats, the melanotropic cell was the most abundant cell
type of the IL ( Schoemaker et al. , 2004 ). In agreement
with the previously mentioned direct neural regulation of
IL secretory activity, the presence of neural elements was
demonstrated in the bovine IL ( Boyd, 1987 ). Evidence is
accumulating that the cells of the pars intermedia are also
involved in the biosynthesis and release of a yet unknown
prolactin-releasing factor. The blood supply of the IL is
poor, and therefore the peptides released from the IL are
thought to mainly act by diffusion in a paracrine manner.
7 . Cells of the Neural Lobe (NL)
The NL contains axonal nerve fibers, often swollen by
being packed with neurosecretory granules. These nerve
fibers and the glial cells (pituicytes) have synaptoid contacts.
The pituicytes play an intermediary role in the regulation
of the release of vasopressin and oxytocin ( Rosso
et al. , 2004 ).
B . Regulation of Pituitary Functions
For the regulation of each of the five major adenohypophyseal
hormone systems (ACTH, LH and FSH, TSH,
GH, and PRL), there is a feedback (closed-loop) system.
AL hormone and hypophysiotropic hormone secretions
are suppressed by the products of target endocrine glands
such as the thyroid, adrenals, and gonads. Apart from this
long-loop feedback, some hormones (e.g., PRL) regulate
their own secretion directly by acting on the hypothalamus
(short-loop feedback). On this powerful feedback
control with primarily blood-borne signals, other signals
are superimposed. These may originate within the central
nervous system (open loop) and can be mediated through
neurotransmitters and hypophysiotropic hormones. Thus,
influences are exerted that represent the environment
(temperature, light-dark), stress (pain, fear), and intrinsic
rhythmicity.
These regulatory factors influence peptide synthesis or
release in adenohypophyseal cells, where each of the steps
in hormone synthesis and ultimate secretion represents a
potential control point in the regulation of circulating hormone
concentrations.
1 . Hypophysiotropic Hormones
The main hypothalamic neurohormones may stimulate or
inhibit the release of a single hormone, or it may affect several
hormone-producing cells ( Fig. 18-3 ). The predominant
influence of the hypothalamic hormones on the pituitary is
stimulatory, and these peptides are known as releasing hormones
( Guillemin, 2005 ). Interference with the integrity of
the hypothalamic-pituitary connections results in decreased
secretion of pituitary hormones. The exception is PRL, the
secretion of which is increased when hypothalamic influence
is removed.
As the complexity of the peptide structures of the hypophysiotropic
hormones increases, species variation in