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

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Chapter | 18 Pituitary Function
TABLE 18-1 Molecular Weights of Adenohypophyseal Hormones and Staining
Properties of Adenohypophyseal Cells
Cell Type Hormone MW
a
Subunits Staining Orange G PAS
b
Color c
Somatotrope GH 22,000 — Acidophil Yellow
Lactotrope PRL 22,500 — Acidophil Yellow
Gonadotrope LH 30,000 α / β Basophil Blue
FSH 32,000 α / β Basophil Blue
Thyrotrope TSH 28,000 α / β Basophil Blue
Corticotrope ACTH 4,500 — Basophil Red
β -End 3,500 — Basophil Red
Melanotrope α -MSH 1,700 — Basophil Red
a
Molecular weight.
b
Periodic-acid-Schiff reaction.
c
Color obtained after costaining with PAS-Orange G and performic acid-Alcian blue.
but not in humans and birds ( Fig. 18-2 ). In humans only
during fetal life is a distinct IL found ( Amar and Weiss,
2003 ), whereas there is some debate about the view that
in adults “ invading ” cells of the posterior lobe are homologous
with the IL cells of lower vertebrates ( McNicol,
1986 ). Unlike reptiles and mammals, birds have no IL
( Batten and Ingleton, 1987 ). Nevertheless the typical “ IL
hormone ” MSH is present in birds, having been found in
the adenohypophysis, where it coexists with ACTH in cells
named corticomelanotrophs ( Iturriza et al. , 1986 ).
The pituitary stalk (infundibulum) and the neurohypophysis
(posterior lobe, neural lobe) develop from the
basal outgrowth of the diencephalon in connection with the
development of Rathke’s pouch. The cells of the diencephalic
outgrowth later develop into glial cells (pituicytes),
whereas nerve fibers from the supraoptic and paraventricular
nuclei grow into the NL.
5 . Cells of the Anterior Lobe (AL)
The peptide hormones secreted by the AL can be divided
into three categories: (1) the somatomammotropic hormones
growth hormone (GH) and prolactin (PRL); (2)
the glycoprotein hormones thyrotropin (TSH), folliclestimulating
hormone (FSH), and luteinizing hormone (LH);
and (3) the corticomelanotropins, which are all derived
from the precursor molecule proopiomelanocortin (POMC),
and the corticomelanotropins include adrenocorticotropin
(ACTH), α -melanotropin ( α -MSH), β -endorphin ( β -END),
and β -lipotropin ( β -LPH).
Identification of the adenohypophyseal cells has been
developed by histochemical staining techniques, immunohistochemical
methods, and ultrastructural studies. Staining
and immune reactions depend on the chemical nature of
the hormones that are stored in granules within the cytoplasm.
A general identification of adenohypophyseal cells
( Batten and Ingleton, 1987 ) is given in Table 18-1 , although
it must be realized that the various cell types do not always
give exactly the same reaction in different animal species.
Having undergone a number of modifications over the
years to improve sensitivity and specificity, immunohistochemistry
remains the single most often used method in the
identification of normal and diseased AL cell types. Apart
from staining for the full spectrum of hormones (GH, PRL,
ACTH, FSH, LH, TSH, and α -subunit), special stains investigate
the diseased AL cell, such as staining with MIB-1, a
cell proliferation marker that recognizes the KI-67 antigen,
or demonstration of the protein p53, the product of a tumor
suppressor gene, as parameters for aggressive behavior
of cells. Development of sophisticated tools of molecular
biology during the 1990s has enabled researchers to study
chromosomal defects, genetic abnormalities such as specific
mutations, the presence of oncogenes or of tumor suppressor
genes, cell membrane receptors, signal transductions mechanisms,
and intracellular changes that affect hormone synthesis
and release. Pituitary cell cultures (normal and diseased,
e.g., murine AtT-20 corticotrope cell line) allow various in
vitro studies to be undertaken. In situ hybridization demonstrates
specific messenger RNAs (mRNAs) at the subcellular
level, which allows the reliable assessment of gene expression.
In situ hybridization can be used to demonstrate the
mRNA of receptors, growth factors, transcription factors,
genes, and other regulatory peptides. Whereas immunocytochemistry
provides information regarding cell hormone content,
in situ hybridization confirms the presence of ongoing
hormone synthesis.
Transmission electron microscopy allows the study
of AL cells at the ultrastructural level. Applications of
immunohistochemical methods at the ultrastructural level
have also contributed to the rapid progress in the field.
Ultrastructural studies of AL cells demonstrate the presence
of distinct varieties of secretory granules and typical