Download The Endocrine System Pituitary Gland (Hypophysis)

G.Histiology(Endocrine system part 1)
Instructor:Dr.Heba F. Hassan
The Endocrine System
Endocrine glands don't have ducts. Their secretions (hormones) are secreted into the
blood stream. Because of this, the hormones can act over long distances, and reach any
organ in the body to co-ordinate activity. The endocrine glands include the pituitary
gland, the thyroid gland, the parathyroid glands, pineal gland, the suprarenal glands (the
adrenals), and islets of Langerhans.
Hormones
Hormones are molecules that function in the body as chemical signals. They are
liberated by specialized cells that are called endocrine cells.
Many endocrine cells, however, produce hormones that act at a short distance. This is
called paracrine secretion.
Another method of secretion is the juxtacrine secretion, by which a molecule is
released into the extracellular tissue, diffuses through the matrix, and acts on cells at a
very short distance.
In autocrine secretion, cells may produce molecules that act on themselves or on cells
of the same type. Insulin-like growth factor (IGF) produced by several cell types may
act on the same cells that produced it.
The tissues and organs on which the hormones act are called target tissues or target
organs.
Endocrine glands are also target organs ,the body is able to control hormone secretion
through a mechanism of feedback and to keep blood hormonal levels within strict
limits.
Pituitary Gland (Hypophysis)
The pituitary (also known as the hypophysis) is found at the base of the brain, about
1cm in diameter, lying beneath the third ventricle in a bony cavity (sella turcica) in the
base of the skull.The pituitary gland produces hormones that regulate growth,
metabolism, and reproduction. Composed 2 part:
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G.Histiology(Endocrine system part 1)
1-The posterior part (Neurohypophysis)
of the pituitary has its embryological
origins in nervous tissue. It is formed from
a downgrowth of the diencephalon that
forms the floor of the third ventricle.
Instructor:Dr.Heba F. Hassan
2-The anterior part ( Adenohypophysis
) is derived from an upgrowth from the oral
ectoderm of the primitive oral cavity
called Rathke's pouch.
Along the posterior part of the the anterior
lobe there is a narrow region called the
pars intermedia, which also has its
embryological origin in Rathke's pounch.
The part of the hypophysis that arises from oral ectoderm is known as the
adenohypophysis (anterior part) and is subdivided into 3 portions: a large pars
distalis, or anterior lobe; a cranial part, the pars tuberalis, which surrounds the neural
stalk; and the pars intermedia.
The neurohypophysis, the part of the hypophysis that develops from nerve tissue,
consists of a large portion, the pars nervosa, and the smaller infundibulum or neural
stalk. The neural stalk is composed of the stem and median eminence.
 Adenohypophysis (Anterior part)
1-Pars Distalis (Anterior lobe)
The main components of the pars distalis
are cords of epithelial cells interspersed
with capillaries .
Common stains allow the recognition of 3
cell types in the pars distalis:
chromophobes and 2 types of chromophils
called basophils and acidophils according
to their affinity for basic and acid dyes, The
hormones produced by the hypophysis have
widespread physiologic activity ; they
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G.Histiology(Endocrine system part 1)
Instructor:Dr.Heba F. Hassan
regulate almost all other endocrine glands, the secretion of milk, and the metabolism of
muscle, bone and adipose tissue.
2-Pars Tuberalis
The pars tuberalis is a funnel-shaped region
surrounding the infundibulum (neural stalk)
of the neurohypophysis . Most of the cells of
the pars tuberalis secrete gonadotropins
(follicle-stimulating hormone and luteinizing
hormone) and are arranged in cords
alongside the blood vessels.
3- Pars Intermedia
The pars intermedia, which developed from
the dorsal portion of Rathke's pouch , in
humans, a rudimentary region made up of
cords and follicles of weakly basophilic cells
that contain small secretory granules. The
function of these cells is not known.
 Neurohypophysis (posterior part)
The neurohypophysis consists of the pars nervosa and the neural stalk. It is composed
of some 100,000 unmyelinated axons of secretory neurons situated in the supraoptic and
paraventricular nuclei . The secretory neurons have all the characteristics of typical
neurons, including the ability to conduct an action potential, but have more developed
Nissl bodies related to the production of the neurosecretory material. The
neurosecretions are transported along the axons and accumulate at their endings in the
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G.Histiology(Endocrine system part 1)
pars nervosa. Here they form structures
known as Herring bodies that are visible
in the light microscope.
Instructor:Dr.Heba F. Hassan
The electron microscope reveals that the
Herring bodies contain neurosecretory
granules that have a diameter of 100200nm are surrounded by a membrane. The
granules are released and enter the
fenestrated capillaries that exist in large
numbers in the pars nervosa; the hormones
are then distributed to the general
circulation.
The neurosecretory material consists of 2
hormones, both cyclic peptides made up of
9 amino acids. The hormones have a
slightly different amino acid composition, which results in very different functions.
They are arginine vasopressin-also called antidiuretic hormone-and oxytocin. Each
hormone is joined to a binding protein (neurophysin).
Cells of the Neurohypophysis
Although the neurohypophysis consists mainly of axons from hypothalamic neurons,
about 25% of the volume of this structure consists of a specific type of highly branched
glial cell called a pituicyte.
Islets of Langerhans
The islets of Langerhans are multihormonal
endocrine micro organs of the pancreas; they
appear as rounded clusters of cells embedded
within exocrine pancreatic tissue. Although
most islets are 100-200µm in diameter and
contain several hundred cells, small islets of
endocrine cells are also found interspersed
among the pancreatic exocrine cells. There
may be more than 1 million islets in the
human pancreas, with a slight tendency for
islets to be more abundant in the tail region.
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G.Histiology(Endocrine system part 1)
In sections, each islet consists of lightly
stained polygonal or rounded cells,
arranged in cords separated by a network
of blood capillaries . In 3-dimensional
reconstructions, islets of Langerhans are
seen as round, compact masses of
secretory epithelial cells pervaded by a
labyrinthine network of blood capillaries.
Both the endocrine cells and the blood
vessels are innervated by autonomic nerve
fibers. A fine capsule of reticular fibers
surrounds each islet, separating it from the
adjacent exocrine pancreatic tissue.
Instructor:Dr.Heba F. Hassan
Routine stains or trichrome stains allow the recognition of acidophils (alpha) and
basophils (beta) .Using immunocytochemical methods four types of cells-A (glucagonproducing), B (insulin-producing), D, and F-have been located in the islets.
Terminations of nerve fibers on islet cells can be observed by light or electron
microscopy.
Pineal Gland
The pineal gland is known as the epiphysis
cerebri, or pineal body.
It is an endocrine gland whose secretions are
influenced by the light and dark periods of the
day. In the adult, it is a flattened conical organ
measuring approximately 5-8mm in length and
3-5mm at its greatest width and weighing about
120mg. It is found in the posterior extremity of
the third ventricle, above the roof of the
diencephalon, to which it is connected by a
short stalk.
The pineal gland is covered by pia mater. Connective tissue septa (containing blood
vessels and unmyelinated nerve fibers originate in the pia mater and penetrate the pineal
tissue. Along with the capillaries, they surround the cellular cords and follicles, forming
irregular lobules.
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G.Histiology(Endocrine system part 1)
Instructor:Dr.Heba F. Hassan
The pineal gland consists of several types of cells, principally pinealocytes and
astrocytes. Pinealocytes have a slightly basophilic cytoplasm with large irregular or
lobate nuclei and sharply defined nucleoli. When impregnated with silver salts, the
pinealocytes appear to have long and tortuous branches reaching out to the vascular
connective tissue septa, where they end as flattened dilatations. These cells produce
melatonin and several other substances (e.g., serotonin) that may influence
reproduction. Melatonin is secreted at night, whereas serotonin is produced during the
day.
The astrocytes of the pineal gland are a
specific type of cell characterized by
elongated nuclei that stain more heavily
than do those of parenchymal cells. They
are observed between the cords of
pinealocytes and in perivascular areas.
These cells have long cytoplasmic
processes that contain a large number of
intermediate filaments 10nm in diameter.
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