HISTOLOGY
Prof.Dr. Huda Al-khateeb
Lec. 2
ENDOCRINE SYSTEM
Pituitary gland (Hypophysis)
Pituitary gland is a small bean-shape organ. Its
dimensions are (12x10x9 mm)
It weighs 0.5gm in male. It is slightly heavier in non-
pregnant and much heavier in pregnant women.
It lies beneath the base of the brain (hypothalamus) to
which it is linked by stalk.
It is enclosed in the sella turcica of the sphenoid bone
(sella turcica is an important radiological land mark).
Pituitary gland has double embryonic origin
1. From the oral ectoderm – the roof of the mouth
cavity will invaginate cranially forming a pouch
called "Rathke's pouch". Later this part detaches
from
the
oral
cavity
and
forms
the
adenohypophysis.
2. From the developing nervous system – the floor of
diencephalon will evaginate caudally. Later it will
form the neurohypophysis.
Therefore pituitary gland can be divided into:
A. Adenohypophysis – subdivided into:
Pars distalis (Anterior lobe)
Pars tuberalis ( Cranial part)
Pars intermedia (Intermediate lobe)
B. Neurohypophysis – subdivided into:
Pars nervosa (Posterior lobe)
Stalk (Infundibulum) – composed of
median eminence and stem
Pars distalis (Ant. Lobe): composed of two types of cells:
a) Chromophobes (50%)
b) Chromophils – subdivided into
i. Acidophils (40%)
ii. Basophils (10%)
Chromophobes:
Light microscopical study of Chromophobes shows:
Small polyhedral cells with pale cytoplasm and small
rounded central nucleus.
Their cytoplasm has no affinity to dyes and has no
granules
Cells are usually found in clusters
Their function is unknown, they are thought to be:
i. Either exhausted degranulated chromophils
ii. Or represent immature chromophils
Chromophils:
Light microscopical study of chromophils manifests:
Large polyhedral cells with large acidophilic (in
acidophils) or basophilic (in basophils) granules.
They have large central rounded nucleus.
Electron microscopic study of chromophils:
Acidophils and basophils are sub-grouped according to
the size of granules (in the cytoplasm) seen in the
electron microscope.
Acidophils – subgrouped into:
1. Somatotrophs – (secretory granules = 300 – 400 nm),
they are responsible for growth hormone (GH)
secretion. GH influences the general body growth and
bone elongation (by its effect on the epiphyseal
cartilage of long bones).
Note: the increase of GH in childhood and adolescent
leads to gigantism, while the increase of GH, in adults,
leads to acromegaly. The reduction in GH in childhood
leads to dwarfism.
2. Mammotrophs – (secretory granules = 200 nm)
They are responsible for prolactin secretion. This
hormone stimulates milk synthesis and secretion from
mammary gland during lactation in female. In male, its
role is not well understood.
Recently, it has been found that these cells secret
luteotrophic hormone (LTH), which stimulates corpus
luteum of the ovary to secret progesterone.
Basophils – are subdivided according to their electron
microscopical features into:
1) Gonadotrophs – (secretory granules = 250 – 400 nm).
The secret the following hormones:
Follicular – stimulating hormone (FSH) – stimulates
folliculogenesis in female and spermatogenesis in male.
Leuteinizing hormone (LH) – in female, it is
responsible for maturation of follicles in ovary,
ovulation and corpus luteum formation.
Interstitial cell – stimulating hormone (ICSH) – in
male, it maintains interstitial cells of Leydig in testis
and stimulates them to secret androgens, particularly,
testosterone which is essential for sperm maturation
and development of secondary sex organs.
2) Thyrotrophs – (secretory granules = 120 – 200 nm)
They are responsible for secretion of thyroid –
stimulating hormone (TSH) which stimulates thyroid
gland to secret tetraiodothyronin (Thyroxine) (T
4
) and
triiodothyronin (T
3
).
3) Corticotrophs – (secretory granules = 400 – 550)
They secret adreno-cortico-trophic hormone (ACTH).
This hormone acts on the adrenal gland cortex, leading
to secretion of corticosteroid hormones (aldosterone
and cortisol) and sex hormones (estrogen, progesterone
and androgen).
Recently, it has been found that these cells are
responsible for the secretion of β-lipotropin hormone
(β-LPH), α-melanocyte-stimulating hormone (α-MSH)
and β-endorphins.
Pars Intermedia:
In human, it is a rudimentary region made up of
cords and follicles of weakly basophilic cells that
contains secretory granules. It might be responsible
for the secretion of α-melanocyte-stimulating
hormone (α-MSH)
Pars Tuberalis:
It is a sleeve like that surround the infundibulum. Its cells
are arranged in cords alongside the blood vessels. Most of
them secret gonadotropins (FSH & LH).
Neurohypophysis: composed of:
Pars nervosa (posterior lobe)
Infundibulum: with its stalk attached to the
hypothalamus at the median eminence
All parts of neurohypophysis contain Pituicytes and axonal
processes of unmyelinated nerve fibers whose cell bodies are
located in the paraventricular and supraoptic nuclei of the
hypothalamus. The axonal processes converge at the median
eminence
forming
a
bundle
called
"hypothalamo-
hypophyseal tract" and pass through the infundibulum to
reach pars nervosa. These nerve fibers are unusual in that
they do not end on other neuron or effector cells, but end
blindly close to the fenestrated capillary plexus of pars
nervosa. The terminal portions of these axonal processes are
commonly expanded and contain secretory granules called
Herring bodies or called neurosecretory bodies. The
neurosecretory
bodies contain numerous membrane-
enclosed granules with either oxytocin or vasopressin bound
to carrier proteins called neurophysin I and II respectively.
Oxytocin and vasopressin are believed to be synthesized in
the cell bodies and then transported through the axons to the
nerve terminals where they are released in response to
hypothalamic nerve impulses. Axons from the supraoptic
nuclei are mainly concerned with vasopressin/ADH
secretion,
whereas
most
of
the
fibers
from
the
paraventricular nuclei are concerned with oxytocin
secretion.
Oxytocin stimulates the contraction of the uterine smooth
muscles during child birth. It also stimulates the contraction
of Myoepithelial cells of the acini of mammary gland leading
to milk ejection.
Vasopressin increases water
permeability of renal collecting
ducts leading to more water reabsorption which cause
increase in blood volume. It stimulates the contraction of
smooth muscles of the small-size arteries and arterioles
leading to increase of blood pressure.
Pituicytes are
Star-shaped glial cells
Supporting
Have cytoplasmic processes that surround the axons.
Blood supply of pituitary gland:
Pituitary gland receives blood from:
i. Right and left superior hypophyseal arteries that
are derived from circle of Willis and supply
infundibulum then anterior lobe by hypophyseal
portal system
ii. Right and left inferior hypophyseal arteries that
are derived from the internal carotid arteries and
supply neurohypophysis.
Blood, from all pituitary lobes, drain into superior and
inferior hypophyseal veins.
Hypophyseal portal system:
Both superior hypophyseal arteries break up into primary
capillary plexus (located near neurosecretory neurons that
contain releasing and inhibiting hormones). Capillaries of
this plexus join to form hypophyseal portal vein that break
up into secondary capillary plexus within adenohypophysis.
Both capillary plexuses are fenestrated with diaphragms.