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Endocrine Organs
Dr. Zahiri
Dr. Zahiri
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The endocrine system includes a number of different cells, structures, and
organs which are not necessarily related at the gross anatomical level.
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1. Histologically the endocrine system includes all structures and organs
that have endocrine secretory cells as the parenchymal tissue.
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2. Components of the endocrine system are functionally related in that
they synthesize and secrete hormones (chemical messengers) which
regulate, coordinate, and integrate functions of cells which are not in
physical contact with the hormone producing cells.
a. Several categories of biological chemicals are used
as hormones.
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(1) Peptides, proteins, and glycoproteins
(2) Steroids
(3) Amino acid analogues and derivatives
b. Hormones are secreted into the tissue fluid in connective tissue, but in
most cases the secretions quickly enter the circulatory system.
B. Endocrine glandular structures are classified
according to cell type and arrangement of cells:
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1. Unicellular endocrine units occur as single cells in another tissue.
2. Neurosecretory endocrine units contain cells which are structurally
similar to neurons.
3. Cord and clump endocrine units contain cells arranged in linear
arrays and/or clusters.
4. Follicular endocrine units contain epithelial cells which form the
walls of epithelial spheres with partially formed secretory product localized
inside sphere.
C. Cellular morphology of endocrine cells
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varies depending on the chemical nature of the secretory product secreted
by the cells.
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1. Protein and glycoprotein secreting cells contain extensive rER,
have well-developed Golgi bodies, and frequently accumulate acidophilic
secretory granules in their peripheral cytoplasm.
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2. Steroid secreting cells contain extensive sER, contain numerous
mitochondria with tubular cristae, and often contain numerous lipid
droplets.
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3. Amino acid and catecholamine secreting cells contain small
secretory or storage granules similar to synaptic vesicles found in axonal
endings in neurons, but lack extensive sER or rER.
E. Components (cells, structures, organs)
comprising the endocrine "system"
1. Endocrine organs
a. Thyroid gland
b. Parathyroid gland
c. Adrenal gland
d. Pituitary gland (hypophysis)
e. Pineal gland
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2. Endocrine cells within organs of other systems
a.
Islets of Langerhans in the pancreas
b.
b. Juxtaglomerular (JG) cells in the kidney
c.
c. Leydig interstitial cells in the testis
d.
d. Granulosa, thecal, and interstitial cells in the ovary
e.
e. Solitary endocrine (or neuroendocrine) cells in the walls of digestive
and respiratory organs and the heart
Pituitary
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Pituitary or hypophysis consists of 2 glands
Pituitary
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Pituitary or hypophysis consists of 2 glands
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Neurohypophysis (posterior pituitary)
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Median eminence
Infundibulum
Pars nervosa
Adenohypophysis
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o
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pars distalis (anterior lobe)
pars tuberalis (around infundibulum)
pars intermedia
Pituitary Vasculature
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Hypothalmo-hypophyseal portal vessels delivers neurosecretory hormones
from median eminence to the secondary capillary plexus of
adenohypophysis
Ending of axons of neurons that originate in various portions of
hypothalamus liberate releasing or inhibiting factors directly into primary
capillary bed, in this way they differ from other nerve ending
Artery/vein system supplying and draining neurohypophysis
Hypothalamic – hypophysial tract
Adenohypophysis: Pars Distalis
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75% of adenohypophysis
is covered by a fibrous capsule and is composed of cords of cells surrounded by
reticular fibers with many fenestrated sinusoidal capillaries
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There are two types of parenchymal cells: Chromophobe and Chromophil cells
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Chromophobe are pale staining or unstained cells that have little cytoplasm that is
not known their function
They may represent stem cells or degranulated chromophils
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Function is unknown but they may support other cells physically
Adenohypophysis: Pars Distalis
 Chromophil cells are those cells that have affinity for dyes
are two types: acidophils and basophils
 Acidophils stain red or orange and more numerous than basophils, they are consist
of:
• Somatotrophs and mammotrophs
 Basophils stain blue and located at periphery of pars distalis, they are consist of:
• Thyrotrophs, gonadotrophs, corticotrophs
Hormone-Producing Cells of the
Adenohypophysis
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Somatotrophs: growth hormone (somatotropin)
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Mammotrophs: prolactin
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Gonadotrophs: follicle stimulating hormone(FSH) and leuteinizing
hormone(LH)
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Thyrotrophs: thyroid stimulating hormone(TSH)
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Corticotrophs: pro- opiomelanocortin(POMC)
adrenocorticotropic hormone and lipotropic hormone
Release and Release-Inhibiting Factors
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Release and inhibiting factors are proteins and amino acid derivatives released into
median eminence that flow to adenohypophysis via portal vessels and control
pituitary cells or they are hormones which are released by target cells
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SRH increases growth hormone
Somatostatin decreases growth hormone
PRH and oxytocin increases prolactin
Dopamine and PIF decreases prolactin
TRH increases tyrotropin (TSH)
T3 and T4 decreases Tyrotropin (TSH)
GnRH increases LH and FSH
Hormones of testis and ovary decreases LH and FSH
CRH increases ACTH
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Pars tuberalis
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Pars tuberalis surrounds infundibulum (hypophysial stalk)
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It is composed of cuboidal to low columnar epithelial cells
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These cells are basophilic and mostly gonadotrophs
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Pars tuberalis do not secrete any specific hormone
Pars intermedia
 Pars intermedia is rudimentary in humans
 It is composed of cuboidal cell which are forming follicles containing colloid
 Sometimes houses cords of basophils which are synthesizing
proopiomelanocortin (POMC) that forms melanocyte stimulating hormone,
melanocyte stimulating hormone (MSH) and ß-endorphin
 In humans function is unknown; well-developed in other mammals
Neurohypophysis
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Includes the infundibulum, median eminence, and pars nervosa
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Pars nervosa is not an endocrine gland
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In pars nervosa distal ends of unmyelinated axon of hypothalamohypophysial tracts
terminate in vicinity of the fenestrated capillaries
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The cell body of neurosecretory cells are located in hypothalamus
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These axons supported by pituicytes which are astrocyte-like glial cells that occupy
25% of the volume of the pars nervosa
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Herring bodies are distentions of the axons not only at their termini but also along
their lenght which are representation of accumulation s of neuroseretory granules
 Vasopressin (ADH), Oxytocin, and a carrier protein known as
neurophysin are produced in nuclei of hypothalamus( supra optic,
paraventricular) and travel down to neurohypophysis
 Vasopressin (ADH) is a vasoconstrictor and also affects collecting ducts
of kidney and cause water resorption
 Oxytocin has effect on smooth muscle cells contraction, especially
during parturition, milk ejection, and sexual intercourse
Diabetes insipidus (DI)
polyuria and polydipsia
deficiency of arginine vasopressin (AVP), also known as antidiuretic hormone
(ADH).
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Adrenal Gland
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Adrenal gland Lies at superior pole of kidney in peri-renal adipose tissue
It has a capsule of dense irregular connective tissue with septa
Stroma is consists of reticular fibers and fibroblasts
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Adrenal gland has two distinct parts that are integrated into one organ Outer cortex
and inner medulla
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Cortex is mesodermal in origin produces steroid hormone
Medulla is from neural crest (ectoderm) produces vasoactive amines
Both areas are highly vascularized with fenestrated capillaries supplied by cortical
arterioles and medullary arterioles with some communication between them
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Adrenal Cortex
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Adrenal cortex is composed of 3 distinct layers; although in humans these are
not always easily distinguished
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Zona glomerulosa: outer layer
Zona fasciculata: middle layer
Zona reticularis: inner layer
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Zona Glomerulosa
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is the thin subcapsular zone constitutes about 13% of gland
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Rounded clusters of small columnar cells surrounded by capillaries
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Cells of this zone have a spherical nuclei, distinct nucleolus
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Acidophilic cytoplasm with few lipid droplets
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Extensive SER, mitochondria with shelf-like cristae, abundant RER, Golgi
complex, some lipid droplets
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These cells secrete mineralcorticoids mainly Aldosterone
This zone merges with the middle layer and is not always a complete layer
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Zona Fasciculata
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is the largest layer of cortex constitutes about 80% of total volume of gland
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Straight cords of polyhedral cells radially oriented separated by sinusoids
which are also arranged longitudinally
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Many lipid droplets which are extracted during tissue processing that make cells
vacuolated and cells known as spongiocytes
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Mitochondria with tubular cristae, extensive SER, some RER, lipofuscin
pigment
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These cells secrete Glucocorticoids ( Cortisol and Corticosterone)and some
androgens
Zona Reticularis
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About 7% of gland volume
Cells are alike to spongiocytes but smaller and have fewer lipid droplets
mitochondria with tubular cristae, large amount of lipofuscin pigments
Dark staining acidophilic cytoplasm
Cells of this zone forming anastomosing cords
Produce some androgens and glucocorticoids
Some macrophages may be present
Adrenal Medulla
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Chromaffin cells arranged in cords or cluster with reticular fiber network and many
capillaries
Cells have large spherical nuclei and many cytoplasmic granules containing
catecholamines: epinephrine or norepinephrine, well- developed Golgi complex,
numerous mitochondria
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About 20% of granules contain epinephrine and norepinephrine and the
remaining contain chromogranin, ATP and enkephalin
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Epinephrine released by sympathetic activation resulting in peripheral
vasoconstriction, hypertension, increased heart rate and increased blood glucose
Suprarenal glands medulla functions as a modified sympathetic ganglion
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Pancreas
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Endocrine part of pancreas is islets of Langerhans scattered among exocrine
acini
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Each islet is a highly vascularized nearly spherical structure which has 100-200
µm in diameter, there is over 1 million islets per pancreas
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Reticular fibers and a few fibroblasts form thin capsule around each islet
which also enter into it and form stroma of islet
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Islets are composed of cords of cells separated by capillaries
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Cords are mainly composed of 4 cell types, none of which can be differentiated
from each other by routine histological technique
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Cell Types in Islets
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Beta cells: 70% of cells, more in center of islets, dense core granule, produce
insulin which is released in response to increased blood glucose (glucose uptake by
cells)
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Alpha cells: 20% of cells, most at islet periphery, produce glucagon, increases blood
glucose via glycogen break down in hepatocytes and gluconeogenesis during fasting
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Delta cells: 5- 10% of cells, produce somatostatin, its paracrine effects is on B and
A cells and inhibits them, endocrinal affects smooth muscle cells of alimentary tracts
and gall bladder
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F cells (PP cells): few in number, produce pancreatic polypeptide, decreases
pancreatic exocrine secretion
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G cells: very few, secrete gastrin that increase HCl secretion and gastric motility
Thyroid Gland
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Thyroid gland forms from gut endoderm located just inferior to larynx
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A thin slender dense irregular connective tissue surrounds the gland
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Septa that are derived from the glands divide the glands into lobules
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The parathyroid glands located in the posterior aspects of thyroid within its
capsule
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Thyroid gland produces, stores and secretes thyroxine and triiodothyronine
which increase metabolic rate
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Thyroid is composed of follicles that are lined by low cuboidal or squamous
epithelium with lumen filled with colloid
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Between follicles are many fenestrated capillaries, some connective tissue
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Parafollicular (C) cells: larger, pale staining, in follicle epithelium or in clusters
between follicles but they don’t reach to the lumen of follicle, they are about 0.1% of
the epithelial cells
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Secrete calcitonin which decreases blood calcium by decreasing bone resorption by
osteoclasts and increasing bone formation by osteoblasts
Parathyroid Glands
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Parathyroid glands are 4 small glands that located on posterior side of
thyroid
Each gland is enveloped by its own thin connective tissue capsule
The paranchymal cells of 3 cell types: Chief cells, Oxyphil cells, and
adipocytes
Adipocytes appear at puberty and gradually increase in number until about
age 40 when most of the gland may be occupied by unilocular adipose
tissue
Chief and oxyphil cells are arranged in cords close to a network of
capillaries
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Chief cell: most common, small polygonal cells with acidophilic cytoplasm
and dark staining small nucleus secrete parathyroid hormone
In healthy adult with normal calcium balance 80% of chief cells are in resting
phase
In secreting phase chief cells contain stack of RER, Golgi complex and
parathyroid hormone granules arranged toward periphery of the cell
In resting phase lipid and glycogen are more but less RER and small Golgi
complex
Parathyroid hormone stimulates increased osteoclast activity leading to
increased blood calcium and phosphate
Parathyroid Gland Cells
•Oxyphil cells are less common, larger than chief cells, acidophilic cytoplasm
due to many mitochondria
•Function unknown, may be a different physiological state of chief cell
•They are rare before puberty, but increasing in numbers in early adult life
Pineal Body
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Pineal gland = epiphysis cerebri
projected from the roof of diencephalon
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It is covered by pia matter that form a capsule around the gland with septa which
divide the gland into incomplete lobule
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Paranchyma is made of Cell cords with many nerve fiber endings
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Pinealocytes: basophilic cytoplasm with large irregular nucleus; RER, SER,
numerous mitochondria , well developed cytoskeleton
Pinealocytes produce melatonin (during darkness) and serotonin (during the day)
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 Inerestitial cells are astroglial cells
 These cells have darker nucleus, long processes with glial filaments
 Pineal gland may have concretions of calcium phosphate known as brain
sand or corpora arenacea
 Brain sands appear during childhood and increase in size
 Postganglionic sympathetic nerve fibers synapses with pinealocytes
 Melatonin inhibits gonad development in prepubertal humans, may regulate
diurnal rhythmus, sleep/wake cycles,
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