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The body has two adrenal (suprarenal) glands, one of which is located superior to each kidney.In the human, they are about 4-6cm long, 1-2cm wide & 0.5cm thick, and both may weigh about 15gm.
Each gland is surrounded by a thick connective tissue capsule that sends septa into the interior of the gland.
They are divisible by embryonic origin, structure, and function into cortex and medulla.
Adrenal Cortex
Embryonic origin:
The adrenal cortex derives from mesoderm.
Structure in adults:
Cells of the adrenal cortex have the characteristic steroid-synthesizing cell structure (polygonal or rounded, with pale- staining central nucleus, acidophilic cytoplasm that contains many lipid droplets, abundant SER , mitochondria containing enzymes),
The suprarenal cortex consists of three concentric layers or zones:
a) zona glomerulosa.
b) zona fasciculate.
c) zona reticularis.
a. Zona glomerulosa :
this outermost cortical layer lies directly under the capsule.constitutes about 15% of adrenal volume.
composed of small cells arranged in form of clusters (glomeruli), separated by thin-walled capillaries.
ultrastructurally the cells contain well developed SER and comparatively little RER with scanty lipid droplets.
the cells of this layer secrete mineralocorticoids.
b. Zona fasciculata :
this middle layer of the cortex.
constitutes about 65% of adrenal volume .
Its cells form straight cords or columns (usually 2-3cells wide) separated by capillaries the columns are perpendicular to the organ surface.
ultrastructurally, the cells have prominent RER, extensive lipid vacuoles and characteristic small round or ovoid mitochondria.
Its cells produce glucocorticoids and some adrenal androgens.
c. Zona reticularis :
this innermost layer of the adrenal cortex.constitutes about 7% of adrenal volume.
consists of an anastomosing network of cell cords with capillary network closely apposed to the cell membranes.
Its cells are smaller and more acidophilic than those in the fasciculata and contain fewer lipid droplets, more mitochondria, many lipofuscin granules and prominent SER.
the reticularis and fasciculata seem to constitute a single functional zone, with the reticularis producing most of the glucocorticoids and adrenal androgens.
Normal function.
The adrenal cortex produces three types of steroid hormones.1. Mineralocorticoids mainly aldosterone: produced by the zona glomerulosa in response to angiotensin II, it stimulates sodium absorption by the distal renal tubules.
2. Glucocorticoids mainly cortisol and corticosterone: produced by the zona reticularis in response to ACTH and by the fasciculata after prolonged stimulation, glucocorticoids control carbohydrate metabolism and suppress the immune response by decreasing circulating lymphocytes and eosinophils.
3. Adrenal androgens: secreted by zona reticularis in response to ACTH and by fasciculata after prolonged stimulation, they produce musculinizing and anabolic effect.
Adrenal Medulla
Embryonic origin.
The adrenal medulla derives from the neural crest.Structure.
The adrenal medulla is composed of epithelial cells supported by a delicate connective tissue richly supplied by blood sinusoids.It contains two major cell types:
a) chromaffin cells.
b) ganglion cells.
a. Chromaffin cells. Also known as pheochromocytes
these are the predominant cell type.they contain large nuclei and cytoplasmic granules that become brown when exposed to chromium salts for this reason, they are sometimes referred to as chromaffin cells.
the chromaffin cells synthesize and secrete norepinephrine and epinephrine in response to stimulation by preganglionic sympathetic nerve fibers, some cells are thought to produce serotonin in addition to epinephrine, the hormones are released by process of exocytosis.
with an electron microscope, the cells shows well developed Golgi complex, few profiles of RER, many oval mitochondria, and membrane bound cytoplasmic granules.
There are two types of chromaffin cells:-
1- cells secrete norepinephrine have dense core granules.
2- light cells secrete epinephrine have granules not so dense.
b. Ganglion cells: few parasympathetic ganglion cells present show typical
morphologic characteristics of autonomic ganglion cells (large, multipolar cells,
randomly distributed throughout ganglion, have large and ovoid nuclei mostly eccentric).
Adrenal Blood Supply :
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1. Arteries:Three main arteries supply each adrenal gland :a. superior suprarenal from the inferior phrenic artery.
b. middle suprarenal from the aorta artery.
c. inferior suprarenal from the renal artery.
These arteries penetrate the capsule separately, and their branches anastomose to form a subcapsular arterial plexus, this plexus gives rise to three groups of arteries:
a. arteries of the capsule.
b. arteries of the cortex which branch to form the cortical capillaries thatpass between the secretory cells and drain into the medullary capillaries.
c. arteries of the medulla, which traverse the cortex without branching
until they reach the medulla, where they form the medullary capillaries.
2. Medullary capillaries: receive a double blood supply from both cortex and medulla and converge to form several medullary veins.
3. Medullary veins converge to form a single large suprarenal vein.
4. Suprarenal vein arises at the core of the medulla and drains into the renal vein or directly into the inferior vena cava.
PARAGANGLIA
Paraganglia are specialized neuroendocrine glands associated with the autonomic nervous system.
Consists of prominent neuroendocrine cells containing neurosecretory vesicles.
It is known that some of the larger paraganglia in the neck and thorax (e.g. carotid body and aortic body) are regarded as part of neuroendocrine system and act as chemoreceptors.
Paraganglia vary in size from clumps of few cells associated with nerves visible by microscope, to structures up to 3mm in diameter which are anatomically distinct.
Paraganglia are composed of: chief cells, sustentacular cells, and rich innervation.
a.Chief cells:
Are neuroendocrine cells,
Arranged in round clumps or nests,
Each clump is surrounded by rich network of capillaries with fenestrated
endothelium and basement membrane.
Are of two types a. Light cells-(have vacuolated cytoplasm ).
b. Dark cells- (have darker staining cytoplasm).
b. Sustentacular (support) cells:
Accounts for 35-45% of paraganglia cells.
Have spindle shaped-nuclei.
Pale staining cytoplasm.
ILL defined cell borders.
Behaves in a similar manner to Schwann cells.
c. Innervation:
The paraganglia are richly innervated by both sympathetic and parasympathetic nervous system.