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Histology 2016-2017
Department of Anatomy &Histology:
Dr.Rajaa Ali
Endocrine system part III
 The adrenal (suprarenal) glands secrete both steroid hormones and
 They have a flattened triangular shape and are embedded in the perirenal fat at the
superior poles of the kidneys.
 The adrenal glands are covered with a thick connective tissue capsule from which
trabeculae extend into the parenchyma , carrying blood vessels and nerves.
 The secretory parenchymal tissue is organized into two distinct regions (Fig.1):
 The cortex is the steroid-secreting portion. It lies beneath the capsule and
constitutes nearly 90% of the gland by weight.
 The medulla is the catecholamine-secreting portion. It lies deep to the
cortex and forms the center of the gland.
Parenchymal cells of the cortex and medulla are of different embryologic origin.
 Embryologically, the cortical cells originate from mesodermal mesenchyme,
whereas the medulla originates from neural crest cells that migrate into the
developing gland .
 Although embryologically distinct, the two portions of the adrenal gland are
functionally related . The parenchymal cells of the adrenal cortex are controlled
in part by the anterior lobe of the pituitary gland and function in regulating
metabolism and maintaining normal electrolyte balance.
Blood Supply:
The adrenal glands are supplied with blood by the superior, middle, and
inferior suprarenal arteries. These vessels branch before entering the capsule,
to produce many small arteries that penetrate the capsule. The vessels form a
system that consists of :
 capsular capillaries that supply the capsule.
 fenestrated cortical sinusoidal capillaries that supply the
cortex and then drain into the fenestrated modularly capillary
 medullary arterioles that traverse the cortex, traveling within
the trabeculae , and bring arterial blood to the medullary
capillary sinusoids.
Lymphatic vessels are present in the capsule and the connective tissue around the
larger blood vessels in the gland.
Cells of the Adrenal Medulla:
Chromaffin cells located in the adrenal medulla are innervated by
presynaptic sympathetic neurons.
The central portion of the adrenal gland, the medulla, is composed of a
parenchyma of large, pale-staining epithelioid cells called chromaffin cells
(medullary cells), connective tissue, numerous sinusoidal blood capillaries, and
The chromaffin cells are, in effect, modified neurons (Fig.1).
Numerous myelinated, presynaptic sympathetic nerve fibers pass directly to the
chromaffin cells of the medulla . When nerve impulses carried by the sympathetic
fibers reach the catecholamine-secreting chromaffin cells, they release their
secretory products.
Therefore, chromaffin cells are considered the equivalent of postsynaptic neurons.
However, they lack axonal processes.
Ganglion cells are also present in the medulla. Their axons extend peripherally to
the parenchyma of the adrenal cortex to modulate its secretory activity and
innervate blood vessels.
Chromaffin cells of the adrenal medulla have a secretory function, the
catecholamines epinephrine and norepinephrine secreted by the chromaffin cells
are produced by different cell types:
 One population of cells contains only large dense core vesicles.
These cells secrete norepinephrine.
 The other population of cells contains vesicles that are smaller, more
homogeneous, and less dense. These cells secrete epinephrine.
 Glucocorticoids secreted in the cortex induce the conversion of norepinephrine
to epinephrine in chromaffin cells. The catecholamines, in concert with the
glucocorticoids, prepare the body for the “fight-or-flight” response.
Zonation of the Adrenal Cortex:
The adrenal cortex is divided into three zones on the basis of the arrangement of
its cells (Fig.1):
• Zona glomerulosa, the narrow outer zone that constitutes up to 15% of the
cortical volume.
• Zona fasciculata, the thick middle zone that constitutes nearly 80% of the
cortical volume.
• Zona reticularis, the inner zone that constitutes only 5% to 7% of the cortical
volume but is thicker than the glomerulosa because of its more central location
Zona Glomerulosa:
The cells of the zona glomerulosa are arranged in closely packed ovoid clusters
and curved columns that are continuous with the cellular cords in the zona fasciculate ,
are relatively small and columnar or pyramidal. Their spherical nuclei appear closely
packed and stain densely.
The zona glomerulosa secretes aldosterone, which functions in the control of
blood pressure.
The cells of the zona glomerulosa secrete mineralocorticoids, compounds that
function in the regulation of sodium and potassium homeostasis and water balance. The
principal secretion, aldosterone, acts on the distal tubules of the nephron in the kidney,
the gastric mucosa, and the salivary and sweat glands to stimulate resorption of sodium
at these sites, as well as to stimulate excretion of potassium by the kidney.
Zona Fasciculata:
The cells of the zona fasciculata are large and polyhedral. They are arranged in
long straight cords, one or two cells thick, that are separated by sinusoidal capillaries.
The cells of the zona fasciculata have a lightly staining spherical nucleus. The principal
secretion of the zona fasciculata is glucocorticoids that regulate glucose and fatty acid
Zona Reticularis:
The zona reticularis produces glucocorticoids and androgens. The cells of the
zona reticularis are noticeably smaller than those of the zona fasciculata, and
their nuclei are more deeply stained. They are arranged in anastomosing cords
separated by fenestrated capillaries. The cells have relatively few lipid droplets.
The cells in this zone are small because they have less cytoplasm than the cells in
the zona fasciculata; thus the nuclei appear more closely packed. They exhibit
features of steroid- secreting cells.
The principal secretions of the zona reticularis are weak androgens. The
principal secretion of the cells in the zona reticularis consists of weak androgens,
mostly dehydroepiandrosterone (DHEA). The cells also secrete some
glucocorticoids, in much smaller amounts than those of the zona fasciculata.
Fig.(4): Adrenal gland A, arteries ; AT, adipose tissue ; BV, blood vessels ; Cap, capsule ; Cort,
cortex ; Med, medulla; ZF, zona fasciculate ; ZG, zona glomerulosa ; ZR, zona reticularis ;
arrows, connective tissue trabeculae ; dashed line, corticomedullary boundary.