Histology 2016-2017 Department of Anatomy &Histology: Dr.Rajaa Ali *********************************************************** Endocrine system part III The adrenal (suprarenal) glands secrete both steroid hormones and catecholamines. 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 sinusoids. 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 nerves. 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 metabolism. 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.