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NORMAL DEVELOPMENT

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The adrenal cortex is of mesodermal origin, whereas the medulla forms from ectodermal elements. The primitive adrenal cortex arises from coelomic mesoderm around the fifth week of gestation. Additional cells migrate to join this initial aggregation at approximately 7 weeks. This early adrenal cortex separates from the peritoneal mesothelium by the end of the eighth week. Adrenocortical differentiation continues after birth, resulting in completed formation of the zona glomerulosa, zona fasciculi, and zona reticularis by about the age of 3 years.

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The adrenal medulla forms from neural crest cells that migrate from the embryonic ectoderm to form the primitive sympathetic ganglia. Some of these differentiate into endocrine cells (chromaffin cells) and migrate to the medial aspect of the developing adrenal cortex at about the 10th week of gestation. There is prominent paraganglionic chromaffin tissue during fetal development. After birth, the majority of the extraadrenal chromaffin tissue atrophies, but small collections of these cells persist adjacent to the origins of the celiac and superior mesenteric arteries and in the organ of Zuckerkandl adjacent to the inferior mesenteric artery.

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Early in fetal development, the adrenal gland is larger than the adjacent kidney. At the time of birth, the adrenal gland of most neonates is approximately one-third the size of the ipsilateral kidney. The cortical portions of the glands rapidly atrophy during the first few weeks of life to assume their mature morphology. In the normal neonate, the adrenal medulla is hyperechoic on sonography, and the cortex is relatively hypoechoic. The entire gland is hypoechoic after the first year of life.

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Development of the adrenal glands is independent to that of the kidneys. Therefore, adrenal gland location is usually normal despite renal ectopia or aplasia. In this situation, the adrenal gland may assume an elongated, flattened, or elliptical shape.1,2 When elliptical, care must be exercised to avoid mistaking the normal adrenal gland for a hypoplastic kidney on diagnostic imaging studies. The abnormally shaped adrenal gland in an infant with renal aplasia or ectopia maintains a normal pattern of echogenicity, with a central echogenic stripe and a peripheral hypoechoic zone.

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The medulla of the adrenal gland synthesizes and secretes the catecholamines epinephrine and norepinephrine. The surrounding cortex consists of 3 zones. The outer zone, the zona glomerulosa, secretes aldosterone, which regulates renal tubular sodium retention and potassium excretion. The inner 2 zones, the zona fasciculata and the zona reticularis, secrete cortisol as well as small amounts of androgenic steroids.

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The hypothalamus and anterior pituitary gland control adrenal steroid secretion. Corticotropin-releasing hormone secreted by the hypothalamus stimulates the release of corticotropin (adrenocorticotropic hormone [ACTH]) in the anterior lobe of the pituitary gland. Circulating corticotropin evokes the release of cortisol and androgens from the adrenal cortex. Prolonged elevation of corticotropin causes enlargement of the adrenal gland.

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Corticotropin plays a minor role in controlling aldosterone secretion. This ...

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