The suprarenal gland develops from two components: (a) a mesodermal portion, which forms the cortex, and (b) an ectodermal portion, which forms the medulla. During the fifth week of development, mesothelial cells between the root of the mesentery and the developing gonad begin to proliferate and penetrate the underlying mesenchyme (Fig. 19.42). Here they differentiate into large acidophilic organs, which form the fetal cortex, or primitive cortex, of the suprarenal gland (Fig. 19.44A). Shortly afterward a second wave of cells from the mesothelium penetrates the mesenchyme and surrounds the original acidophilic cell mass. These cells, smaller than those of the first wave, later form the definitive cortex of the gland (Fig. 19.44, A and B). After birth the fetal cortex regresses rapidly except for its outermost layer, which differentiates into the reticular zone. The adult structure of the cortex is not achieved until puberty.
While the fetal cortex is being formed, cells originating in the sympathetic system (neural crest cells) invade its medial aspect, where they are arranged in cords and clusters. These cells give rise to the medulla of the suprarenal gland. They stain yellow-brown with chrome salts and hence are called chromaffin
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