Cytodifferentiation of the Chick Pineal Gland, With Special Reference to the Photosensory and Secretory Elements

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Abstract
Cytodifferentiation of the chick pineal gland throughout the embryonic development was investigated with light and electron microscopy. The chick pineal anlage appears first as a small evagination in the diencephalic roof at 60 h of incubation (27–30 somites). Until day 5 of incubation, pineal anlage cells are undifferentiated and appear similar to ventricular ependymal cells. Subsequently, pinealocytes and supporting cells are first distinguishable at 7–8 days, and parafollicular cells are distinguishable at 12 days of incubation. Pigment-containing cells after 6 days and nerve cells after 17 days of incubation gradually increase, especially in the posterior wall of the pineal recess. During embryonic development, the chick pineal gland has both photosen-sory and secretory elements: viz. the former, mitochondria-laden apical protrusions, synaptic ribbons, lamellar whorl-like cilia of the pinealocytes, and adjacent appearance of the pigment-containing cells and the nerve cells; and the latter, dense-cored vesicles of the pinealocytes and dense bodies of the supporting cells. Moreover, nuclear invaginations having a large lipid droplet nearby and some aggregations of glycogen are found in the pinealocytes and are transitory changes in structure restricted to certain days of incubation.