An immunohistochemical study of pinealocytes of chicks and some other lower vertebrates by means of visinin (retinal cone-specific protein)-immunoreactivity.

Visinin-like immunoreactivity was recognized in a large population of cells in the pineal organs of submammals including goldfish, frogs, turtles and chicks. In the chick pineal organ two discrete immunoreactive cells--follicular and parafollicular cells--were identified at young stages of development. Judging from the detailed morphology of the individual cells revealed by visinin-immunoreactivity and a morphological comparison with the retina and brain, it is supposed that the former represents an intermediate form closer to the photoreceptor cell and the latter yet another intermediate form closer to the neuroendocrine type. At older stages of the development both cell types assume a morphology similar to the neuroendocrine type. This finding infers that the developmental change in the morphology of the chick pinealocytes represents a condensed expression of the phylogenic development in the ontogeny.