Postnatal development of the dog pineal gland: electron microscopy.

The ultrastructure of the dog pineal gland from the first postnatal day to the seventh month is described. In the first postnatal stages, pineal parenchyma only shows immature proliferative cells with abundant cytoplasmic glycogen. Nerve fibers are seen in the pineal connective tissue spaces. The differentiation of the dog pineal cell types begins in the first postnatal week. Both pinealocytes and pigmented cells are first seen on the fourth postnatal day. The pineal astrocytes are observed on the tenth day. Immature cells are still found in the pineal gland of 1 mo-old dogs. The differentiation of the dog pineal cell types is completed by the second postnatal month.