Evidence has been accumulating which supports the hypothesis that through phylogenetic evolution, the mammalian pinealocyte has evolved from the pineal photoreceptor-cell which is characteristic of certain lower vertebrates. In the present study, developing pineal glands in fetal rats and hamsters were studied at light and electron microscopic levels to determine whether or not the ontogenetic development of pineal cells affords clues to their phylogenetic origin. Pineal development in fetal rats occurs during the last eight days of gestation (14-22) and in fetal hamsters, during the last five days of gestation (11-16). The pineal gland exhibits many similarities in the two species as it develops from a tubular evagination of the diencephalic roof into a compact cellular mass. Different cellular types could not be readily distinguished in fetal material from either species. In both animals, as long as lumina persist, cells bordering on these lumina exhibit surface modiiications reminiscent of developing photoreceptor-cells, i.e., an "ellipsoid-like" apical cytoplasmic bulge which often contains one or two centrioles and bears a ciliary derivative exhibiting a 9 + 0 tubular configuration. As pineal tissue proliferates, the lumina and ciliumlike structures disappear. The findings, when considered with phylogenetic studies, strengthen the hypothesis that the mammalian pineal gland contains cells derived from the photoreceptor-cell line.

The Harderian glands of the golden hamster exhibit a sexual dimorphism manifested by acinar cells of one type and the presence of porphyrin granules in the female, and by acinar cells of two types and the absence of porphyrin in the male. Castration of male hamsters produces Harderian glands of the female type. In the present study, it was found that a similar effect is not produced by pineal-induced testicular regression and that blinding inhibits the usual castration-induced change in the Harderian glands. Combinations of procedures involving castration, blinding, and pinealectomy of adult male hamsters indicate that the pineal gland is responsible for this inhibition. The findings support the hypothesis that elevated gonadotropin levels are responsible for producing the female type of Harderian glands in castrated males.

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Examination of Harderian glands of adult male and female golden hamsters by appropriate histochemical techniques reveals that adrenergic nerves are associated only with the blood vessels. Acetylcholinesterase-positive fibers are present in the connective tissue surrounding the gland, along the ducts, and among the acini.

The effects of blinding on cellular structure and porphyrin pigment content were studied in Harderian g1and.s of pinealectomized and nonpinealectomized adult female golden hamsters. Uterine weights were used as a criterion for pineal activity. Five weeks post-operatively, no significant changes were observed in the Harderian glands or in the uterine weights of animals in either group. Eleven weeks post-operatively, the blinded hamsters with intact pineal glands had significantly lowered uterine weights and Harderian glands that exhibited diminished amounts of porphyrin pigments and cellular characteristics of the male hamster. Harderian glands and reproductive organs from the 11-week blinded-pinealectomized hamsters did not differ from those of untreated controls.