[Light microscopic observations on intrapineal neurons and myelinated fibers distributed in the pineal gland and its capsule in cotton rats (Sigmodon hispidus) at various postnatal ages].

From birth to 300 days, neurons are mainly encountered in the distal third of the pineal gland. Neurons are scarce in the middle or proximal third; their number is fewer in the proximal third. Nuclear size of neurons in all pineal regions increases until 100-200 day. Since there are regional differences in size of nuclei of intrapineal neurons in cotton rats until 200 days, intrapineal neurons of this animal may be composed of different populations of cells. The observation that age changes in size of nuclei of intrapineal neurons are similar to those of pinealocytes suggests that such neuronal changes may be related to the functional differentiation of the organ. In all pineal regions, the number of neurons reduces to half from birth to 100 days. The enlargement of neuronal nuclei and the decrease in neuronal numbers proceed simultaneously with advancing age. Half the number of neurons may degenerate and disappear until 100 days. Myelinated fibers in the pineal gland and its capsule appear first at 20 days and their density increases with age. Myelinated fibers are mainly distributed in the distal pineal region and the capsule covering its posteroventral area. Since neurons are mainly located in the posteroventral area of the distal pineal region, a close topographical relation exists between myelinated fibers and intrapineal neurons. The myelinated fibers in the capsule may usually run from distal to proximal regions of the pineal gland; some of these fibers may enter the organ.