Abstract
Intrapineal neurons and intracapsular and intrapineal myelinated fibers were studied by light microscopy in male cotton rats (Sigmodon hispidus) at various postnatal ages. Until 300 days of age, neurons are encountered mainly in the distal third of the organ; neuronal nuclei are largest in this region. The remaining regions contain only a few neurons with smaller nuclei. In all regions, nuclear sizes of the neurons increase, and neuronal numbers reduce by half until 100-200 days. Neurons thus consist of different populations of cells, and the aging changes in neuronal nuclei seem to be related to the differentiation of the organ. The disappearance of neurons and the enlargement of the nuclei of remaining neurons proceed simultaneously with age. Intracapsular and intrapineal myelinated fibers appear at 20 days and their density increases with age. As the myelinated fibers are distributed mainly in the distal region and its capsule, these fibers are related topographically to intrapineal neurons. The intracapsular myelinated fibers continue into bilateral bundles of myelinated fibers found under the transverse sinuses, corresponding to the nervi conarii. Since the intracapsular fibers and the fibers traced between the capsule and the parenchyma bifurcate in a proximal direction, the intracapsular fibers are considered to run proximally, some entering the organ.
