Comparative Anatomy of the Mammalian Hypothalamic Suprachiasmatic Nucleus

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Abstract
A detailed analysis of the cytoarchitecture, retinohypothalamic tract (RHT) pro jections, and immunohistochemical localization of major cell and fiber types within the hy pothalamic suprachiasmatic nuclei (SCN) was conducted in five mammalian species: two species of opossum, the domestic cat, the guinea pig, and the house mouse. Cytoarchitectural and immunohistochemical studies were conducted in three additional species of marsupial mammals and in the domestic pig. The SCN in this diverse transect of mammalian taxonomy bear striking similarities. First, the SCN are similar in location, lying close to the third ventricle (3V) dorsal to the optic chiasm (OC), with a cytoarchitecture characterized by small, tightly packed neurons. Second, in all groups studied, the SCN receive bilateral retinal input. Third, the SCN contain immu nohistochemically similar elements. These similarities suggest that the SCN developed char acteristic features early in mammalian phylogeny. Some details of SCN organization vary among the species studied. In marsupials, vaso pressin-like immunoreactive (VP-LI) and vasoactive intestinal polypeptide-like immunoreac tive (VIP-LI) cells codistribute primarily in the dorsomedial aspects of the SCN, while in eutherians, VP-LI and VIP-LI cells are separated into SCN subnuclei. Furthermore, the marsupial RHT projects to the periventricular dorsomedial region, whereas the eutherian RHT projects more ventrally in the SCN into the zone that typically contains VIP-LI perikarya.