Time of neuron origin in preoptic and septal areas of the mouse: An autoradiographic study

Abstract Times of neuron origin were determined for septal and preoptic nuclei of adult mice given a single injection of H 3 ‐thymidine at some stage of development. In the septal area, the sequence of neuron origin for its constituent nuclei roughly follows phylogenetic trends. Mediolateral and lateromedial gradients in septal and preoptic areas, respectively, seem to be opposites but are both “outside‐in,” as is a rostrocaudal septal gradient when viewed sagittally. An “inside‐out” gradient characterizes the olfactory tubercle and possibly the periventricular proptic region. These findings strengthen the impression that cells of nuclear areas undergo final division in an “outside‐in” sequence; those of layered structures arise “inside‐out.” A dorsoventral gradient in periventricular preoptic region parallels that in hypothalamus. The significance of these gradients is unknown, but they often presage corresponding gradients in nuclear differentiation, as in the preoptic area. Cell birthdates for nucleus accumbens resemble those in the striatum more than in the septum, where the nucleus is usually catalogued. To help resolve the issue of whether the preoptic area is telencephalic or diencephalic, it was compared with neighboring regions. No conclusions were reached, since similarities exist in both directions. The validity of a telodiencephalic boundary based on cytoarchitectonics. circuitry, or function is considered; such separation of the forebrain, except for topographical convenience, appears unwarranted.