Neurogenesis in the Rat Amygdala

Neurogenesis in the rat amygdala was examined with 3H-thymidine radiography. The animals were the offspring of pregnant females given two injections of 3H-thymidine on consecutive days in an overlapping series: Embryonic day (E) 12 + E13, E l 3 + E14. . . E21 + E22. On 60 days of age, the percentage of labelled cells and the proportion of cells added during each day of formation were determined a t several anatomical levels within the following components of the amygdala: anterior amygdaloid area, bed nuclei of the lateral and accessory olfactory tracts, central, medial, anterior cortical, posterolateral cortical, posteromedial cortical, basomedial, basolateral, and lateral nuclei, the amygdalo-hippocampal area, and the intercalated masses. All large and many small neurons originate in most nuclei between E l3 and E17, those in the intercalated masses between E l 5 and E19, those in the amygdalo-hippocampal area between El6 and E19. The anterior amygdaloid area, intercalated masses, central, medial, posterolateral cortical, posteromedial cortical, basomedial, basolateral, and lateral nuclei have strong rostral-to-caudal intranuclear gradients. There are five additional intranuclear gradients: 1) medial to lateral in the central nucleus, anterior amygdaloid area, and anterior intercalated masses; 2) lateral to medial in the bed nucleus of the lateral olfactory tract and basolateral nucleus; 3) superficial to deep in the amygdalohippocampal area, posterolateral, and posteromedial cortical nuclei; 4) ventral to dorsal in the medial nucleus; and 5 ) dorsal to ventral between the small and large-celled parts of the lateral nucleus. Only the bed nucleus of the accessory olfactory tract and the anterior cortical nucleus do not have intranuclear gradients. Between 10 and 15% of the total cell population in most nuclei are very small neurons and/or glia which originate simultaneously between E l8 and E20. This population is absent in the ventral part of the medial, anterior cortical, and anterior basomedial nuclei; these contiguous areas may form a distinct subunit in the amygdala. In contrast to the pronounced intranuclear gradient, internuclear gradients are weak. Neurogenesis in the central nucleus and corticomedial and basolateral complexes appears to take place both concurrently and independently. There are groups of early-originating neurons in the central, medial, and basolateral nuclei located near the periphery of the amygdala. Each of these groups is surrounded by younger neurons farther within the interior. The youngest cells are in the centrally placed intercalated masses. These settling patterns suggest that cells in the amygdala arise simultaneously from more than one neuroepithelial source during morphogenesis. The chronology of neurogenesis in the amygdala can be related to some of its anatomical connections. Rostral-to-caudal gradients in the corticomedial complex may be timed to coincide with early vs. late arrival of olfactory fibers. The subdivisions based on intranuclear gradients in the central, medial, and basolateral nuclei match the subdivisions based on patterns of anatomical connec-