GENESIS AND FATE OF THE PERIRETICULAR THALAMIC NUCLEUS DURING EARLY DEVELOPMENT

A striking feature of the internal capsule during early development is that it is full of small neurones. Later, this group of neurones, cal led the perireticular thalamic nucleus, appears to have reduced in siz e, and only a few scattered cells are seen. In an effort to understand better the developmental history of the perireticular nucleus this st udy examines: i) the period of cell generation in the nucleus, ii) the magnitude of cell loss in the nucleus, and iii) the subsequent fate o f cells in the nucleus during development. The perireticular cells are generated very early in development, being among the first generated in the thalamus (rats: E13-14; cats: E21-30). In rats, the first perir eticular cells are generated at about; the same developmental stage as the first subplate cells, which are among the first generated cells o f the cortex: in cats, the first perireticular cells are generated wel l before those in the subplate (E24-30). In rats, the number of perire ticular cells during development peaks at P5 (similar to 30,000) and t hen declines sharply(similar to 98%) by P15 (similar to 750), when adu lt-like patterns are seen. This dramatic loss of perireticular cells i s due to both cell death and a migration of cells into the adjacent gl obus pallidus. The majority of the perireticular cells which migrate i nto the globus pallidus, however, are likely to die also. The presence of pyknotic profiles (indicators of dying cells) in the rat periretic ular nucleus points to cell death, as a contributor to the reduction i n cell number during development. In this study, a period of relativel y high pyknotic profile incidence (number of pyknotic cells per 1,000 ''living'' cells) is recorded in the perireticular nucleus over a 5 da y period, from P2 to P7 (13.5-15.5). Similar values and patterns are r ecorded in the reticular nucleus and globus pallidus, except that in t hese structures, a period of relatively high pyknotic profile incidenc e (15-20) occurs over a shorter period (3 days; P2-5). Previous studie s have suggested that some perireticular cells migrate into and settle within the adjacent globus pallidus. This study, with the use of long -term survivals after tracer injections in rats, shows that none (or v ery few) of these perireticular cells which migrate into the globus pa llidus survive into more mature postnatal stages. Tracer (biotinylated dextran) was injected into tile sensory nuclei of the dorsal thalamus at early stages (P7) and the rats were allowed to survive for either a day thereafter (to P8) or until well after the period of cell death was complete (to P16 or P21). In the short-term survivals ito P8), the re are many dextran-labelled cells seen in the globus pallidus and in the perireticular nucleus. In the long-term survivals (to P16 or P21), by contrast, there are no dextran-labelled cells apparent in the glob us pallidus or in the perireticular nucleus. It is likely that these c ells in the globus pallidus, as with those in the perireticular nucleu s, undergo cell death during development. (C) 1996 Wiley-Liss, Inc.