DEVELOPMENT OF THE THALAMIC RETICULAR NUCLEUS IN FERRETS WITH SPECIALREFERENCE TO THE PERIGENICULATE AND PERIRETICULAR CELL GROUPS

This study describes the development of the ferret thalamic reticular nucleus from Nissl-stained and from parvalbumin-immunostained sections . From early stages [embryonic day (E) 23 - E25], there is a large gro up of ventral thalamic cells which lies between the dorsal thalamus an d the primordial internal capsule. This group of cells, the primordial reticular nucleus, gives rise to the main body of the reticular nucle us, the perigeniculate nucleus and the perireticular nucleus. In the r eticular nucleus, there are two waves of parvalbumin expression during development. The first wave begins prenatally in small cells which ar e seen rarely after birth. Their fate is not clear: they may have lost immunoreactivity, migrated elsewhere, or died. At the end of the firs t wave, a second wave begins in a distinct group of larger ovoid retic ular cells, which appear to remain into adulthood. At about birth, the dorsocaudal pole of the reticular nucleus first forms the perigenicul ate nucleus. During this developmental stage, cells which make up the reticular and perigeniculate nuclei are the only parvalbumin-immunosta ined structures in the thalamus. Thus, rather than develop from the do rsal thalamus, the perigeniculate nucleus seems to have its origins in the ventral thalamus together with the reticular nucleus. During deve lopment, the reticular nucleus is associated closely with a targe mass of cells located in the internal capsule, called the perireticular nu cleus. Later, the perireticular nucleus is dramatically reduced in siz e: that is, there is a large reduction in the number of perireticular cells seen per section and in the extent of the nucleus across the int ernal capsule. There are two cytoarchitectonically distinct groups of perireticular cells. One group of cells, called the large-celled perir eticular zone (LPR), enters the internal capsule from early prenatal d evelopment (E25). Many of these cells reach the globus pallidus and ex tend as far as the cortical subplate zone. The LPR together with the s ubplate form an extensive neuronal network in the white matter during early development, which disappears later in development (about postna tal day 20). The second group of perireticular cells is made up of sma ller cells and is called the small-celled perireticular zone (SPR). Th ese small cells enter the internal capsule from the reticular nucleus just prior to birth. Many of the cells in the SPR remain in the adult.