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Notes: 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 group of ventral thalamic cells which lies between the dorsal thalamus and the primordial internal capsule. This group of cells, the primordial reticular nucleus, gives rise to the main body of the reticular nucleus, the perigeniculate nucleus and the perireticular nucleus. In the reticular nucleus, there are two waves of parvalbumin expression during development. The first wave begins prenatally in small cells which are seen rarely after birth. Their fate is not clear: they may have lost immunoreactivity, migrated elsewhere, or died. At the end of the first wave, a second wave begins in a distinct group of larger ovoid reticular cells, which appear to remain into adulthood. At about birth, the dorsocaudal pole of the reticular nucleus first forms the perigeniculate nucleus. During this developmental stage, cells which make up the reticular and perigeniculate nuclei are the only parvalbumin-immunostained structures in the thalamus. Thus, rather than develop from the dorsal thalamus, the perigeniculate nucleus seems to have its origins in the ventral thalamus together with the reticular nucleus. During development, the reticular nucleus is associated closely with a large mass of cells located in the internal capsule, called the perireticular nucleus. Later, the perireticular nucleus is dramatically reduced in size: 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 internal capsule. There are two cytoarchitectonically distinct groups of perireticular cells. One group of cells, called the large-celled perireticular zone (LPR), enters the internal capsule from early prenatal development (E25). Many of these cells reach the globus pallidus and extend as far as the cortical subplate zone. The LPR together with the subplate form an extensive neuronal network in the white matter during early development, which disappears later in development (about postnatal day 20). The second group of perireticular cells is made up of smaller cells and is called the small-celled perireticular zone (SPR). These 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.