Observation on the development of the striatum in mice and rats.

The morphology and development of the striautum (caudate-putamen) have been examined in normal and reeler mice of the C57 BL/6J strain and in Sprague-Dawley rats. The striatum of an adult mouse contains approximately 690,000 neurons on each side and of these about 97% are medium-sized cells and just under 3% are large cells. In adult rats the percentage of large- and medium-sized neurons is about the same, but the total number of neurons is in excess of 1,500,000. Tritiated (3H) thymidine autoradiographic studies indicate that neuronogenesis in the mouse striatum occurs both in the neuroepithelium of the ventricular zone and in the underlying subventricular zone, between the 12th day of gestation and the first few days postnatally, with the peak period of neuronogenesis being on the 14th and 15th embryonic day. In both mice and rats striatal neurons are generated along three distinct gradients: from caudal to rostral, from lateral to medial, and from ventral to dorsal. The large neurons are among the earliest cells generated. A major proportion of the medium-sized neurons is also generated during this period, but proliferation of these neurons continues for at least several days. In the immediate postnatal period there appears to be some turnover of neurons in the striatum, some cells degenerating while others are still being formed. Cell counts indicate that in the rat there may be as many as 2,000,000 neurons in the striatum at postnatal day 4, but that this number is reduced to the adult figure by day 8.