Postnatal development of the telencephalon of the tammar wallaby (Macropus eugenii). An accessible model of neocortical differentiation.

The sequence of development of cell layers in the neocortex of the tammar has been followed from 24 days gestation to 213 days postnatal. The tammar is born at 27 days gestation and the major period of its development occurs during the subsequent 250 days, most of this time being spent within the pouch. Although the pattern of differentiation of the cell layers appears to resemble that described for many Eutherian mammals, the neocortex is at an embryonic 2 layered stage at birth and a cortical plate is not present throughout the telencephalon until 10-15 days postnatal. A transient subplate zone, presenting a characteristic appearance with widely spaced rows of cells aligned parallel to the cortical surface, develops between 20 and 70 days postnatal, but no secondary proliferative region is seen in the subventricular zone of the dorso-lateral wall. Preliminary experiments with (3H)-thymidine injections indicate that the cortical plate follows the "inside-out" pattern of development described in many Eutherian mammals and that the oldest neurons are found in the parallel cell rows of the subplate zone. The importance of the late differentiation of the neocortex in relation to the time of birth and the resulting usefulness of the tammar as an experimental model of cortical development is discussed.