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.
The postnatal development of somatostatin (SRIF)-immunoreactive neurons, previously labeled with [3H]thymidine on embryonic days E14-E22, has been studied in the rat occipital cortex. Immunocytochemistry combined with autoradiography showed an "inside-out" maturation pattern. Only SRIF neurons generated at E14 were present in layer VI in newborn rats. Later generated SRIF neurons appeared progressively higher in the cortex until about postnatal day 12 when SRIF neurons from E21 appeared in layer II. At 2 weeks of age, therefore, all SRIF neurons from E14-E21 were present. Most of these had been generated between E15 and E17 with a moderate number at E14 and rapidly diminishing numbers from E18 to E21. Although an overall layered distribution was apparent at peak production, there was a tendency for diffuse distribution most noticeable at E17. Diffusely distributed neurons were more likely to be below their appropriate layer than above it, thus contributing extra SRIF neurons to layer VI. At 3, 4, and 5 weeks, progressively fewer SRIF neurons were seen with a consequent reduction in the number of double-labeled neurons. It is suggested that the transient population of SRIF neurons thus revealed plays a role in cortical development.
The postnatal development of neuropeptide Y (NPY)-immunoreactive neurons, previously labeled with [3H]thymidine on embryonic days E14-E21, has been studied in the rat occipital cortex. Immunohistochemistry combined with autoradiography showed evidence of a modified "inside-out" pattern of maturation. NPY-neurons are generated between E14 and E20 and are found in layers II-VI of the cortex and the subcortical white matter. NPY neurons from all these birthdates are overproduced at first, although cells generated at E16 produce the greatest excess, followed by E15 and E17. Some of these transient neurons are found in the "wrong" layer for their birthdates, and their elimination produces a more "correct" alignment at maturity. However, most of the NPY neurons that survive are generated at E17, and these cells are found throughout layers II-VI with a preponderance in layer VI. This evidence is strongly suggestive of cell death rather than merely cessation of production of NPY.
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