The formation and growth of the cortical layers in the cerebellum of the opossum.

The development of the cerebellar cortex in the opossum was analyzed in Nissl-stained sections using qualitative and quantitative methods. The young of the opossum are born 12-13 days after conception and mature for approximately 85 days in an external pouch providing an excellent model for embryological studies. Qualitative observations of cerebellar growth were made from birth to postnatal day (PN) 19. At birth the opossum cerebellar anlage can be divided into two layers, a ventricular layer and an intermediate layer; histologically his is comparable to the rat cerebellar anlage at embryonic day 13 (Altman and Bayer 1978) and the human cerebellar anlage prior to the seventh embryonic week (Rakic and Sidman 1970). By PN 3 the cerebellar anlage consists of five layers: the ventricular layer, the ventral intermediate layer, the acellular layer, the dorsal intermediate layer and the marginal layer. The external granular layer begins migrating over the dorsal surface of the cerebellum at PN 5. The immature Purkinje cell layer is first seen at PN 12 and is subsequently arranged as four clusters between PN 12 and PN 22. At PN 19 the opossum cerebellum is comparable to the rat cerebellum at birth (Korneliussen 1968c). A quantitative analysis of cerebellar growth was performed between PN 17 and PN 77 using vermal sections. The area and thickness of each of the cortical layers was determined from five vermal sagittal sections using two methods; a Zeiss Videoplan and a point counting system. The external granular layer increases in area from PN 17 to PN 75, however its maximal width is achieved between PN 19 and PN 33. The persistence of the EGL until after PN 105 suggests that synaptic contacts between granule cell axons and Purkinje cells may continue to form after PN 77 when the Purkinje cell is mature based on Golgi and fine structural features (Laxson and King 1983). Between PN 17 and PN 77 the area of the molecular layer and the area of the internal granular layer increase at a more rapid rate than the other cerebellar layers. The maturation of the cerebellum in the opossum is a lengthy process lasting approximately 77 days in comparison to rodent cerebellar growth which requires about 25 days (Korneliussen 1968c). Also, the entire process of cortical lamination occurs after birth while the opossum is maturing in an external pouch.