Early development of the inferior olivary complex in pouch young opossums. I. A. Light microscopic study

The inferior olivary complex (IOC) in the opossum is not present as a discernible nuclear group at birth (12–13 days post-conception), but it is seen by the third day in the pouch (postnatal day 3). At this age the IOC is a homogeneous mass of cells with no apparent nuclear subdivisions. Furthermore, cells destined for the IOC can be seen in migratory positions termed the marginal and submarginal migratory strands similar to those described for the rat (Altman and Bayer, '78a, b; Ellenberger et al., '69). By pouch day 5–7 the marginal migratory strand is no longer present; however, the submarginal stream of cells is still evident. As in 3-day-old opossums no nuclear subdivisions can be identified in the 5–day old animal. Analysis of both Nissl and Golgi preparations from 5–7 day old opossums reveals that olivary neurons are 5–7 μ in diameter and exhibit relatively short dendrites with infrequent spines. By 10–14 days in the pouch the rostral IOC has begun to separate into major nuclear subdivisions, whereas intermediate and caudal areas have not separated. Individual olivary cell bodies at this age are 6–9 μm dendritic shafts exhibit irregular contours and moniliform threads at their terminations, and spines arising from the dendrites are either pedunculated or sessile. In the 21–25-day-old opossum individual principal and accessory nuclei can be identified in a form similar to those in the adult; however, the entire IOC is less that one-fourth the length of its adult counterpart (Bowman and King, '73). Neuronal perikarya have increased in size to 10–12 μm, while the dendrites are varicose and often end as moniliform threads. Spines have similarly increased in number and complexity since the previous age and include sessile, pedunculated, multilobed, and elongate forms. Dendritic spines reach their greatest density in 36–40-day-old animals and decrease in number by pouch day 65–68. At this latter age the nerve cell bodies measure 15–22 μm in diameter. The time span between pouch days 21–25 and days 65–68 is characterized by tremendous growth as the length of the IOC increases to three-fourths of the adult size. The cerebellum was also examined from day 1 through day 68. At birth a rudimentary cerebellum contains darkly stained cells dorsal to the mitotically active ventricular epithelium. By pouch days 5–7 the immature cerebellum retains its plate-like appearance, but includes a superficial band of darkly stained cells. Two dense bands of cells, one superficial and one deeper, are present in the cerebellum by pouch days 10–14. Throughout pouch days 10–14 to 65–68 the developing cerebellum is acquiring its adult laminar appearance; however, an external granular layer is still conspicuous at 68 days of age. The present results indicate that the development of the IOC, from its initial appearance in the ventral medulla through its separation into discrete subnuclei, is markedly extended in the opossum when compared to the rat (Altman and Bayer, '78b). Moreover, the adult nuclear configuration of the opossum IOC is apparent prior to the appearance of distinct cortical laminae in the cerebellum. The significance of some of the morphological aspects of IOC development, e.g., the temporal relationship between spine density and the arrival of certain major olivary afferents, is discussed.