THE EARLY DEVELOPMENT OF MAJOR PROJECTIONS FROM CAUDAL LEVELS OF THE SPINAL-CORD TO THE BRAIN-STEM AND CEREBELLUM IN THE GRAY SHORT-TAILED BRAZILIAN OPOSSUM, MONODELPHIS-DOMESTICA
Yq. Qin et al., THE EARLY DEVELOPMENT OF MAJOR PROJECTIONS FROM CAUDAL LEVELS OF THE SPINAL-CORD TO THE BRAIN-STEM AND CEREBELLUM IN THE GRAY SHORT-TAILED BRAZILIAN OPOSSUM, MONODELPHIS-DOMESTICA, Developmental brain research, 75(1), 1993, pp. 75-90
The Brazilian short-tailed opossum, Monodelphis domestica, is born 14-
15 days after copulation and is available for experimentation at stage
s of development corresponding to those which occur in utero in placen
tal mammals. In the present study, we took advantage of the opossum's
embryology to study the development of projections from caudal levels
of the spinal cord to the brainstem and cerebellum using axonal tracin
g methods. In all cases, a 2-3 day survival time was used for axonal t
ransport. When injections of Fast blue (FB) were made into caudal leve
ls of the thoracic cord at postnatal day (PD) 1 or 2, axonal labeling
could not be identified at supraspinal levels. When injections were ma
de at PD3, however, labeled axons were found in the fasciculus gracili
s at caudal medullary levels, within the ventrolateral medulla and pon
s, within an incipient inferior cerebellar peduncle, and within the ce
rebellar anlage. The dorsal root origin of at least some of the axons
within the fasciculus gracilis was evidenced by the transganglionic tr
ansport of cholera toxin conjugated to horseradish peroxidase from the
hindlimbs. After FB injections at PD7, a few labeled axons could be t
raced from the fasciculus gracilis into the nucleus gracilis and from
the ventrolateral pathway to the inferior olive. Generally comparable
results were obtained using wheat germ agglutinin conjugated to horser
adish peroxidase (WGA-HRP). In cases injected with FB at PD9, the patt
ern of brainstem labeling was adult-like. Although labeled axons were
present within the cerebellum of animals injected with FB on PD3, they
were limited to the marginal zone. Axonal labeling was present within
an identifiable internal granular layer in cases injected with either
FB or WGA-HRP at PD16, and it appeared to be limited to specific band
s which foreshadowed those seen at later stages of development and in
the adult animal. In some cases, labeled axons were present within the
molecular layer where they were not seen in the adult animal. Our res
ults provide a timetable for the normal development of projections fro
m caudal levels of the spinal cord to the brainstem and cerebellum in
Monodelphis and show that such development occurs postnatally rather t
han prenatally, as in placental mammals.