David Tay

The localization of motoneuron pools innervating wing muscles in the chick

SummaryMotoneuron pools supplying principal muscles of the shoulder girdle and wing were localized in the chicks 2–7 days post hatching with the use of retrograde axonic transport of horse radish peroxidase (HRP). HRP was injected into selected muscles and the animals sacrificed after 24 h survival. Labelled motoneuron pools representing individual muscles were found to be clustered in longitudinal columns along the brachial spinal cord segments 13–16. Muscles derived from the dorsal muscle mass were innervated by motoneurons located in the ventro-lateral portion of the horn, while those originating from the ventral muscle mass received innervation from neurons occupying the dorsomedial portion within the ventral horn of the spinal cord. The rostrocaudal extent of the motoneuron pools could be correlated with the proximodistal position of wing muscles.The observed orderly topographical relationships between clusters of motoneurons of the brachial spinal cord and the muscles they innervate will be used as baseline data for experiments where the limb innervation is perturbed.

Temporo-nasal asymmetry in the accretion of retinal ganglion cells in late larval and postmetamorphic Xenopus

SummaryThe spatial pattern of cell production and retinal growth were studied in Xenopus between stage 60 and two months after metamorphosis using 3H-proline and 3H-thymidine autoradiography. The position and the number of the ganglion cells labelled with 3H-thymidine were determined. The area of the unlabelled retina due to growth since 3H-proline administration at stage 60 was measured. Both retinal area measurements and counts of labelled ganglion cells showed 30–40% higher values in the temporal than in the nasal retinal half. The greater cell production and area accretion were even more pronounced between the temporal and the nasal retinal quadrants. The results on the temporoventral crescentic retinal growth rule out the possibility that from midlarval stages onwards the retinal and the tectal growth patterns are matched.

The development of the diencephalon in Xenopus

SummaryThe development of the diencephalon and the time of origin of neurons of thalamic nuclei were determined in Xenopus with 3H-thymidine autoradiography Isotope was administered into embryos, tadpoles and metamorphic animals and sacrificed after survival time between 24 hours and 5 months. The position and the number of heavily labeled cells, corresponding to terminal mitoses at the time of isotope injection were established on transverse and sagittal sections of the frog brain. Neurons in the diencephalon were distributed in a spatiotemporal manner such, that cells generated earliest were located in the caudo-ventro-lateral portion of the diencephalon followed in a rostro-dorso-medial sequence by cells formed at later stages. The waves of cell generation resulted in three apparent developmental gradients in the caudo-rostral, latero-medial and in the ventro-dorsal directions in the diencephalon. Consequently neurons generated latest were found in the rostro-dorso-medial portion of the diencephalon. the overall rostro-dorso-medial diencephalic growth and the spatiotemporal generation of its neurons are the reverse of the tectal growth and cell generation reported in Xenopus which occurs in a rostrolateral to caudomedial direction.The findings of the present observations appear to indicate that the mirror-image reversal of the retinotectal and retinodiencephalic projections along the temporo-nasal retinal axis is the consequence of the divergent growth of the diencephalon and the tectum from the common embryonic di-mesencephalic junction. These observations furthermore suggest that the orientation of the retinal maps is ensured by the differential maturity gradients in the tectum and diencephalon, respectively, presumably expressed in molecular terms.

The development of the nucleus isthmi in Xenopus: an autoradiographic study.

The time of origin of the neurons of the nucleus isthmi (NI) in Xenopus has been investigated with [3H] thymidine autoradiography. The first cells of the NI were formed at stage 20 and they continued to be formed until stage 62. The majority of the neurons of the NI were generated between stages 45 and 55. The cells in the NI were distributed in a spatiotemporal manner. Cells generated earliest were located in the rostroventral pole of the NI followed in a dorsocaudal sequence by neurons formed later.

Segregation of optic fibre projections into eye-specific bands in dually innervated tecta in Xenopus

The restoration of the retinotectal projections following metamorphic bilateral optic nerve sections was studied in Xenopus 20 weeks after operation, using [3H]proline autoradiography and electrophysiological mapping. Optic fibre projections from each eye to both tecta segregated into eye-specific termination bands with some overlap at the borders. The bands were arranged rostrocaudally in the tecta such that each eye had an alternating representation. Despite the segregation of the optic fibres, each eye had a complete visual (retinal) projection with the normal nasotemporal and dorsoventral polarities of the visual field maintained. The results suggest competition for preferred terminal sites between the two sets of optic fibres in dually innervated tecta.

Aberrant retinotectal pathways induced by larval unilateral optic nerve section in Xenopus

Following unilateral optic nerve section in Xenopus tadpoles and toadlets, the reformation of visual pathways and retinotectal projections was analyzed using [3H]proline autoradiography. Bilateral retinotectal projections were found in 61% of the animals operated on between stages 42 and 58. In these animals, the projections were established through multiple aberrant pathways which included the oculomotor nerve, the trigeminal nerve and the posterior commissure. In contrast, in animals with optic nerve section at stage 62 or after metamorphosis, the regenerated optic fibres arrived at the contralateral and ipsilateral tecta through normal visual pathways.

Changes in retinotectal projection in adult Xenopus following partial retinal ablation

Abstract The retinotectal projection was mapped electrophysiologically and autoradiographically after surgical removal of the nasal or temporal retinal quadrants in Xenopus . The immediate result of retinal ablation was a corresponding temporal or nasal scotoma in the visual field and the absence of part of the optic fibre projection from the tectum. Thirty to fourty days after operation the visual field scotoma still persisted but the vacated tectal area became reinnervated by collateral sprouting of optic fibres from the fringe of the residual retinal projection. It is concluded that the retinotectal connection pattern can be altered by retinal ablations in adult toads.

Retinodiencephalic projections from compound eyes in Xenopus

The contralateral diencephalic projections from compound eyes, made by the early embryonic fusion of two nasal (NN), two temporal (TT) or two ventral (VV) retinal fragments, have been studied in young toads using [(3)H] proline autoradiography. After intraocular isotope injection into compound eyes which represent only half of the normal naso-temporal or dorso-ventral sets of ganglion cells, the total projection to the nucleus of Bellonci (NB), corpus geniculatum thalamicum (CGT) and pretectal nucleus (PN) was found to be the same as in normal eyes. These results show the spreading of a half-retinal projection to the whole of the diencephalic visual centres, indicating that the pattern of retinodiencephalic projections may be modified during embryogenesis.