Dkc Tay

The postnatal development of the optic nerve in hamsters: an electron microscopic study.

The postnatal development of the optic nerve in the golden hamster has been examined using the electron microscope. The number of the optic fibres present in the optic nerve showed an initial increase during the first day after birth but it declined afterwards and tapered off after Day 16. At its peak on postnatal Day 1, there was an average of 314,629 axons in the optic nerve but when the animal reached adulthood only 109,587 fibres remained amounting to about 65% loss of the total fibre population. The period of axon loss coincided with the appearance of large patches of degenerated profiles in the optic nerve. The occurrence of the optic fibre loss has been implied to correlate with the time when the retinal projections were undergoing a dynamic reorganization at the target sites during the process of establishing the adult patterns of retinofugal connections.

Elimination of transient dendritic spines in ipsilaterally projecting retinal ganglion cells in rats with neonatal unilateral thalamotomy.

Using the DiI and intracellular Lucifer Yellow labeling techniques in the rat, we have demonstrated that the unilateral neonatal thalamotomy does not result in retention of transient dendritic spines of ipsilaterally projecting retinal ganglion cells (IPRGCs), although the thalamotomy is known to retain the normally transient IPRGCs (Chan et al., Dev. Brain Res., 49 (1989) 265-274). These results suggest that the process of elimination or retraction of transient dendritic spines occurs in retinal ganglion cells during development regardless of whether they make connections with appropriate or inappropriate loci in the visual targets, and/or a decrease in interactions with neighboring retinal ganglion cells.

Differential expression of glutamate decarboxylase isoforms in the horizontal cells of human and rat retinas

Brain development is a very complicated process in which neurones are born in the germinal layer of the neural tube and then migrate away to form the different parts of the brain including the cerebral cortex. In hydrocephalus the cerebrospinal fluid (CSF) pathway is blocked and fluid accumulates in the ventricles. Fluid accumulation results in a rise in intracranial pressure. CSF is actively produced within the brain and does not switch off when a blockage occurs. Our recent work demonstrates that fluid blockage results in immediate effects on cell division in the germinal layer resulting in fewer neurones being produced. We have also shown that the fluid which accumulates in the ventricles prevents cell division of neural precursors grown outside the brain in culture. Fluid accumulation does not immediately raise pressure nor lead to brain damage but affected individuals do have less cortical thickness/mass than normal. We investigated this using the substance BrDu that allows us to identify neurones born at a particular time during development. This demonstrates that the number of cells being generated by the germinal layer is much less than normal and that fewer cells arrive in the cortex. Accumulated CSF contains signals which prevent normal cell division in affected brains. We tested this by taking cells from normal and abnormal brains and growing them in artificial medium. Released from the effects of the accumulated fluid, cells from affected brains divide faster than those from normal brains. Fluid from normal brains has no effect on the division of neural precursors but fluid from affected brains prevents cell division. These data show that the flow of CSF through and over the brain is important to the normal development of the brain and, particularly, the cerebral cortex.