Proceedings of The Physiological Society

Puerto de la Cruz, Tenerife (2003) J Physiol 548P, P144

Poster Communications

Early development of the thalamic reticular and perireticular nuclei in the human brain

I. Bystron*†, C. Blakemore*, V. Otellin† and Z. Molnar‡

*University Laboratory of Physiology, Oxford, UK, †Department of Morphology IEM, St Petersburg, Russia and ‡Department of Human Anatomy and Genetics, Oxford, UK

Growing axons of the nervous system often contact cellular groups, which are thought to play a part in their guidance to their ultimate targets. In several species, thalamic fibres have been shown to associate with cells of the thalamic reticular nucleus (TRN) and perireticular nucleus (PRN) as they leave the diencephalon and negotiate towards and through the internal capsule (Metin & Godement 1996; Earle & Mitrofanis 1996). Although equivalent transient neuronal groups have been described in the human embryonic forebrain (Letinic & Kostovic, 1996; Ulfig et al. 1998), little is known about their early development and their relationship with thalamocortical fibres. By means of the carbocyanine fluorescent dye DiI and antibodies against microtubule-associated protein MAP2, calretinin (CR) and calbindin (CB), we studied neuronal populations along the path of developing thalamocortical and corticofugal axons in humans.

Embryos and early fetuses from Carnegie stages 16-23 (37-56 days post-fertilization) were obtained after legal abortions, following national guidelines in Russia. Retrograde tracing of axons from the dorsal thalamus at stage 17 revealed back-labelled cells in the ventral thalamus, hypothalamus and a region presumed to be the primordium of the TRN, which contained CR-, CB- and MAP2-immunopositive cells. We also found a distinct group of cells projecting to the thalamus in the mantle zone of the medial part of the basal telencephalon at stage 16-17, prior to the formation of the internal capsule. They showed a strong resemblance to the PRN described in other mammalian species. At stage 18, few thalamocortical axons have reached the ventral telencephalon, although numerous axons of the putative reticular and perireticular nuclei have entered the dorsal thalamus. By stage 21, thalamocortical axons have increased in density and have reached the lateral ganglionic eminence, and by stage 23 they have invaded the intermediate zone beneath the ventrolateral cortex. It seems, then, that, as thalamic fibres arrive at each cell group along their path, axonal projections from that region have already reached the dorsal thalamus. It is conceivable that these thalamopetal projections, and the cell groups from which they arise, play a part in the navigation of thalamocortical fibres.

This work was supported by the MRC and the Royal Society.

Where applicable, experiments conform with Society ethical requirements