Electrophysiological and cytological studies of brain homografts in the anterior chamber of the eye: Maturation of cerebellar cortex in oculo

Abstract

The intraocular transplantation technique was used to study maturation of cerebellar homografts with electrophysiological and cytological methods. Taking of the transplants was optimal using donors with a gestational age of 15–17 days, whereafter it declined markedly. The time-table for the cytological maturation mimicked the in situ maturation surprisingly well, being retarded at the most by 5 days. External granule cells appeared 5 days postoperatively, and had completely disappeared about 5 weeks later. Internal granule cells appeared from about day 12. Purkinje cells were identified in clusters from day 8. They slowly fanned out to form a monolayer after about 3 weeks in oculo, situated between an internal granule cell layer and a molecular layer. The molecular layer was clearly recognized from the third week and often contained apical dendrites from the Purkinje cells. The overall cytological organization of the mature transplants was very similar to the normal cerebellum, also including some foliation and always including typical trialaminar areas. The intraocular transplantation technique is thus very suitable for producing cytologically homotypical cerebellar transplants. The electrophysiological maturation of the transplants was also followed using extracellular recordings. A spontaneous discharge pattern strongly suggesting the presence of Purkinje cells mimicked the well known in situ activity both regarding time course and way of firing. Thus, few single spikes were recorded 12 days postoperatively having very few high frequency bursts. The interspike intervals and the inter- and intraburst intervals then gradually decreased to reach values very close to those of normal adult cerebellum about 4 weeks postoperatively. Interspike and interburst interval histograms are presented to show these similarities. Also, potentials evoked by stimulating the base or the surface of the transplants showed action potential responses similar to those of normal cerebellum. Thus, (1) stimulating the base of the transplant, a short latency all-or-none single spike was elicited. (2) When stimulating the surface of the transplant often 1–3 driven spikes over a 5–10 msec period could be elicited. This response was invariably graded. (3) Stimulation of the surface often produced a short latency inhibition of spontaneous discharge lasting 40–120 msec. This response was also graded. The inhibition was shown to be independent of adrenergic fiber influence as shown by superior cervical ganglionectomy or parenteral reserpine administration. The inhibition was antagonized by the GABA-blocker bicuculline when administered parenterally. These 3 responses closely resemble the antidromic spike response, the parallel fiber excitation response and the basket-stellate cell inhibition response respectively, as seen in the normal adult cerebellum in situ. It is concluded that the intraocular cerebellar homografts develop and mature in a very homotypical way as shown by means of electrophysiology and histology in spite of the complete lack of normal connections with the rest of the brain and underlines the importance of intrinsic regulatory mechanisms in CNS ontogeny.