Expression of cholinergic phenotype by embryonic ventral horn neurons transplanted into the spinal cord in the rat

Solid grafts of E12 embryonic spinal ventral horn were transplanted into motoneuron-depleted adult lumbar spinal cord in the rat. A muscle was implanted parallel to the vertebral column with its nerve inserted into the lumbar cord at the site of transplantation so as to provide a target for innervation by the grafted neurons. Previous retrograde labelling studies have shown that modest numbers of grafted motoneuron-like cells participate in the muscle's reinnervation and these are often found outside the graft within the host spinal cord. However, Nissl stained sections show that larger numbers of neurons survive within tissue recognisable as being of graft origin. In this study we have examined the expression of acetylcholinesterase (AChE) and choline acetyltransferase (ChAT) by neurons within the graft. These enzymes are involved in cholinergic neurotransmission and are characteristic of motoneurons. Thirty-four to seventy days following transplantation the grafts contained numerous neurons with acetylcholinesterase (AChE) activity. Different patterns of AChE staining were observed which probably reflected the degree of differentiation and maturation within the graft. AChE positive neurons were found in isolation or in groups resembling developing motor pools. Most of the AChE-positive neurons appeared immature with scant cytoplasm. However, neurons could be found which appeared relatively mature with a regularly shaped nucleus, prominent nucleolus and Nissl bodies. The grafts contained few AChE-positive axons and no dense plexuses of varicose fibres around the neurons such as are found around motoneurons in the mature ventral horn. Comparisons between the size of AChE-positive neurons in the graft and the size of AChE-positive neurons in the developing ventral horn found that the size of grafted neurons to be intermediate between the sizes of spinal motoneurons at E19 and P0. Far fewer grafted neurons were found to be immunoreactive for choline acetyltransferase (ChAT) than histochemically reactive for AChE. This was consistent with our findings in the spinal cord during normal development where we found that fixation and staining procedures which labelled adult motoneurons failed to reliably demonstrate ChAT immunoreactivety in normal motoneurons prenatally, although AChE histochemical reactivity could be demonstrated as early as E16. We conclude that the grafts contain numbers of immature motoneurons which fail to proceed beyond a certain stage of development, perhaps because of a failure to form appropriate efferent and afferent connections.