Two and three dimenstional ultrastructure of boutons and glial cells on the motoneuronal surface in the cat spinal cord.

Abstract

AbstractA method is presented for individually mounting serial sections on one‐hole specimen holders. Also a technique for constructing stereograms based upon serial sections is described including a method for obtaining perspective foreshortening. Ultrastructural findings revealed the surface of the motoneuron to be covered with synaptic boutons which frequently pressed against each other. Surface area not contacted by boutons was covered with astroglial processes. The latter processes wedged around and sometimes between the boutons. On occasion finger‐like astrocytic processes (spines) were found to invaginate the terminal boutons. No synaptic thickenings were found between adjacent boutons (axo‐axonal contacts) on the perikaryal surface. However, in view of the enormous size of the motoneuron contrasted to the small volume of tissue thus far investigated, this observation is regarded as inconclusive. Three‐dimensional studies revealed an astrocytic process with disproportionately large amounts of surface membrane in the form of many interconnected pockets. The initial unmyelinated segment of a large anterior horn cell was encountered and found to have synaptic boutons on its proximal half. Its distal half was surrounded by an extracellular space containing electron dense material similar to that found in the goldfish by Robertson, Bodenheimer, and Stage ('63).