DEVELOPMENT OF FINE STRUCTURE OF SPINAL CORD IN MONKEY FETUSES .2. PRE-REFLEX PERIOD TO PERIOD OF LONG INTERSEGMENTAL REFLEXES

A series of 13 macaque fetuses of estimated ovulation ages 42 to 51 days (17 to 41 mm) was subjected to behavioral analysis with umbilical cord intact (in and out of the amnion), and to light and electron microscopic analysis of the cervical spinal cord. This period is a critical one in terms of onset and early development of spontaneous activity and of cutaneous reflexes. Behavioral criteria, reinforced by well‐defined microscopic characteristics, suggested a grouping of specimens into three major stages, namely, a pre‐reflex group (Stage 1: 17–22 mm); a group in the period of onset of primitive spontaneous activity and of precocious local cutaneous reflexes in the trigeminal‐cervical region (Stage 2: 24 to 28 mm); and a group characterized by the development of more vigorous activity, spontaneous and otherwise, and of long intersegmental and crossed reflexes (Stage 3: 32 to 41 mm). The evidence suggests that junctional differentiation and occurrance of a cluster of agranular spheroid synaptic vesicles characterize the primitive synaptic knobs which appear on motoneuron dendrites coincident with earliest reflexes. Onset of active long intersegmental reflexes in Stage 3 is coincident with development of axosomatic synaptic knobs, and of F type (flattened) synaptic vesicles in a rapidly increasing fraction of all synaptic bulbs in the motoneuron neuropil. Total volume of all synaptic bulbs increases gradually as a fraction of the volume of the motoneuron neuropil until birth, but a steep increase in proportionate number of F type synaptic knobs occurs in Stage 3. Copyright © 1968 The Wistar Institute of Anatomy and Biology