Synaptogenesis in the intermediate gray region of the lumbar spinal cord in the postnatal rat

Mid-thoracic spinal cord transection produces dramatically different behavioral results depending upon a rat's age at the time of surgery. The present study was initiated to determine whether the synaptic development in the gray matter of the normal, developing spinal cord differs before and after the period when maximal behavioral recovery occurs. The L6 segments from 10 groups of animals, 0--30 days of age, taken at 3 day intervals (4 animals/group) were studied by light microscopy. Areal measurements of the gray matter were made using an integrating x-y tablet interfaced to a computer. Cell size, cell density and area of neuropil were evaluated in the lateral portions of the intermediate gray matter, laminae VI and VII. Electron microscopic analyses of synaptogenesis were performed on material from the same region in animals 3, 12, 15, 21 and 30 days old using similar morphometric methods while taking note of vesicle, junctional, and mitochondrial morphology. A 60% increase in area of neuropil paralleled a linear increase, of comparable magnitude, in area of the gray matter until 15 days of age when both curves reached plateau. Neuronal perikaryal size remained constant (congruent to 200 sq. microns in plane of nucleolus) throughout development and so could not have contributed to the increase in area of gray matter. Areal measurements of the size and counts of the number of vesicle containing profiles demonstrated a 50% increase in density of axon terminals between 3 and 12 days of age and a steady decline thereafter. The size of vesicle-containing profiles in laminae VI and VII remained constant at a small value (congruent to 0.35 sq microns) until 12 days of age, showed rapid growth to 0.54 sq. microns between 12 and 15 days of age, followed by a more moderate increase in sectional area after 15 days. These results suggest that during the period when recovery of function follows spinal injury, synaptogenesis in the intermediate gray region of the lumbar spinal cord proceeds rapidly, while at stages when little recovery of function follows spinal transection, synaptogenesis is essentially complete.