Postnatal development of presynaptic terminals in the gigantocellular tegmental field (FTG) of the rat

Presynaptic boutons of the gigantocellular tegmental field (FTG) of the brain stem reticular formation were analyzed by stereological techniques for changes in numerical and volumetric density during early postnatal development. Electron microscopic analysis revealed that presynaptic boutons increased in numerical density and volumetric fraction during the first 15 postnatal days. By day 15, the presynaptic boutons had reached the numerical density and volumetric fraction seen in the adult tissue. Myelination was not evident until day 10 and by day 30 contributed over 45% to the total volume of the FTG tissue. Presynaptic boutons were differentiated into those with spherical vesicles and those with predominantly polymorphic (i.e. oblong) vesicles. Both types of presynaptic boutons were present at day 1, but over the next 30 days the percentage of boutons with polymorphic vesicles increased to over 40%. These results revealed that the FTG region of the rat demonstrates its synaptogenic period during the earliest postnatal days. The differentiation of the synapse into two types within the FTG region suggests a morphological basis for alterations in function during early maturation.