Synaptogenesis in the ventromedial hypothalamus of the prenatal mouse.

Mouse embryos from day 13 to day 19 of gestation (E13 through E19) were removed by Caesarean section and their brains were prepared for electron microscopy. Coronal sections were examined in three planes through the ventromedial hypothalamus: anterior, the level at which the optic tracts pass posterolaterally to become partially enclosed in the major brain mass; middle, the level at which the floor of the third ventricle begins to widen and flatten; posterior, the level at which the most anterior infundibulum appears. Three types of junctional complexes were examined. Close junctions are identified as straight, parallel areas of two apposed membranes which appear more electron-dense than immediately adjacent regions. The membranes are separated by a clearly visible cleft. Unlike synapses, no clear synaptic vesicles are found in either of the adjacent profiles unless randomly distributed and accompanied by ribosomes or glycogen. Close junctions are seen most frequently on day E15, then decrease in number through E19. Their participation in synaptogenesis is discussed from temporal, morphological, distributional and quantitative perspectives and is provisionally rejected. Immature synapses show only the minimal membrane specialization found in close junctions, but vesicles are present, ribosomes absent in at least one of the adjacent cytoplasms. Their appearance peaks on E17-E18, paralleling and slightly preceding that of the mature synapses. Their evolution from close junctions has only weak temporal support. Mature synapses display the cytoplasmic densities which immature synapses lack. They attain their greatest prenatal numbers on E18, then decrease in number on E19. The conclusion is advanced that synaptogenesis does--or at least can--occur without the prior appearance of avesicular regions of increased membrane density.