Neuritic growth cone and ependymal gap junctions in the feline subfornical organ during early development.

Intercellular contacts in the subfornical organ (SFO) of kittens 3, 16, and 29 days old were studied in thin sections and by the freeze-etch method. Gap junctions appeared between growing nerve processes and target cells. The junctions were interspersed between immature synapses lacking mitochondria as well as full pre- and postsynaptic membrane specializations. Gap junctions were seen on filopodia as well as on more mature processes. The morphology of these junctions was typical of those described earlier but they were of small size (0.2-0.3 micron). Gap junctions of peculiar form were also seen between ependymal elements in the SFO at 16 days. These were of large size (0.5-0.8 micron) and were often of segmented character. This segmentation consisted of bands 3-4 particles in width with a center-to-center spacing of 90 nm with particle free corridors between corresponding to the width of about two rows of particles. The margin of the group might be circumscribed by a row of particles. Although gap junctions of large size were seen between ependymal cells in thin section, features corresponding to the particle free corridors have not been observed to date.