Formation and maturation of axo-glandular synapses and concomitant changes in the target cells of the rat subcommissural organ.

Abstract

Synapse formation and maturation in the subcommissural organ (SCO) of Wistar rats were studied from birth to the end of the first month. Modifications of the secretory ependyma were analyzed over the same period. On the 1st postnatal day, the large varicosities in contact with the SCO ependymocytes appeared immature (absence or low density of vesicular population, no synaptic membrane differentiation). The synaptic contacts were formed from the 3rd postnatal day, near the glandular cell nuclei (0.1 micron distance); progressively, the content of the axonal boutons and the pre- and post-synaptic specializations became similar to those of adults. From the 21st day on, the axo-glandular innervation was considered analogous to that in the adult. Using immunocytochemistry, it was found that the increase in the serotonin-immunoreactive fiber density in the whole organ was time-dependent. Light and electron microscopy demonstrated changes in the morphology of SCO ependymocytes during the first postnatal weeks, notably in the endoplasmic reticulum and content ot apical protrusions. On postnatal day 14, two types of ependymal cells, neonatal-like and adult-like, coexisted. The evolution of SCO ependymocytes coincided with the progressive onset and maturation of axo-glandular innervation taking place after birth.