Synapse formation during neuron differentiation: An in situ study of the myenteric plexus during murine embryonic life

Ultrastructural steps characterizing synapse formation in vivo and appearan ce in neuroblasts of properties suggestive of synaptic function acquisition have scarcely been studied. Synapse formation and proteosynthetic apparatu s organization were thus studied under transmission electron microscope in mouse myenteric neurons from embryonic day 12.5 (E12.5) until birth. Expres sion of Ret and p75(NTR), markers of neural crest cells, as well as that of neuron-specific enolase (NSE), synaptophysin (SY), and synaptosomal-associ ated protein (SNAP), markers of synaptic function acquisition, were immunoh istochemically evaluated. At E12.5 many cells were Ret- and p75(NTR)-immuno reactive (IR), whereas a few were NSE-IR and had neuronal ultrastructural c haracteristics. Two types of contacts between poorly or nondifferentiated c ells and axons of presumed extrinsic (synapse-like contacts) or local (imma ture synapses) origin were identified, along with SY-IR elements. By E16.5, many cells had developed a proteosynthetic apparatus, synapse-like contact s were no longer present, and immature synapses were gradually differentiat ing. Concurrently, there was an increase in NSE-IR cells, some of which wer e also SNAP-IR, and in SY-IR varicosities. At E18.5, ultrastructurally matu re neurons and synapses had increased in number as had NSE-IR and SNAP-IR c ells and SY IR varicosities. These data indicate that 1) one type of contac t (synapse-Like) is present at E12.5 between very immature cells and presum ed vagal fibers, with a possible transient role for the onset of the differ entiative process of these cells; and 2) another type of contact (typical s ynapses) lasts until E18.5, with a similar but long-lasting role that progr essively shifts to the classical function (neurotransmission) as the synaps e matures and the embryo reaches the day of birth. J. Comp. Neurol. 425: 36 9-381, 2000. (C) 2000 Wiley-Liss, Inc.