The development of the chick gizzard and its innervation have been studied using histochemical techniques and light and electronmicroscopy. In Hamburger-Hamilton stage 28-29 (6-day old) embryos, the primordium of the gizzard consists of a vascularised mesenchymal thickening of the gut. Auerbach's plexus is clearly established; cholinesterase (ChE)-positive and negative neuroblasts are present, but catecholamine containing nerves are absent. The innervation of the musculature is not extensive. By stage 30-31 (day 7), myofilaments, in association with dense bodies, are present within myoblasts. Auerbach's plexus extends into the underlying musculature. Fibroblasts become active by stage 35-36 (day 9) and collagen is laid down, thus delineating the myoblasts into bundles. At this time, ChE-positive axon bundles accompany the arterial vessels over their entire length; the proximal branches of the coeliac artery are also accompanied by bundles of noradrenergic axons. Dense areas along the cytoplasmic side of the unit membranes of myoblasts are established by stage 36-37 (day 10), and associated with them, external laminae and a uniform intercellular gap are developed. The outer longitudinal muscle primordium is lost at this time. Between stage 37-42 (days 11-16) noradrenergic axons become evident in Auerbach's plexus, and the axons in the plexus and musculature contain large granular vesicles. The bundles of axons innervating the muscle become ChE-positive and large numbers of agranular vesicles appear within them. The envelopment of axon bundles by Schwann cells does not occur prior to stage 43 (day 17) and myelination of individual axons apparently does not occur until three days after hatching. The most active period of myoblast division is around stage 28-29 (day 6) when cells are undifferentiated, but division of myofilament-containing myoblasts does occur. © 1969 Springer-Verlag.
