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The histochemical localization of specific acetylcholinesterase (AcChoEase) and nonspecific cholinesterase (BtChoEase) is described during the early morphogenesis of the whole chicken head with main emphasis on the visual system. It is found that: (i) Expression of AcChoEase is an early differentiation event in the entire brain. Its pattern of first appearance on the external part of the neuroepithelium correlates with the general spatio-temporal pattern of differentiation. AcChoEase thus represents an early differentiation marker. (ii) The late pattern of AcChoEase (at E18), reflecting at least partially the distribution of synaptic AcChoEase shows no direct correlation to the distributions found at early stages when synapses are not yet formed. This argues for a nonsynaptic function of the early appearing AcChoEase. (iii) BtChoEase in early nervous tissue is diffusely localized on the ventricular side of the neural tube. At later stages it becomes concentrated on the ependymal layer as well as along fibers reaching from this inner layer outwards. Minor activities appear in specific external layers of tectum and retina. (iv) During the course of differentiation the enzymes express pronounced graded distributions within the areas where they are detectable. (v) Mesenchymal and epidermal BtChoEase is abundant in the entire head. Prominent amounts of activity are expressed on the rostral epidermis, along the eye cup next to the sclera, and on the rostro-dorsal surrounding of the optic nerves. The results are discussed in the light of possible morphogenetic functions of these enzymes.

Comparative localization of acetylcholinesterase and pseudocholinesterase during morphogenesis of the chicken brain.