[Cholinesterase in the development of the chick limb : I. Phases of cholinesterase activity in the early bud and in the demarkating cartilage and Muscle Anlagen].

1. In the chick limb bud, histochemical Cholinesterase (ChE) activity has been studied from stage 13 of Hamburger and Hamilton to stage 29 (6 days). 2. In stage 13 the neural crest cells on their intraectodermal migration into the limb field are revealed by the ChE reaction (Fig. 1). 3. In stage 14-16 the ectoderm of the lateral body wall covering the Wolffian ridge exhibits ChE activity (Fig. 2). 4. In the following stage the apical ectodermal ridge is formed from this positive ectoderm. Between the wing and leg bud ChE activity disappears. So, in the early limb bud ChE activity is confined to the apical ectodermal ridge and the ectoderm. The mesodermal core is negative (Fig. 3). 5. From stage 20 onwards ChE activity becomes demonstrable in the preaxial mesoderm, too. The activity is accompanied by a loosening and vascularisation of the tissue (Fig. 4-6). With further development it spreads distally. In the distal region the asymmetrical ChE distribution within the mesoderm (positive preaxially and negative postaxially) is maintained until stage 27. 6. In stage 25 the mesodermal ChE activity reaches the distal tip of the wing. Concommitantly, activity disappears from the apical ectodermal ridge (Fig. 8). 7. As cartilage condensations are formed in the loosened central mesoderm, ChE activity increases further in the condensing cells. Finally it disappears from the differentiated cartilage (Figs. 7-10). 8. In the presumptive soft tissue ChE activity disappears temporarily. After the cartilage condensation has formed centrally myoblasts with high activity develop in the peripheral tissue (Figs. 8-10). 9. The invading nerve fibers also exhibit a high ChE activity. They make their way in the negative zone between cartilage and muscle (Figs. 8-10). 10. Like in the development of other organs, ChE activity in the chick limb bud is correlated to morphogenetic changes of the reacting cells. Concommitantly, ChE activity is the first indication of the onset of organ formation and cytodifferentiation.