IMMUNOHISTOLOGICAL AND ULTRASTRUCTURAL-STUDY OF THE DEVELOPING TENDONS OF THE AVIAN FOOT

The aim of the present report is to provide a detailed description of the morpho,oenesis and initial differentiation of the long tendons of the chick foot, the long autopodial tendons (LAT), from day 6 to day 1 1 of development. The fine structure of the developing LAT was studied by light and transmission electron microscopy. The characterization b y immunofluorescent techniques of the extracellular matrix was perform ed using laser scanning confocal (tenascin, elastin, fibrillin, emilin , collagen type I, II, III, IV and VI) or routine fluorescence (tenasc in, 13F4) microscopy. In addition, cell proliferation in pretendinous blastemas was analyzed by the detection of BrdU incorporation by immun ofluorescence. The light microscopic analysis permitted the identifica tion of different stages during LAT morphogenesis. The first stage is the formation of a thick ectoderm-mesenchyme interface along the digit al rays, followed by the differentiation of the ''mesenchyme lamina'', an extracellular matrix tendon precursor, and ending with the formati on and differentiation of the cellular condensation that forms the ten don blastema around this lamina. The immunofluorescence study revealed the presence and arrangement of the different molecules analyzed. Ten ascin and collagen type VI are precocious markers of the developing te ndons and remain present during the whole process of tendon formation. Collagen type I becomes mainly restricted to the developing tendons f rom day 7.5. Collagens type II and IV are never detected in the develo ping tendons, while a faint labeling for collagen type III is first de tected at day 7. The analysis of the distribution of the elastic matri x components in the developing tendons is a major contribution of our study. Elastin was detected in the periphery of the tendons from day 8 and also in fibrils anchoring the tendons to the skeletal elements. A t the same stage, emilin strongly stains the core of the tendon rods, while fibrillin is detected a little later. Our study indicates the ex istence of an ectoderm-mesoderm interaction at the first stage of tend on formation. In addition, our results show the different spatial and temporal pattern of distribution of extracellular matrix molecules in developing tendons. Of special importance are the findings concerning the tendinous elastic matrix and its possible role in tendon maturatio n and stabilization.