Localisation of extracellular matrix components in the embryonic human notochord and axial mesenchyme.

The unsegmented axial mesenchyme is the first anlage of the embryonic vertebral column in amniotes. Its development and differentiation are influenced by the notochord. The appearance of extracellular matrix (ECM) components which may regulate the developmental processes in the human embryonic axial mesenchyme and the notochord was studied in 5 human embryos between Carnegie stages 12 and 14 (4th and early 5th gestational wk) by using Alcian blue histochemistry and immunohistochemistry with antibodies against types I and II collagens, the proteoglycans aggrecan, biglycan and decorin, the glycoproteins fibronectin, laminin and tenascin and the growth factor TGF-beta. The results demonstrate that the notochordal sheath contains high amounts of sulphated glycosaminoglycans, hyaluronic acid and the glycoproteins fibronectin, laminin and tenascin during all developmental stages examined. Additionally type II collagen and the small proteoglycan decorin were found. Type I collagen and the small proteoglycan biglycan could be detected in notochordal cells. The number of TGF-beta-immunoreactive notochordal cells increased between stages 12 and 14. The ECM of the axial mesenchyme is rich in sulphated glycosaminoglycans, hyaluronic acid, fibronectin and tenascin. Strong immunoreactions for collagen type I were found only during the early stage 12. The absence of aggrecan and keratan sulphate indicates that large aggregating proteoglycan complexes are missing. Epithelial-mesenchymal interactions which may be influenced by TGF-beta and the ECM components demonstrated are discussed.