Distribution and expression of two interactive extracellular matrix proteins, cytotactin and cytotactin‐binding proteoglycan, during development of Xenopus laevis . II. Metamorphosis

During metamorphosis of Xenopus laevis the extracellular matrix (ECM) proteins cytotactin and cytotactin‐binding (CTB) proteoglycan, and the cell adhesion molecules N‐CAM and Ng‐CAM, appear in highly restricted patterns determined by immunofluorescence histology. During limb development, cytotactin appears from the earliest stages in a meshwork of ECM fibrils associated with migrating mesenchymal cells forming the limb bud. Cytotactin also appears in the ECM between the apical limb ectoderm and mesenchyme. Later, both cytotactin and CTB proteoglycan appear co‐localized within the central (prechondrogenic) limb mesenchyme. During chondrogenesis in these areas, cytotactin becomes restricted to perichondrium, while CTB proteoglycan is expressed throughout the cartilage matrix. The premyogenic mesenchyme surrounding the chondrogenic areas expresses N‐CAM. Later, N‐CAM is concentrated at the myogenic foci where cytotactin appears at sites of nerve/muscle contact and in tendons. Expression of these molecules in the blastemas of regenerating limbs was also studied, and during development of the central nervous system, stomach, and small intestine. Analysis of the expression patterns of cytotactin and CTB proteoglycan throughout development and metamorphosis reveals several consistent themes. The expression of these molecules is highly dynamic, often transient, and associated with key morphogenetic events. Cytotactin appears at multiple sites where cells undergo a transition from an undifferentiated, migratory phenotype to a differentiated phenotype. One or both molecules appear at several sites of border formation between disparate cell collectives, and CTB proteoglycan expression is associated with chondrogenesis.