Cell-Cell Interactions in the Mesenchyme of Limb Bud and Blastema

Cell-cell interaction is known to be important for the various processes of morphogenesis. In limb bud morphogenesis, cell-cell interaction of mesenchyme is crucial (Fig. 1). Cell differentiation of the limb bud mesenchyme proceeds from the proximal to the distal region. In the distal-most region, the progress zone (PZ), within 250-350 μm from tip, the cells are in an undifferentiated state under the control of the tip ectoderm, apical ectodermal ridge (AER) (Summerbell et al., 1973). The mesenchymal cells in the PZ are relatively isolated from each other and surrounded by extracellular matrix molecules such as Type I collagen, fibronectin and proteoglycan M (versican) (Shinomura and Kimata, 1990). Since the limb bud grows distalward and the influence of AER is limited, the cells emerge from the PZ during limb development and undergo differentiation to cartilage or soft connective tissue. This differentiation is thought to be caused by the removal of extracellular matrix which has previously prevented cell-cell interaction. In the core region, the mesenchymal cells condense, which promotes cellcell interaction and thus, chondrogenic differentiation occurs to form the humerus in the forelimb and the femur in the hindlimb. In the peripheral region, the cell density remains relatively low, and the formation of soft tissues such as dermis and tendon occurs. Thus, the mesenchymal condensation is crucial for the cell differentiation of the limb bud. Another type of cell-cell interaction occurs in the limb morphogenesis. The mesenchymal tissue of the chick limb bud, especially in the PZ, has a high regulation capacity, and removal or addition of the mesenchyme results in normal development. When a square piece of mesenchymal tissue is removed from the center of the young chick limb bud to produce a hole from ventral to dorsal surface, a limb of normal size and with a normal cartilage pattern is formed (Summerbell, 1981). Thus, the limb bud cells are necessary to recognize their positions and adjacent cells to recover the missing area. This type of interaction seems to depend on "positional values". The limb cartilage pattern is specified in the PZ (Summerbell, 1974, 1981), in which cells are thought to receive anteroposterior positional values which are specified by the polarizing region at the posterior margin of the limb bud; proximodistal positional values which change autonomously under the influence of AER during the limb bud development are also specified (Summerbell et al., 1973; Fig. 1). Thus, cells in the PZ seem to be different from each other both temporally and spatially. Recent findings on homeobox gene expression and targeting in the limb bud supports the notion that homeobox genes are closely related to positional values (Nohno et al., 1991; Yokouchi et al., 1991; IzpusiaBelmonte et al., 1991; Davis et al., 1995). Since local cellcell interaction is necessary for limb morphogenesis, the heterogeneity in homeobox gene expression might be reflected in cell surface differences. Recently, by sorting-out experiments we have found such cell surface differences in the chick limb bud cells which are related to the positional values along the limb bud axes (Wada et al., 1993; Ide et al., 1994; Wada and Ide, 1994). Similar positional value-related cell-cell interaction has been reported in amphibian limb regeneration (Nardi and Stocum, 1983; Crawford and Stocum, 1988). In this paper, I will review briefly those experiments on the cell surface difference in the limb bud and regenerating blastema from the aspect of positional value-related cell-cell interaction in limb morphogenesis. Cell-substratum interaction in the limb mesenchyme is also important for limb morphogenesis and is closely related to cell-cell interaction, because inhibition of cell-substratum attachment often prompts the cell-cell attachment. Epithelio-mesenchymal interaction is also important for limb morphogenesis (Ide, 1990). However, in this paper, I will focus on the cell-cell interaction of the mesenchyme.