Apoptosis and epithelium-mesenchyme interaction during mammalian morphogenesis

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Apoptosis is a common event during mammalian morphogenesis, which is morphologically known as programd cell death. The cell death in interdigital spaces of mammalian developing limb and the disappearance of the midline epithelial seam during the fusion process of the mammalian secondary palate are classical examples of morphogenetic programd cell death. For the formating of various structure during development, apoptosis has been considered to play a key role by elimination unnecessary cells to achieve complicated histogenesis and organogenesis. Our recent studies showed that apoptosis in the limb and palate development plays more aggressive roles in addition to the elimination of unnecessaery cells. In mammalian limb development, apoptosis may play a role in preventing the formation of extra digits and reducing the size of the first digit as well as in the digit separation and joint formation (Mori et al., 1995 ; Mori, 1995). In the process of palate fusion, apoptotic cell death may be required for the midline epithelial fusion of opposing palatal shelves (Mori et al., 1994). In addition, we examined the cause of limb malformations induced by BrdU in fetal mice and found that the altered expression of several genes including apoptosis-related genes, Hox genes and growth factor genes at the anterior region of the hindlimb bud in the formation of preaxial polydactyly and triphalangism of the first digit in BrdU-treated mouse fetuses. Moreover, our analysis of apoptosis in the testes of some testis-specific gene deficient mice showed that an additional increased level of apoptosis during the first wave of spermatogenesis, in pachytene spematocytes (Mori et al., 1997). further studies will be necessary to define clearly the significance of apoptosis during mammalian morphogenesis.