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A comparative review of the sites of histoenzymatic dephosphorylating activities and of gene expression in developing limb buds of mammalian and avian species.

The sites of dephosphorylating activities histochemically demonstrated in developing limb buds of mammalian and avian species have been reviewed and compared with the pattern of gene expression or of other biochemical properties reported at similar stages in the corresponding sites. Alkaline phosphatase, acid phosphatase, 5' nucleotidase and ATP-phosphohydrolase reactions were studied in mouse, rat and chick embryos. Alkaline phosphatase only was detected in mole limb buds whereas only 5' nucleotidase and ATP-phosphohydrolase were revealed in limb rudiments of macacus rhesus embryos. Five decisive periods or events of limb morphogenesis have been considered successively: (1) the early stages during which the prospective limb constituents acquire limb forming properties and give rise to the young limb buds, (2) the invasion of the limb bud mesoderm by myogenic cells of somitic origin, (3) the ectoderm-mesoderm interactions with particular emphasis on the properties displayed by the apical ectodermal ridge and by the underlying subridge mesoderm of the progress zone, (4) the period of growth and pattern formation along the proximo-distal, anterior-posterior and dorso-ventral axes, with special attention to the properties of the zone of polarizing activity, and (5) the period of tissular predifferentiation particularly as concerns prospective skeletal, musculo-tendinous and connective tissues, with brief comments about growing nerves and blood vessels. At least during the morphogenetic period, most dephosphorylating properties appear independently associated with gene expression or other regional biochemical properties. Many sites of dephosphorylating activity may therefore be considered as interesting markers of ungoing morphogenetic events among which tissue interaction and signalling are frequently concerned.