HISTOGENETIC POTENTIAL OF RAT HINDLIMB INTERDIGITAL TISSUES PRIOR TO AND DURING THE ONSET OF PROGRAMMED CELL-DEATH

The histogenetic potential of interdigital tissues isolated from the a utopod of rat embryonic hind-limbs between 14.5 and 16.5 days was inve stigated. A wedge of tissue containing ectoderm and mesoderm was excis ed from between the developing digits and grafted beneath the kidney c apsule of adult rats for two weeks. We have previously demonstrated th at the renal capsule is an excellent site for permitting limb tissues to proliferate and differentiate (Chan et al.: J. Exp. Zool., 260:74-8 3, 1991). At 14.5 days, when cell death (revealed with neutral red sta ins) within the interdigital zone was limited to the apical ectodermal ridge (AER), the interdigital mesoderm was capable of developing into bone, cartilage, and loose connective tissue in the kidney. It was es timated that the skeletal elements occupied approximately 38% of the o verall area of the grafts. In addition, the ectoderm was able to produ ce keratinized epithelium, hair follicles, and sebaceous glands. In 15 .5 day autopod, necrosis was present both in the AER and the mesoderm between the AER and marginal sinus. Interdigital mesoderm obtained fro m this stage of development formed cartilage but not as extensively as that derived from 14.5 day autopod (4% as compared with 38%). Necroti c cells were present in all of the interdigital zones at 16.5 days. Te n explants were introduced into the kidney at this stage, but only 4 g rafts were recovered after 2 weeks. In all cases, the explants did not produce cartilage. Only a small amount of keratinized epithelium and loose connective tissue was found. In summary the interdigital mesoder m has the potential to develop bone, cartilage, and loose connective t issue, but this ability is progressively lost during morphogenesis.