Blastema formation and cockroach leg regeneration.

The cockroach limb shows a remarkable ability to regenerate after amputation or grafting together of normally non-adjacent tissues. For this reason, it is a useful tool in the study of tissue growth and pattern formation. In this thesis a careful study has been made of the behaviour of the epidermal cells in a number of situations in which regeneration occurs, in order to discover some of the general processes involved and to test some predictions of models of pattern formation. The situations studied were: regeneration from the trochanter-femur breakage plane, amputating either before or after the critical point in the moult cycle, regeneration from the tibia-tarsus breakage plane, regeneration from the mid-tibia level, intercalation in the tibia and the formation of supernumerary limbs. Light microscopy was used to study the epidermal cells and both colchicine and 3H-thymidine were used to investigate the patterns of cell division. Scanning electron microscopy was used to show the cuticular changes that were visible after regeneration and hence which structure had been regenerated. The results showed that during terminal regeneration, epidermis proximal to the level of amputation becomes 'activated' and forms a blastema in which all the epidermal cells appear to be dividing rapidly. The blastema grows and differentiates into the new leg with differentiation probably occurring in a disto-proximal sequence. During intercalation and supernumerary limb production the situation is less clear, although blastemas are clearly produced during supernumerary formation. The results are compared with those of regeneration in other animals and the importance of blastema formation to limb regeneration is discussed.