Morphogenesis of shell and scutes in the turtle Emydura macquarii

Formation of the scutes and dermis of the embryonic shell of the turtle Emy dura macquarii was studied using light and electron microscopy. Shell morph ogenesis begins at embryonic stage 15 and the shape of the shell is mostly completed by embryonic stage 19. The carapace anlagen arises as a thickenin g of the skin in the dorsal part of the mid-trunk region between the anteri or and posterior limbs. This thickening extends ventro-laterally to form ri dges at the margins of the carapace. Each ridge forms as a thick epidermal placode over a condensation of mesenchymal cells. The epidermis behind the advancing margins of the carapace is cuboidal or columnar but does not form placodes. The margins of the carapace expand rapidly in all directions. Th e plastron anlagen is derived from epidermal cells localised in the latero- ventral regions between the fore- and hind-limbs. Plastron placodes are pre sent laterally, while the mid-ventral and central epidermis remains cuboida l or columnar but does not form placodes at embryonic stage 16. The plastro n thickening rapidly moves from a latero-ventral position to a flat ventral position between embryonic stages 16 and 19. Dermal-epidermal anchoring co mplexes occur throughout placodes of both the carapace and plastron, but ar e rare in non-placode areas. The accumulation of a dense mesenchyme beneath the shell epidermis forms a dermal cushion that surrounds the body cavity. The superficial dermis close to the epidermis is made of mesenchymal fibro blasts at embryonic stage 19, although the inner-most areas contain bipolar fibroblasts and extracellular fibrils. Scutes with set-rations at their bo rders form as invaginations of the epidermis into the dermis in the mid-dor sal areas of the embryo at embryonic stages 18-19. Dermal-epidermal anchori ng complexes are located around the infoldings that form the scutes of the hinge region. The epidermis of the shell has 2-3 suprabasal cells at embryo nic stages 19-22, and lacks keratinisation before embryonic stage 22 when i t has 4-6 suprabasal layers with 2-3 external layers made of flat cells. Th e dermis thickens and has numerous collagen fibrils after embryonic stage 1 9. The formation of dermal bones begins at embryonic stage 18-19 in the pla stron. Only small areas of the carapace near to the bridge have begun to fo rm dermal bone at embryonic stage 19. Calcification begins at embryonic sta ge 19, but is still incomplete at embryonic stages 24-25.