The differentiation of epidermis

When the whole skin of embryonic chickens is grown in culture, epidermal differentiation quite closely resembles that in vivo (Miszurski, 1937; Fell, 1957; Wessells, 1961). Separated epidermis, however, lacking its connective tissue, develops less well; in hanging drop cultures it may be maintained occasionally as an epithelial sheet which does not differentiate (Champy, 1914; Drew, 1922; McLoughlin, 1961a), while in organ culture the epidermis curls up and either remains undifferentiated (Gomot, 1958; Matoltsy, 1960) or becomes largely necrotic (Wessells, 1962). Some degree of cytodifferentiation was observed by McLoughlin (1961a), however; she found that in isolated limb epidermis of 5day embryonic chickens, while many cells degenerated, the live cells organized themselves into nodules and underwent squamous changes, but there were very few mitoses. In none of the above work did isolated epidermis survive and differentiate as an organized, keratinizing tissue. On the other hand, when epidermis was combined in culture with its own mesenchyme, it survived and differentiated as in whole skin grown under the same conditions (McLoughlin, 19616; Wessells, 1962). Thus, for its histodifferentiation, the epidermis is dependent upon the underlying connective tissue. In addition to its involvement in epidermal histodifferentiation, the underlying mesenchyme acts on the epidermis in the morphogenesis of integumentary appendages, as in the development of the preen gland (Gomot, 1958), feathers (Sengel, 1958; Rawles, 1963) and scale, beak and spur (Rawles, 1963). A similar tissue interrelationship has been demonstrated in other developing organs— e.g. mouse salivary gland (Grobstein, 1953, 1956) and pancreas (Golosow & Grobstein, 1962), ureter of mouse (Grobstein, 1955) and chicken (Calame, 1961), and lung of chicken (Dameron, 1961) and mouse (Alescio & Cassini, 1962)— which shows that for their continued growth and differentiation, embryonic epithelia usually depend upon their subjacent mesenchyme. 1 Author's address: Department of Zoology, University of Bristol, Bristol, U.K.