Avian Spur Development: Abnormal Morphogenesis and Keratinization in the Scaleless (sc/sc) Mutant

Abstract

We have examined morphogenesis and biochemical differentiation of the metatarsal spur in normal and scaleless chickens. Unlike previous studies suggesting that the spur is a modified reticular scale, we find that keratins produced by the metatarsal spur are identical to those of scutate scales. Gross observations show that although the scaleless mutant develops spurs at the normal site, they are abnormal in their size, morphology, and time of appearance. Histologically, the epidermis and dermis of the spur in both normal and scaleless embryos are similar, yet electrophoretic data show that some alpha-keratins and all beta-keratins of normal spurs are absent from the epidermis of scaleless spurs. Thus, the spur's response to the scaleless gene differs from that observed for either scutate or reticulate scales, emphasizing the point that the tissue interactions that regulate morphogenesis and biochemical differentiation of the skin and its appendages may differ, depending on the structure being studied. Stratification is characteristic of the epithelia of all vertebrates and, except for fishes, all vertebrate epithelia exhibit some form of keratinization (Parakkal & Alexander, 1972). Keratin, a fibrous protein, has two forms identifiable on the basis of their X-ray diffraction patterns (Baden & Maderson, 1970; Kemp & Rogers, 1972; Parakkal & Alexander, 1972), and recently the alpha and beta forms of keratin have been characterized further according to their molecular weights and labeling with 14C-iodoacetate (Dhouailly et al., 1978; O'Guin & Sawyer, 1982; McAleese & Sawyer, 1982). The avian feather and beak elaborate the beta form of keratin (Baden & Maderson, 1970), while the reticulate scales of the metatarsal footpads produce the alpha form (O'Guin & Sawyer, 1982; Sawyer & Borg, 1979, 1980; Spearman, 1966). Scutate scales of the anterior metatarsus show an outer scale surface which consists of the beta form and an inner scale surface and hinge region which consists of the alpha form (Baden & Maderson, 1970; O'Guin & Sawyer, 1982; O'Guin et al., 1982; Sawyer, 1982), very similar to the crocodilian scale (Baden & Maderson, 1970). The diverse structures of the avian integument arise by interaction between their mesodermal and ectodermal components (Rawles, 1963; Sawyer, 1982; Sengel, 1976; Wessells, 1977), and it is becoming more evident that gene 1 This study constitutes a thesis presented in partial fulfillment of the requirements of the Master of Science Degree, and was supported in part by a National Science Foundation Grant PCM-8011745 to R.H.S. The authors thank Debra Chavis for typing the manuscript. 2 Publication costs, in part, are being met by a grant from the Spencer-Tolles Fund of the American Microscopical Society. 3 Present address: Animal Science Department, Clemson University, Clemson, South Carolina