Keratogenic metaplasia of the avian chorionic epithelium: Absence of the Beta stratum which characterizes the epidermis of the avian scutellate scale

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Abstract
Upon exposure to air, the avian chorionic epithelium (CE) transforms into a keratinizing, stratified squamous epithelium. The overall morphology of this keratogenic CE is unlike any keratinized epidermis of the normal embryo. The basal cells are squamous, not like those of normal avian epidermis which are cuboidal or columnar, and the basal lamina is poorly developed. Although desmosomes are present throughout this keratogenic epithelium, hemidesmosomes do not form in the basal cells. Peridermal and subperidermal layers likewise were not observed. This ultrastructural study does show, however, that the cells of the intermediate layers and stratum corneum resemble very closely those observed in the Alpha Stratum of normal avian epidermis. Yet, in no case does a Beta Stratum characteristic of the normal avian scutellate scale epidermis ever form. Evidently, a rather non‐specific cue can bring about the formation of an Alpha Stratum, whereas induction of a Beta Stratum requires a more specific cue. The hypothesis that the formation of the Beta Stratum of scutellate scales does require a specific dermis and that the dermis itself requires a prior association with an “epidermal placode” is discussed.