Squamous metaplasia and keratinization of chorionic epithelium of the chick embryo in egg and in culture

Abstract

1. 1. Experimental production in ovo and in vitro of a keratogenic metaplasia in chorionic epithelium of chick embryos is described. The metaplasia manifested itself as an alteration of the one-cell-thick, respiratory, chorioepithelial membrane into a stratified, squamous, highly keratinized epithelium. 2. 2. In ovo, keratogenic metaplasia occurred regularly in fenestrated eggs, provided with shell openings on the 8th to 14th day and incubated under conditions of near-saturation or saturation-humidity until the 18th day. The degree and extent of change were directly related to over-all size of the openings, i.e., to the facility of interchange between inside and outside air. 3. 3. In vitro, metaplastic changes occurred in explants grown for 8 days upon a plasma clot by the organ culture method, without further experimental interference. 4. 4. There were no keratogenic changes in fenestrated, well-sealed eggs which indicated that mere detachment of the chorionic epithelium from the shell, without provision of direct access to outside air, did not provide conditions necessary for this metaplastic alteration. 5. 5. Keratogenic metaplasia was expedited and intensified by slight trauma to the exposed chorionic epithelium and by small tissue grafts placed upon it. However, without provision of shell openings, these promoting conditions did not, by themselves cause typical keratogenic changes. 6. 6. Other types of morphogenetic changes in metaplastic chorionic epithelium, such as movements of epithelial cells into the underlying mesenchyme and their formative activities these, are briefly described. 7. 7. The developmental state of chorionic cells at the time of their response to the effective conditions, and the possible nature of the direct stimulus, are discussed. It is suggested that alteration in the gaseous environment of chorionic epithelium, consequent upon its exposure to atmospheric oxygen:carbon dioxide proportions, may have initiated the chain of events conducive to keratogenic metaplasia.