differentiation and protein synthesis 107 



The Avian Secreted Keratins 



The secreted keratins, the horny lining of birds' gizzards and the egg- 

 shell membranes, have not been as extensively studied as their unusual 

 extracellular location would seem to warrant. We lack an exact knowledge 

 of their structure and mode of formation. Even the type of fibrous protein 

 found in egg-shell membranes is disputed. According to Champetier and 

 Faure-Fremiet (1938) the material gives a collagen pattern. On the other 

 hand, the chemical analysis of purified membranes reported by Calvery 

 (1933) shows it to be of the keratin type. The purification involved an 

 enzymatic digestion aimed at removing constituents less resistant than 

 keratin (mucin, collagen, etc.) which may account for the different findings. 

 The present writer has made electron micrographs of sections of hen's egg 

 membranes (Plate 18B) which revealed a felt- work of fibrils each of which 

 consists of two distinct components. By staining methods and the light 

 microscopy two separate fibrillar systems are usually distinguished (Moran 

 and Hale, 1936): mucin and keratin. It would appear that actually the 

 fibrils of one system (probably keratin from its stronger reaction with 

 osmium tetroxide) are enveloped by a deposit of a less reactive material 

 (mucin?). 



No study has been made of the fine cytology of the cells whose secretion 

 coats the eggs with their membranes during their passage through the 

 oviduct. 



The tough, thick lining of the gizzard, which protects the cellular 

 surfaces of this muscular mill against wear and tear, consists of coherent 

 sheets of protein mixed with mucin. The material gives a strong and 

 specific reaction for disulphide groups and is accepted as a keratin (Broussy, 

 1932). The layer is birefringent, but irregularly oriented. Longley (1950) 

 reports that it gives an X-ray pattern consisting of three haloes cor- 

 responding to spacings of 10, 4*7 and 3*7 A which would suggest it is a 

 /S-keratin (feather keratin?). 



The layer is secreted by the cells of a columnar epithelium resting on a 

 typical basement membrane. The secretion collects as rounded liquid-like 

 inclusions in the apex of the cells and escapes to flow down a typical 

 glandular lumen to join the overlying material (Broussy, 1932 and Fell 

 (private communication)). An electron microscopic study of these cells in 

 21 and 17-day-old chicken embryos made by the writer shows their fine 

 cytology to resemble that of epidermal cells. RNP granules are common 

 but a basophilic (particle covered) reticulum is also present (Plate 19). 

 Clusters of smooth membranes (y-cytomembranes) were conspicuous. 

 A structureless basal membrane underlies the epithelium; there are few 

 desmosomes and no intracellular tonofibrils. In the body of the cytoplasm 

 and towards the cell apex the secretion appears as structureless rounded 

 droplets of various sizes (Plate 19) which stain strongly with PTA 



