Cytogenic Study of Embryonic Chick Pancreas. I. Exocrine Tissue

Chick-embryo exocrine pancreas was examined at various days of incubation with both the light and electron microscopes. Tissue prepared for electron microscopy was fixed in either Palade's or Dalton's fluids. Some ultrathin sections were stained with lead salts before examination. The development of the acinar cells as observed by electron microscopy is described with relation to temporal and spatial changes and compared with light microscopic observations. The young acinar cells are similar in many respects to the cuboidal duct cells with which they are closely associated. One of the earliest characteristics distinguishing the young acinar cells is the acquisition of a large nucleolus. The mitochondria, endoplasmic reticulum, zymogen granules, Golgi components, and lipide content exhibit a progressive and distinctive change during ontogenic development. Evidence from this and previous developmental studies indicates that production of increasing amounts of secretory material (zymogen) is concomitant with the observation of increased amounts of organized and closely spaced ergastoplasm in the acinar cells. A few small zymogen granules are present at 6 days of incubation, though the increase in secretion and hyperplasia of the cells and cell system at about 11 to 15 days of incubation follows the general pattern described for the chick embryo in other studies. Mature cyto-logical configurations are observed at approximately the time of hatching. Exocrine cell components, which do not change dramatically throughout development, are characterized. Among these are the microvilli, plasma membranes, and terminal bars.