Fine structure and absorption patterns of intestinal epithelial cells in rainbow trout alevins

Summary The intestinal epithelium in the rainbow alevins at the period of initial feeding is composed of typical columnar cells with a striated border and goblet cells with mucigen droplets. The columnar epithelial cells are provided with well organized cell organelles which are found in the intestinal absorptive cells of vertebrates in general. Remarkable differences are seen in some morphological aspects of columnar epithelial cells between the antero-mid intestine and the posterior intestine. Those in the antero-mid intestine are loaded with considerable quantities of fat droplets, whereas those in the posterior intestine are characterized by containing vacuoles with less dense materials which resemble engulfed protein in general feature. No pinocytotic process is found in the former, but vigorous pinocytosis occurs in the latter. The membranous lamellar structures are developed in the mid-basal portion of the columnar epithelial cells. Those in the basal portion of the cells in the mid intestine are arranged in parallel to the longitudinal cell axis with associated groups of mitochondria.The cellular morphology of the intestinal epithelium suggests the possibility that absorption of nutrients derived from external foods occurs in the rainbow alevins still having a small yolk sac.I am grateful to Dr. Kiyomatsu Matsubara of our laboratory, Dr. Reuben Lasker of the U.S. Bureau of Commercial Fisheries, La Jolla, and Dr. Carl L. Hubbs of the Scripps Institution of Oceanography, La Jolla, for much valuable criticisms and encouragement. Thanks are due to Mr. Tomomi Watabe of the Japan Electron Optics Laboratory for his generous help.