Ultrastructure of the granule cells in the small intestine of the rainbow trout (Salmo gairdneri) before and after stratum granulosum formation

The ultrastructure of the small intestinal granule cells (GC) of the rainbow trout (Salmo gairdneri) was studied in fry and fingerlings before the cells organized to form the stratum granulosum (SG) and in 12-month-old juveniles which possess a well-developed SG several cell diameters in thickness. The GC of juvenile trout exhibit a striking reduction in nucleolar numbers and in the quantity of rough endoplasmic reticulum relative to the GC of younger fish, but contain even greater numbers of large, electron-dense cytoplasmic granules. The final stages of cellular maturation therefore take place in the GC after their organization to form a layer, and the cells appear to lose their capacity to synthesize protein(s) for the cytoplasmic granules. Since the latter structures are, nevertheless, maintained in large numbers in mature GC, it is suggested that they function intact within the cell. No precursor to the granulated form of the GC was identified in the intestine, a result which suggests that the earliest developmental stages of the GC locate within another organ.