The fine localization of acid phosphatase activity in the unvacuolated notochordal cells of the early chick embryo

Summary The electron microscopical localization of acid phosphatase activity was investigated in ultra-thin and semi-thin sections of unvacuolated notochordal cells of chick embryos from stages 9 to 14 (as defined by Hamburger & Hamilton). At stage 9, many notochordal cells show a lightly positive reaction for acid phosphatase activity. Thereafter, the acid phosphatase-positive cells of the notochord increase in number and, at stage 14, the reaction products for the enzyme are distributed throughout almost all the cisternae of the nuclear envelope and a well-differentiated endoplasmic reticulum, the parallel cisternal and reticular parts of the Golgi complex, and various lysosomes in nearly all notochordal cells. In the cisternae of the nuclear envelope and endoplasmic reticulum, the acid phosphatase reaction products are in a fine granular form. In the outermost layer of the cisternal parts of the Golgi complex, faint lead deposits similar to those in the endoplasmic reticulum are found, but in other cisternal and reticular regions which may correspond to the GERL, considerable amounts of reaction products are present. Knob-like projections are also seen protruding from the reticular parts of the Golgi complex. These results suggest that, at least up to stage 14, the notochordal cells are actively synthesizing acid phosphatase which is directly transported from the endoplasmic reticulum to the Golgi complex. The enzyme may be accumulated by the Golgi complex from which primary lysosomes are formed. Furthermore, the pattern of the ultrastructural localization of acid phosphatase activity in embryonic notochordal cells of the chick differs from that of adult cells of other animals.