Electron microscopic histochemistry of alkaline phosphatase activity in the developing chick retina

The present paper describes ultrastructural localization of non-specific alkaline phosphatase (ALPase) activity in the chick retina during late embryonic development and after hatching. In the newly-hatched chick retina, intense ALPase activity was found in the outer plexiform layer (OPL), and a less intense reaction was found in the ganglion cell layer and at the outer limiting membrane. Electron microscopic histochemistry revealed that bipolar and horizontal dendrites within the OPL displayed intense reaction product along their plasma membranes. Only horizontal cells showed reaction product on almost their whole circumference. Müller cells also showed reaction product on the plasma membrane in the ganglion cell layer and on the microvilli at their apical portion. The reaction product in the OPL was first recognizable in the retina of the 13- to 14-day-old embryonic chick, and its intensity increased during further development. Little reaction product was found in Müller cells at the embryonic stage. The function of ALPase activity is discussed in relation to the development of the OPL and the nutrient supply for the neural retina.