MORPHOLOGY OF BASAL OPTIC TRACT TERMINALS IN THE TURTLE, PSEUDEMYS-SCRIPTA ELEGANS

The morphologies of axon terminals of retinal ganglion cells projectin g to the basal optic nucleus (BON) via the basal optic tract (BOT) wer e studied in the red-eared turtle. The BOT was visualized on the ventr al surface of the brainstem in vitro, and either biotinylated dextran amine was injected extracellularly or neurobiotin was injected into ph ysiologically identified axons during intracellular recordings. Up to 16 hours after tracer injection, the brains were fixed, sectioned para sagittally, and stained for biotin and Nissl substance. The diameters and depths of extracellularly tilled axons were measured at three BON sites. Fourteen axons were reconstructed from serial sections with the aid of appropriate computer software. Analysis of extracellularly fil led retinal axons revealed that about three times more axons were pres ent just inside the rostral border of the BON compared with its caudal border. Thick (2-4 mu m) axons were located within 100 mu m from the ventral border, whereas thin (<2 mu m) axons were found throughout the nucleus. Only the thinnest axons (<1 mu m) extended caudally from the nucleus, indicating that some extracellularly labelled fibers passed through the BON. The intracellularly filled axons were more similar to the thick axons filled extracellularly and arborized entirely within the BON. All of the intracellularly filled axons had thick ventral tru nks from which many thin branches extended dorsally and obliquely with in the BON. The thin branches bifurcated repeatedly to form bead-like varicosities or boutons that often formed clusters within regions of 1 50 mu m(3) or less. These clusters may reflect areas of focused synapt ic contact on BON cells with specific direction preferences. (C) 1998 Wiley-Liss, Inc.