QUANTITATIVE-ANALYSIS OF SYNAPTOGENESIS IN THE INNER PLEXIFORM LAYER OF MACAQUE MONKEY FOVEA

Synaptogenesis has been tracked by using quantitative electron microsc opic methods in the inner plexiform layer (IPL) of the developing Maca ca monkey fovea from fetal day (Fd) 55 to Fd132. Vesicle-containing pr ofiles were classified according to whether (1) they contained a ribbo n indicating that they originated from a bipolar cell, or (2) the prof ile formed a junction. Group 2 was further subdivided by morphological characteristics into (2a) amacrine, (2b) bipolar, or (2c) unknown pro files. Ribbon-containing bipolar profiles are clearly identifiable at Fd55 when they occur at a density of 0.9/100 mu m(2). Bipolar synapses increase rapidly to 4.7/100 mu m(2) by Fd88, similar to their density at Fd132. Identifiable amacrine profiles forming a junction are rare at Fd55-68. By Fd88, amacrine synaptic density has jumped to 6.7/100 m u m(2) and continues to increase to 9.5/100 mu m(2) at Fd132. These qu antitative data strongly suggest that, at the Macaca fovea, bipolar sy naptogenesis both begins and ends before amacrine synaptogenesis. The large number of immature amacrine synaptic profiles and densities at F d132 suggests that amacrine synapses continue to form after Fd132. Thi s study confirms that cone-dominated monkey fovea has a different sequ ence of synaptogenesis than the rod-dominated peripheral retina (Nishi mura and Rakic, [1985] J. Comp. Neurol 241:420-434). The data support the concept that synaptic developmental sequence is determined by the type of photoreceptor which dominates a particular retinal region or s pecies. Bipolar ribbon synapses are observed in the outer half of the IPL at Fd55, are present in the inner IPL at Fd60, and then, with incr easing age, are found throughout the IPL. This pattern strongly sugges ts that vertical OFF bipolar pathways form earlier than ON pathways in the IPL. In contrast, amacrine profiles are found throughout the IPL at the youngest ages, with an adult-like banding pattern present by Fd 132. (C) 1995 Wiley-Liss, Inc.