J. Crooks et al., QUANTITATIVE-ANALYSIS OF SYNAPTOGENESIS IN THE INNER PLEXIFORM LAYER OF MACAQUE MONKEY FOVEA, Journal of comparative neurology, 360(2), 1995, pp. 349-362
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.