Journal of Neuroscience, Vol 5, 562-577, Copyright © 1985 by Society for Neuroscience
Starburst amacrine cells: morphological constancy and systematic variation in the anisotropic field of rabbit retinal neurons
EV Famiglietti
Starburst amacrine cells of rabbit retina have been characterized
previously in terms of their highly distinctive and regular dendritic
geometry. They have been identified as probable cholinergic neurons of the
retina and have been shown to direct output solely to ganglion cells. The
objectives of this paper are to chart the variation of starburst amacrine
cells across the retina, to register the morphological features which are
held constant for individual cells, and to examine factors which may remain
invariant for the population with change in retinal position. Starburst
amacrine cells occur as two completely segregated mirror-symmetrical
populations, type a and type b cells, separately serving OFF and ON
pathways, respectively. They are treated here as two distinct
subpopulations with very similar features. A characteristic morphological
feature of both types, related to branching pattern and best seen in flat
view, is the location of boutons in the distal annular zone. This is the
effective zone of synaptic output, which is constant at 50 to 60% of
dendritic field area, regardless of the cell's retinal location. Both type
a and type b cells exhibit systematic increase in cell body size and
dendritic field diameter, and systematic decrease in frequency of branching
and of synaptic boutons with perpendicular distance from the visual streak.
These rates of increase or decrease fall off considerably at distances
greater than about 1.5 mm dorsal and ventral to the visual streak, but at
this distance, the dendritic field diameters of cells in dorsal retina are
about 65% larger than the diameters of cells in ventral retina. When type a
and type b cells are closely compared, they are seen to differ in several
respects. Branching patterns of type a and type b cells differ slightly,
the latter being more highly branched, and the normalized branching
frequency histograms, characteristic for each type, remain constant with
changing retinal position. At the same retinal location type a cells always
have larger dendritic field diameters than type b cells. This difference is
significant in ventral retina, out to a distance of at least 4.5 mm from
the streak. The maximum percentage difference in size occurs not at
mid-visual streak, but about 1.5 mm ventral to the streak. The population
statistics of dendritic field overlap and areal dendritic coverage have
been calculated using published data on cell densities. It is concluded
that overlap is extraordinarily high (k greater than 25), more than 10
times that calculated for retinal ganglion cells.(ABSTRACT TRUNCATED AT 400
WORDS)
