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Journal of Neuroscience, Vol 3, 644-657, Copyright © 1983 by Society for Neuroscience
Fine structural distribution of acetylcholine receptors at developing mouse neuromuscular junctions
JA Matthews-Bellinger and MM Salpeter
The distribution of acetylcholine receptors (AChRs) in neuromuscular
junctions of embryonic and newborn mice (strain 129/ReJ) was examined on
the fine structural level using [125I]alpha-bungarotoxin labeling and
quantitative electron microscope autoradiography. Changes in junctional
receptor site density were related to changes in the structure of the
postjunctional membrane, in particular the differentiation of
morphologically distinct regions of thickened membrane and the formation of
junctional folds. The following sequence of development is described: (1)
At the earliest age examined, embryonic day 16 of gestation (i.e., at
approximately the time when junctional receptor accumulations are first
detected), subneural receptor aggregates have poorly defined boundaries and
seem to extend beyond the region of direct axonal or Schwann cell contact.
At that time the subneural AChR site density is somewhat variable but
averages approximately 2000 to 3000 sites/micron 2, and the subneural
muscle membrane shows discontinuous membrane specialization (membrane
thickening plus a cytoplasmic amorphous layer). There seems to be no
preferential labeling of contact regions rich in such specialization
compared with those where membrane thickening was less obvious or absent.
(2) By birth, junctional AChRs are strongly correlated with morphologically
specialized membrane, at a constant density of approximately 9000/micron 2
(comparable to that in adult animals of this strain of mouse). During the
first postnatal week, a large fraction of the primary cleft is covered by
Schwann cell rather than axon terminal. Receptor-rich dense membrane and
incipient junctional folds are found under axon terminals and, to a lesser
extent, under Schwann cell extensions, but they are not seen beyond the
edge of the "junctional complex." Coated vesicles are a prominent feature
of the subjunctional sarcoplasm in neonatal junctions, and their number
declines sharply during the first week. (3) Junctional folds differentiate
mainly after the first postnatal week. Development of folds is associated
with a 1.5- to 1.8-fold increase in length of specialized thickened
membrane and a parallel increase in number of receptor sites per endplate.
Since this is also a period of rapid overall endplate growth (Steinbach,
J.H. (1981) Dev. Biol. 84: 267-276) the total number of endplate receptors
increases greater than 30-fold during the maturation period following the
first week while the concentration of receptors/micron 2 of thickened
membrane remains constant.
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