Freeze-etched neuronal growth cones from rat cerebral cortex at birth: plasma membrane morphology in relation to synapse formation

Specificity of membrane specializations in mechanoreceptors of birds--A freeze-etching study

The detailed knowledge of the molecular process of mechanotransduction is still an unsolved question. The investigation of the intramembranous structure of the cutaneous mechanoreceptors may play an important role in elucidating this problem. In this relation, Herbst sensory corpuscles in ducks were studied for the first time using the freeze-etching and thin sectioning techniques. Herbst corpuscles have the basic structural components valid for most of the encapsulated mechanoreceptors in mammals: a capsule made of perineural cells, a lamellar complex of modified Schwann cells, surrounding the non-myelinated part of the receptor nerve fiber and its ending. Freeze-etching replicas reveal that the plasmalemmae of the capsule cells, modified Schwann cells and axolemmae of parts of the nerve fiber differ in both density and pattern of distribution of intramembranous particles (IMPs) as well as IMP size. On all the plasmalemmae the IMP density is higher on the P-face (2000-3300 microm(-2)) than the respective E-face (800-1500 microm(-2)). The axolemma of the ending of the receptor nerve fiber expresses higher density of IMPs than its shaft. The mean IMP size for all the plasmalemmae varies between 5.5 and 7.5 nm. Many tight junctions occur between the capsule cells. These results indicate that the non-myelinated axolemma as well as the plasmalemmae of other components of Herbst corpuscles are specialized in terms of content and distribution of IMPs. The IMPs may represent various kinds of mechanosensitive channel proteins or related membrane-bound proteins participating in the process of mechanotransduction.

Synaptic membrane morphology in the rat cerebral cortex during development: image analysis of freeze-etching replicas of isolated synapses and synapses in situ

Both synaptic sites in situ from rat cerebral cortex and isolated ones in synaptosomal and growth cone fractions derived from it were studied during postnatal development. Freeze-etching technique and image analysis were used to determine the size of the intramembranous particles in the pre- and postsynaptic sites. At each age investigated, the greatest mean particle size was established on the E-face of the postsynaptic sites and ranges from 6.2 nm (day 0) to nearly 10 nm (day 90). The continuous mean particle size increase from birth to maturity shows the same rate but a different rhythm for the two synaptic sites. The results indicate that fractionation do not disturb the correlation between the particle size and age thus outlining the stability of the developing synaptic membrane morphology to physical treatments.