The fine structure of freeze-fractured intercellular junctions in the guinea pig inner ear

Abstract

The distribution, size and configuration of tight and gap junctions in the guinea pig inner ear were examined in aldehyde-fixed and non-fixed freeze-etched tissue preparations.

Generally, freeze fracture of the zonulae occludentes which seal the endolymphatic space reveals a band of five to eight interconnecting strands. The zonulae occludentes of the cochlear and vestibular sensory epithelia consist of numerous strands and represent the “very tight” type of junction. Possible pathological alterations of the tight junctions in some inner ear diseases are discussed.

The supporting cells of all inner ear sensory epithelia as well as all other epithelia lining the endolymphatic space are coupled by many gap junctions of different sizes. Gap junctions are a common finding on the plasma membranes of the fibrocytes of the spiral ligament and-as previously reported-of all cells of the stria vascularis. Rather seldom they can be observed connecting the cells of the tympanic covering layer.

The different particle densities of inner ear cell membranes are described in detail. At the base of the stereocilia, both in the hair cells of the organ of Corti and in the vestibular hair cells particle aggregations can be found which may be the morphological equivalents of ionic permeability sites and may mediate the first step of the mechano-electric transduction mechanism. The kinocilia of the vestibular hair cells exhibit a “ciliary necklace” (Gilula and Satir, 1972).

On the A fracture faces of the supporting cells in all inner ear sensory epithelia rectangular arrays of small particles occur.

Preliminary results are reported concerning the hair cell synapses of the cochlea and of the vestibular labyrinth. The A fracture faces of the inner ear hair cells show 80–90 Å particles arranged in a thumb-print-like pattern. On the A-fracture faces of cochlear and vestibular hair cells rows of particles can be seen. Some synaptic cell membrane features of the inner ear hair cell membranes are similar to those observed in the central nervous system at the sites of chemical transmission, e.g. aggregations of membrane particles and small protuberances occuring on the B fracture faces of presynaptic membranes (Pfenninger et al., 1972) which are interpreted as temporary fusion sites between synaptic vesicles and the presynapatic membrane. The postsynaptic membranes of the nerve calyx which surround the vestibular hair cell type I show a high particle density on their A fracture faces. These particles are interpreted as transmitter receptors.

The fine structure of freeze-etched inner ear blood vessels is described, e.g. their tight junctions, particle densities and micropinocytotic vesicles.

The physiological relevance of the findings is discussed with special respect to the barriers which seal the different compartments of the inner ear (perilymphatic spaces, endolymphatic space, stria vascularis) as well as to the blood-perilymph-barrier.