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The freeze-cleave technique has revealed particles on internal aspects of plasma membranes which appear to be glycoprotein and lipoprotein complexes (1, 2, 12, 18, 20) . The particle size most commonly observed is 85-90 A but in certain sites the particles may be as small as 60 A or as large as 300 A in diameter (9, 11, 17, 22, 25) . For most membrane systems the granules are most numerous on the cytoplasmic fracture face (Face A), but in certain circumstances the polarity is reversed and the granules are more numerous on the external leaflet (Face B) (3, 5) . In the specialized areas of close intercellular contact recognized as gap junctions (14, 21), for which there is good evidence of involvement in intercellular communication (7, 16, 22), the intramembranous granules are arranged in arrays frequently displaying a hexagonal lattice with an interparticle spacing of 90-100 A (8, 14) . It is characteristic of gap junctions that the outer fracture face (Face B) shows a complementary hexagonal lattice of pits corresponding to the granular lattice on the cytoplasmic leaflet (Face A) (14) . Free membrane surfaces may display special aggregation patterns of intramembranous particles associated with specific regions of highly specialized cells such as spermatozoa (6), sites of secretion product discharge (23), forming micropinocytotic vesicles (15) or the bases of motile cilia (27) . Also, granule patterns may be altered experimentally in immune reactions (24), by changes in pH and temperature (19), by exposure to low concentrations of proteolytic enzymes (18), and by physiologic activation (6) . All of these patterns are distinct from the closely packed hexagonal arrays that characterize gap junctional complexes . The current study has revealed a specialized configuration of intramembranous particles at the external surface of normal Hydra epithelial cells which closely resembles the arrangement in gap junctions. This specialization appears to be confined to epidermal cells as it has not been observed on gastrodermal cells included in the same replicas. The purpose of this communication is to describe this specialized arrangement and to discuss its possible significance .

A CLOSELY PACKED ARRAY OF MEMBRANE INTERCALATED PARTICLES AT THE FREE SURFACE OF HYDRA