Formation and growth of gap junctions in mouse myocardium during ontogenesis: a freeze-cleave study.

The freeze-cleave technique demonstrates the presence of gap junctions at early stages of mouse cardiac muscle ontogenesis. The formation and growth of these junctions were studied at 4 stages of development: 10, 14, 18 days post-coitum (dpc) and at the adult stage. The diverse aspects of the gap junctions are interpreted as different steps in their formation. The first indication of this formation seems to be the presence of linear arrays of 9-nm particles on PF faces. At one end of these arrays a small aggregate of particles appears which acts as nucleation site and grows by incorporating individual gap particles and/or linear arrays. Nexuses with arms and/or central particle-free zones would represent intermediate steps in the formation of junctions. The largest nexuses could be formed by fusion of smaller ones and/or by accretion of gap particles. Analysis of the size distribution of gap junctions shows their growth during their development. At 10 dpc the surface area (S) of nexuses ranges from 0.1 to 3 x 10(-2) micrometer2, at 14 dpc from 0.1 to 15 x 10(-2) micrometer2, at 18 dpc from 0.1 to 26 x 10(-2) micrometer2, and at the adult stage from 0.1 to 54 x 10(-2) micrometer2. The percentage of large nexuses (Sgreater than 0.5 x 10(-2) micrometer2) steadily increases from 10 dpc to the adult stage. Fixation by glutaraldehyde before glycerol infiltration does not induce any modification in the size distribution of adult heart gap junctions.