Developmental changes in P-face and E-face particle densities of Xenopus cardiac muscle plasma membrane.

P- and E-face particle densities (PPD and EPD) were measured in electron micrographs of freeze-fractured cardiac sarcolemma from eight developmental stages of Xenopus laevis (stages 33/34 (33 post-otic somite embryos) to 66 (fully metamorphosed juvenile toad], using stereo-imaged replicas. We found striking progressive increases in PPD and EPD, most rapid between stages 33/34 and 37/38; that PPD was significantly greater than EPD at all stages; that both PPD and EPD of stereo-imaged replicas were about X2 greater than corresponding values not stereo-imaged; and that sarcolemmal PPD of late anuran embryonic and post-metamorphosis hearts were significantly greater than our previously determined PPD values for chick late embryo and adult mammalian sarcolemma. We suggest that PPD and EPD depend on how membrane particles segregate during freeze-fracture and on the relative contributions of membrane-spanning and non-membrane-spanning integral membrane protein complexes to each fracture face.