Scanning electron microscopy of cell aggregation: cardiac and mixed retina-cardiac cell suspensions

Reaggregates of cells from 7-day embryonic chick hearts, 10-day neural retinas and respective cell mixtures were examined by scanning electron microscopy (SEM). Cardiac cell aggregates formed during the first 2 hours were composed of rounded cells arranged in linear or branched arrays. By further collection of single cells and accretion of cell clusters, the aggregates increased in size and formed large grape-like masses. By 12 hours, heart aggregates assumed a spherical shape with partial sorting-out of myogenic from non-myogenic cells; the muscle elements occupied the interior of the aggregates, whereas flattened, squamous-like cells, of a non-muscle character, covered the surface in a multilayered epithelium. Single rounded cells were still found on the surface of 12 to 24-hour aggregates; however, they were absent by 48 hours, suggesting that the collection of free cells by the cardiac aggregates ceased during the second day. Early aggregates (2 hours) of retina cells showed initial stages of axonal and dendritic outgrowth characteristic of neural tissue. Examination of heterotypic aggregates of neural retina and myocardial cells after 2 to 6 hours showed small groups of retina cells attached to the surface of the heart cell clusters. The retina cells did not appear to be randomly distributed within the early aggregates but formed small tissue specific clusters even by 2 hours in culture. These results indicate that SEM should be a valuable tool in the further analysis of homo- and heterotypic cellular aggregation.