Myoblasts obtained by trypsin dissociation of 6-day chick-embryo hearts re-aggregated in rolling tubes and formed a pulsating mass within 1 h.

Newly formed intercalated discs (adhesion plaques) were the most frequent type of intercellular contact in the earlier stages of culture.

Desmosomes were also present.

Focal tight junctions were rare and difficult to identify.

In advanced aggregates more extended regions of very close plasma membranes apposition could be observed; lanthanum infiltration did not reveal obliteration of the intercellular gap.

The most striking feature, even after 12 days of culture, is the disorder of the contractile units within the sarcoplasm and the irregularity of the cell outlines. The ineffectiveness of the factors responsible for the orientated disposition of myofibrils in culture conditions has been emphasized.

The smooth endoplasmic reticulum never attains a regular arrangement in relationship with the myofibrillar banding. Subsarcolemmal cysterns containing a finely granular matrix (peripheral couplings) have been found after the first 24 h of culture; it has been observed that they reach their mature form only in the last stages of culture.

The T-system is lacking. Numerous pits, often complicated by beading, are present in the peripheral sarcoplasm.

All these features may affect the physiological response that can be recorded from these cells.

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