AbstractHuman embryonic myocardial tissues ranging from eight to eighteen weeks of development were prefixed and dissected in buffered formaldehyde, postfixed in buffered OsO4and processes for Epon embedding. At these stages of development the myocardial cells contain an abundance of sarcoplasm with a sparse random distribution of myofibrils that frequently appear branched. The A, I and Z bands exhibited in the adult myocardial cells are also observed in these stages of development. However, the M line is only slightly apparent, while no H band was observed. The mitochondria are not arranged in parallel arrays as found in the adult, but appear to be randomly distributed throughout the interfibrillar sarcoplasm. An array of tubules, cisternae and vesicles that comprise the sarcoplasmic reticulum is also distributed throughout the interfibrillar sarcoplasm. In some areas this tubular system appears as vesicles with small dense granules along its outer surface, while in other areas it appears smooth. The distribution and orientation of the sarcoplasmic reticulum is far less pronounced in the embryonic stages examined in this study than that reported for the adult myocardium. Dense granules (150 to 350 Å) appear as clusters or aggregates of granules in the sarcoplasm and are similar to the glycogen granules described by other investigators. The intercalated disc is observed at the junction of two opposing cells and occurs with less frequency than the intercalated disc in the adult myocardium
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