Development of specific granules in embryonic cardiac myocytes of the human auricular wall.

Atrial specific granules first appear in the human auricle by about 7 weeks of fetal development. They are rapidly formed between the 7 and 8 week stages. Granules are fairly evenly spread at the onset of their appearance. As they increase in number they soon aggregate in the juxtanuclear and subsarcolemmal positions. The electron dense matrix of granules of young cardiomyoblasts is condensed and/or synthesized within the Golgi cisternae. Large and electron dense cisternae are sometimes observed in close connection with mature specific granules. Bristle coated vesicles develop by a "budding off" process from the lateral margins of the Golgi cisternae. A fusion between bristle coated vesicles and specific granules is indicated.