[Ultrastructural aspects in the development of the human heart. II. The endocrine cells of the embryonic heart].

Electronmicroscopic studies performed on atrial and ventricular tissue fragments from 9 human embryos, 8 to 15 weeks old, showed that in certain myocardiocytes, especially in the atrial ones, from the 10 weeks old embryos, there appear granules of secretion, very similar in morphology with those from adults, which had been demonstrated to contain the atrial natriuretic factor. The precocious appearance of this peptide represents a functional adaptation by which the embryo attempts at counterbalancing the actions of renin and angiotensin from the maternal circulation.