Quantitative study of the myocardium in human embryos.

We studied six human embryos of the second gestational month (postsomitic period, from stages 15 to 23). They were fixed in 4% formaldehyde and serially sectioned. Stereological determinations were made from the compact layer of the ventricular myocardium: a) volume density of the myocardial parts: myocytes (Vv[myocyte]) and interstitium (Vv[interstitium]), b) numerical density of the myocytes (Nv[myocyte] mm3) calculated from six optical disector pairs per embryo, c) total number of myocytes (N[myocyte]), d) volume of the myocytes (V[myocyte] micron 3). In embryos from stages 15 to 19 the quantities of the myocytes and interstitium remained practically unchanged (no statistical difference was found). However, the volume of the ventricular myocardium mass increased more than 5 times during this period. Comparing embryos of stages 15 and 23, the mean value of the Nv[myocyte] decreased about 30 per cent, while N[myocyte] increased almost 2,000 per cent in the same period. Simultaneously, the volume of the ventricular myocardial mass increased almost 30 times, and Vv[myocyte] and Vv[interstitium] showed a small increase in the myocyte component (about 20 per cent), with a decrease of the interstitial component (about 70 per cent). In the early post-somitic period the human myocardium has a relatively small number of small myocytes, in the late post-somitic period it is composed of large and relatively abundant cardiac myocytes. The conspicuous increase in the ventricular myocardial volume observed in stage 23 seems not to be related to the increase in the interstitial portion of the myocardium. These arguments suggest both the enlargement and the division of the cardiac myocytes during the post-somitic period.