QUANTITATIVE STUDY OF THE MYOCARDIUM IN HUMAN EMBRYOS

We studied six human embryos of the second gestational month (postsomi tic period, from stages 15 to 23). They were fixed in 4% formaldehyde and serially sectioned. Stereological determinations were made from th e compact layer of the ventricular myocardium: a)volume density of the myocardial parts: myocytes (Vv[myocyte]) and interstitium (Vv[interst itium]), b) numerical density of the myocytes (Nv[myocyte] mm(3)) calc ulated from six optical disector pairs per embryo, c) total number of myocytes (N[myocyte]), d) volume of the myocytes (V[myocyte] mu m(3)). In embryos from stages 15 to 19 the quantities of the myocytes and in terstitium remained practically unchanged (no statistical difference w as found). However, the volume of the ventricular myocardium mass incr eased more than 5 times during this period. Comparing embryos of stage s 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 per iod. Simultaneously, the volume of the ventricular myocardial mass inc reased almost 30 times, and Vv[myocyte] and Vv[interstitium] showed a small increase in the myocyte component (about 20 per cent), with a de crease of the interstitial component (about 70 per cent). In the early post-semitic period the human myocardium has a relatively small numbe r of small myocytes, in the late postsomitic period it is composed of large and relatively abundant cardiac myocytes. The conspicuous increa se in the ventricular myocardial volume observed in stage 23 seems not to be related to the increase in the interstitial portion of the myoc ardium. These arguments suggest both the enlargement and the division of the cardiac myocytes during the post-semitic period.