STEREOLOGY OF THE MYOCARDIUM IN EMBRYOS, FETUSES AND NEONATES OF THE RAT

We studied with quantitative methods the myocardium of 32 specimens of rats divided into three age groups: embryos, fetuses and neonates. Da ys of gestation were counted from the day following an overnight matin g that was considered day 1 of gestation and only one animal per litte r was used. The hearts were fixed in Bouin's fixative, sectioned and s tained by routine methods. Stereological determinations were made on v entricular myocardium: (1) volume density of the myocytes [Vv(myocyte) ] and interstitium [Vv(interstitium)], and (2) numerical density of th e myocytes [Nv(myocyte) 1/mm(3)] calculated from fifteen optical disse ctor pairs per specimen. The total number of cardiac myocytes [N(myocy te)] was calculated as the product of Nv(myocyte) and the cardiac volu me. The Nv(myocyte) increased from embryos to neonates, differences be tween embryos and fetuses and between embryos and neonates were statis tically significant. The Vv(myocyte) increased from embryos to neonate s (from 80.0 to 94.0%). During this period the Vv(interstitium) decrea sed from 21.0 to 5.5%. Differences of the Vv(myocyte) and Vv(interstit ium) were significant comparing embryos with neonates and comparing fe tuses with neonates. The N(myocyte) is roughly (mean +/- standard erro r of the mean) 9,297 +/- 487 in fetuses and 38,438 +/- 612 in neonates . This represents an increase of about 3.1 times from fetuses to neona tes while the cardiac weight increased about 2.2 times in the same per iod. Coefficients of error for the Nv(myocyte) estimates averaged abou t 7.3%, for the Vv(myocyte) estimates averaged about 1.8%, and for the Vv(interstitium) estimates: averaged about 9.1%. These results sugges t a high mitotic activity in the rat myocardium during prenatal life a nd after birth.