Estimation of the Cell Number of Heart Muscles in Normal Rats

The total number of heart muscle cells of rats was estimated by a histometrical method. It was approximately 5 millions immediately after birth and rapidly increased to over 19 millions in the first month of life. Then it gradually increased to 20-22 millions by the age of 2 years. The total number of interstitial cells was approximately 3.5 millions immediately after birth, and increased progres-sively to 28 millions in a month, to 68 millions in a year and to 72 millions in 2 years. The number of heart muscle cells per 1mm3 of heart muscle was approximately 330, 000 immediately after birth and decreased with growth to about 1/9 of the initial number in 2 years. It was concluded that the heart muscle cells as well as the interstitial cells increased by cell division even after birth, and that the heart muscle cells not only increased in number, but also grew in size during the normal growth of the organ.