The predominance of binucleation in isolated rat heart myocytes

Abstract

Myocytes of the heart of the newborn rat are mononucleated, whereas myocytes of the heart of growing, maturing rats become predominantly binucleated. This appears to be explained by mitotic division shortly after birth without cell division, i.e., karyokinesis without cytokinesis. Myocytes isolated from hearts of adult guinea pig and pigeon are also predominantly binucleated. Although only about an eighth of the cells of adult rat hearts are myocytes, most of the increase in size of the heart from birth to six months can be accounted for by change in size of these cells.

Morphometric study of early postnatal development in the left and right ventricular myocardium of the rat. I. Hypertrophy, hyperplasia, and binucleation of myocytes.

“…Although the percentages of binucleate myocytes calculated here from the numbers of binucleate cells per unit myocardial volume all agree well with the results of other investigators (Katzberg et al, 1977), similar morphometric measurements from adult rat myocardium (Loud et al, 1978;Anversa et al, 1979) have been significantly lower (2-11%) than the estimates made from myocytes isolated in cell suspension in different laboratories (Katzberg et al, 1977). The controversy seems to revolve about the questions whether isolated myocytes constitute a representative sampling of the whole myocyte population and whether binucleate pairs are recognizable microscopically in proportion to their true frequency in longitudinal tissue sections.…”

“…The myocyte population of the left ventricular free wall increased 100% by the addition of 7.3 million cells, whereas that of the right ventricle added only 1.5 million cells. In agreement with previous observations (Katzberg et al, 1977), less than 3% of all myocytes were binucleate at 1 day, 15-20% at 5 days, and 45-50% at 11 days. During this period, the average cardiac myocyte also increased 2.44-to 2.73-fold in volume, doubled in length, and increased its mean transverse cross-sectional area by 21-35%.…”

“…It has been shown that mitotic activity in cardiac myocytes, which is high during the earliest stages of histogenesis, declines rapidly during the postnatal period, and cell enlargement becomes the main growth mechanism underlying the further development of the heart (Zak, 1974;Katzberg et al, 1977). DNA synthesis in these cells has been reported to cease completely sometime between 2 and 6 weeks after birth (Zak, 1974;Claycomb, 1975;Dowell and McManus, 1978).…”

“…Average size of myocytes also increases in parallel with postnatal body growth, and myocytes possess the capacity for additional hypertrophy in response to an added workload (Anversa et al, 1978(Anversa et al, , 1979Korecky and Rakusan, 1978;Loud et al, 1978). In addition, the population of myocytes shows a rapidly increasing percentage of binculeate cells during early postnatal growth (Katzberg et al, 1977). Recently developed morphometric methodology for measuring in situ the average cell size, number, and percentage of binucleate myocytes (Anversa et al, 1978(Anversa et al, , 1979Loud et al, 1978) has been applied in the present study to investigate these properties of myocytes and their differential growth rates in the left and right ventricular myocardium in the early postnatal period.…”

“…The combi-nation of these effects induces a faster growth in the left ventricular myocardium leading to its relatively larger muscle mass characteristic of the adult heart. Myocardial growth is primarily the result of an increasing volume of contactile cells (Katzberg et al, 1977;Korecky and Rakusan, 1978). Cardiac myocytes in the rat retain some capacity for proliferation up to about the age of weaning (Dowell and McManus, 1978), although significant hyperplasia may cease earlier (Claycomb, 1975).…”