Nuclear and cellular size of myocytes in different segments of the developing rat heart

Abstract

Background In the embryonic heart, the individual cardiac segments show different growth rates. For the analysis of changing form in relation with changing function, data on number and shape of cardiomyocytes are necessary. Such data will give insight into the process of hypertrophy and/or hyperplasia as they may take place in the myocardium in the embryonic period. Methods We have measured the volumes of the nuclei and myocytes as well as the surface areas of the nuclear envelope and cellular membrane using stereological tools in rat embryos from 11 days postcoitum to 17 days postcoitum. From the data of the cellular volume of the myocytes and the myocardial volume of the individual segments, we have calculated the total number of myocytes during the developmental period. Results It is shown that the sinus venosus, sinu‐atrial junction, and atrium increase their cellular volume during development, whereas the other cardiac segments show no difference in cellular volume. Similarly, the surface area of the cell membrane of the sinus venosus and sinu‐atrial junction had increased during development. The nuclear volume and the surface area of the nuclear envelope did not differ during the period studied. The total number of myocytes showed a conspicuously smaller increase in the atrio‐ventricular canal and distal outlet segment than in the other segments. Conclusions The increase of the cellular volume in the segments sinus venosus and sinu‐atrial junction seems to be due to a late differentiation process. In general, however, the increase of the myocardial volume in the individual cardiac segments is caused by hyperplasia of the cardiomyocytes in these segments and not by hypertrophy. The surface area of cells has a fixed relationship with cell volume, indicating that no important changes take place in the developmental period studied. © 1996 Wiley‐Liss, Inc.

Postnatal growth of cardiomyocytes in the left ventricle of the rat

“…Several have used disector counting to estimate total numbers of myocyte nuclei in human and rat hearts (Austin et al, 1995;Mandarim-de-Lacerda et al, 1997;Mayhew et al, 1997). A few research teams have determined sizes of myofibers (termed "fibers" in the present report) in mammalian hearts using stereological sampling designs that fully deal with the anisotropy of myocardial structure (Knaapen et al, 1995(Knaapen et al, , 1996Tang et al, 1996Tang et al, , 1999Brü el et al, 2002a,b).…”

Ultrastructural changes of the myocardium in the embryonic rat heart

“…The transitional zones, atrioventricular canal, and distal outlet segment do not increase their myocardial volume after day 15 , indicating their diminishing role as sphincters (Goerttler, 1955;Jaffee, 1965). However, after the time of septation, the segments destined to be the working atrial and ventricular myocardium show a rapid increase in myocardial mass, which is caused by hyperplasia (Knaapen et al, 1996). Because the ventricular pressure increases during development , as the heart rate does (Broekhuizen et al, 1993), the working myocardium needs to increase its contractile force.…”

“…Obviously, at this time of development, important physiological alterations take place in the embryonic heart, and the mass of the working myocardium increases rapidly. Even when the temporary decline of cellular myofibrils is accounted for, the increasing number of myocytes (Knaapen et al, 1996) still leads to a continuous increase of total ventricular myofibrillar mass. Myocyte division continues until 2 weeks after birth, and we have compared our data whith the data of investigators who have measured the cellular organelles in the postnatal period (Fig.…”

“…SI is the number of estimated intercepts hitting the structure of interest, and SP is the number of points hitting the reference space. The data on the cellular volumes, presented previously (Knaapen et al, 1996), were used to calculate the absolute values of the ultrastructural parameters in this study.…”

“…The embryonic heart consists of eight different cardiac segments and intersegmental junctions that develop differently with respect to their individual myocardial volumes and to their number of myocytes (Knaapen et al, , 1996. Functionally, the intersegmental junctions, also described as transitional zones , are described as having specific tasks during early development before valves have developed (Goerttler, 1955;Jaffee, 1965).…”