The research team, based at Brigham and Women’s Hospital, followed thirty wild-type pups from three litters through four developmental milestones: postnatal day 0 (P0), day 8 (P8), day 21 (P21), and eight weeks of age. Using the VisualSonics Vevo F2 high-frequency ultrasound system with transducers operating between 25 and 71 MHz, the investigators captured images of hearts beating at rates of 400 to 600 beats per minute with exceptional spatial and temporal resolution. At birth, when a pup weighs barely more than a gram and its left ventricle measures roughly 1.5 millimeters across, imaging was performed without anesthesia or hair removal, with animals gently restrained on a heated platform. From P8 onward, a standardized isoflurane protocol maintained heart rates above 400 beats per minute while body temperature was held at 37 degrees Celsius, minimizing the hemodynamic variability that anesthesia can otherwise introduce.

The structural findings chart an astonishing trajectory of growth. Body weight rose from 1.25 grams at birth to nearly 21 grams at adulthood, while the left ventricular end-diastolic volume expanded almost tenfold, from about 6.4 microliters to 59 microliters. Ventricular walls thickened in parallel, with the systolic anterior wall more than doubling from birth to weaning and continuing to grow into adulthood. Perhaps most intriguing was the behavior of body-weight-indexed left ventricular mass, which surged during the early postnatal and juvenile periods before declining toward adult values. This transient overshoot, the authors note, mirrors the well-described postnatal transition in which cardiomyocytes shift from proliferative expansion to hypertrophic growth, with the heart briefly outpacing the body before mass and body weight rescale proportionally at maturity.

Counterintuitively, conventional indices of pumping performance declined steadily across development. Ejection fraction fell from 69 percent at birth to 60 percent at eight weeks, and fractional shortening followed a similar downward path. Rather than signaling weakening hearts, the authors explain, this decline reflects geometry and loading: as the ventricular cavity expands disproportionately relative to stroke volume, and as systemic afterload rises with maturation, volumetric efficiency measures naturally decrease even as the heart muscle itself grows stronger. Crucially, the strain measurements told a different story. Global circumferential strain, endocardial circumferential strain, and global radial strain all remained statistically stable from P8 through adulthood, indicating that the intrinsic contractile machinery of the myocardium is established early and preserved intact through the storms of growth and remodeling.

Strain analysis, enabled by speckle-tracking software that follows the natural acoustic markers within heart tissue frame by frame, quantifies how much the ventricular wall deforms during each beat. Longitudinal strain captures shortening along the base-to-apex axis, circumferential strain measures shortening around the chamber’s circumference, and radial strain records wall thickening toward the cavity. Because these indices depend on the coordinated behavior of individual contractile units rather than on chamber geometry, they are considered exquisitely sensitive detectors of early dysfunction, often flagging impairment in disease models long before ejection fraction budges. The one strain parameter that did change was myocardial global longitudinal strain, which became significantly more negative, from minus 8.8 percent at P8 to minus 11.9 percent at P21, suggesting selective maturation of the longitudinal contractile pathway during the juvenile period.

Diastolic function, the heart’s capacity to relax and fill, matured along its own timetable. From P8 onward, early and late transmitral filling velocities climbed steadily, with the E/A ratio peaking at day eight before stabilizing. Tissue Doppler velocities at the mitral annulus, including the early relaxation velocity e-prime, the late atrial contraction velocity a-prime, and the systolic annular velocity s-prime, all increased progressively into adulthood. Right ventricular function, tracked through tricuspid annular plane systolic excursion, nearly doubled from 0.40 millimeters at P8 to 0.78 millimeters at eight weeks. Left atrial areas grew in step with the rest of the heart, though fractional area change fluctuated without a consistent directional trend. These trajectories align with known postnatal refinements in calcium-handling proteins such as SERCA2a and ryanodine receptor 2, and with the metabolic switch from glycolysis to fatty-acid oxidation that newborn cardiomyocytes undergo shortly after birth.

One of the study’s most striking discoveries emerged when the data were split by sex. Most conventional parameters showed no sex-specific developmental patterns, but strain imaging revealed a clear divergence in late adolescence. By eight weeks, males exhibited significantly more negative longitudinal strain and higher radial strain than females, and systolic annular velocity showed the same pattern, with males reaching 21.5 millimeters per second versus 17.3 in females. Stroke volume also displayed a significant time-by-sex interaction. The timing is telling: the divergence between P21 and week eight coincides with pubertal maturation and the surge in circulating androgens, and it is consistent with prior work showing that isolated cardiomyocytes from adult male mice contract more vigorously than those from females, likely through testosterone-driven modulation of calcium handling and myofilament function.

Notably, this mouse pattern inverts the human picture, where women typically show greater myocardial strain values than men. The authors point to fundamental species differences as likely explanations: adult mouse ventricles predominantly express the fast-kinet ics alpha myosin heavy chain, whereas human ventricles express the slower beta isoform, and the tenfold difference in resting heart rate between the species fundamentally alters contraction-relaxation dynamics. Sex differences in cardiac function are also dynamic across the lifespan; aging studies suggest the male advantage seen at eight weeks may reverse later in life, with females maintaining better contractility into old age. For researchers translating murine findings to human cardiology, the message is unambiguous: sex- and species-specific normative values are essential, and extrapolating adult or human reference ranges to neonatal and juvenile mice risks misjudging both disease severity and therapeutic efficacy.

The study also functions as a technical manual for a notoriously difficult measurement. The authors document a systematic 12.3 percent overestimation of ejection fraction by short-axis M-mode derived from the Teichholz formula compared with B-mode Simpson’s monoplane volumetry, a bias that persisted at every developmental stage and that they recommend accounting for, particularly in disease models with regional wall motion abnormalities. They further emphasize maintaining normothermia in neonatal pups, which cannot thermoregulate and are prone to hypothermia-induced bradycardia, transferring maternal scent to gloved hands before handling to prevent rejection by dams, positioning the M-mode cursor perpendicular to the septum at the mid-papillary level while avoiding papillary muscle inclusion, and keeping Doppler angles below 45 degrees to prevent velocity underestimation. A printable echocardiography quicksheet summarizing key reference values accompanies the publication as a bench-side tool.

The limitations are candidly acknowledged: the data derive exclusively from Jackson Laboratory C57BL/6 mice, whose developmental trajectories may differ across strains; volumetric parameters rest on geometric assumptions that strain measurements avoid; and the extreme technical constraints of imaging newborn hearts meant that strain, Doppler, and right ventricular assessments could not be performed at P0. Future directions include comprehensive right ventricular and large-vessel characterization, integration with invasive pressure-volume loop analysis, and extending the serial observations into aging to trace how the early-life sex differences evolve across the lifespan. For now, the study stands as the definitive developmental atlas of the laboratory mouse heart, one that promises to sharpen the interpretation of countless cardiac disease models and, by reducing misclassification of phenotypes, to trim both animal usage and cost in preclinical research.

Subject of Research: Postnatal cardiac development and myocardial strain in C57BL/6 mice assessed by high-frequency echocardiography

Article Title: Prospective echocardiographic characterization of postnatal cardiac development in C57BL/6 mice: Strain analysis between birth and adulthood

Article References: Jo, S., Spyropoulos, G., Hart, L. G., Covington, T. A., Pandey, A. K., Michel, T., & Spyropoulos, F. (2026). Prospective echocardiographic characterization of postnatal cardiac development in C57BL/6 mice: Strain analysis between birth and adulthood. Physiological Reports, 14(18), Article e71113. https://doi.org/10.14814/phy2.71113

Image Credits: AI Generated

DOI: 10.14814/phy2.71113

Keywords: echocardiography, strain analysis, C57BL/6 mice, cardiac development, speckle-tracking, ejection fraction, diastolic function, sexual dimorphism, left ventricle, normative values, preclinical imaging, Physiological Reports