Scammon’s Growth Curves and the Developing Heart

The human body does not grow at the same rate throughout childhood. The brain, bones, reproductive organs and many internal organs each follow different patterns of development.

Almost a century ago, anatomist Richard E. Scammon developed a simple way of showing these differences. His growth curves became one of the best-known models of human growth and are still discussed today.

The heart is particularly interesting because it follows what Scammon described as the “general” pattern of growth. But modern research has revealed something Scammon could not have known in 1930: as the heart grows, the way its individual cells grow changes dramatically.

At AtheroCare Scammon’s growth curves provide an interesting historical starting point for looking at how the heart develops from infancy into adulthood.

What are Scammon’s growth curves?

Richard E. Scammon was an American anatomist who studied human growth and development.

In 1930, his work appeared in the book The Measurement of Man. Scammon grouped the growth of different tissues and organs into four broad patterns: general, neural, lymphoid and genital growth.

Rather than assuming that every part of the body grows at the same speed, Scammon showed that different systems have very different developmental timelines.

The four traditional patterns are:

General growth — This includes much of the body, such as height, weight, muscles, skeleton and many internal organs.

Neural growth — The brain and other parts of the nervous system develop particularly quickly during the first years of life.

Lymphoid growth — Lymphatic tissues grow rapidly during childhood, can temporarily become larger than their adult relative size, and then decrease toward adult proportions.

Genital growth — Reproductive tissues develop relatively slowly during childhood before accelerating around puberty.

The curves were never intended to show disease risk. They were a way of illustrating the different rates at which parts of the human body develop.

Where does the heart fit?

The heart belongs to Scammon’s general growth category.

Historical descriptions of the model include the heart and pulmonary vessels alongside other structures following the general growth pattern. Later researchers examining Scammon’s classification have likewise used heart and liver growth as examples of the general or visceral growth type.

The general pattern is roughly S-shaped.

Growth is relatively rapid during infancy. It becomes slower and steadier through much of childhood, accelerates again around adolescence, and eventually approaches adult size.

That makes intuitive sense when thinking about the heart. A newborn does not simply have a miniature adult cardiovascular system that remains unchanged apart from size. The heart must grow and adapt as the body becomes larger and its circulatory demands increase.

Modern research, however, has shown that this process is considerably more complicated than a single curve can represent.

The developing heart begins before birth

The heart starts developing very early in embryonic life.

During fetal development, cardiac muscle cells called cardiomyocytes multiply rapidly. This increase in cell number is a major mechanism by which the developing heart becomes larger.

Researchers describe this as proliferative growth.

The fetal heart must continually expand while chambers, valves, blood vessels and other structures are forming. Development of the heart therefore involves not only an increase in size but major changes in shape, structure and function.

Studies of cardiac development show that fetal heart growth depends heavily on cardiomyocyte proliferation. This differs significantly from how most heart growth occurs later in life.

Birth represents an important turning point.

What changes after birth?

At birth, the cardiovascular system undergoes one of the most dramatic transitions it will ever experience.

Before birth, oxygen comes through the placenta. After birth, the lungs begin providing oxygen and the circulation reorganises to support life outside the womb.

The heart must adapt rapidly to this new environment.

Cardiomyocytes undergo structural, metabolic and electrical changes as they mature. Their mitochondria develop, their contractile machinery becomes more organised and the way they produce energy changes.

At approximately the same period, cardiomyocytes dramatically reduce their ability to divide.

This changes the way the heart grows.

Before and around birth, increasing the number of cardiomyocytes makes an important contribution to heart growth.

Later, growth increasingly occurs by existing cardiomyocytes becoming larger.

This is known as physiological hypertrophy.

In this context, “hypertrophy” simply means enlargement of the cells. Normal developmental hypertrophy should not be confused with pathological cardiac hypertrophy associated with conditions such as long-standing high blood pressure or certain forms of heart disease.

Growing a bigger heart without simply making more heart cells

This transition is one of the most interesting parts of human heart development.

For many tissues, growth can involve producing large numbers of new cells.

The heart becomes different.

Research indicates that the human heart establishes most of its cardiomyocyte population around the perinatal period. Cardiomyocyte renewal continues after birth, particularly during early childhood, but the rate falls considerably with age.

As a child grows, existing cardiomyocytes therefore become larger and more mature.

The heart itself becomes larger because it must support a larger body and pump a greater volume of blood. The blood vessels and the rest of the cardiovascular system develop alongside it.

This overall increase in cardiovascular size broadly resembles the “general” growth pattern described by Scammon.

But Scammon’s curve cannot show what is happening inside individual heart cells.

That part of the story required technologies and biological knowledge that did not exist when his model was developed.

The heart continues maturing through childhood

Heart development does not suddenly finish after infancy.

Research into human cardiomyocyte maturation suggests that particularly rapid changes occur during the first year after birth, while some features of adult cardiac muscle may continue developing through much of the first decade of life.

The growing cardiovascular system must adapt to changes in:

• body size
• blood volume
• metabolic demand
• physical activity
• hormones
• blood pressure
• lung function

Adolescence brings another major period of physical growth.

This is one reason the heart was placed within Scammon’s general growth pattern. Like height, body mass, skeletal muscle and several internal organs, cardiovascular growth continues through childhood and changes substantially around adolescence.

Research examining the historical Scammon model has found an adolescent growth peak in structures classified within the general growth category, including heart-growth data.

What happens when the heart reaches adulthood?

By adulthood, the heart has largely completed its normal developmental growth.

Adult cardiomyocytes have very limited ability to divide compared with fetal cardiac cells.

Human studies suggest that some cardiomyocyte renewal continues throughout life, but at a relatively low rate. Research using carbon-14 dating of human heart cells found cardiomyocyte exchange to be highest during early childhood and to decline to less than one per cent per year in adulthood.

This has major implications for cardiovascular medicine.

Skin, blood and several other tissues have substantial abilities to replace damaged cells.

Heart muscle has a much more limited regenerative capacity.

When large numbers of adult cardiomyocytes are lost through significant injury, the heart cannot simply replace them as efficiently as many other tissues can replace their cells.

That difference has made cardiac regeneration an important area of scientific research.

Why scientists study the developing heart

One of the major questions in cardiovascular research is why young, developing cardiomyocytes can proliferate while mature adult cardiomyocytes largely cannot.

Researchers are studying the molecular signals that cause cardiomyocytes to leave the cell cycle, mature and increase in size after birth.

Understanding those mechanisms could provide clues for future approaches to cardiac repair.

Current research is investigating pathways involved in cardiomyocyte proliferation, metabolism, maturation and regeneration. The goal is to better understand whether some of the biological processes active during early heart development could eventually be used to encourage repair after cardiac injury.

This remains an active field of research. Understanding the developmental biology of the heart does not mean scientists can currently make an adult human heart regenerate like a developing heart.

But it explains why the earliest stages of cardiac growth remain relevant to cardiovascular research today.

Was Scammon right?

Scammon’s curves were groundbreaking for their time, but they should not be treated as a precise modern model of organ development.

The original curves simplified an enormous amount of biological complexity into four patterns.

Later researchers have questioned whether those four categories accurately represent every aspect of human growth. Modern attempts to mathematically analyse the curves have suggested that some categories overlap more than Scammon’s original illustration implied.

The heart itself demonstrates the limitations of such a simple model.

Its overall size may broadly follow a general pattern of body growth, but underneath that curve are major biological changes involving:

• cardiomyocyte proliferation
• cardiomyocyte enlargement
• maturation of cardiac muscle
• changes in energy metabolism
• development of blood vessels
• changes in electrical activity
• adaptation to increasing circulatory demands

A single line on a graph cannot capture all of this.

Scammon’s model is therefore best viewed as a historical framework for understanding the timing of human growth rather than a detailed description of cardiovascular biology.

A simple curve with a much bigger story

Scammon’s growth curves helped demonstrate an important principle: the human body does not develop as one uniform structure.

Different tissues have different developmental schedules.

The heart broadly follows the general growth pattern, becoming larger as the body grows from infancy through childhood and adolescence.

Modern science has taken the story much further.

We now know that the developing heart changes not only in size but in the fundamental behaviour of its cells. Early cardiomyocytes can multiply. As the heart matures, those cells largely leave the cell cycle, become larger and develop the specialised characteristics needed to keep the adult heart beating.

Nearly a century after Scammon published his famous curves, studying how the heart grows remains important.

Not because an old growth curve can predict future heart health, but because understanding how the heart is built, how its cells mature and why their regenerative ability changes may help scientists better understand one of the body's most important organs.

References and further reading

Scammon RE. “The Measurement of the Body in Childhood.” In The Measurement of Man. University of Minnesota Press, 1930.

Fujii K. “Scammon's Growth Curve & Growth and Development.” Sports and Health Science Research, 2013.

Bergmann O, et al. “Dynamics of Cell Generation and Turnover in the Human Heart.” Cell, 2015.

Dimasi CG, Darby JRT, Morrison JL. “A Change of Heart: Understanding the Mechanisms Regulating Cardiac Proliferation and Metabolism Before and After Birth.” The Journal of Physiology, 2023.

Pervolaraki E, et al. “Adapting to a New Environment: Postnatal Maturation of the Human Cardiomyocyte.” The Journal of Physiology, 2023.

Ivanovitch K, Esteban I, Torres M. “Growth and Morphogenesis During Early Heart Development in Amniotes.” Journal of Cardiovascular Development and Disease, 2017.

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