Growth, Differentiation and Percentile Charts

A multicellular organism is made of many cells that work together. If every cell remained unspecialised, the organism could not carry out all the different functions it needs. Differentiation enables cells to become suited to specific roles.

For example, as a cell develops into a nerve cell, its cell membrane and cytoplasm extend to form long connections. This structure enables communication over long distances within the body. The important link is between structure and function: differentiation develops a structure that helps the cell do its job.

Most cells in an organism contain the same genetic information, but different cell types have different genes active. During differentiation, this difference in gene activity leads to the development of different cell structures. A specialised cell usually does not need a different set of genes; it uses different parts of the shared genetic information.

In animals, most differentiation happens early in development. Once mature, most specialised animal cells cannot change into other cell types. Cell division in mature animals is mainly used for repair and replacement, although some cells retain the ability to differentiate and supply replacement cells.

Plants differ because many plant cells retain the ability to differentiate throughout the plant's life. This helps plants continue developing new specialised cells as they grow.

What a percentile chart tells you

Growth can be monitored by taking measurements repeatedly and comparing them with a reference population. For babies, measurements include mass, length and head circumference. Growth charts compare these measurements with those of other children of the same age. Boys and girls use different charts because their usual growth patterns differ.

A percentile line shows a position within the comparison group. A child on the 25th percentile for mass is heavier than approximately 25% and lighter than approximately 75% of children of the same age in that group. The 50th percentile is the median: approximately half the group has a lower mass and half has a higher mass.

A percentile is therefore a comparison, not a target that every child should reach. Healthy children can have different masses and follow different percentile lines.

Reading a growth chart

On a mass-for-age chart, age is on the horizontal axis and mass is on the vertical axis. To find a child's percentile:

1. Find the child's age on the horizontal axis.
2. Find their mass on the vertical axis.
3. Locate the point where these two values meet.
4. Identify the percentile line closest to that point. If it falls between lines, describe it as between those percentiles.

The following chart is a constructed teaching illustration, not a clinical growth chart.

Data for Reading an illustrative mass-for-age percentile chart

In this illustration, a fictional child aged 6 months with a mass of 6.5 kg lies on the 25th-percentile line. Their mass is therefore greater than that of approximately one quarter of the comparison group of the same age.

If this child previously had a mass of 5 kg at 3 months, both measurements lie on the same percentile line. Their mass has increased, but their position relative to the comparison group has remained similar. Following a percentile line does not mean staying the same mass: the line rises as children grow.

Using measurements over time

Repeated measurements show a growth pattern more clearly than one measurement. Children may fluctuate around a percentile line, particularly when young. However, a marked change away from their previous pattern can be a reason for further assessment.

Growth charts can help identify possible concerns such as malnutrition or obesity. Comparing different measurements can also reveal an unusual pattern, such as a relatively large body size alongside a small head circumference. A chart highlights a possible concern; it does not diagnose the cause on its own.

For real monitoring, use an appropriate growth chart based on reference data, rather than the constructed illustration here. The same reading method applies: locate the measurement, identify its percentile and compare successive measurements to assess the trend.

• Animals: cell division increases cell number; differentiation produces specialised cells.
• Plants: cell division, elongation and differentiation contribute to growth and development.
• Differentiation develops structures suited to a cell's function. Different cell types have different genes active, usually within the same genetic information.
• Most animal cells differentiate early in development; many plant cells retain this ability throughout life.

Percentile charts

• Read age on the horizontal axis and the measured quantity on the vertical axis; locate their intersection and identify the nearest percentile line.
• 25th percentile: approximately 25% of the same-age comparison group has a lower measurement and 75% has a higher measurement.
• 50th percentile: the median.
• Use the appropriate chart and compare repeated measurements. Following one percentile line can still mean increasing in size or mass.
• Changes in growth pattern can prompt assessment, but a percentile alone is not a diagnosis.