How do infants' and young children's bones differ from those of adults?

As we all know, newborn babies are soft and sticky, which makes people feel that they are "afraid of falling if held in the hand, and afraid of melting if held in the mouth". In addition, it is normal for children to fall down, and they usually just brush off the dust on their bodies, but if the same thing happens to the elderly, it often causes irreversible damage or even more serious consequences.

So what exactly causes these differences? And how do infants' and toddlers' bones differ from those of adults?

(Original picture)

Under the regulation of growth hormone (GH), the functions of various parts of infants and young children undergo a series of changes from low-level to high-level, from simple to complex. Take teeth as an example. Usually, the baby's deciduous teeth begin to erupt 6 months after birth, and all of them are in place by the age of 3, with a total of 20. Children's deciduous teeth begin to fall off around the age of 6 and are gradually replaced by permanent teeth, with a total of 32 permanent teeth.

Seeing this, one may wonder, “Similarly, do adults have more bones than infants?” On the contrary, infants actually have more bones than adults. Adults have 206 bones, which can be divided into skull, trunk, and limb bones. Among them, there are 29 skull bones, 51 trunk bones, and 126 limb bones. Children have 217-218 bones, and newborns have as many as 305 bones [1].

From a morphological point of view, the difference between the skeleton of infants and young children and that of adults is still very obvious.

For example, the facial skull of a newborn baby accounts for 1/8 of the total skull, while that of an adult is 1/4, so a baby's head will appear larger than that of an adult, making it cuter. Another example is that the limb bones of an adult are usually much thicker than those of an infant. This is because the epiphyses of long bones begin to form bones around the time of birth, making the long bones longer, and the peripheral periosteum continues to form bones, making the bone shaft thicker.

(Original picture)

Structurally, the skeletons of infants and young children are "imperfect" compared to those of adults.

When observed from the top of the skull, the skull of a newborn is pentagonal in shape. The bones of the skull have not yet fully developed. There are large gaps at the junction of multiple bones, forming the anterior fontanelle, posterior fontanelle, sphenoid fontanelle, and mastoid fontanelle. As the baby ages, the fontanelles gradually shrink and eventually close completely. The larger anterior fontanelle usually closes at 1-2 years old, and the other fontanelles close soon after birth. At the same time, the hip bones, sacrum, coccyx, etc., are all formed by the continuous growth and development of "juvenile" and "dispersed" infant bones, which eventually fuse to form a complete bone [2].

In terms of nature, the bones of infants and young children are more "powerful".

First of all, the bone marrow of infants and young children is all red bone marrow, which contains red blood cells and other immature blood cells at different developmental stages, is red in color, and has hematopoietic and immune functions. After the age of 5, the red bone marrow in the long bone shaft will gradually be replaced by fat and become yellow bone marrow without hematopoietic ability. For adults, only when there is excessive blood loss or severe anemia can the yellow bone marrow be transformed into red bone marrow and the hematopoietic function can be restored.

Secondly, the chemical composition and physical properties of infant bones are also very different from those of adult bones. In early childhood, the ratio of organic matter to inorganic matter in bones is 1:1. Bones are relatively elastic, soft, and easily deformed. They are not prone to fractures under external forces, or to "green branch fractures" that break but never break, like the tender branches of plants in spring. The ratio of organic matter to inorganic matter in adult bones is about 3:7, so bones have both hardness and elasticity, which is the most suitable. The inorganic matter in the bones of the elderly accounts for a larger proportion, and the brittleness increases. As the body's hormone levels decrease, the absorption and deposition of calcium and phosphorus are affected, the bones become porous and loose, and the brittleness of the bones further increases, making them more prone to fractures [3-6].

References

[1] Zhang Hong. Health education and women’s bone health: Pay attention to bone health from infancy to life (I) [J]. Health Guide, 2019, 25(05): 4-6.

[2] Why is the coccyx vulnerable to injury? [N]. China Family News, 2022-02-28(015). DOI:10.44197/n.cnki.nzgjt.2022.000059.

[3] Liang Yuan. Six tips for bone health[N]. Health Times, 2021-10-15(007). DOI:10.28434/n.cnki.njksb.2021.000348.

[4] Xi Nianchu. Balanced nutrition helps bone health[N]. China Family News, 2021-07-01(013). DOI:10.44197/n.cnki.nzgjt.2021.000458.

[5] Fang Wenhuan, Mai Wei, Yan Zhijian, Huang Xiaowei, Wei Xinjian. Correlation between serum estrogen and pro-adipocyte factor-1 levels and bone metabolism and bone mineral density in postmenopausal patients with osteoporotic vertebral compression fractures[J]. Chinese Journal of Osteoporosis, 2021, 27(12): 1763-1766+1773.

[6] Li Zige, Wu Jingjing, Zhou Ruoyu, Tang Shengyao, Jiang Ziwei. Retrospective study of 301 cases of fragile hip fractures and co-occurrence analysis of literature[J]. Chinese Journal of Osteoporosis, 2021, 27(12): 1810-1814.

Author: Song Shiyao, head nurse, Bachelor of Medicine from Sichuan University.