Issue link: https://hi.iaq.net/i/670243
Chapter 3: In the beginning: protecting our future generations 3.1 Early human development To understand why fetuses, babies and children are more susceptible to external insults, including the effects of air pollution, it is useful to understand, at least in outline, aspects of early human development and growth. 3.1.1 Development and growth: a short background to early development Development implies more than just growth; it includes changes in the nature and structure of tissues. In biological terms, it requires differentiation of cells into specialised types with varying functions, eg heart cells, which contract rhythmically, or cells in the gut, which secrete liquid and absorb nutrients. Growth is increase in size; in the body, this means an increase in the number of cells in a particular tissue, rather than the cells themselves enlarging. The amazing transformation of the fertilised human egg – a single cell – into a complex organism in a very short time requires the integrated and precisely controlled coordination of differentiation and growth of cells. These processes are controlled by the switching on and off of genes in a specific sequence in a timely manner. It is believed that over half the genes in the human genome are used only for early development. The majority of organs are formed by about 10 weeks after fertilisation (12 weeks of pregnancy). The heart The heart is one of the first tissues in the body to develop into a functional organ; it starts to beat and to pump blood from around 3–4 weeks' gestation. The lungs The lungs will be the first point of contact for air pollution throughout life and, therefore, are particularly vulnerable to its adverse effects. The bronchial tree (air passages in the lungs) is completely formed by 16 weeks of pregnancy (Fig 6). The air sacs (alveoli) start developing at 28 weeks and about half the final adult number of alveoli are present at birth (40 weeks). Most of the remainder develop by about 2 years of age, but it is likely that they continue to develop through adolescence until body growth ceases. The peak of lung function is not achieved until the early to mid-twenties, making the lung vulnerable for many years, but perhaps maintaining an ability to recover some lost function. The brain and central nervous system The primitive brain and spinal cord are present from very early pregnancy, but development of the nervous system is an ongoing process that continues throughout fetal life and after birth to adolescence. Most of the nerve cells have developed by the middle of pregnancy and afterwards they go on to develop 36 © Royal College of Physicians 2016