Growth and Development

Growth — increasing in size — sounds simple, but it is one of the most precisely regulated processes in the body. A newborn weighs about 3–4 kg and is about 50 cm long. By adulthood, they will weigh 60–80 kg and stand 150–190 cm tall. This 20-fold increase in mass happens in a predictable pattern, controlled by hormones, genetics, and nutrition working together. The main hormone controlling growth is Growth Hormone (GH), produced by the anterior pituitary gland. Growth hormone does not act on bones and muscles directly — it stimulates the liver to produce IGF-1 (insulin-like growth factor 1), which then acts on bones, muscles, and organs to stimulate growth. Growth hormone is released in pulses, mostly during deep sleep. This is one reason sleep is so important for children and teenagers — most physical growth happens at night. Stress, starvation, and poor sleep all reduce GH release. Genetics sets the upper limit: Your genes determine your potential height (roughly — your mid-parental height, calculated from your parents' heights). But nutrition, health, and hormones determine whether you reach that potential. Growth charts are used in paediatrics (child medicine) to plot a child's height and weight over time. A child consistently following the 50th centile (median) is normal. A child who suddenly drops centiles or stops growing may have a growth hormone deficiency, chronic illness, or nutritional problem.

Bones do not grow uniformly — they grow from specific regions called growth plates (epiphyseal plates), located near the ends of long bones like the femur (thigh bone) and tibia (shin bone). The growth plate is made of cartilage — softer and more flexible than bone. New cartilage is constantly produced at the growth plate, and the older cartilage is gradually replaced by bone, making the bone longer. This process is driven by: - Growth hormone / IGF-1 — stimulates cartilage cell proliferation in the growth plate - Thyroid hormone — needed for normal growth plate function (thyroid deficiency in childhood causes stunted growth) - Sex hormones (oestrogen and testosterone) at puberty — cause a growth spurt by stimulating rapid bone growth, but then cause the growth plate to close (ossify — turn to bone) permanently Growth plate closure: At the end of puberty (typically 16–18 in females, 18–21 in males), oestrogen causes the growth plate cartilage to ossify completely. Once the growth plate has closed, the bone cannot grow longer — adult height is fixed. This is why puberty that starts early (precocious puberty) can paradoxically result in shorter adult height — the bones grow fast initially but close early. Clinical connection: - Fractures through the growth plate in children are serious — damage to the growth plate can disrupt bone growth, causing leg length inequality or angulation. - Achondroplasia — the most common cause of dwarfism — is caused by a mutation in a receptor that normally slows growth plate activity. The mutation makes the receptor overactive, stopping growth plate cartilage from proliferating properly → very short limbs (though the spine grows more normally).

Puberty is not just about reproductive development — it is a period of rapid overall growth and dramatic change in body composition. The pubertal growth spurt: At peak growth velocity during puberty, children can grow 8–12 cm per year — nearly 1 cm per month. Girls typically reach peak growth velocity around age 12; boys around age 14 (later than girls, which is why boys often seem shorter in early teens but end up taller on average). The growth spurt is driven by the combined effects of: - Growth hormone levels rising during puberty - Oestrogen (in girls) and testosterone (in boys) acting on growth plates - IGF-1 levels peaking during mid-puberty Changes in body composition: - Girls: increased proportion of body fat (especially subcutaneous fat — under the skin — around hips and breasts). Muscle mass also increases, but less dramatically than in boys. - Boys: dramatic increase in muscle mass and bone density under the influence of testosterone. Boys typically have about 10% more muscle mass than girls at the same height by adulthood. Testosterone also stimulates red blood cell production — giving boys higher haemoglobin levels and therefore greater oxygen-carrying capacity (one reason for male advantage in many aerobic sports). Adrenarche — around age 6–8 in girls, 7–9 in boys, the adrenal glands begin producing adrenal androgens (DHEA and androstenedione). These cause pubic and underarm hair and body odour — often the earliest signs of puberty in girls. This happens separately from, and earlier than, the gonadal changes driven by GnRH.

After growth stops at the end of puberty, the body begins a very slow process of change that we call ageing. Ageing is not a disease — it is a normal biological process. But understanding it helps explain why older people are more vulnerable to certain diseases. Bone: Peak bone density is reached in the late 20s. After about age 35, bone is broken down slightly faster than it is replaced — bone density slowly falls. In women, the rapid fall in oestrogen after menopause dramatically accelerates bone loss for about 10 years. This leads to osteoporosis — bones become fragile and fracture easily (especially the hip, spine, and wrist). Treatment includes calcium and vitamin D supplements, weight-bearing exercise, and drugs called bisphosphonates (which slow bone breakdown). Muscle: Muscle mass peaks in the 30s and declines with age — a process called sarcopenia (literally "poverty of flesh"). By age 70, most people have lost 25–30% of their peak muscle mass. This contributes to weakness, falls, and loss of independence. Regular resistance exercise (weights, bodyweight exercises) is the most effective way to slow sarcopenia. Cardiovascular: Blood vessels become stiffer with age (arteriosclerosis — different from atherosclerosis which involves plaques). The heart has to work harder to pump against stiffer vessels → blood pressure tends to rise with age. The heart's maximum rate decreases (roughly: max HR ≈ 220 minus age). Immune: The immune system becomes less effective with age (immunosenescence). New T cells are produced in the thymus — which shrinks progressively from childhood. Vaccines are less effective in the elderly because the immune response is weaker. This is why flu and COVID-19 vaccines often use higher doses or adjuvants in older patients.

Growth hormone disorders: Growth hormone deficiency (GHD) in children: Without enough GH, children grow slowly and end up short. GHD can be congenital (from birth) or acquired (from a pituitary tumour or brain injury). Treated with daily injections of synthetic GH — remarkably effective if started early, enabling near-normal adult height. Growth hormone excess — Gigantism and Acromegaly: Caused by a GH-secreting tumour (usually a benign pituitary adenoma). If it occurs before growth plate closure → gigantism — extremely tall stature. If after growth plate closure (so bones cannot lengthen) → acromegaly — characteristic changes: large hands and feet, prominent brow and jaw, widely spaced teeth, coarsened facial features, and excess sweating. Also causes diabetes (GH is anti-insulin) and cardiovascular disease. Treated by surgery to remove the tumour, or drugs that suppress GH. Thyroid hormone and growth: Thyroid hormone is essential for normal growth and brain development in childhood. Congenital hypothyroidism (born with insufficient thyroid hormone) causes severe intellectual disability and growth failure if untreated — this is why all newborns are tested for thyroid function (the heel-prick test in the first week of life). Treated with daily thyroxine tablets. Insulin and growth: Insulin has growth-promoting effects — it stimulates protein synthesis and nutrient uptake into cells. Children with uncontrolled type 1 diabetes grow poorly because without insulin, cells cannot take up nutrients properly even when food is plentiful. Good blood sugar control restores normal growth. Nutrition and growth: Severe malnutrition in childhood (particularly protein deficiency) causes stunting — permanent short stature due to growth failure during critical developmental windows. The first 1,000 days (from conception to age 2) are the most critical for brain and body growth.