A human embryo spends its first week dividing without growing at all
After fertilisation, the single-celled zygote splits again and again inside a protective protein shell, yet a week later the embryo is no bigger than when it started. Only when it hatches from that shell and embeds in the uterine wall does real growth begin, launching a journey that runs through birth, childhood and puberty to adulthood.
Each sperm and egg carries 23 chromosomes, one copy of each. When they fuse, the zygote has the full 46 and a genetic mix unique to it. If a faulty gamete carries an extra copy of one chromosome, the result is trisomy, three copies instead of two. The early rapid divisions, called cleavage, happen inside the zona pellucida; after about a week the embryo breaks out and attaches to the uterine lining, whose cells multiply around it until it is buried in the tissue. This germinal stage ends around ten days after fertilisation.
From the third week, the embryo organises into three germ layers. The outer ectoderm will become skin and the nervous system; the middle mesoderm forms muscles, blood cells, most of the circulatory system and the reproductive and urinary organs; the inner layer builds much of the gut. Meanwhile other cells form the membranes and placenta, which ferries oxygen and nutrients, removes waste, helps fight infection and makes hormones that sustain pregnancy. By convention the embryo becomes a fetus eight weeks after fertilisation, when all major organs are present though not yet finished, a boundary that is somewhat arbitrary because development is a continuum.
Childhood lengthens the bones at growth plates near their ends, under the direction of growth hormone, vitamin D and a messenger called IGF-1 that drives new bone-cell formation. Developmental psychologists divide the years into toddlerhood, early childhood, middle childhood and adolescence, and the Tanner stages can estimate a child's age from physical development.
Puberty begins when the brain signals the ovaries or testicles to raise hormone output. Girls typically start around 10 to 11 and finish around 15 to 17; boys start around 11 to 12 and finish around 16 to 17. Children now reach puberty earlier than in the 19th century. Biologically, adulthood means sexual maturity, though legal adulthood usually arrives at 18, and the two do not always coincide.
Source: Development of the human body
