Human development (biology)

"Developmental" redirects here. For other uses, see Development.

Human development is the process of growing to maturity. In biological terms, this entails growth from a one-celled zygote to an adult human being.

Biological development

General aspects

Sperm fertilizing an egg

From a biological standpoint, human development is a continuum, starting with the germ cells (ovum and spermatozoon), through fertilization, prenatal development, birth, and growth to adulthood. The germinal stage, refers to ovum (egg) prior to fertilization, through the development of the early embryo, up until the time of implantation. During this stage, the fertilization creates a single-celled zygote, which is defined as an embryo because it contains a full complement of genetic material. Prior to implantation, the embryo remains in a protein shell, the zona pellucida, and undergoes a series of cell divisions. A week after fertilization the embryo still has not grown in size, but hatches from its protein shell and adheres to the lining of the mother's uterus. This induces a decidual reaction, wherein the uterine cells proliferate and surround the embryo thus causing it to become embedded within the uterine tissue. The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the fetal membranes and the placenta. In humans, the embryo is referred to as a fetus in the later stages of prenatal development. The transition from embryo to fetus is arbitrarily defined as occurring either 8 weeks after fertilization or 8 weeks after implantation. In comparison to the embryo, the fetus has more recognizable external features, and a set of progressively developing internal organs. A nearly identical process occurs in other species, especially among Chordates.

Childbirth is the process in which the baby is born. Age is defined relative to this event in most cultures.

Physical stages

There are no universal definitions for terms of age-related physical development stages, but following are some approximate age ranges:

Stages in prenatal development.

• Prenatal (sperm fertilizes egg - birth)
  • Embryo - fertilization - 8 weeks after fertilization)
    • Zygote, the single cell stage which occurs after fertilization
    • Blastocyst, the stage prior to implantation, when the embryo is a hollow sphere
    • Post-implantation embryo, the period 1 – 8 weeks after fertilization (3 to 10 weeks gestation)
  • Fetus, (10th week of pregnancy - birth)

Approximate outline of development periods in child development.

Adult hands holding the foot of a baby

• Childhood/Juvenile (Childbirth)(0 - 19)

Further information: Child development and Child development stages

• • Neonate (newborn) (0 – 30 days)
  • Infant (baby) (0 month - 12 months)
  • Toddler (1 – 3 years)
  • Play age (4–5 years)
  • Primary school age (middle childhood also called prepubescence) (4-12)
    • Elementary school age (6-12)
    • Preadolescence (preteen.The child in this and the previous phase are called schoolchild (schoolboy or schoolgirl), when still of primary school age.) (10 – 12 years)
  • Adolescence and puberty (13 – 19 years)
    • Peripuberty (8-10 until 15-17)
• Adulthood (20+ years)
  • Young adulthood (20 – 39 years)
  • Middle adulthood (40 – 59 years)
  • Advanced adulthood/Senior citizen (60+ years)
• Death (occurs at various ages, depending on person)
  • Decomposition (breakdown of the body after death)

Also sometimes used are terms that specify one's age in numbers, such as:

• Young child (0-9)
• Pre-teenager (10-12)
• Teenager (13-19)
• Twentysomething (20-29)
• Thirtysomething (30-39)
• Fortysomething (40-49) (formerly also Quadragenarian, rarely used since 1980)
• Quinquagenarian (50-59)
• Sexagenarian (60-69)
• Septuagenarian (70-79)
• Octogenarian (80-89)
• Nonagenarian (90-99)
• Centenarian (100-109)
• Supercentenarian (110+)

The Tanner stages can be used to approximately judge a child's age based on physical development.

See also

• Auxology
• Child development
• Developmental biology
• Embryogenesis
• Life-history theory
• Mammalian embryogenesis