Development of the human body

Sperm fertilizing an egg

Fertilization occurs when the sperm successfully enters the ovum's membrane. The chromosomes of the sperm combine with those of the egg to form a single cell, called a zygote, and the germinal stage of embryogenesis commences.[1] The germinal stage refers to the time from fertilization, through the development of the early embryo, up until implantation. The germinal stage is over at about 10 days of gestation.[2]

The zygote contains a full complement of genetic material, with all the biological characteristics of a single human being, and develops into the embryo. Briefly, embryonic development have four stages: the morula stage, the blastula stage, the gastrula stage, and the neurula stage. Prior to implantation, the embryo remains in a protein shell, the zona pellucida, and undergoes a series of cell divisions, called mitosis. A week after fertilization the embryo still has not grown in size, but hatches from the zona pellucida 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 8 weeks after fertilization. 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.

Human embryogenesis refers to the development and formation of the human embryo. It is characterised by the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, human development entails growth from a one-celled zygote to an adult human being. Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of prenatal development commences.[1] Embryogenesis covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus.

The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days.[3] During this stage, the zygote begins to divide, in a process called cleavage. A blastocyst is then formed and implanted in the uterus. Embryogenesis continues with the next stage of gastrulation, when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow.

In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. The entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression, cell growth and cellular differentiation. A nearly identical process occurs in other species, especially among chordates.

A fetus is a stage in the human development considered to begin nine weeks after fertilization.[4][5] In biological terms, however, prenatal development is a continuum, with many defining feature distinguishing an embryo from a fetus. A fetus is also characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final location.

Stages in prenatal development

The fetus and embryo develop within the uterus, an organ that sits within the pelvis of the mother. The process the mother experiences whilst carrying the fetus or embryo is referred to as pregnancy. The placenta connects the developing fetus to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother's blood supply; to fight against internal infection; and to produce hormones which support pregnancy. The placenta provides oxygen and nutrients to growing fetuses and removes waste products from the fetus's blood. The placenta attaches to the wall of the uterus, and the fetus's umbilical cord develops from the placenta. These organs connect the mother and the fetus. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.[6] The homology of such structures in various viviparous organisms is debatable, and in invertebrates such as Arthropoda, is analogous at best.

Childhood is the age span ranging from birth to adolescence.[7] In developmental psychology, childhood is divided up into the developmental stages of toddlerhood (learning to walk), early childhood (play age), middle childhood (school age), and adolescence (puberty through post-puberty). Various childhood factors could affect a person's attitude formation.[7]

• Prepubescence
  • Neonate (newborn)
  • Infant (baby)
  • Toddler
  • Play age
  • Elementary school age, may coincide with preadolescence

Approximate outline of development periods in child development.

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

Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sex organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when an adult body has been developed. Until the maturation of their reproductive capabilities, the pre-pubertal physical differences between boys and girls are the external sex organs.

On average, girls begin puberty around ages 10–11 and end puberty around 15–17; boys begin around ages 11–12 and end around 16–17.[8][9][10][11][12] The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12 and 13;[13][14][15][16] for males, it is the first ejaculation, which occurs on average at age 13.[17] In the 21st century, the average age at which children, especially girls, reach puberty is lower compared to the 19th century, when it was 15 for girls and 16 for boys.[18] This can be due to any number of factors, including improved nutrition resulting in rapid body growth, increased weight and fat deposition,[19] or exposure to endocrine disruptors such as xenoestrogens, which can at times be due to food consumption or other environmental factors.[20][21] Puberty which starts earlier than usual is known as precocious puberty, and puberty which starts later than usual is known as delayed puberty.

Notable among the morphologic changes in size, shape, composition, and functioning of the pubertal body, is the development of secondary sex characteristics, the "filling in" of the child's body; from girl to woman, from boy to man.

Biologically, an adult is a human or other organism that has reached sexual maturity. In human context, the term adult additionally has meanings associated with social and legal concepts. In contrast to a "minor", a legal adult is a person who has attained the age of majority and is therefore regarded as independent, self-sufficient, and responsible. The typical age of attaining adulthood is 18, although definition may vary by legal rights and country.

Human adulthood encompasses psychological adult development. Definitions of adulthood are often inconsistent and contradictory; a person may be biologically an adult, and have adult behavior but still be treated as a child if they are under the legal age of majority. Conversely, one may legally be an adult but possess none of the maturity and responsibility that may define an adult, the mental and physical development and maturity of an individual is proven to be greatly influenced by the circumstances in which they exist.