Development of the human body

Development of the human body is the process of growth to maturity. The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through cell proliferation and differentiation, and the resulting embryo then implants in the uterus, where the embryo continues development through a fetal stage until birth. Further growth and development continues after birth, and includes both physical and psychological development that is influenced by genetic, hormonal, environmental and other factors. This continues throughout life: through childhood and adolescence into adulthood.

Development before birth, or prenatal development (from Latin natalis 'relating to birth') is the process in which a fertilized egg develops into a zygote, an embryo, and then a fetus during the course of gestation. Prenatal development starts with fertilization and the formation of the zygote, the first stage in embryonic development which continues in fetal development until birth.

Fertilization occurs when the sperm successfully enters the ovum's membrane and the two cells fuse to form a single cell zygote. Each gamete (sperm and egg) are haploid, meaning they have half the number (23) of chromosomes present in a typical human cell, one copy of each chromosome. When they fuse, the resulting zygote is diploid, with 46 chromosomes, and has unique genetic material that is a combination of both parents. Some gametes form improperly and retain two copies of one or more chromosomes. When one of these improper gametes fuses with a regular gamete, they create a zygote with trisomy, a disorder in which a human has three copies of a certain chromosome instead of two.

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.

The zygote contains a full complement of genetic material with all the biological characteristics of a single human being, and develops into the embryo. Embryonic development has 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 rapid mitotic cell divisions called cleavage. 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 embryonic development 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. Fertilization 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. The embryonic stage 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. During this stage, the zygote begins to divide, in a process called cleavage. A blastocyst is then formed and implanted in the uterus. Embryonic development 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 embryonic development 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.