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Linear growth is a complex process regulated by the growth hormone (GH) – insulin-like growth factor-1 axis, the thyroxine/triiodothyronine axis, androgens, estrogens, vitamin D, glucocorticoids and possibly leptin. [25] GH is secreted by the anterior pituitary gland in response to hypothalamic, pituitary and circulating factors.
Tissue growth is the process by which a tissue increases its size. In animals, tissue growth occurs during embryonic development, post-natal growth, and tissue regeneration. The fundamental cellular basis for tissue growth is the process of cell proliferation, which involves both cell growth and cell division occurring in parallel. [1] [2] [3] [4]
In biological terms, the development of the human body 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 the single cell zygote and the germinal stage of development commences.
Adult development encompasses the changes that occur in biological and psychological domains of human life from the end of adolescence until the end of one's life. Changes occur at the cellular level and are partially explained by biological theories of adult development and aging. [1]
A newborn's head is very large in proportion to the body, and the cranium is enormous relative to his or her face. While the adult human skull is about one seventh of the total body length, the newborn's is about 1 ⁄ 4. Normal head circumference for a full-term infant is 33–36 cm at birth. [6]
[5] [6] After growth from the zygote to the adult, cell division by mitosis allows for continual construction and repair of the organism. [7] The human body experiences about 10 quadrillion cell divisions in a lifetime. [8] The primary concern of cell division is the maintenance of the original cell's genome.
The mammary ridge is primordial for the mammary glands on the chest in humans, and is associated with mammary gland and breast development.. In human embryogenesis, the mammary ridge usually appears as a narrow, microscopic ectodermal thickening during the first seven weeks of pregnancy and grows caudally as a narrow, linear ridge. [1]
It is also a portmanteau term that describes all biological aspects of the human body, typically using the human body as a type organism for Mammalia, and in that context it is the basis for many undergraduate University degrees and modules. [4] [5] Most aspects of human biology are identical or very similar to general mammalian biology.