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| Human growth and development |
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| Stages |
| Biological milestones |
| Development and psychology |
Development of the human body is the process ofgrowth tomaturity. The process begins withfertilization, where anegg released from theovary of a female is penetrated by asperm cell from a male. The resultingzygote develops through cellproliferation anddifferentiation, and the resultingembryo thenimplants in theuterus, where the embryo continues development through afetal stage untilbirth. Further growth and development continues after birth, and includes bothphysical andpsychological development that is influenced bygenetic,hormonal,environmental and other factors. This continues throughoutlife: throughchildhood andadolescence intoadulthood.[1]
Development before birth, orprenatal development (from Latin natalis 'relating to birth')[2] is the process in which a fertilized egg develops into azygote, anembryo, and then afetus during the course ofgestation.Prenatal development starts withfertilization and the formation of thezygote, the first stage inembryonic development which continues in fetal development untilbirth.
Fertilization occurs when the sperm successfully enters the ovum's membrane and the two cells fuse to form a single cellzygote. Eachgamete (sperm and egg) are haploid, meaning they have half the number (23) ofchromosomes present in a typical human cell, one copy of each chromosome. When they fuse, the resultingzygote is diploid, with 46 chromosomes, and has unique genetic material that is a combination of both parents.[3] 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 withtrisomy, a disorder in which a human has three copies of a certain chromosome instead of two.[4]
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.[5]
Thezygote contains a full complement ofgenetic material with all the biological characteristics of a single human being, and develops into the embryo. Embryonic development has four stages: themorula stage, theblastula stage, thegastrula stage, and theneurula stage. Prior to implantation, theembryo remains in a protein shell, thezona pellucida, and undergoes a series of rapid mitotic cell divisions calledcleavage.[6] 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'suterus. This induces adecidual 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 thefetal membranes and theplacenta. In humans, the embryo is referred to as afetus 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 humanembryo. It is characterised by the process ofcell division andcellular differentiation of theembryo that occurs during the early stages ofdevelopment. In biological terms, human development entails growth from a one-celledzygote to an adulthuman being.Fertilization occurs when thesperm cell successfully enters and fuses with an egg cell (ovum).[5] The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage ofprenatal development commences. The embryonic stage covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed afetus.
The germinal stage refers to the time from fertilization through the development of the early embryo untilimplantation is completed in theuterus. The germinal stage takes around 10 days.[5] During this stage, the zygote begins to divide, in a process calledcleavage. Ablastocyst is then formed and implanted in theuterus. Embryonic development continues with the next stage ofgastrulation, when the threegerm layers of the embryo form in a process calledhistogenesis, and the processes ofneurulation andorganogenesis 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 ingene expression,cell growth andcellular differentiation. A nearly identical process occurs in other species, especially amongchordates.
Afetus is a stage in the human development considered to begin nine weeks after fertilization.[7][8] In biological terms, however, prenatal development is a continuum, with many defining features 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.
The fetus and embryo develop within theuterus, an organ that sits within the pelvis of themother. The process the mother experiences whilst carrying the fetus or embryo is referred to aspregnancy. Theplacenta connects the developingfetus to theuterine 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' blood. The placenta attaches to the wall of the uterus, and the fetus'umbilical cord develops from the placenta. These organs connect the mother and the fetus. Placentas are a defining characteristic ofplacental mammals, but are also found inmarsupials and some non-mammals with varying levels of development.[9] Thehomology of such structures in variousviviparous organisms is debatable, and in invertebrates such asArthropoda, isanalogous at best.
Childhood is the age span ranging from birth toadolescence.[10] Indevelopmental psychology, childhood is divided up into the developmental stages oftoddlerhood (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.[10]
TheTanner stages can be used to approximately judge a child's age based on physical development.
| For North American, Indo-Iranian (India, Iran) and European girls | For North American, Indo-Iranian (India, Iran) and European boys |
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Puberty is the process of physical changes through which achild'sbody matures into anadult body capable ofsexual reproduction. It is initiated byhormonal signals from thebrain to thegonads: theovaries in a girl, thetesticles in a boy. In response to the signals, the gonads produce hormones that stimulatelibido and the growth, function, and transformation of the brain,bones,muscle,blood,skin,hair,breasts, andsex 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.[11][12][13][14][15] The major landmark of puberty for females ismenarche, the onset of menstruation, which occurs on average between ages 12 and 13;[16][17][18][19] for males, it is the firstejaculation, which occurs on average at age 13.[20] 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.[21] This can be due to any number of factors, including improved nutrition resulting in rapid body growth, increased weight and fat deposition,[22] or exposure toendocrine disruptors such asxenoestrogens, which can at times be due to food consumption or other environmental factors.[23][24] Puberty which starts earlier than usual is known asprecocious puberty, and puberty which starts later than usual is known asdelayed puberty.
Notable among themorphologic changes in size, shape, composition, and functioning of the pubertal body, is the development ofsecondary sex characteristics, the "filling in" of the child's body; from girl to woman, from boy to man.[clarification needed]
Biologically, an adult is a human or other organism that has reached sexual maturity. In human context, the term adult has additional meanings associated with social and legal concepts. In contrast to alegal 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 legal majority is 18 years in most contexts, although the definition of majority may vary by legal rights and country.
Human adulthood encompasses psychological adult development. Definitions of adulthood are often inconsistent and contradictory; an adolescent may be biologically an adult and display adult behavior but still be treated as a child if they are under the legal age of majority. Conversely, a legal adult may possess none of the maturity and responsibility that is supposed to define them; the mental and physical development and maturity of an individual has been proven to be greatly influenced by their life circumstances.
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| Development oforgan systems |
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Humanorgans andorgan systems develop in a process known asorganogenesis. This begins in the third week of embryonic development, when the gastrula forms three distinctgerm layers, theectoderm,mesoderm andendoderm. The ectoderm will eventually develop into theouter layer of skin andnervous system. The mesoderm will formskeletal muscles,blood cells, thereproductive system, theurinary system, most of thecirculatory system, and theconnective tissue of the torso. The endoderm will develop into theepithelium of therespiratory andgastrointestinal tracts and severalglands.[25]
During childhood, the bones undergo a complex process of elongation that occurs in a specific area called theepiphyseal growth plates (EGP). This process is regulated by varioushormones and factors, including the growth hormone, vitamin D, and others. These hormones promote the production of insulin-like growth factor-1 (IGF-1), which plays a key role in the formation of new bone cells. Adequatenutrient intake is essential for the production of these hormones, which are critical for proper bone growth. However, a lack of proper nutrition can hinder this process and result instunted growth.
Linear growth takes place in the epiphyseal growth plates (EGP) of long bones.[26] In the growth plate,chondrocytes proliferate, hypertrophy and secrete cartilage extracellular matrix. New cartilage is subsequently remodeled into bone tissue, causing bones to grow longer.[27] Linear growth is a complex process regulated by the growth hormone (GH) – insulin-like growth factor-1 (IGF-1) axis, the thyroxine/triiodothyronine axis, androgens, estrogens, vitamin D, glucocorticoids and possibly leptin.[28] GH is secreted by the anterior pituitary gland in response to hypothalamic, pituitary and circulating factors. It affects growth by binding to receptors in the EGP,[26] and inducing production and release of IGF-1 by the liver.[29] IGF-1 has six binding proteins (IGFBPs), exhibiting different effects on body tissues, whereIGFBP-3 is most abundant in human circulation.[30] IGF-1 initiates growth through differentiation and maturation of osteoblasts, and regulates release of GH from the pituitary through feedback mechanisms.[31] The GH/IGF-1 axis is responsive to dietary intake and infections. The endocrine system seems to allow for rapid growth only when the organism is able to consume sufficient amounts of nutrients and signaling from key nutrients such as amino acids and zinc to induce production of IGF-1 is present.[28] At the same time inflammation and increased production of pro-inflammatory cytokines may cause GH resistance and a decrease in circulating IGF-1 and IGFBP-3 which in turn reduces endochondrial ossification and growth.[28][32] However, the EGP appears to conserve much growth capacity to allow for catch-up growth.[33] Concerns have been raised about associations between catch-up growth and increased risk of non-communicable diseases in adulthood.[34] In a large study based on 5 birth cohorts in Brazil, Guatemala, India, the Philippines and South Africa, faster linear growth at 0–2 years was associated with improvements in adult stature and school performance, but also an increased likelihood of overweight (mainly related to lean mass) and a slightly elevated blood pressure in young adulthood.[35]
This article incorporatestext by Marianne Sandsmark Morseth available under theCC BY-SA 3.0 license.
