Development of the human body

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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 mitosis and cell 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. [1]

Contents

Before birth

Development before birth, or prenatal development (from Latin natalis 'relating to birth') is the process in which a zygote, and later an embryo, and then a fetus develops during 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

Sperm fertilizing an egg Sperm-egg.jpg
Sperm fertilizing an egg

Fertilization occurs when the sperm successfully enters the ovum's membrane. The chromosomes of the sperm are passed into the egg to form a unique genome. The egg becomes a zygote and the germinal stage of embryonic development begins. 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. 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. [3] 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.

Embryonic development

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). [2] 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. [2] 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.

Fetal development

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 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.

Stages in prenatal development Prenatal development table.svg
Stages in prenatal development

Maternal influences

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' 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 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.

After birth

Infancy and childhood

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]

Approximate outline of development periods in child development Child development stages.svg
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.

For North American, Indo-Iranian (India, Iran) and European girlsFor North American, Indo-Iranian (India, Iran) and European boys
  • Thelarche (breast development) 10.5y (8y–13y)
  • Pubarche (pubic hair) 11y (8.5y–13.5y)
  • Growth spurt 11.25y (10y–12.5y)
  • Menarche (first menstrual bleeding) 12.5y (10.5y–14.5y)
  • Wisdom tooth eruption 15.5y (14y–17y)
  • Adult height reached 15.5y (14y–17y)
  • Gonadarche (testicular enlargement) 11.5y (9.5y–13.5y)
  • Pubarche (pubic hair) 12y (10y–14y)
  • Growth spurt 13y (11y–18.5y)
  • Spermarche (first ejaculation) 13.5y (11.5y–15.5y)
  • Wisdom tooth eruption 17y (15y–19y)
  • Completion of growth 17y (15y–19y)

Puberty

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 testicles 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.[ clarification needed ]

Adulthood

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 a legal 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.

Organ systems

Human organs and organ systems develop in a process known as organogenesis. This begins in the third week of embryonic development, when the gastrula forms three distinct germ layers, the ectoderm, mesoderm and endoderm. The ectoderm will eventually develop into the outer layer of skin and nervous system. The mesoderm will form skeletal muscles, blood cells, the reproductive system, the urinary system, most of the circulatory system, and the connective tissue of the torso. The endoderm will develop into the epithelium of the respiratory and gastrointestinal tracts and several glands. [22]

Linear growth

During childhood, the bones undergo a complex process of elongation that occurs in a specific area called epiphyseal growth plates (EGP). This process is regulated by various hormones 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. Adequate nutrient 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 in stunted growth.

Linear growth takes place in the epiphyseal growth plates (EGP) of long bones. [23] 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. [24] 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. [25] 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, [23] and inducing production and release of IGF-1 by the liver. [26] IGF-1 has six binding proteins (IGFBPs), exhibiting different effects on body tissues, where IGFBP-3 is most abundant in human circulation. [27] IGF-1 initiates growth through differentiation and maturation of osteoblasts, and regulates release of GH from the pituitary through feedback mechanisms. [28] 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. [25] 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. [25] [29] However, the EGP appears to conserve much growth capacity to allow for catch-up growth. [30] Concerns have been raised about associations between catch-up growth and increased risk of non-communicable diseases in adulthood. [31] 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. [32]

See also

Related Research Articles

<span class="mw-page-title-main">Ontogeny</span> Origination and development of an organism

Ontogeny is the origination and development of an organism, usually from the time of fertilization of the egg to adult. The term can also be used to refer to the study of the entirety of an organism's lifespan.

<span class="mw-page-title-main">Pregnancy (mammals)</span> Period of reproduction

In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus, and ends once it leaves the uterus.

<span class="mw-page-title-main">Uterus</span> Female sex organ in mammals

The uterus or womb is the organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic and fetal development of one or more embryos until birth. The uterus is a hormone-responsive sex organ that contains glands in its lining that secrete uterine milk for embryonic nourishment.

<span class="mw-page-title-main">Embryo</span> Multicellular diploid eukaryote in its earliest stage of development

An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm cell. The resulting fusion of these two cells produces a single-celled zygote that undergoes many cell divisions that produce cells known as blastomeres. The blastomeres are arranged as a solid ball that when reaching a certain size, called a morula, takes in fluid to create a cavity called a blastocoel. The structure is then termed a blastula, or a blastocyst in mammals.

<span class="mw-page-title-main">Placenta</span> Organ that connects the fetus to the uterine wall

The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the fetus via the umbilical cord, and on the opposite aspect to the maternal uterus in a species-dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.

<span class="mw-page-title-main">Blastulation</span> Sphere of cells formed during early embryonic development in animals

Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula is a hollow sphere of cells known as blastomeres surrounding an inner fluid-filled cavity called the blastocoel. Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.

<span class="mw-page-title-main">Blastocyst</span> Structure formed around day 5 of mammalian embryonic development

The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the embryoblast which subsequently forms the embryo, and an outer layer of trophoblast cells called the trophectoderm. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. In the late blastocyst, the trophectoderm is known as the trophoblast. The trophoblast gives rise to the chorion and amnion, the two fetal membranes that surround the embryo. The placenta derives from the embryonic chorion and the underlying uterine tissue of the mother.

<span class="mw-page-title-main">Female reproductive system</span> Reproductive system of human females

The female reproductive system is made up of the internal and external sex organs that function in the reproduction of new offspring. The human female reproductive system is immature at birth and develops to maturity at puberty to be able to produce gametes, and to carry a fetus to full term. The internal sex organs are the vagina, uterus, fallopian tubes, and ovaries. The female reproductive tract includes the vagina, uterus, and fallopian tubes and is prone to infections. The vagina allows for sexual intercourse and childbirth, and is connected to the uterus at the cervix. The uterus or womb accommodates the embryo which develops into the fetus. The uterus also produces secretions which help the transit of sperm to the fallopian tubes, where sperm fertilize ova produced by the ovaries. The external sex organs are also known as the genitals and these are the organs of the vulva including the labia, clitoris, and vaginal opening.

<span class="mw-page-title-main">Trophoblast</span> Early embryonic structure that gives rise to the placenta

The trophoblast is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the placenta. They form during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg to become extraembryonic structures that do not directly contribute to the embryo. After blastulation, the trophoblast is contiguous with the ectoderm of the embryo and is referred to as the trophectoderm. After the first differentiation, the cells in the human embryo lose their totipotency because they can no longer form a trophoblast. They become pluripotent stem cells.

<span class="mw-page-title-main">Male reproductive system</span> Reproductive system of the human male

The male reproductive system consists of a number of sex organs that play a role in the process of human reproduction. These organs are located on the outside of the body, and within the pelvis.

<span class="mw-page-title-main">Human reproductive system</span> Organs involved in reproduction

The human reproductive system includes the male reproductive system which functions to produce and deposit sperm; and the female reproductive system which functions to produce egg cells, and to protect and nourish the fetus until birth. Humans have a high level of sexual differentiation. In addition to differences in nearly every reproductive organ, there are numerous differences in typical secondary sex characteristics.

Prenatal development includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal development until birth.

<span class="mw-page-title-main">Human reproduction</span> Procreative biological processes of humanity

Human reproduction is sexual reproduction that results in human fertilization to produce a human offspring. It typically involves sexual intercourse between a sexually mature human male and female. During sexual intercourse, the interaction between the male and female reproductive systems results in fertilization of the ovum by the sperm to form a zygote. While normal cells contain 46 chromosomes, gamete cells only contain 23 single chromosomes, and it is when these two cells merge into one zygote cell that genetic recombination occurs and the new zygote contains 23 chromosomes from each parent, giving it 46 chromosomes. The zygote then undergoes a defined development process that is known as human embryogenesis, and this starts the typical 9-month gestation period that is followed by childbirth. The fertilization of the ovum may be achieved by artificial insemination methods, which do not involve sexual intercourse. Assisted reproductive technology also exists.

<span class="mw-page-title-main">Puppy</span> Juvenile dog

A puppy is a juvenile dog. Some puppies can weigh 1–1.5 kg (2.2–3.3 lb), while larger ones can weigh up to 7–11 kg (15–24 lb). All puppies display primary altriciality and healthy puppies grow quickly after birth. A puppy's coat color may change as the puppy grows older, as is commonly seen in breeds such as the Yorkshire Terrier. Puppy refers specifically to young dogs, while pup may be used for other animals such as wolves, seals, giraffes, guinea pigs, rats or sharks.

<span class="mw-page-title-main">Implantation (embryology)</span> First stage of pregnancy

Implantation, also known as nidation, is the stage in the embryonic development of mammals in which the blastocyst hatches, attaches, adheres, and invades into the wall of the female's uterus. Implantation is the first stage of gestation, and, when successful, the female is considered to be pregnant. An implanted embryo is detected by the presence of increased levels of human chorionic gonadotropin (hCG) in a pregnancy test. The implanted embryo will receive oxygen and nutrients in order to grow.

<span class="mw-page-title-main">Sexual differentiation in humans</span> Process of development of sex differences in humans

Sexual differentiation in humans is the process of development of sex differences in humans. It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination. Sexual differentiation includes development of different genitalia and the internal genital tracts and body hair plays a role in sex identification.

<span class="mw-page-title-main">Human embryonic development</span> Development and formation of the human embryo

Human embryonic development or human embryogenesis is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. 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. Embryonic development in the human, covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. The eight weeks has 23 stages.

A fetus or foetus is the unborn offspring that develops from an animal embryo. Following embryonic development, the fetal stage of development takes place. In human prenatal development, fetal development begins from the ninth week after fertilization and continues until birth. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, a fetus is 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 anatomical location.

Hormonal regulation occurs at every stage of development. A milieu of hormones simultaneously affects development of the fetus during embryogenesis and the mother, including human chorionic gonadotropin (hCG) and progesterone (P4).

<span class="mw-page-title-main">Mammalian reproduction</span> Most mammals are viviparous, giving birth to live young

Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.

References

Creative Commons by-sa small.svg  This article incorporates text by Marianne Sandsmark Morseth available under the CC BY-SA 3.0 license.

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