Tissue growth is the process by which ''a tissue increases its size''. In animals, tissue growth occurs during

embryonic development In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm, sperm cell (spermat ...

, post-natal growth, and tissue regeneration. The fundamental cellular basis for tissue growth is the process of cell proliferation, which involves both

cell growth Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

and cell division occurring in parallel. How cell proliferation is controlled during tissue growth to determine final tissue size is an open question in biology. Uncontrolled tissue growth is a cause of

cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...

. Differential rates of cell proliferation within an organ can influence proportions, as can the orientation of cell divisions, and thus tissue growth contributes to shaping tissues along with other mechanisms of tissue morphogenesis.

Mechanisms of tissue growth control in animals

Mechanical control of tissue growth in animal skin

For some animal tissues, such as mammalian

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...

, it is clear that the growth of the

is ultimately determined by the size of the body whose surface area the skin covers. This suggests that cell proliferation in skin stem cells within the basal layer is likely to be mechanically controlled to ensure that the skin covers the surface of the entire body. Growth of the body causes mechanical stretching of the skin, which is sensed by skin stem cells within the basal layer and consequently leads to both an increased rate of cell proliferation as well as promoting the planar orientation of stem cell divisions to produce new skin stem cells, rather than only producing differentiating supra-basal daughter cells. Cell proliferation in skin stem cells within the basal layer can be driven by the mechanically-regulated YAP/ TAZ family of transcriptional co-activators, which bind to TEAD-family DNA binding transcription factors in the nucleus to activate target

gene expression Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...

and thereby drive cell proliferation. For other animal tissues, such as the bones of the skeleton or the internal mammalian organs intestine,

pancreas The pancreas (plural pancreases, or pancreata) is an Organ (anatomy), organ of the Digestion, digestive system and endocrine system of vertebrates. In humans, it is located in the abdominal cavity, abdomen behind the stomach and functions as a ...

kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

, it remains unclear how developmental gene regulatory networks encoded in the

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

lead to organs of such different sizes and proportions.

Hormonal control of tissue growth in the entire animal body

Although different animal tissues grow at different rates and produce organs of very different proportions, the overall growth rate of the entire animal body can be modulated by circulating hormones of the

Insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the insulin (''INS)'' gene. It is the main Anabolism, anabolic hormone of the body. It regulates the metabol ...

/ IGF-1 family, which activate the PI3K/AKT/mTOR pathway in many cells of the body to increase the average rate of both

and cell division, leading to increased cell proliferation rates in many tissues. In mammals, production of IGF-1 is induced by another circulating hormone called Growth Hormone. Excessive production of Growth Hormone or IGF-1 is responsible for giantism while insufficient production of these hormones is responsible for dwarfism.

Developmental control of tissue growth during adult tissue homeostasis

Adult animal tissues such as

or intestine maintain their size but undergo constant turnover of cells by proliferation of stem cells and progenitor cells while undergoing an equivalent loss of differentiated daughter cells via sloughing off. Gradients of Wnt signaling pathway activity appear to have a fundamental role in maintaining proliferation of stem and progenitor cells, at least in the intestine, and possibly also in skin.

Regenerative tissue growth after wounding or other types of damage

Upon tissue damage, there is an upregulation in the activity of many pathways that control tissue growth, including the YAP/ TAZ pathway, Wnt signaling pathway, and

growth factors A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for regu ...

that activate the PI3K/AKT/mTOR pathway.

References

{{DEFAULTSORT:Tissue growth) Developmental biology Cell biology Cell cycle Cellular processes