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 In biology, regeneration is the process of renewal, restoration, and tissue growth that makes genomes, cells, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of rege ...

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

cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...

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 abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...

. 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 Morphogenesis (from the Greek ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of deve ...

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 de ...

, 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 In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of ...

within the

basal layer The ''stratum basale'' (basal layer, sometimes referred to as ''stratum germinativum'') is the deepest layer of the five layers of the epidermis, the external covering of skin in mammals. The ''stratum basale'' is a single layer of columnar or ...

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

within the

and consequently leads to both an increased rate of cell proliferation as well as promoting the planar

orientation Orientation may refer to: Positioning in physical space * Map orientation, the relationship between directions on a map and compass directions * Orientation (housing), the position of a building with respect to the sun, a concept in building de ...

of stem cell divisions to produce new skin

, rather than only producing differentiating supra-basal daughter cells. Cell proliferation in skin

within the

can be driven by the mechanically-regulated

YAP Yap ( yap, Waqaab) traditionally refers to an island group located in the Caroline Islands of the western Pacific Ocean, a part of Yap State. The name "Yap" in recent years has come to also refer to the state within the Federated States of Micr ...

/ TAZ family of transcriptional co-activators, which bind to TEAD-family DNA binding

transcription factors In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fun ...

in the

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

to activate target gene expression 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 is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an en ...

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a ve ...

, it remains unclear how developmental

gene regulatory networks A gene (or genetic) regulatory network (GRN) is a collection of molecular regulators that interact with each other and with other substances in the cell to govern the gene expression levels of mRNA and proteins which, in turn, determine the fun ...

encoded in the

genome In the fields of molecular biology and genetics, 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 g ...

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

, leading to increased cell proliferation rates in many tissues. In mammals, production of IGF-1 is induced by another circulating hormone called

Growth Hormone Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in h ...

. Excessive production of

or IGF-1 is responsible for

giantism Gigantism ( el, γίγας, ''gígas'', "giant", plural γίγαντες, ''gígantes''), also known as giantism, is a condition characterized by excessive growth and height significantly above average. In humans, this condition is caused by ove ...

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

and progenitor cells while undergoing an equivalent loss of differentiated daughter cells via sloughing off. Gradients of

Wnt signaling pathway The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling ...

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

/ TAZ 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 reg ...

that activate the PI3K/AKT/mTOR pathway.

References

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