View source for Developmental biology

{{short description|Study of how organisms develop and grow}}
{{For|the journal|Developmental Biology (journal){{!}}''Developmental Biology'' (journal)}}
{{TopicTOC-Biology}}
'''Developmental biology''' is the study of the process by which [[animal]]s and [[plant]]s grow and develop. Developmental biology also encompasses the biology of [[Regeneration (biology)|regeneration]], [[asexual reproduction]], [[metamorphosis]], and the growth and differentiation of [[stem cell]]s in the adult organism.

== Perspectives ==
The main processes involved in the [[embryogenesis|embryonic development]] of animals are: tissue patterning (via [[regional specification]] and patterned [[cellular differentiation|cell differentiation]]); [[tissue growth]]; and tissue [[morphogenesis]].

* [[Regional specification]] refers to the processes that create the spatial patterns in a ball or sheet of initially similar cells. This generally involves the action of [[cytoplasmic determinant]]s, located within parts of the fertilized egg, and of inductive signals emitted from signaling centers in the embryo. The early stages of [[regional specification]] do not generate functional differentiated cells, but cell populations committed to developing to a specific region or part of the organism. These are defined by the expression of specific combinations of [[transcription factors]]. 
* [[Cell differentiation]] relates specifically to the formation of functional cell types such as nerve, muscle, secretory epithelia, etc. Differentiated cells contain large amounts of specific proteins associated with cell function. 
* [[Morphogenesis]] relates to the formation of a three-dimensional shape. It mainly involves the orchestrated movements of cell sheets and of individual cells. Morphogenesis is important for creating the three germ layers of the early embryo ([[ectoderm]], [[mesoderm]], and [[endoderm]]) and for building up complex structures during organ development. 
* [[Tissue growth]] involves both an overall increase in tissue size, and also the differential growth of parts ([[allometry]]) which contributes to morphogenesis. Growth mostly occurs through [[cell proliferation]] but also through changes in cell size or the deposition of extracellular materials. 

The development of plants involves similar processes to that of animals. However, plant cells are mostly immotile so morphogenesis is achieved by differential growth, without cell movements. Also, the inductive signals and the genes involved are different from those that control animal development.

===Generative biology===
'''Generative biology''' is the [[generative science]] that explores the dynamics guiding the development and evolution of a biological morphological form.<ref name="Goodwin">{{cite book |last1=Webster |first1=Gerry |last2=Goodwin |first2=Brian |title=Form and Transformation: Generative and Relational Principles in Biology |date=13 November 1996 |publisher=Cambridge University Press |isbn=978-0-521-35451-6 |chapter-url=https://books.google.com/books?id=jeaoWY5MYHsC&dq=%22generative+biology%22&pg=PA231 |language=en |chapter=Chapter 9 - Generative Biology}}</ref><ref name="Amgen2022">{{cite news |title=Generative Biology: Designing Biologic Medicines with Greater Speed and Success |url=https://www.amgen.com/stories/2022/06/generative-biology--designing-biologics-with-greater-speed-and-success |access-date=5 April 2024 |work=Amgen |date=June 7, 2022 |language=en}}</ref><ref>{{cite web |title=Generative Biology: Learning to Program Cellular Machines |url=https://www.youtube.com/watch?v=0oG7OMbSWZQ |publisher=NIH |access-date=5 April 2024 |language=en |date=Mar 15, 2024}}</ref>

== Developmental processes ==

=== Cell differentiation ===

[[File:Slack Essential Dev Biol Fig 14.12a.jpg|thumb|The Notch-delta system in neurogenesis (Slack Essential Dev Biol Fig 14.12a)]]

[[Cell differentiation]] is the process whereby different functional cell types arise in development. For example, neurons, muscle fibers and hepatocytes (liver cells) are well known types of differentiated cells. Differentiated cells usually produce large amounts of a few proteins that are required for their specific function and this gives them the characteristic appearance that enables them to be recognized under the light microscope. The genes encoding these proteins are highly active. Typically their [[chromatin]] structure is very open, allowing access for the transcription enzymes, and specific transcription factors bind to regulatory sequences in the DNA in order to activate gene expression.<ref>{{cite journal | vauthors = Li B, Carey M, Workman JL | title = The role of chromatin during transcription | journal = Cell | volume = 128 | issue = 4 | pages = 707–19 | date = February 2007 | pmid = 17320508 | doi = 10.1016/j.cell.2007.01.015 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Heintzman ND, Stuart RK, Hon G, Fu Y, Ching CW, Hawkins RD, Barrera LO, Van Calcar S, Qu C, Ching KA, Wang W, Weng Z, Green RD, Crawford GE, Ren B | display-authors = 6 | title = Distinct and predictive chromatin signatures of transcriptional promoters and enhancers in the human genome | journal = Nature Genetics | volume = 39 | issue = 3 | pages = 311–8 | date = March 2007 | pmid = 17277777 | doi = 10.1038/ng1966 | s2cid = 1595885 }}</ref> For example, [[NeuroD]] is a key transcription factor for neuronal differentiation, [[myogenin]] for muscle differentiation, and [[HNF4]] for hepatocyte differentiation.
Cell differentiation is usually the final stage of development, preceded by several states of commitment which are not visibly differentiated. A single tissue, formed from a single type of progenitor cell or stem cell, often consists of several differentiated cell types. Control of their formation involves a process of lateral inhibition,<ref>{{cite journal | vauthors = Meinhardt H, Gierer A | year = 2000 | title = Pattern formation by local self-activation and lateral inhibition | url = http://www.me.ucsb.edu/~moehlis/APC514/2002_1.pdf | journal = BioEssays | volume = 22 | issue = 8| pages = 753–760 | doi = 10.1002/1521-1878(200008)22:8<753::aid-bies9>3.0.co;2-z | pmid = 10918306 | url-status = live | archive-url = https://web.archive.org/web/20171027025156/https://me.ucsb.edu/~moehlis/APC514/2002_1.pdf | archive-date = 2017-10-27 | citeseerx = 10.1.1.477.439 }}</ref> based on the properties of the [[Notch signaling pathway]].<ref>{{cite journal | vauthors = Sprinzak D, Lakhanpal A, Lebon L, Santat LA, Fontes ME, Anderson GA, Garcia-Ojalvo J, Elowitz MB | display-authors = 6 | title = Cis-interactions between Notch and Delta generate mutually exclusive signalling states | journal = Nature | volume = 465 | issue = 7294 | pages = 86–90 | date = May 2010 | pmid = 20418862 | pmc = 2886601 | doi = 10.1038/nature08959 | bibcode = 2010Natur.465...86S }}</ref> For example, in the neural plate of the embryo this system operates to generate a population of neuronal precursor cells in which NeuroD is highly expressed.

=== Regeneration ===
[[Regeneration (biology)|Regeneration]] indicates the ability to regrow a missing part.<ref>{{cite book | vauthors = Carlson BM | date = 2007 | title = Principles of Regenerative Biology. | publisher = Academic Press | location = Burlington MA }}</ref> This is very prevalent amongst plants, which show continuous growth, and also among colonial animals such as hydroids and ascidians. But most interest by developmental biologists has been shown in the regeneration of parts in free living animals. In particular four models have been the subject of much investigation. Two of these have the ability to regenerate whole bodies: ''[[Hydra (genus)|Hydra]]'', which can regenerate any part of the polyp from a small fragment,<ref>{{cite journal | vauthors = Bosch TC | title = Why polyps regenerate and we don't: towards a cellular and molecular framework for Hydra regeneration | journal = Developmental Biology | volume = 303 | issue = 2 | pages = 421–33 | date = March 2007 | pmid = 17234176 | doi = 10.1016/j.ydbio.2006.12.012 | doi-access = free }}</ref> and [[planarian]] worms, which can usually regenerate both heads and tails.<ref name="Reddien P.W., Alvarado A.S. 2004 725–757">{{cite journal | vauthors = Reddien PW, Sánchez Alvarado A | s2cid = 1320382 | title = Fundamentals of planarian regeneration | journal = Annual Review of Cell and Developmental Biology | volume = 20 | pages = 725–57 | year = 2004 | pmid = 15473858 | doi = 10.1146/annurev.cellbio.20.010403.095114 }}</ref> Both of these examples have continuous cell turnover fed by [[stem cells]] and, at least in planaria, at least some of the stem cells have been shown to be [[cell potency|pluripotent]].<ref>{{cite journal | vauthors = Wagner DE, Wang IE, Reddien PW | title = Clonogenic neoblasts are pluripotent adult stem cells that underlie planarian regeneration | journal = Science | volume = 332 | issue = 6031 | pages = 811–6 | date = May 2011 | pmid = 21566185 | pmc = 3338249 | doi = 10.1126/science.1203983 | bibcode = 2011Sci...332..811W }}</ref> The other two models show only distal regeneration of appendages. These are the insect appendages, usually the legs of hemimetabolous insects such as the cricket,<ref>{{cite journal | vauthors = Nakamura T, Mito T, Bando T, Ohuchi H, Noji S | title = Dissecting insect leg regeneration through RNA interference | journal = Cellular and Molecular Life Sciences | volume = 65 | issue = 1 | pages = 64–72 | date = January 2008 | pmid = 18030418 | doi = 10.1007/s00018-007-7432-0 | pmc = 11131907 }}</ref> and the limbs of [[urodele amphibians]].<ref>{{cite journal | vauthors = Simon A, Tanaka EM | title = Limb regeneration | journal = Wiley Interdisciplinary Reviews. Developmental Biology | volume = 2 | issue = 2 | pages = 291–300 | year = 2013 | pmid = 24009038 | doi = 10.1002/wdev.73 | s2cid = 13158705 }}</ref> Considerable information is now available about amphibian limb regeneration and it is known that each cell type regenerates itself, except for connective tissues where there is considerable interconversion between cartilage, dermis and tendons. In terms of the pattern of structures, this is controlled by a re-activation of signals active in the embryo.
There is still debate about the old question of whether regeneration is a "pristine" or an "adaptive" property.<ref>{{cite book | vauthors = Slack JM | date = 2013 | title = Essential Developmental Biology | chapter = Chapter 20 | publisher = Wiley-Blackwell | location = Oxford }}</ref> If the former is the case, with improved knowledge, we might expect to be able to improve regenerative ability in humans. If the latter, then each instance of regeneration is presumed to have arisen by natural selection in circumstances particular to the species, so no general rules would be expected.

== Embryonic development of animals ==
{{Main |Embryogenesis}}
[[File:Slack Essential Dev Biol Fig 02-08.jpg|thumb|Generalized scheme of embryonic development. Slack "Essential Developmental Biology". Fig. 2.8.]]
[[File:HumanEmbryogenesis.svg|thumb|300px|The initial stages of [[human embryogenesis]]]]
The sperm and egg fuse in the process of fertilization to form a fertilized egg, or [[zygote]].<ref>{{cite journal | vauthors = Jungnickel MK, Sutton KA, Florman HM | title = In the beginning: lessons from fertilization in mice and worms | journal = Cell | volume = 114 | issue = 4 | pages = 401–4 | date = August 2003 | pmid = 12941269 | doi = 10.1016/s0092-8674(03)00648-2 | doi-access = free }}</ref> This undergoes a period of divisions to form a ball or sheet of similar cells called a [[blastula]] or [[blastoderm]]. These cell divisions are usually rapid with no growth so the daughter cells are half the size of the mother cell and the whole embryo stays about the same size. They are called [[cleavage (embryo)|cleavage]] divisions.

Mouse [[epiblast]] primordial [[germ cell]]s (see Figure: "The initial stages of human [[embryonic development|embryogenesis]]") undergo extensive [[epigenetics|epigenetic]] reprogramming.<ref name="pmid23223451">{{cite journal | vauthors = Hackett JA, Sengupta R, Zylicz JJ, Murakami K, Lee C, Down TA, Surani MA | title = Germline DNA demethylation dynamics and imprint erasure through 5-hydroxymethylcytosine | journal = Science | volume = 339 | issue = 6118 | pages = 448–52 | date = January 2013 | pmid = 23223451 | pmc = 3847602 | doi = 10.1126/science.1229277 | bibcode = 2013Sci...339..448H }}</ref> This process involves [[genome]]-wide [[DNA demethylation]], [[chromatin]] reorganization and [[epigenetics|epigenetic]] imprint erasure leading to [[cell potency|totipotency]].<ref name="pmid23223451" /> DNA demethylation is carried out by a process that utilizes the DNA [[base excision repair]] pathway.<ref name="pmid20595612">{{cite journal | vauthors = Hajkova P, Jeffries SJ, Lee C, Miller N, Jackson SP, Surani MA | title = Genome-wide reprogramming in the mouse germ line entails the base excision repair pathway | journal = Science | volume = 329 | issue = 5987 | pages = 78–82 | date = July 2010 | pmid = 20595612 | pmc = 3863715 | doi = 10.1126/science.1187945 | bibcode = 2010Sci...329...78H }}</ref>

Morphogenetic movements convert the cell mass into a three layered structure consisting of multicellular sheets called [[ectoderm]], [[mesoderm]] and [[endoderm]]. These sheets are known as [[germ layers]]. This is the process of [[gastrulation]]. During cleavage and gastrulation the first regional specification events occur. In addition to the formation of the three germ layers themselves, these often generate extraembryonic structures, such as the mammalian [[placenta]], needed for support and nutrition of the embryo,<ref>{{cite book | veditors = Steven DH | date = 1975 | title = Comparative Placentation. | publisher = Academic Press | location = London }}</ref> and also establish differences of commitment along the anteroposterior axis (head, trunk and tail).<ref>{{cite journal | vauthors = Kimelman D, Martin BL | title = Anterior-posterior patterning in early development: three strategies | journal = Wiley Interdisciplinary Reviews. Developmental Biology | volume = 1 | issue = 2 | pages = 253–66 | year = 2012 | pmid = 23801439 | pmc = 5560123 | doi = 10.1002/wdev.25 }}</ref>

[[Regional specification]] is initiated by the presence of [[cytoplasmic determinant]]s in one part of the zygote. The cells that contain the determinant become a signaling center and emit an inducing factor. Because the inducing factor is produced in one place, diffuses away, and decays, it forms a concentration gradient, high near the source cells and low further away.<ref>{{cite journal | vauthors = Slack JM | year = 1987 | title = Morphogenetic gradients - past and present | journal = Trends in Biochemical Sciences | volume = 12 | pages = 200–204 | doi=10.1016/0968-0004(87)90094-6}}</ref><ref>{{cite journal | vauthors = Rogers KW, Schier AF | s2cid = 21477124 | title = Morphogen gradients: from generation to interpretation | journal = Annual Review of Cell and Developmental Biology | volume = 27 | pages = 377–407 | year = 2011 | pmid = 21801015 | doi = 10.1146/annurev-cellbio-092910-154148 }}</ref> The remaining cells of the embryo, which do not contain the determinant, are competent to respond to different concentrations by upregulating specific developmental control genes. This results in a series of zones becoming set up, arranged at progressively greater distance from the signaling center. In each zone a different combination of developmental control genes is upregulated.<ref>{{cite journal | vauthors = Dahmann C, Oates AC, Brand M | title = Boundary formation and maintenance in tissue development | journal = Nature Reviews. Genetics | volume = 12 | issue = 1 | pages = 43–55 | date = January 2011 | pmid = 21164524 | doi = 10.1038/nrg2902 | s2cid = 1805261 }}</ref> These genes encode [[transcription factors]] which upregulate new combinations of gene activity in each region. Among other functions, these transcription factors control expression of genes conferring specific adhesive and motility properties on the cells in which they are active. Because of these different morphogenetic properties, the cells of each germ layer move to form sheets such that the ectoderm ends up on the outside, mesoderm in the middle, and endoderm on the inside.<ref>{{cite journal | vauthors = Hardin J, Walston T | title = Models of morphogenesis: the mechanisms and mechanics of cell rearrangement | journal = Current Opinion in Genetics & Development | volume = 14 | issue = 4 | pages = 399–406 | date = August 2004 | pmid = 15261656 | doi = 10.1016/j.gde.2004.06.008 }}</ref><ref>{{cite journal | vauthors = Hammerschmidt M, Wedlich D | title = Regulated adhesion as a driving force of gastrulation movements | journal = Development | volume = 135 | issue = 22 | pages = 3625–41 | date = November 2008 | pmid = 18952908 | doi = 10.1242/dev.015701 | doi-access = free }}</ref> 

[[File:AxialTwistSchema.png|thumb|upright=1|Schema of the development of the axial twist in vertebrates]]
Morphogenetic movements not only change the shape and structure of the embryo, but by bringing cell sheets into new spatial relationships they also make possible new phases of signaling and response between them. In addition, first morphogenetic movements of embryogenesis, such as gastrulation, [[epiboly]] and [[chirality#Biology|twisting]], directly activate pathways involved in endomesoderm specification through mechanotransduction processes.<ref>{{Cite journal |last1=Farge |first1=Emmanuel |year=2003 |title=Mechanical induction of twist in the Drosophila foregut/stomodeal primordium|journal=Current Biology |volume=13 |issue=16 |pages=1365–1377 |doi=10.1016/s0960-9822(03)00576-1 |pmid=1293230 |doi-access=free }}</ref><ref>{{Cite journal|last1=Brunet|first1=Thibaut|last2=Bouclet|first2=Adrien |last3=et|first3=al |year=2013|title=Evolutionary conservation of early mesoderm specification by mechanotransduction in Bilateria|journal=Nature Communications|volume=4|pages=2821|doi=10.1038/ncomms3821 |pmid= 24281726 |pmc=3868206|bibcode=2013NatCo...4.2821B }}</ref> This property was suggested to be evolutionary inherited from endomesoderm specification as mechanically stimulated by marine environmental hydrodynamic flow in first animal organisms (first metazoa).<ref>{{Cite journal|last1=Nguyen|first1=Ngoc-Minh|last2=Merle|first2=Tatiana |last3=et|first3=al |year=2022|title=Mechano-biochemical marine stimulation of inversion, gastrulation, and endomesoderm specification in multicellular Eukaryota|journal=Frontiers in Cell and Developmental Biology|volume=10|page=992371 |doi=10.3389/fcell.2022.992371 |pmid= 36531949 |pmc=9754125 |doi-access=free }}</ref> Twisting along the body axis by a left-handed chirality is found in all [[chordates]] (including vertebrates) and is addressed by the [[axial twist theory]].<ref name="Lussanet2012">{{cite journal | first1=M.H.E. | last1=de Lussanet | first2=J.W.M. | last2=Osse | year=2012 | title=An ancestral axial twist explains the contralateral forebain and the optic chiasm in vertebrates | journal=Animal Biology | volume=62 | issue=2 | pages=193–216 | doi=10.1163/157075611X617102 | arxiv=1003.1872 | s2cid=7399128}}</ref>

[[Human development (biology)|Growth]] in embryos is mostly autonomous.<ref>{{cite book | vauthors = O'Farrell PH | year = 2003 | chapter = How metazoans reach their full size: the natural history of bigness. | title = Cell Growth: Control of Cell Size | veditors = Hall MN, Raff M, Thomas G | pages = 1–21 | publisher = Cold Spring Harbor Laboratory Press }}</ref> For each territory of cells the growth rate is controlled by the combination of genes that are active. Free-living embryos do not grow in mass as they have no external food supply. But embryos fed by a placenta or extraembryonic yolk supply can grow very fast, and changes to relative growth rate between parts in these organisms help to produce the final overall anatomy.

The whole process needs to be coordinated in time and how this is controlled is not understood. There may be a master clock able to communicate with all parts of the embryo that controls the course of events, or timing may depend simply on local causal sequences of events.<ref>{{cite journal | vauthors = Moss EG, Romer-Seibert J | title = Cell-intrinsic timing in animal development | journal = Wiley Interdisciplinary Reviews. Developmental Biology | volume = 3 | issue = 5 | pages = 365–77 | year = 2014 | pmid = 25124757 | doi = 10.1002/wdev.145 | s2cid = 29029979 }}</ref>

=== Metamorphosis ===
Developmental processes are very evident during the process of [[metamorphosis]]. This occurs in various types of animal such as insects, amphibians, some fish, and many marine invertebrates.<ref>{{Cite journal |last=Bishop |first=C. D. |last2=Erezyilmaz |first2=D. F. |last3=Flatt |first3=T. |last4=Georgiou |first4=C. D. |last5=Hadfield |first5=M. G. |last6=Heyland |first6=A. |last7=Hodin |first7=J. |last8=Jacobs |first8=M. W. |last9=Maslakova |first9=S. A. |last10=Pires |first10=A. |last11=Reitzel |first11=A. M. |last12=Santagata |first12=S. |last13=Tanaka |first13=K. |last14=Youson |first14=J. H. |date=2006-12-01 |title=What is metamorphosis? |url=https://academic.oup.com/icb/article-abstract/46/6/655/702188 |journal=Integrative and Comparative Biology |volume=46 |issue=6 |pages=655–661 |doi=10.1093/icb/icl004 |issn=1540-7063}}</ref> Well-known examples are seen in frogs, which usually hatch as a tadpole and metamorphoses to an adult frog, and certain insects which hatch as a larva and then become remodeled to the adult form during a pupal stage.

All the developmental processes listed above occur during metamorphosis. Examples that have been especially well studied include tail loss and other changes in the tadpole of the frog ''Xenopus'',<ref>{{cite journal | vauthors = Tata JR | year = 1996 | title = Amphibian metamorphosis: an exquisite model for hormonal regulation of postembryonic development in vertebrates | journal = Development, Growth and Differentiation| volume = 38 | issue = 3| pages = 223–231 | doi=10.1046/j.1440-169x.1996.t01-2-00001.x| pmid = 37281700 | s2cid = 84081060 }}</ref><ref>{{cite journal | vauthors = Brown DD, Cai L | title = Amphibian metamorphosis | journal = Developmental Biology | volume = 306 | issue = 1 | pages = 20–33 | date = June 2007 | pmid = 17449026 | pmc = 1945045 | doi = 10.1016/j.ydbio.2007.03.021 }}</ref> and the biology of the imaginal discs, which generate the adult body parts of the fly ''Drosophila melanogaster''.<ref>{{cite book | vauthors = Cohen SM | date = 1993 | chapter = Imaginal Disc Development. | veditors = Bate M, Martinez-Arias M | title = The Development of Drosophila melanogaster | publisher = Cold Spring Harbor Press }}</ref><ref>{{cite journal | vauthors = Maves L, Schubiger G | title = Transdetermination in Drosophila imaginal discs: a model for understanding pluripotency and selector gene maintenance | journal = Current Opinion in Genetics & Development | volume = 13 | issue = 5 | pages = 472–9 | date = October 2003 | pmid = 14550411 | doi = 10.1016/j.gde.2003.08.006 }}</ref>

== Plant development ==
{{further|Plant development}}
Plant '''development''' is the process by which structures originate and mature as a plant grows. It is studied in [[plant anatomy]] and [[plant physiology]] as well as plant morphology.

Plants constantly produce new tissues and structures throughout their life from [[meristem]]s<ref>{{cite journal | vauthors = Bäurle I, Laux T | title = Apical meristems: the plant's fountain of youth | journal = BioEssays | volume = 25 | issue = 10 | pages = 961–70 | date = October 2003 | pmid = 14505363 | doi = 10.1002/bies.10341 | department = Review }}</ref> located at the tips of organs, or between mature tissues. Thus, a living plant always has embryonic tissues. By contrast, an animal [[embryo]] will very early produce all of the body parts that it will ever have in its life. When the animal is born (or hatches from its egg), it has all its body parts and from that point will only grow larger and more mature.

The properties of organization seen in a plant are [[emergence|emergent properties]] which are more than the sum of the individual parts. "The assembly of these tissues and functions into an integrated multicellular organism yields not only the characteristics of the separate parts and processes but also quite a new set of characteristics which would not have been predictable on the basis of examination of the separate parts."<ref>{{cite book | vauthors = Leopold AC | title = Plant Growth and Development | url = https://archive.org/details/plantgrowthdevel00leoprich | url-access = registration | page = [https://archive.org/details/plantgrowthdevel00leoprich/page/183 183] | location = New York | publisher = McGraw-Hill | date = 1964 }}</ref>

=== Growth ===
A [[vascular plant]] begins from a single celled [[zygote]], formed by [[fertilisation]] of an egg cell by a sperm cell. From that point, it begins to divide to form a plant [[embryo]] through the process of [[embryogenesis]]. As this happens, the resulting cells will organize so that one end becomes the first root, while the other end forms the tip of the shoot. In [[seed]] plants, the embryo will develop one or more "seed leaves" ([[cotyledon]]s). By the end of embryogenesis, the young plant will have all the parts necessary to begin its life.

Once the embryo [[germination|germinates]] from its seed or parent plant, it begins to produce additional organs (leaves, stems, and roots) through the process of [[organogenesis]]. New roots grow from root [[meristem]]s located at the tip of the root, and new stems and leaves grow from shoot [[meristem]]s located at the tip of the shoot.<ref>{{cite journal | vauthors = Brand U, Hobe M, Simon R | title = Functional domains in plant shoot meristems | journal = BioEssays | volume = 23 | issue = 2 | pages = 134–41 | date = February 2001 | pmid = 11169586 | doi = 10.1002/1521-1878(200102)23:2<134::AID-BIES1020>3.0.CO;2-3 | s2cid = 5833219 | department = Review }}</ref> Branching occurs when small clumps of cells left behind by the meristem, and which have not yet undergone [[cellular differentiation]] to form a specialized tissue, begin to grow as the tip of a new root or shoot. Growth from any such meristem at the tip of a root or shoot is termed [[primary growth]] and results in the lengthening of that root or shoot. [[Secondary growth]] results in widening of a root or shoot from divisions of cells in a [[Cambium (botany)|cambium]].<ref>{{cite journal | vauthors = Barlow P | title = Patterned cell determination in a plant tissue: the secondary phloem of trees | journal = BioEssays | volume = 27 | issue = 5 | pages = 533–41 | date = May 2005 | pmid = 15832381 | doi = 10.1002/bies.20214 }}</ref>

In addition to growth by [[cell (biology)|cell]] division, a plant may grow through '''cell elongation'''.<ref>{{cite journal | vauthors = Pacifici E, Di Mambro R, Dello Ioio R, Costantino P, Sabatini S | title = Arabidopsis root | journal = The EMBO Journal | volume = 37 | issue = 16 | date = August 2018 | pmid = 30012836 | pmc = 6092616 | doi = 10.15252/embj.201899134 }}</ref> This occurs when individual cells or groups of cells grow longer. Not all plant cells will grow to the same length. When cells on one side of a stem grow longer and faster than cells on the other side, the stem will bend to the side of the slower growing cells as a result. This directional growth can occur via a plant's response to a particular stimulus, such as light ([[phototropism]]), gravity ([[gravitropism]]), water, ([[hydrotropism]]), and physical contact ([[thigmotropism]]).

Plant growth and development are mediated by specific [[plant hormone]]s and plant growth regulators (PGRs) (Ross et al. 1983).<ref name="ross">{{cite book | vauthors = Ross SD, Pharis RP, Binder WD | date = 1983 | chapter = Growth regulators and conifers: their physiology and potential uses in forestry. | pages = 35–78 | veditors = Nickell LG | title = Plant growth regulating chemicals | volume = 2 | publisher = CRC Press | location = Boca Raton, FL }}</ref> Endogenous hormone levels are influenced by plant age, cold hardiness, dormancy, and other metabolic conditions; photoperiod, drought, temperature, and other external environmental conditions; and exogenous sources of PGRs, e.g., externally applied and of rhizospheric origin.

=== Morphological variation ===
Plants exhibit natural variation in their form and structure. While all organisms vary from individual to individual, plants exhibit an additional type of variation. Within a single individual, parts are repeated which may differ in form and structure from other similar parts. This variation is most easily seen in the leaves of a plant, though other organs such as stems and flowers may show similar variation. There are three primary causes of this variation: positional effects, environmental effects, and juvenility.

=== Evolution of plant morphology ===
Transcription factors and transcriptional regulatory networks play key roles in plant morphogenesis and their evolution. During plant landing, many novel transcription factor families emerged and are preferentially wired into the networks of multicellular development, reproduction, and organ development, contributing to more complex morphogenesis of land plants.<ref name="MBE_1767">{{cite journal | vauthors = Jin J, He K, Tang X, Li Z, Lv L, Zhao Y, Luo J, Gao G | display-authors = 6 | title = An Arabidopsis Transcriptional Regulatory Map Reveals Distinct Functional and Evolutionary Features of Novel Transcription Factors | journal = Molecular Biology and Evolution | volume = 32 | issue = 7 | pages = 1767–73 | date = July 2015 | pmid = 25750178 | pmc = 4476157 | doi = 10.1093/molbev/msv058 | url =http://mbe.oxfordjournals.org/content/32/7/1767.full| url-status = live | archive-url = https://web.archive.org/web/20160602063847/http://mbe.oxfordjournals.org/content/32/7/1767.full | archive-date = 2016-06-02 }}</ref>

Most land plants share a common ancestor, multicellular algae. An example of the evolution of plant morphology is seen in charophytes. Studies have shown that charophytes have traits that are homologous to land plants. There are two main theories of the evolution of plant morphology, these theories are the homologous theory and the antithetic theory. The commonly accepted theory for the evolution of plant morphology is the antithetic theory. The antithetic theory states that the multiple mitotic divisions that take place before meiosis, cause the development of the sporophyte. Then the sporophyte will development as an independent organism.<ref>{{cite journal | vauthors = Pires ND, Dolan L | title = Morphological evolution in land plants: new designs with old genes | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 367 | issue = 1588 | pages = 508–518 | date = February 2012 | pmid = 22232763 | pmc = 3248709 | doi = 10.1098/rstb.2011.0252 }}</ref>

== Developmental model organisms ==

Much of developmental biology research in recent decades has focused on the use of a small number of [[model organisms]]. It has turned out that there is much conservation of developmental mechanisms across the animal kingdom. In early development different vertebrate species all use essentially the same inductive signals and the same genes encoding regional identity. Even invertebrates use a similar repertoire of signals and genes although the body parts formed are significantly different. Model organisms each have some particular experimental advantages which have enabled them to become popular among researchers. In one sense they are "models" for the whole animal kingdom, and in another sense they are "models" for human development, which is difficult to study directly for both ethical and practical reasons. Model organisms have been most useful for elucidating the broad nature of developmental mechanisms. The more detail is sought, the more they differ from each other and from humans.

===Plants===
* Thale cress (''[[Arabidopsis thaliana]]'')<ref name="Friedman-1999">{{cite journal | last=Friedman | first=William E. | title=Expression of the cell cycle in sperm of ''Arabidopsis'': implications for understanding patterns of gametogenesis and fertilization in plants and other eukaryotes | journal=[[Development (journal)|Development]] | publisher=[[The Company of Biologists]] | volume=126 | issue=5 | year=1999 | issn=0950-1991 | pmid=9927606 | pages=1065–75 | doi=10.1242/dev.126.5.1065 | s2cid=13397345}}</ref>

===Vertebrates===
* Frog: ''[[Xenopus]]''<ref name="Friedman-1999" /> (''[[Xenopus laevis|X. laevis]]'' and ''[[xenopus tropicalis|X. tropicalis]]'').<ref>{{cite book | vauthors = Nieuwkoop PD, Faber J | year = 1967 | title = Normal table of ''Xenopus laevis'' (Daudin) | location = North-Holland, Amsterdam }}</ref><ref>{{cite journal | vauthors = Harland RM, Grainger RM | title = Xenopus research: metamorphosed by genetics and genomics | journal = Trends in Genetics | volume = 27 | issue = 12 | pages = 507–15 | date = December 2011 | pmid = 21963197 | pmc = 3601910 | doi = 10.1016/j.tig.2011.08.003 }}</ref> Good embryo supply. Especially suitable for microsurgery.
* [[Zebrafish]]: ''Danio rerio''.<ref>{{cite journal | vauthors = Lawson ND, Wolfe SA | title = Forward and reverse genetic approaches for the analysis of vertebrate development in the zebrafish | journal = Developmental Cell | volume = 21 | issue = 1 | pages = 48–64 | date = July 2011 | pmid = 21763608 | doi = 10.1016/j.devcel.2011.06.007 | doi-access = free }}</ref> Good embryo supply. Well developed genetics.
* Chicken: ''Gallus gallus''.<ref>{{cite journal | vauthors = Rashidi H, Sottile V | title = The chick embryo: hatching a model for contemporary biomedical research | journal = BioEssays | volume = 31 | issue = 4 | pages = 459–65 | date = April 2009 | pmid = 19274658 | doi = 10.1002/bies.200800168 | s2cid = 5489431 }}</ref> Early stages similar to mammal, but microsurgery easier. Low cost.
* Mouse: ''Mus musculus''.<ref>{{cite book | vauthors = Behringer R, Gertsenstein M, Vintersten K, Nagy M | date = 2014 | title = Manipulating the Mouse Embryo. A Laboratory Manual | edition = Fourth | location = Cold Spring Harbor, NY | publisher = Cold Spring Harbor Laboratory Press }}</ref> A mammal<ref name="Friedman-1999" /> with well developed genetics.

===Invertebrates===

* Fruit fly: ''[[Drosophila melanogaster]]''.<ref>{{cite journal | vauthors = St Johnston D | title = The art and design of genetic screens: Drosophila melanogaster | journal = Nature Reviews. Genetics | volume = 3 | issue = 3 | pages = 176–88 | date = March 2002 | pmid = 11972155 | doi = 10.1038/nrg751 | s2cid = 195368351 }}</ref> Good embryo supply. Well developed genetics. 
* Nematode: ''[[Caenorhabditis elegans]]''.<ref>{{cite book | vauthors = Riddle DL, Blumenthal T, Meyer BJ, Priess JR | year = 1997 | title = C.elegans II. | publisher = Cold Spring Harbor Laboratory Press | location = Cold Spring Harbor, NY }}</ref> Good embryo supply. Well developed genetics. Low cost.

===Unicellular===
* Algae: ''[[Chlamydomonas]]''<ref name="Friedman-1999" />
* Yeast: ''[[Saccharomyces]]''<ref name="Friedman-1999" />

===Others===

Also popular for some purposes have been [[sea urchin]]s<ref>{{cite book | vauthors = Ettensohn CA, Sweet HC | title = Current Topics in Developmental Biology Volume 50 | year = 2000 | chapter = Patterning the early sea urchin embryo | chapter-url = https://archive.org/details/currenttopicsind00gera/page/1 | journal = Curr. Top. Dev. Biol. | volume = 50 | pages = [https://archive.org/details/currenttopicsind00gera/page/1 1–44] | doi = 10.1016/S0070-2153(00)50002-7 | pmid = 10948448 | isbn = 9780121531508 | chapter-url-access = registration | publisher = Academic Press }}</ref><ref name="Friedman-1999" /> and [[Ascidiacea|ascidians]].<ref>{{cite journal | vauthors = Lemaire P | title = Evolutionary crossroads in developmental biology: the tunicates | journal = Development | volume = 138 | issue = 11 | pages = 2143–52 | date = June 2011 | pmid = 21558365 | doi = 10.1242/dev.048975 | doi-access = free }}</ref> For studies of regeneration [[urodele amphibians]] such as the [[axolotl]] ''Ambystoma mexicanum'' are used,<ref>{{cite journal | vauthors = Nacu E, Tanaka EM | title = Limb regeneration: a new development? | journal = Annual Review of Cell and Developmental Biology | volume = 27 | pages = 409–40 | year = 2011 | pmid = 21801016 | doi = 10.1146/annurev-cellbio-092910-154115 }}</ref> and also planarian worms such as ''[[Schmidtea mediterranea]]''.<ref name="Reddien P.W., Alvarado A.S. 2004 725–757"/> [[Organoid]]s have also been demonstrated as an efficient model for development.<ref>{{cite journal | vauthors = Ader M, Tanaka EM | title = Modeling human development in 3D culture | journal = Current Opinion in Cell Biology | volume = 31 | pages = 23–8 | date = December 2014 | pmid = 25033469 | doi = 10.1016/j.ceb.2014.06.013 }}</ref> Plant development has focused on the thale cress ''[[Arabidopsis thaliana]]'' as a model organism.<ref>{{cite book | vauthors = Weigel D, Glazebrook J | date = 2002 | title = Arabidopsis. A Laboratory Manual. | publisher = Cold Spring Harbor Laboratory Press | location = Cold Spring Harbor, NY }}</ref>

== See also ==
{{columns-list|colwidth=18em|
* [[Blastocyst]]
* [[Body plan]]
* [[Cell signaling]]
* [[Cell signaling networks]]
* [[Embryology]]
* [[Enhancer (genetics)]]
* [[Fish development]]
* [[Gene regulatory network]]
* [[Homology (biology)]]
* [[Ontogeny]]
* [[Plant evolutionary developmental biology]]
* [[Promoter (biology)]]
* [[Signal transduction]]
* [[Synthetic biology]]
* [[Teratology]]
}}

== References ==
{{reflist|30em}}

== Further reading ==
{{refbegin}}
* {{cite book |vauthors=Gilbert SF, Barresi M |author-link1=Scott F. Gilbert |title=Developmental Biology |edition=13th |location=NY |language=en |publisher=[[Oxford University Press]] |year=2023 |isbn=9780197574591}}
* {{cite book |vauthors=Slack JM |date=2013 |title=Essential Developmental Biology |location=Oxford |publisher=Wiley-Blackwell}}
* {{cite book |vauthors=Wolpert L, Tickle C |date=2011 |title=Principles of Development |location=Oxford and New York |publisher=Oxford University Press}}
{{refend}}

== External links ==
{{Wikibooks}}
{{Commons category}}
* [http://www.sdbonline.org/ Society for Developmental Biology]
* [http://www.sdbcore.org/ Collaborative resources]
* [http://10e.devbio.com/ Developmental Biology - 10th edition]
* [http://bcs.wiley.com/he-bcs/Books?action=index&bcsId=7612&itemId=0470923512 Essential Developmental Biology 3rd edition]
* [https://embryo.asu.edu/ Embryo Project Encyclopedia]

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{{Developmental biology}}
{{Embryology}}
{{Human development}}
{{Branches of biology}}
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[[Category:Developmental biology| ]]
[[Category:Philosophy of biology]]