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Developmental biology is the study of the process by which
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s and
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s grow and develop. Developmental biology also encompasses the biology of
regeneration Regeneration may refer to: Science and technology * Regeneration (biology), the ability to recreate lost or damaged cells, tissues, organs and limbs * Regeneration in humans, the ability of humans to recreate, or induce the regeneration of, lost ...
,
asexual reproduction Asexual reproduction is a type of that does not involve the fusion of s or change in the number of . The offspring that arise by asexual reproduction from either unicellular or s inherit the full set of genes of their single parent. Asexual rep ...
,
metamorphosis Metamorphosis is a biological process Biological processes are those processes that are vital for an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are ...
, and the growth and differentiation of
stem cell In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties o ...
s in the adult organism.


Perspectives

The main processes involved in the
embryonic development An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to a unicellular organism. All species of animals, Embryophyte, la ...

embryonic development
of animals are: tissue patterning (via
regional specification In the field of developmental biology Developmental biology is the study of the process by which animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...
and patterned
cell differentiation Cellular differentiation is the process in which a cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...
);
tissue growth Tissue growth is the process by which ''a tissue increases its size''. In animals, tissue growth occurs during embryonic development ''Embryonic'' is the twelfth studio album by experimental rock Experimental rock, also called avant-rock, is a ...
; and tissue
morphogenesis Morphogenesis (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is a ...
. *
Regional specification In the field of developmental biology Developmental biology is the study of the process by which animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...
refers to the processes that create spatial pattern in a ball or sheet of initially similar cells. This generally involves the action of cytoplasmic determinants, located within parts of the fertilized egg, and of inductive signals emitted from signaling centers in the embryo. The early stages of
regional specification In the field of developmental biology Developmental biology is the study of the process by which animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...
do not generate functional differentiated cells, but cell populations committed to develop to a specific region or part of the organism. These are defined by the expression of specific combinations of
transcription factors In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...

transcription factors
. *
Cell differentiation Cellular differentiation is the process in which a cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...
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 the cell function. *
Morphogenesis Morphogenesis (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is a ...
relates to the formation of 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 The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrate Vertebrates () comprise all species of animal Animals (also ...

ectoderm
,
mesoderm The mesoderm is the middle layer of the three germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoa ...

mesoderm
and
endoderm Endoderm is the innermost of the three primary germ layer A germ layer is a primary layer of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small r ...
) and for building up complex structures during organ development. *
Tissue growth Tissue growth is the process by which ''a tissue increases its size''. In animals, tissue growth occurs during embryonic development ''Embryonic'' is the twelfth studio album by experimental rock Experimental rock, also called avant-rock, is a ...
involves both an overall increase in tissue size, and also the differential growth of parts (
allometry Allometry is the study of the relationship of body size to shape, anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient ...
) which contributes to morphogenesis. Growth mostly occurs through
cell proliferation Cell proliferation is the process by which ''a cell grows and divides to produce two daughter cells''. Cell proliferation leads to an exponential growth, exponential increase in cell number and is therefore a rapid mechanism of tissue growth. C ...

cell proliferation
but also through changes of 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.


Developmental processes


Cell differentiation

Cell differentiation Cellular differentiation is the process in which a cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...
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 Chromatin is a complex of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecu ...
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. For example,
NeuroDNeuroD, also called Beta2, is a basic helix loop helix transcription factor expressed in certain parts of brain, beta pancreatic cells and enteroendocrine cells. It is involved in the differentiation of nervous system and development of pancreas. It ...
is a key transcription factor for neuronal differentiation,
myogenin Myogenin (myogenic factor 4), also known as MYOG, is a gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) ...
for muscle differentiation, and
HNF4 HNF4 (Hepatocyte Nuclear Factor 4) is a nuclear receptor of a heterodimer of the nuclear receptors PPAR-γ (green) and RXR-α (cyan) bound to double stranded DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) ...
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, based on the properties of the
Notch signaling pathway The Notch signaling pathway is a highly conserved cell signaling In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular ...

Notch signaling pathway
. 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 Regeneration may refer to: Science and technology * Regeneration (biology), the ability to recreate lost or damaged cells, tissues, organs and limbs * Regeneration in humans, the ability of humans to recreate, or induce the regeneration of, lost ...
indicates the ability to regrow a missing part. 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'', which can regenerate any part of the polyp from a small fragment, and
planarian A planarian is one of many flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), of ...
worms, which can usually regenerate both heads and tails. Both of these examples have continuous cell turnover fed by
stem cells In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties o ...
and, at least in planaria, at least some of the stem cells have been shown to be
pluripotent Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potential within a cell, which like a continuum, ...
. 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, and the limbs of urodele amphibians. 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. 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

The sperm and egg fuse in the process of fertilization to form a fertilized egg, or
zygote A zygote (, ) is a eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

zygote
. This undergoes a period of divisions to form a ball or sheet of similar cells called a
blastula Blastulation is the stage in early animal embryonic development An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, li ...

blastula
or
blastodermA blastoderm ( germinal disc, blastodisc) is a single layer of embryonic epithelial tissue that makes up the blastula. It encloses the fluid filled blastocoel. Gastrulation follows blastoderm formation, where the tips of the blastoderm begins the for ...
. 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 Cleavage may refer to: Science * Cleavage (crystal), in mineralogy and materials science, a process of splitting a crystal * Cleavage (geology), the foliation perpendicular to stress as a result of ductile deformation * Cleavage (embryo), in embr ...
divisions. Mouse
epiblast In amniote animal embryology, the epiblast (also known as the primitive ectoderm) is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst or from the blastodisc in reptiles and birds. It derives the embryo proper ...
primordial
germ cell A germ cell is any biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to a ...
s (see Figure: “The initial stages of human
embryogenesis An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...
”) undergo extensive
epigenetic In biology, epigenetics is the study of heritability, heritable phenotype changes that do not involve alterations in the DNA sequence. The Ancient Greek, Greek prefix ''wikt:epi-, epi-'' ( "over, outside of, around") in ''epigenetics'' implies f ...
reprogramming. This process involves
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
-wide
DNA demethylation For molecular biology in mammals, DNA demethylation causes replacement of 5-methylcytosine (5mC) in a DNA sequence by cytosine (C) (see figure of 5mC and C). DNA demethylation can occur by an active process at the site of a 5mC in a DNA sequence ...
,
chromatin Chromatin is a complex of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecu ...
reorganization and
epigenetic In biology, epigenetics is the study of heritability, heritable phenotype changes that do not involve alterations in the DNA sequence. The Ancient Greek, Greek prefix ''wikt:epi-, epi-'' ( "over, outside of, around") in ''epigenetics'' implies f ...
imprint erasure leading to
totipotency Cell potency is a Cell (biology), cell's ability to Cellular differentiation, differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potentia ...
. DNA demethylation is carried out by a process that utilizes the DNA
base excision repair Base excision repair (BER) is a cellular mechanism, studied in the fields of biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided ...
pathway. Morphogenetic movements convert the cell mass into a three layered structure consisting of multicellular sheets called
ectoderm The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrate Vertebrates () comprise all species of animal Animals (also ...

ectoderm
,
mesoderm The mesoderm is the middle layer of the three germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoa ...

mesoderm
and
endoderm Endoderm is the innermost of the three primary germ layer A germ layer is a primary layer of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small r ...
. These sheets are known as
germ layers A germ layer is a primary layer of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse live ...

germ layers
. This is the process of
gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of Cell (biology), cells) is reorganized into a multilayered structure known as the gastrula. Before gastrulat ...

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 The placenta is a temporary fetal organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. ...

placenta
, needed for support and nutrition of the embryo, and also establish differences of commitment along the anteroposterior axis (head, trunk and tail).
Regional specification In the field of developmental biology Developmental biology is the study of the process by which animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...
is initiated by the presence of cytoplasmic determinants 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. 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. These genes encode
transcription factors In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...

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. 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. Growth in embryos is mostly autonomous. 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.


Metamorphosis

Developmental processes are very evident during the process of
metamorphosis Metamorphosis is a biological process Biological processes are those processes that are vital for an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are ...
. This occurs in various types of animal. 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'', and the biology of the imaginal discs, which generate the adult body parts of the fly ''Drosophila melanogaster''.


Plant development

Plant development is the process by which structures originate and mature as a plant grows. It is studied in
plant anatomy Plant anatomy or phytotomy is the general term for the study of the internal structure A structure is an arrangement and organization of interrelated elements in a material object or system A system is a group of Interaction, interacting or ...
and
plant physiology Plant physiology is a subdiscipline of botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. ...

plant physiology
as well as plant morphology. Plants constantly produce new tissues and structures throughout their life from
meristem The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa du ...
s located at the tips of organs, or between mature tissues. Thus, a living plant always has embryonic tissues. By contrast, an animal
embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

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
emergent properties In philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about reason, Metaphysics, existence, Epistemology, knowledge, Ethics, values, Philosophy of mind, mind, and Philosophy of language, ...

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."


Growth

A
vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''phutá'' 'plants'), form a large group of plants ( 300,000 ...
begins from a single celled
zygote A zygote (, ) is a eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

zygote
, formed by
fertilisation Fertilisation or fertilization (see American and British English spelling differences#-ise.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

fertilisation
of an egg cell by a sperm cell. From that point, it begins to divide to form a plant
embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryo
through the process of
embryogenesis An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

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 A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was released to generally positi ...

seed
plants, the embryo will develop one or more "seed leaves" (
cotyledon A cotyledon (; ; ; , gen. (), ) is a significant part of the embryo within the seed of a plant, and is defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germination, germinating see ...
s). By the end of embryogenesis, the young plant will have all the parts necessary to begin its life. Once the embryo from its seed or parent plant, it begins to produce additional organs (leaves, stems, and roots) through the process of
organogenesis Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In ma ...
. New roots grow from root
meristem The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa du ...
s located at the tip of the root, and new stems and leaves grow from shoot
meristem The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa du ...
s located at the tip of the shoot. Branching occurs when small clumps of cells left behind by the meristem, and which have not yet undergone
cellular differentiation Cellular differentiation is the process in which a cell changes from one cell type A cell type is a classification used to distinguish between morphologically or phenotypically distinct cell forms within a species In biology, a sp ...
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 In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Anc ...
results in widening of a root or shoot from divisions of cells in a
cambium A cambium (plural cambia or cambiums), in plants, is a tissue layer that provides partially undifferentiated cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic ...
. In addition to growth by cell division, a plant may grow through cell elongation. 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 File:Phototropism Diagram.svg, thumbnail, Auxin distribution controls phototropism. 1. Sunlight strikes the plant from directly above. Auxin (pink dots) encourages growth straight up. 2, 3, 4. Sunlight strikes the plant at an angle. Auxin is conc ...

phototropism
), gravity (
gravitropism Gravitropism (also known as geotropism) is a coordinated process of differential growth by a plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all ...
), water, ( hydrotropism), and physical contact (
thigmotropism Thigmotropism is a directional growth movement which occurs as a mechanosensory response to a touch stimulus. Thigmotropism is typically found in twining plants and tendrils, however plant biologists have also found thigmotropic responses in flowe ...
). Plant growth and development are mediated by specific
plant hormone Plant hormones (or phytohormones) are signal molecule In biology, cell signaling (cell signalling in British English), or cell-cell communication, governs the basic activities of cell (biology), cells and coordinates multiple-cell actions. A si ...
s and plant growth regulators (PGRs) (Ross et al. 1983). 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. 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.


Developmental model organisms

Much of developmental biology research in recent decades has focused on the use of a small number of
model organisms A model organism (often shortened to model) is a non-human species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is o ...
. 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 ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from ...

Arabidopsis thaliana
'')


Vertebrates

* Frog: ''
Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In bi ...

Xenopus
'' ('''' and '' X. tropicalis''). Good embryo supply. Especially suitable for microsurgery. *
Zebrafish The zebrafish (''Danio rerio'') is a freshwater fish are common freshwater fish throughout temperate Eurasia. Freshwater fish are those that spend some or all of their lives in fresh water, such as river A river is a natural flowing wate ...

Zebrafish
: ''Danio rerio''. Good embryo supply. Well developed genetics. * Chicken: ''Gallus gallus''. Early stages similar to mammal, but microsurgery easier. Low cost. * Mouse: ''Mus musculus''. A mammal with well developed genetics.


Invertebrates

* Fruit fly: ''''. Good embryo supply. Well developed genetics. * Nematode: ''
Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes a ...

Caenorhabditis elegans
''. Good embryo supply. Well developed genetics. Low cost.


Unicellular

* Algae: ''
Chlamydomonas alt=, 329x329px, Cross section of a ''Chlamydomonas reinhardtii'' cell ''Chlamydomonas'' is a genus of green algae consisting of about 325 speciesSmith, G.M. 1955 ''Cryptogamic Botany Volume 1. Algae and Fungi'' McGraw-Hill Book Company Inc a ...

Chlamydomonas
'' * Yeast: ''
Saccharomyces ''Saccharomyces'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), ci ...
''


Others

Also popular for some purposes have been
sea urchin Sea urchins () are spine (zoology), spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone — from the intertidal seashore down to . The spherical, ha ...

sea urchin
s and
ascidians Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class (biology), class in the subphylum Tunicate, Tunicata of sac-like marine (ocean), marine invertebrate filter feeders. Ascidians are ...
. For studies of regeneration urodele amphibians such as the
axolotl The axolotl (; from nci, āxōlōtl ), ''Ambystoma mexicanum'', also known as the Mexican walking fish, is a neotenic Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing o ...

axolotl
''Ambystoma mexicanum'' are used, and also planarian worms such as '' Schmidtea mediterranea''. Organoids have also been demonstrated as an efficient model for development. Plant development has focused on the thale cress ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from ...

Arabidopsis thaliana
'' as a model organism.


See also


References


Further reading

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External links


Society for Developmental Biology

Collaborative resources

Developmental Biology - 10th edition

Essential Developmental Biology 3rd edition
{{Authority control Philosophy of biology