Developmental biology

Developmental biology is the study of the process by which animals and plants grow and develop, and is synonymous with in vitro fertilization, now widely used in fertility treatment, the understanding of risks from substances that can damage the fetus (teratogens), and the creation of various animal models for human disease which are useful in research. Developmental Biology has also help to generate modern stem cell biology which promises a number of important practical benefits for human health.

Perspectives

The main processes involved in the embryonic development of animals are: 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 the cell function. Growth involves both an overall increase in size, and also the differential growth of parts (allometry) which contributes to morphogenesis. Growth mostly occurs through cell division but also through changes of cell size and the deposition of extracellular materials. The control of timing of events and the integration of the various processes with one another is the least well understood area of the subject. It remains unclear whether animal embryos contain a master clock mechanism or not. 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 in plant development are different from those that control animal development.

Developmental model organisms

Much of developmental biology research in recent decades has focused on the use of a small number of

Vertebrates:

Frog: Xenopus (X.laevis and tropicalis).^[1][2] Good embryo supply. Especially suitable for microsurgery. Zebrafish: Danio rerio.^[3] Good embryo supply. Well developed genetics. Chicken: Gallus gallus.^[4] Early stages similar to mammal, but microsurgery easier. Low cost. Mouse: Mus musculus.^[5] A mammal with well developed genetics.

Invertebrates:

Fruit fly: Drosophila melanogaster.^[6] Good embryo supply. Well developed genetics. Nematode: Caenorhabditis elegans.^[7] Good embryo supply. Well developed genetics. Low cost.

Also popular for some purposes have been

Human biological and psychological development Pre- and perinatal Embryo Fetus Postnatal development Infant Toddler Preschool / Early childhood Elementary school Preadolescence Adolescence Middle age Old age Legal and general definitions Minor Infancy Child Childhood Adolescence Age of majority Adult Developmental events and phases Gestational age Prenatal development Birth Child development (stages) Cognitive development of infants Human development Puberty Ageing Senescence Death Development and psychology Pre- and perinatal psychology Infant and child psychology Adolescent psychology Youth development Young adult psychology Adult development Developmental psychology Psychological maturity Theorists and theories Bowlby (Attachment theory) Bronfenbrenner (Ecological systems theory) Erikson (Psychosocial development) Freud (Psychosexual development) Kohlberg (Kohlberg's stages of moral development|Moral development) Piaget (Cognitive development) Vygotsky (Cultural-historical psychology) Evolutionary developmental psychology

Developmental biology Ageing Compartment Embryogenesis Human Drosophila Fish polarity Embryology Metamerism Metamorphosis Model organisms Ontogeny Puberty Regeneration Human Teratology Evolutionary developmental biology

• Society for Developmental Biology
• Collaborative resources
• Developmental Biology - 8th edition
• Essential Developmental Biology 3rd edition

External links

• Gilbert, S. F. (2013). Developmental Biology. Sunderland, Mass.: Sinauer Associates Inc.
• Slack, J. M. W. (2013). Essential Developmental Biology. Oxford: Wiley-Blackwell.
• Wolpert, L. and Tickle, C. (2011). Principles of Development. Oxford and New York: Oxford University Press.

Further reading

References

See also

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,^[34][35] and the biology of the imaginal discs, which generate the adult body parts of the fly Drosophila melanogaster.^[36][37]

Developmental processes are very evident during the process of metamorphosis. This occurs in various types of animal. Well known are the examples of the frog, which usually hatches 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.

Metamorphosis

Regeneration indicates the ability to regrow a missing part.^[27] 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,^[28] and planarian worms, which can usually regenerate both heads and tails.^[29] 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 pluripotent.^[30] 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,^[31] and the limbs of urodele amphibians.^[32] 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.^[33] If the former then 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.

Regeneration

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,^[25] based on the properties of the Notch-Delta signaling system.^[26] 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.

for hepatocyte differentiation. HNF4 for muscle differentiation, and myogenin is a key transcription factor for neuronal differentiation, NeuroD For example, ^[24][23] 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.chromatin 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 cell. 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 Cell differentiation

Cell differentiation

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.^[22]

Growth in embryos is mostly autonomous.^[21] 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.

Regional specification 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.^[16][17] 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.^[18] 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.^[19][20] 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.

The sperm and egg fuse in the process of fertilization to form a fertilized egg, or zygote.^[13] 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 divisions. Morphogenetic movements convert the cell mass into a three layered structure consisting of multicellular sheets called ectoderm, mesoderm and endoderm, which 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,^[14] and also establish differences of commitment along the anteroposterior axis (head, trunk and tail).^[15]

Embryonic development of animals

Developmental Processes

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