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Developmental biology

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"Views of a Fetus in the Womb", Leonardo da Vinci, ca. 1510-1512. The subject of prenatal development is a major subset of developmental biology.

Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis," which is the process that gives rise to tissues, organs and anatomy. Developmental biology is that branch of life science, which deals with the study of the process by which organisms grow and develop.

Embryology is a subfield, the study of organisms between the one-cell stage (generally, the zygote) and the end of the embryonic stage. Embryology was originally a more descriptive science until the 20th century. Embryology and developmental biology today deal with the various steps necessary for the correct and complete formation of the body of a living organism.

The related field of evolutionary developmental biology was formed largely in the 1990s and is a synthesis of findings from molecular developmental biology and evolutionary biology which considers the diversity of organismal form in an evolutionary context.

Perspectives

Developmental genetics is a very helpful process. It studies the effect that genes have in a phenotype. The findings of developmental biology can help to understand developmental malfunctions such as chromosomal aberrations, for example, Down syndrome. An understanding of the specialization of cells during embryogenesis may yield information on how to specialize stem cells to specific tissues and organs, which could lead to the specific cloning of organs for medical purposes. Another biologically important process that occurs during development is apoptosis - programmed cell death or "suicide". For this reason, many developmental models are used to elucidate the physiology and molecular basis of this cellular process. Similarly, a deeper understanding of developmental biology can foster greater progress in the treatment of congenital disorders and diseases, e.g. studying human sex determination can lead to treatment for disorders such as congenital adrenal hyperplasia.

Developmental model organisms

Often used model organisms in developmental biology include the following:

Studied phenomena

Cell differentiation

Differentiation is the formation of cell types, from what is originally one cell – the zygote or spore. The formation of cell types like nerve cells occurs with a number of of intermediary, less differentiated cell types. A cell stays a certain cell type by maintaining a particular pattern of gene expression.[4] This depends on regulatory genes, e.g. for transcription factors and signaling proteins. These can take part in self-perpetuating circuits in the gene regulatory network, circuits that can involve several cells that communicate with each other.[5] External signals can alter gene expression by activating a receptor, which triggers a signaling cascade that affects transcription factors. For example, the withdrawal of growth factors from myoblasts causes them to stop dividing and instead differentiate into muscle cells.[6]

Embryonal development

Embryogenesis is the step in the life cycle after fertilisation – the development of the embryo, starting from the zygote (fertilised egg). Organisms can differ drastically in the how embryo develops, especially when belong to different phyla. For example, embryonal development in placental mammals starts with cleavage of the zygote into eight uncommited cells, which then form a ball (morula). The outer cells become the trophectoderm which will form the fetal part of the placenta, while inner cells become the inner cell mass that will form all other organs. In contrast, the fruit fly zygote first forms a sausage-shaped syncytium, which is still one cell but with many cell nuclei.[7]

Patterning is important for determining which cells develop which organs. This is mediated by signaling between adjacent cells by proteins on their surfaces, and by gradients of signaling molecules.[8] An example is retinoic acid, which forms a gradient in the head to tail direction in animals. Retinoic acid enters cells and activates Hox genes in a concentration-dependent manner – Hox genes differ in how much retinoic acid they require for activation. As Hox genes code for transcription factors, this causes discrete segments in the head to tail direction.[9] This is important for e.g. the segmentation of the spine in vertebrates.[8]

Embryonal development does not always go right, and errors can result in birth defects or miscarriage. Often the reason is genetic (mutation or chromosome abnormality), but there can be environmental influence (teratogens).[10] Abnormal development is also of evolutionary interest as it provides a mechanism for changes in body plan (see evolutionary developmental biology).[11]

Growth

Growth is the enlargement of a tissue or organism. Growth continues after the embryonal stage, and occurs through cell proliferation, enlargement of cells or accumulation of extracellular material. In plants, growth results in an adult organism that is strikingly different from the embryo. The proliferating cells tend to be distinct from differentiated cells (see stem cell and progenitor cell). In some tissues proliferating cells are restricted to specialised areas, such as the growth plates of bones.[12] But some stem cells migrate to where they are needed, such as mesenchymal stem cells which can migate from the bone marrow to form e.g. muscle, bone or adipose tissue.[13] The size of an organ frequently determines its growth, as in the case of the liver which grows back to its previous size if a part is removed. Growth factors, such as fibroblast growth factors in the animal embryo and growth hormone in juvenile mammals, also control the extent of growth.[12]

Metamorphosis

Most animals have a larval stage, with a body plan different from that of the adult organism. The larva abrubtly develops into an adult in a process called metamorphosis. For example, butterfly larvae (caterpillars) are specilised for feeding whereas adult butterflies (imagos) are specilised for flight and reproduction. When the caterpillar has grown enough, it turns into an immobile pupa. Here, the imago develops from imaginal discs found inside the larva.[14]

Regeneration

Regeneration is the reactivation of development so that a missing body part grows back. This phenomenon has been studied particularily in salamanders, where the adults can reconstruct a whole limb after it has been amputated.[15] Researchers hope to one day be able to induce regeneration in humans (see regenerative medicine).[16] There is little spontaneous regeneration in adult humans, although the liver is a notable exception. Like for salamanders, the regeneration of the liver involves dedifferentiation of some cells to a more embryonal state.[15]

Developmental systems biology

Computer simulation of multicellular development is a research methodology to understand the function of the very complex processes involved in the development of organisms. This includes simulation of cell signaling, multicell interactions and regulatory genomic networks in development of multicellular structures and processes (see French flag model or Biological Physics of the Developing Embryo for literature). Minimal genomes for minimal multicellular organisms may pave the way to understand such complex processes in vivo.

See also

References

  1. ^ Haffter P, Nüsslein-Volhard C (1996). "Large scale genetics in a small vertebrate, the zebrafish". Int. J. Dev. Biol. 40: 221–7. PMID 8735932.
  2. ^ Amaya E (2005). "Xenomics". Genome Res. 15 (12): 1683–91. PMID 16339366.
  3. ^ Keller G (2005). "Embryonic stem cell differentiation: emergence of a new era in biology and medicine". Genes Dev. 19 (10): 1129–55. PMID 15905405.
  4. ^ Wolpert L, Beddington R, Jessell T, Lawrence P, Meyerowitz E, Smith J (2002). Principles of development (2nd ed.). Oxford university press. pp. 293–295. ISBN 0-19-879291-3.{{cite book}}: CS1 maint: multiple names: authors list (link)
  5. ^ Ben-Tabou de-Leon S, Davidson EH (2007). "Gene regulation: gene control network in development". Annu Rev Biophys Biomol Struct. 36: 191. doi:10.1146/annurev.biophys.35.040405.102002. PMID 17291181.
  6. ^ Wolpert L, Beddington R, Jessell T, Lawrence P, Meyerowitz E, Smith J (2002). Principles of development (2nd ed.). Oxford university press. pp. 304–307. ISBN 0-19-879291-3.{{cite book}}: CS1 maint: multiple names: authors list (link)
  7. ^ Wolpert L, Beddington R, Jessell T, Lawrence P, Meyerowitz E, Smith J (2002). Principles of development (2nd ed.). Oxford university press. pp. 41–50, 493. ISBN 0-19-879291-3.{{cite book}}: CS1 maint: multiple names: authors list (link)
  8. ^ a b Christ B, Schmidt C, Huang R, Wilting J, Brand-Saberi B (1998). "Segmentation of the vertebrate body". Anat. Embryol. 197 (1): 1–8. PMID 9462855. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  9. ^ Marshall H, Morrison A, Studer M, Pöpperl H, Krumlauf R (1996). "Retinoids and Hox genes". FASEB J. 10 (9): 969–78. PMID 8801179. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  10. ^ Holtzman NA, Khoury MJ (1986). "Monitoring for congenital malformations". Annu Rev Public Health. 7: 237–66. doi:10.1146/annurev.pu.07.050186.001321. PMID 3521645.
  11. ^ Fujimoto K, Ishihara S, Kaneko K (2008). "Network evolution of body plans". PLoS ONE. 3 (7): e2772. doi:10.1371/journal.pone.0002772. PMC 2464711. PMID 18648662.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)
  12. ^ a b Wolpert L, Beddington R, Jessell T, Lawrence P, Meyerowitz E, Smith J (2002). Principles of development (2nd ed.). Oxford university press. pp. 467–482. ISBN 0-19-879291-3.{{cite book}}: CS1 maint: multiple names: authors list (link)
  13. ^ Chamberlain G, Fox J, Ashton B, Middleton J (2007). "Concise review: mesenchymal stem cells: their phenotype, differentiation capacity, immunological features, and potential for homing". Stem Cells. 25 (11): 2739–49. doi:10.1634/stemcells.2007-0197. PMID 17656645. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  14. ^ Gilbert SF (2003). Developmental biology (7th ed.). Sinauer. pp. 575–585. ISBN 0-87893-258-5.
  15. ^ a b Gilbert SF (2003). Developmental biology (7th ed.). Sinauer. pp. 592–601. ISBN 0-87893-258-5.
  16. ^ Stocum DL (2002). "Development. A tail of transdifferentiation". Science. 298 (5600): 1901–3. doi:10.1126/science.1079853. PMID 12471238. {{cite journal}}: Unknown parameter |month= ignored (help)
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