samedi 30 avril 2016

Annexe 39 : biologie du développement (anglais) (vu)



Developmental biology

From Wikipedia, the free encyclopedia
"Developmental genetics" redirects here. For the journal formerly known as Developmental Genetics, see Genesis (journal).
For the journal, see Developmental Biology (journal).

Views of a Fetus in the Womb, Leonardo da Vinci, c. 1510 - 1512. The subject of prenatal development is a major subset of developmental biology.
Developmental biology is the study of the process by which animals and plants grow and develop, and is synonymous with ontogeny. In animals most development occurs in embryonic life, but it is also found in regeneration, asexual reproduction and metamorphosis, and in the growth and differentiation of stem cells in the adult organism. In plants, development occurs in embryos, during vegetative reproduction, and in the normal outgrowth of roots, shoots and flowers. Practical outcomes from the study of animal developmental biology have included 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 helped to generate modern stem cell biology which promises a number of important practical benefits for human health.

Contents

Perspectives

The main processes involved in the embryonic development of animals are: regional specification, morphogenesis, cell differentiation, growth, and the overall control of timing. Regional specification 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 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. Morphogenesis 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, mesoderm and endoderm) and for building up complex structures during organ development. 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 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.
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:

Also popular for some purposes have been sea urchins[8] and ascidians.[9] For studies of regeneration urodele amphibians such as the axolotl Ambystoma mexicanum are used,[10] and also planarian worms such as Schmidtea mediterranea.[11] Plant development has focused on the thale cress Arabidopsis thaliana as a model organism.[12]

Developmental processes

Embryonic development of animals


Generalized scheme of embryonic development. Slack "Essential Developmental Biology" Fig.2.8

The initial stages of human embryogenesis.
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]
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.
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.
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]

Cell differentiation


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 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 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.[23][24] 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,[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.

Regeneration

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 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.

Metamorphosis

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.
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]

See also

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vendredi 29 avril 2016

Annexe 38 : Multicellular organism (anglais) (vu)



Multicellular organism



Tetrabaena socialis
Caenorhabditis elegans
Dictyostelium.
cell division
Colonial organisms

Evolutionary history

Occurrence

Multicellularity has evolved independently at least 46 times,[5][6] including in some prokaryotes, like

cyanobacteria,
myxobacteria,
actinomycetes,
Magnetoglobus multicellularis
Methanosarcina.


Complex multicellular organisms evolved only in six eukaryotic groups: -animals,
-fungi,
-brown algae,
-red algae,
-green algae, and
-land plants.[7]

It evolved
- repeatedly for 
Chloroplastida (green algae and land plants),
- once or twice for animals,
- once for brown algae,
- three times in the fungi (chytrids, ascomycetes and basidiomycetes)[8]
- perhaps several times for slime molds and red algae.[9]

The first evidence of multicellularity is from
- cyanobacteria-like organisms that lived 3-3.5 billion years ago.[5]

germ cells (i.e. sperm and egg cells), an issue that is studied
in developmental biology.
Animals have evolved a greater diversity of cell types in a multicellular body (100–150 different cell types),
compared with 10–20 in plants, fungi, and protoctists.[10]

Loss of multicellularity

Loss of multicellularity occurred in some groups.[11]
Fungi are predominantly multicellular, though early diverging lineages are largely unicellular (e.g., Microsporidia)
and there have been numerous reversions to unicellularity across fungi
(e.g., Saccharomycotina, Cryptococcus, and other yeasts).[12][13]

It may also have occurred in some
red algae (e.g., Porphyridium), but it is possible that they are primitivelly unicellular).[14]
Loss of multicellularity is also considered probable in some
green algae (e.g., Chlorella vulgaris and some Ulvophyceae).[15][16]

In other groups, generally parasites, a reduction of multicellularity occurred, in number or types of cells
(e.g., the myxozoans, multicellular organisms, early thought to be unicellular, are probably extremely reduced cnidarians).[17]

Cancer

Multicellular organisms, especially long-living animals, face the challenge of
cancer, which occurs when cells fail to regulate their growth within the normal program of development.
Changes in tissue morphology can be observed during this process. Cancer in animals (metazoans) ( id est animals) has often been described as a loss of multicellularity.[18] There is a discussion about the possibility of existence of cancer in other multicellular organisms[19][20] or even in protozoa.[21]
For example, plant galls have been characterized as tumors,[22] however, other authors argue that plants do not develop cancer.[23]

Separation of somatic and germ cells

In some multicellular groups, which are called
Weismannists, a separation between a sterile somatic cell line and a germ cell line evolved. However, Weismannist development is relatively rare (e.g., vertebrates, arthropods, Volvox), as great part of species have the capacity for somatic embryogenesis (e.g., land plants, most algae, many invertebrates).[24][25]

Hypotheses for origin

There are various mechanisms by which multicellularity could have evolved.
One hypothesis is that a group of function-specific cells aggregated into a slug-like mass called a grex, which moved as a multicellular unit. This is essentially what slime molds do. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a syncytium. A membrane would then form around each nucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected cells in one organism (this mechanism is observable in Drosophila). A third hypothesis is that as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues. This is what plant and animal embryos do as well as colonial choanoflagellates.[26][27]
Because the first multicellular organisms were simple, soft organisms lacking bone, shell or other hard body parts, they are not well preserved in the fossil record.[28] One exception may be the demosponge, which may have left a chemical signature in ancient rocks. The earliest fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon (Gabonionta).[29] The Doushantuo Formation has yielded 600 million year old microfossils with evidence of multicellular traits.[30]
Until recently phylogenetic reconstruction has been through anatomical (particularly embryological) similarities. This is inexact, as living multicellular organisms such as animals and plants are more than 500 million years removed from their single-cell ancestors. Such a passage of time allows both divergent and convergent evolution time to mimic similarities and accumulate differences between groups of modern and extinct ancestral species. Modern phylogenetics uses sophisticated techniques such as alloenzymes, satellite DNA and other molecular markers to describe traits that are shared between distantly related lineages.
The evolution of multicellularity could have occurred in three ways, of which the latter, the colonial theory, is most credited by the scientific community:

The symbiotic theory

This theory suggests that the first multicellular organisms occurred from symbiosis (cooperation) of different species of single-cell organisms, each with different roles. Over time these organisms would become so dependent on each other they would not be able to survive independently, eventually leading to the incorporation of their genomes into one multicellular organism.[31] Each respective organism would become a separate lineage of differentiated cells within the newly created species.
This kind of severely co-dependent symbiosis can be seen frequently, such as in the relationship between clown fish and Riterri sea anemones. In these cases, it is extremely doubtful whether either species would survive very long if the other became extinct. However, the problem with this theory is that it is still not known how each organism's DNA could be incorporated into one single genome to constitute them as a single species. Although such symbiosis is theorized to have occurred (e.g., mitochondria and chloroplasts in animal and plant cells – endosymbiosis), it has happened only extremely rarely and, even then, the genomes of the endosymbionts have retained an element of distinction, separately replicating their DNA during mitosis of the host species. For instance, the two or three symbiotic organisms forming the composite lichen, while dependent on each other for survival, have to separately reproduce and then re-form to create one individual organism once more.

The cellularization (syncytial) theory

This theory states that a single unicellular organism, with multiple nuclei, could have developed internal membrane partitions around each of its nuclei.[32] Many protists such as the ciliates or slime molds can have several nuclei, lending support to this hypothesis. However, the simple presence of multiple nuclei is not enough to support the theory. Multiple nuclei of ciliates are dissimilar and have clear differentiated functions. The macronucleus serves the organism's needs, while the micronucleus is used for sexual reproduction with exchange of genetic material. Slime molds syncitia form from individual amoeboid cells, like syncitial tissues of some multicellular organisms, not the other way round. To be deemed valid, this theory needs a demonstrable example and mechanism of generation of a multicellular organism from a pre-existing syncytium.

The colonial theory

The third explanation of multicellularisation is the Colonial Theory proposed by Haeckel in 1874. This theory claims that the symbiosis of many organisms of the same species (unlike the symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism. At least some, it is presumed land-evolved, multicellularity occurs by cells separating and then rejoining (e.g., cellular slime molds) whereas for the majority of multicellular types (those that evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division.[33] The mechanism of this latter colony formation can be as simple as incomplete cytokinesis, though multicellularity is also typically considered to involve cellular differentiation.[34]
ColonialFlagellateHypothesis.png
The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently in 16 different protoctistan phyla. For instance, during food shortages the amoeba Dictyostelium groups together in a colony that moves as one to a new location. Some of these amoeba then slightly differentiate from each other. Other examples of colonial organisation in protista are Volvocaceae, such as Eudorina and Volvox, the latter of which consists of up to 500–50,000 cells (depending on the species), only a fraction of which reproduce.[35] For example, in one species 25–35 cells reproduce, 8 asexually and around 15–25 sexually. However, it can often be hard to separate colonial protists from true multicellular organisms, as the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular".[3]

The Synzoospore theory

Some authors suggest that the origin of multicellularity, at least in Metazoa, occurred due to a transition from temporal to spatial cell differentiation, rather than through a gradual evolution of cell differentiation, as affirmed in Haeckel’s Gastraea theory.[36]

GK-PID

In January 2016, scientists reported that, about 800 million years ago,[37] a minor genetic change in a single molecule called GK-PID may have allowed organisms to go from a single cell organism to one of many cells.[38]

The role of Viruses

Genes borrowed from Viruses have recently been identified as playing a crucial role in the differentiation of multicellular tissues and organs and even in sexual reproduction, in the fusion of egg cell and sperm. Such fused cells are also involved in metazoan membranes such as those which prevent chemicals crossing the placenta and the brain body separation. Two viral components have been identified. The first is syncytin, which came from a virus. The second identified in 2007 is called EFF1, which helps form the skin of Caenorhabditis elegans, part of a whole family of FF proteins. Felix Rey, of the Pasteur Institute in Paris has constructed the 3D structure of the EFF1 protein[39] and shown it does the work of linking one cell to another, in Viral infections. At the moment all cell fusion molecules are viral in origin. The fact that the first two cellular fusion proteins are viral in origin it is highly suggestive. Fasseli Coulibaly of Monash University, Australia, suggests that without the ability of cellular fusion, colonial multicellularity could have formed, but anything as complex as even a sponge would have been impossible. Viruses thus appear to have been vitally important to the inter-cellular communication systems that have made multicellularity possible[40]

Experimental evidence

The evolution of multicellularity from unicellular ancestors has been modeled in the laboratory, in evolution experiments using predation as the selective pressure[citation needed]..

Advantages

Multicellularity allows an organism to exceed the size limits normally imposed by diffusion: single cells with increased size have a decreased surface-to-volume ratio and have difficulty in absorbing sufficient nutrients and transporting them throughout the cell. This confers multicellular organisms with the competitive advantages of an increase in size. It also permits increasing complexity by allowing the differentiation of numerous cellular lineages within an organism.[citation needed] Being multicellular also benefits with having a longer life span, thus still living even when a single-cell dies.

See also


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mercredi 27 avril 2016

Annexe 37 : époques géologiques et évolutions des vivants ( vu les 3 premiers éons)




Chronologie simple de l'évolution



_________________________________________________________________________



ÉON HADÉEN : 600 Ma
de 4,567 et 3,8  Ga (milliards d'années)
période de la formation de la Terre,
la « chimie de la vie », sans compter des
chocs majeurs ( entre Gaïa et Théia, à l'origine de la Lune.)

ÉON ARCHÉEN: 1500 Ma (point de départ des vivants)
ÉON PROTÉROZOÏQUE  : 1959 MA ( avant les plantes et les animaux)
ÉON PHANÉROZOÏQUE : 542 Ma ( abondance de plantes et d'animaux)

               Lignées passées au milieu terrestre :
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