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In the third week of intrauterine life is triggered by the process of gastrulation cell movements and hierarchical integrated holders achieves the goal of rearranging the cellular compartments of the bilaminar germ disc. At this stage, gastrula stage, cells in the mesodermal compartment migration acquire new positions to establish the basic body plan with the formation of an embryonic disc trimalinar also defines and allows specific molecular interactions between the embryonic tissues.
The achievement of migration and invagination of the mesoderm into its final position depends on the formation of three structures: the primitive streak, Hensen's knot and the plate precordal. The first two structures depend on the mesoderm, while the last of endoderm. All three have, through their molecular products, a regulatory role on mesoderm invagination and initial differentiation: chordal mesoderm or notochord, lateral mesoderm and mesoderm cadiogénico pluripotencial.La primitive streak and Hensen knot through its products regulate the formation of molecular left-right axis in two levels: global, for all organs and structures ILATERAL provision and one specific national level for the elements of asymmetrical arrangement.
It is a stage of embryonic development, which occurs after the formation of the blastula, ie following the segmentation or cleavage, and aims at the formation of the fundamental layers of the embryo (germ layers)
Process-oriented movements and cell migration to the generation of 3 layers blastoderm. These early cell movements of gastrulation are similar in all animals but gastrulation mechanisms highly dependent on the amount of yolk available. Cell movements enable the contact of previously distant cells, allowing the inductive interactions that determine the neurulation and organogenesis. He begins to express the paternal genome. Occurs between days 15 to 17 days, begins with the formation of the primitive streak or line.
This is formed on the surface of the epiblast to the opposite side where the plate was formed precordal, thus forming in what will be the caudal end of
embryo. The primitive streak results from a thickening of the epiblast, due to proliferation and migration of epiblast cells towards the midplane of the embryonic disk. The primitive streak is extended in a cranial direction and form a bulge in its cranial end: the primitive node (or Hensen's node). Simultaneously, along the primitive streak develops a groove: the primitive groove, which becomes more evident at the primitive streak, forming a depression: the fovea primitive. The appearance of the primitive streak determines the identification of:
The cranio-caudal axis.
The cranial and caudal ends of the embryo.
Its dorsal and ventral surfaces, and
Left and right sides
Established by both levels of body asymmetry. The primitive groove and dimple are the result of invagination ventral (inward) of epiblast cells
As the cells invaginating epiblast will continue well-established paths in dddress cranial and lateral. Epiblast cells that more are opening up the coat hipoblasto reach, gets between their cells and moved to the suburbs, going to form the roof of the yolk sac and forming the endoderm Intra. Moreover, other cells of the epiblast form a loose network of embryonic tissue called mesenchyme or nephroma, which will result in Intra. Some of the mesenchymal cells reach the mesoderm lateral edges, contributing to the formation of extraembryonic mesoderm. Epiblast cells that are introduced into the primitive streak following a cranial direction up to the plate precordal, where they stop, forming a cellular cord called the notochord process. Epiblast cells that arise from this layer, will form the ectoderm. The migration of epiblast cells to form the mesenchyme, reaching the entire surface of the embryonic disk of the plate except precordal or oropharyngeal membrane and a small area caudal to the primitive streak: the cloacal membrane.
This process of gastrulation determines the change of bilaminar embryonic disc to a trilaminar embryonic disc, with its three blastoderm layers: ectoderm, mesoderm and endoderm.
The primitive streak, during the 4th week, decreases in relative size, eventually disappearing at the end of 4th week.
The involution is not the primitive streak leads to the formation of a large tumor coccygeus sacral level, known as sacrococcygeal teratoma. At this stage we can identify specific areas of cells and "determined" to form a particular type of tissue.
In vertebrate gastrulation stage is followed by neurulation
Gastrulation is intussusception: a wall of the blastula sinks toward the opposite wall, and results in: a) reduces the blastula cavity (blastocoel), b) are two walls in the embryo or embryonic layers (ectoderm and endoderm), and c) form a new cavity (limited by the endoderm) called Archenteron, which communicates with the exterior through the blastopore.
Mesoderm and coelom formation:
With gastrulation, the embryo becomes Diploblasty (with two embryonic layers: ectoderm and endoderm). The development of an animal does not pass the state Diploblasty (coelenterates are the sponges and cnidarians). But in the remaining third germ layer develops between the ectoderm and the endoderm: the mesoderm, in which the coelom is formed or cavity celomática. These are called Triploblasty or coelomates.
The formation of mesoderm and coelom may occur by three processes:
Enterocoelous: Archenteron the sides and at the expense of endoderm (types of gastrulation by invagination, as echinoderms and Anfioxus).
Esquizocelia: at the expense of two bodies of blastomeres that form at the boundary between ectoderm and endoderm around the blastopore (in total egg segmentation and unequal, as mollusks and annelids).
Immigration: proliferating cells at the edges or lips of the blastopore and migrate to lie between ectoderm and endoderm in the dorsal part of the latter. So are there two lateral strands are split transversely into several segments or somites. The coelom is caused by hollowing of the cell cords. This process occurs in vertebrates.
Formation of mesoderm and Coelom BY AND ESQUIZOCELIA Enterocoelous

It is the third stage of embryonic development. It involves the formation of various organs from each of the embryonic layers. We can distinguish the following organs and tissues:
Derivatives of the ectoderm
a) The epidermis and glands attached to it (such as sweat), and mucous membranes of the body's natural openings (mouth, nostrils, etc.)..
b) The central nervous system (which is formed by thickening and collapse of the longitudinal midline of the ectoderm: invertebrate ventral and dorsal in vertebrates).
Endoderm derivatives:
a) The gut and its glands attached.
b) The inner lining of some organs like the lungs.
Derived from mesoderm
a) The dermal layer of skin and connective tissue elsewhere in the body.
b) The circulatory system.
c) The excretory system and gonads.
d) The muscular system.
e) The skeleton of chordates.
Once formed the blastula, there is a displacement of surface cells into the blastocoel This generates invagination and consequent shrinkage of the blastocoele cavity. This is accomplished by entering cells (which initially could be attributed ectodermal features) by the blastopore, in a move similar to turn over a sock. The point of entry of these cells forms an opening called the blastopore. While the blastocoel cavity decreases, there arises a new cavity called gastrocele Archenteron or that later become the gut mitotic activity, intense along the segmentation decreases steadily while never completely.
The blastomeres, or groupings of them, undertake considerable migrations originating from cell segregation into two types, one of which will cover the other. Or ectoblasto The outer layer (ectoderm), covers the inner layer or endoblasto (endoderm). But the germ Diploblasty gastrula is not only in the Porifera and coelenterates (cnidarians and ctenophores <h; in all other metazoans, a middle layer or nephroma (mesoderm is sandwiched between two layers above.
2.3 Training of the gastrula
In the blastula a part of the blastomeres begins to invaginate, forming the blastopore. The place where this occurs, is regulated genetically. The invagination progresses and invades all parts of the blastocoel being scaled viewing Archenteron the increased or new cavity is formed, which has the distinction of being in contact with the outside through the blastopore
At this stage, the embryo is called a gastrula and give rise to layers of the embryo described above. Through the process of gastrulation have formed two layers of blastomeres, a contact abroad or ectoderm and the other in contact with the endoderm and Archenteron or between the two with the blastocoel blastocoele fluid
3. TYPES OF gastrulation
The process of gastrulation occurs differently depending on the egg and its subsequent segmentation. The main types of gastrulation are:
3.1 gastrulation BY EPIBOLIA
Higher cells divide, spread, and slip forming the embryo. It occurs when the egg has a moderate amount of yolk located in the vegetative pole, and the segmentation produces large macrómeros yolk. In this case, the blastocoel is more or less virtual (estereoblástula) and intussusception is difficult because the vegetative pole macrómeros have no mobility.
For this reason, micrómeros located at the animal pole divide, proliferate, sink and surrounding macrómeros until the blastopore formed in the vegetative pole.
They move the animal pole macrómeros which multiply by mitosis and move involving the macrómeros. You can give two results:
"The micrómeros fail to stick together in the vegetative pole. In the gastrula the outer cell layer (micrómeros) are the ectoderm and the inner cells (macrómeros) will be the endoderm. They have a small but Archenteron and blastopore disappears blastocoel
"The micrómeros join in the vegetative pole, the germ layers are the same but there is not no Archenteron blastocoel. The anime will complete digestive tract but is formed in later stages.
Gastrulation in amphibian
The eggs of amphibians are often far yolk with bilateral symmetry. Half vegetative yolk is called white. Peripherally the animal has a yolk or yolk lightly pigmented brown.
The blastula formed after segmentation is a small blastocoel blastula with displaced, as the wall has at least two layers of cells being considerably higher hemisphere of the plant. These cells prevent a complete invagination during gastrulation. Morphogenetic movements of gastrulation begin with. It starts in the back of vitelline field.
Gastrulation begins with the invagination and continues through a process of involution in the range of the gray crescent. Intussusception of migrating cells form the Archenteron. The notch which distinguishes the blastopore now called dorsal blastopore lip. These movements are known around the embryo full circle. While such movements continue invagination and involution, the epibolia of germ ring cells eventually isolating the cells of the yolk within the limits of the blastopore, in an area known as yolk plug.
The blastopore is now surrounded by lips dorsal, ventral and lateral. The growing layer of cells that forms the roof of Archenteron give rise to endoderm and mesoderm, whereas for some time the floor will be occupied by Archenteron large yolk cells. The cells of the outer surface of the embryo are now the ectoderm. As gastrulation proceeds, the ventral lip of the blastopore begins to involute and gradually invaginated endoderm proliferates to close the tip and complete the Archenteron. The third layer, the mesoderm, develops between the endoderm and ectoderm.
During primary neurulation of the embryo within the primitive gut endoderm cell is surrounded and they are located around the mesoderm. Dorsal midline in the notochord is transformed into a rod around which the blade is thickened mesoderm forms the somites. In the ectodermal layer nervous system cells begin to organize. The edges of the neural plate and form aneural fold elevation of a longitudinal neural groove. The edges rise and unite in the midline to form a complete neural tube. Which begins forming in the upper levels of the spinal cord and is closing to the caudal. At this point begin to differentiate a new group of ectodermal cells between neural tube and ectoderm that form the neural crest.
Gastrulation in ANELO
Approximately 15,000 species, marine, freshwater and terrestrial. Between 1 mm and 7 m in length, coloration variable. Body metamerizado: linear repetition of parts throughout the body due to the segmentation of the mesoderm. Three parts: Prostomio (anterior, preoral, not a segment); Metastomio (set of all segments, the first: peristome) Pygidium(posterior to the anus, not a segment). The formation of terminal segments is
3.2 gastrulation BY EMBOLISM
By invagination
The celoblástulas suffer with central blastocoel. Vegetative pole cells fold inward and are introduced into the blastocoel by a process called intussusception. This creates a cavity surrounded by ectoderm called Archenteron and will be the future digestive tract. The pore entrance is called Blastopore Archenteron depending on the subsequent development, the blastopore will lead to the anus, mouth or closed.
They form two germ layers, the outer layer is the ectoderm (epithelium, leading to the body surface and the nervous system) and the innermost, which is the Archenteron is the endoderm (which forms the lining of the digestive tract). The outgrowths of Archenteron form the mesoderm (third layer to form embryonic muscle and reproductive system).
Most animal phyla have holoblástica segmentation, and in this case, the blastula looks like a hollow ball (celoblástula) that delimits the cavity called blastocoel. One such blastula undergoes gastrulation by invagination process, ie that part of the ectodermal cells invaginates into the blastocoel to form the second germ layer (endoderm) and define a new cavity, which communicates with the Archenteron exterior through the blastopore. It's the kind that has been detailed in the "Sequence"
Gastrulation in fish
At the edges of the blastodisc on the yolk is epibolia. Will also produce an involution in this edge that will affect the deep blastomeres. So defined by the embryo and hipoblasto epiblast, forms a ring around the blastodisc germ.
Simultaneously, there will be a convergence of blastomeres to the area of the disc, forming the embryonic shield, where the embryo develops with an anteroposterior axis.
In amphibians, the yolk plug cells were yolk-rich vegetables. The epibolia continues until the yolk is completely covered. At the end of yolk plug is formed, composed exclusively of yolk.
By involution
Step by cell from the outside through a fold. When the amount of egg yolk is so large that the segmentation is meroblástico type forms a small cap of cells or gastrulation discoblástula and then usually occurs by involution. This process is that cells at the periphery of the disk and head sink back beneath the surface layer and form a double layer. Is the typical case of cephalopods. The record of animal pole cells multiply and form another layer inside. The embryo has ectoderm and endoderm but nothing more.
Dorsal lip involution and cell epibolia side
Top: After dorsal lip cells are constantly changing. The first involuting cells give rise to the pharynx and the others give rise to notochord cells (cordamesodermo). Bottle cells invaginate but do not regress. It other mesodermal involution that are close. With the regression is forming the Archenteron, who moves the blastocoel extended to one end of the blastula. The Archenteron endodermal cells will be coated.
To delamination
From a cellular sheet, forming two. It is a rare type. It also happens when you form a estereoblástula. The endoderm is formed from the ectodermal cell division, and migration and subsidence of the same; gastrulation is completed with separation of two layers of cells, an external and internal, and no blastopore, but the Archenteron cavity opens through a process of some secundario.Gastrulación own Celentinmates. For mitosis celuloblástula cells are separated into two layers. The mitotic spindles are radial segmentation and planes occur parallel to the surface of the egg. The monoestratificada blastula becomes a double germ cell layer, forming an ectoderm and endoderm. The latter surrounds the Archenteron left as a remnant of the blastocoel. No blastopore. It should open a gap, but secondarily, it is not comparable with the blastopore. In animals with this Archenteron, other primary cavity, it will secrete a gelatinous substance between the endoderm and ectoderm, forming a structure called the mesoglea, full of cells that can pass through it (cnidarians). It is a rare type. It also happens when you form a estereoblástula. The endoderm is formed from the ectodermal cell division, and migration and subsidence of the same; gastrulation is completed with separation of two layers of cells, an external and internal, and no blastopore, but the Archenteron cavity was opened for a child process.
Gastrulation by internal migration and immigration, some cells of the blastoderm is going to go off and grow in depth form at first forming a mass called parenquimula stage or phase and these cells give rise to endoderm and ectoderm. This internal migration can start from a single point of blastoderm and the subject of immigration from unipolar and if two or more areas of the blastoderm is spoken multipolar immigration. Once you have produced these leaves begin to acquire different structural characters. The embryo is placed in contact with the external environment through the ectoderm and the endoderm on the other hand will perform very basic functions nutricción. When two leaves have formed little evolved in animals (jellyfish) morphogenesis that has formed the gastrula has completed most of the adult. In the most advanced adult structures are more complex to occur there need to be further cell divisions and further growth, this leads to the formation of the mesoderm.
Some animals, coelenterates poríferosy maintain this stage, being animals Diploblasty (with two leaves blastoderm). For example polyps <have two layers and can look like an inverted gastrula, being the equivalent mesoglea blastocoel and the inner cavity in contact with the outside Archenteron equivalent, but not the gastroporo with the blastopore, as they have different embryonic origins. These animals are representatives of a very simple level of organization that have not been anything like organs but tissues
To have formed bodies has had to develop a third sheet blastoderm, but so too it does not increase the volume, along the lines described above.
In the gastrula Triploblasty, endoderm cells invaginate to form pockets that are expanded to the pursuit of a third sheet included blastoderm or mesoderm between the endoderm and ectoderm, with two layers, one close to somatopleura ectoderm and the endoderm other nearby esplacnopleura .
The mesoderm delimits together with the mediastinum (which will lead to a cavity or coelom mesentery Animals with three leaves are called Triploblasty blastoderm, both acoelomate, eucelomados pseudocoelomate as mesoderm formation by the process described above is called Enterocoelous, but not the only mechanism for training is the same (see mesoderm).
It is a set of movements of parts of the early embryo (morphogenetic movements) and is intended to outline the organs of the embryo. So at least there is an embryo with three layers of fully differentiated cells. They move the blastomeres. Dan ectodermal organs (the surface), mesoderm (middle portion) and endodermal (inside).
Seven key features
The rearrangement of the cells by morphogenetic movements.
Rate of cell division.
Little or no growth (the cells are segmented but not grow a lot). It modifies the metabolism, increasing oxidation.
Increases the activity of the nucleus to control the processes that occur in cells emmbrionarias.
Begins the synthesis of new proteins.
Gastrulation DISORDERS

. Situs inversus.
. SITUS isomers.
. Holoprosencephaly.
. TERATOMA sacrococcygeal: Remnants of the primitive streak - Tumor
As derived from stem cells, these tumors contain different types of tissue (hair, nails, bone, etc.).
It is the most common tumor in infants (1:35,000 RNV) 80% are women.

2. During this period, fish larvae develop mainly food and respiratory organs. The yolk sac provides the material necessary for their growth and development. The size of the yolk sac decreases slowly until complete resorption, shortly after the larva begins to eat outside. The duration of this period depends, therefore, the initial size of the yolk sac, which varies from species to species and larval development rate, which varies mainly with the water temperature. For each species there is an optimal thermal range, similar to that defined for the incubation of eggs. The duration of the larval rearing period is defined in terms ofdegree days (gd), very similar to the incubation period. This corresponds roughly to three to four days to almost all warm water fish, although it is somewhat longer for colder water fish.

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