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Embryonic induction
1. Secondary embryonic induction involves one group of cells, known as the inducer, directing the development of another group of cells or responder tissue. For example, the optic vesicle induces the formation of the lens. 2. Induction requires two components - the inducer, which produces signals to change the responder tissue, and the responder tissue which must be competent to receive the signals from the inducer. Competence is conferred by proteins like Pax6. 3. Signaling between tissues can occur through paracrine factors which diffuse short distances, or juxtacrine interactions which involve direct cell contact. This signaling activates gene expression through signal transduction pathways in competent responder tissues.
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Embryonic induction
- 1. Name Naveen Gul TopicSecondary Embryonic Induction Contents 1 Induction and Competence Inducer Responder 2 Optic Vesical As inducer 3 Competence Pax6 protein as competence factor 4 Paracrine and Juxtacrine signaling 5 Signal Transduction pathway 6 Reciprocal and Sequential induction events 7 Rciprocal events Optic cup formation by lens Cornea formation from lens 8 Sequential events Hyaloronidase Thyroxine signal 9 Instructive Induction 10 Permissive Induction 11 Epithelial Mesenchyma Interaction 12 Regional Specificity of Induction 13 Conclusion 14 References
- 2. Secondary Embryonic induction Embryonicinduction describes the embryonic process in which one group of cells, the inducing tissues directs the development of another group of cells or the responding tissue. Secondary induction directs the development of various tissues and organs in most animal embryos for example the eye lens and the heart. Induction and Competence Organs are complex structures composed of numerous types of tissues. In the vertebrate eye for example light is transmitted through the transparent corneal tissue and focused by the lens tissue. The precise arrangement of tissues in this organ cannot be disturbed without impairing its function. Such coordination in the construction of organs is accomplished by one group of cells changing the behavior of an adjacent set of cells, thereby causing them to change their shape, mitotic rate, or fate. These kind of interactions are known as induction. Components of Inductive interaction There are at least two components to every inductive interaction Inducer The first component is the inducer the tissue that produces a signals that changes the cellular behavior of the other tissue. Responder The second component, the tissue being induced is the responder. Not all tissues can respond to the signal being produced by the inducer. Optic Vesicle As Inducer If the optic vesicle (presumptive retina) of vertebrates is placed under the head ectoderm it will induce that ectoderm to form lens tissue. Only the optic vesicle appears to be able to do this therefore it is an inducer. However, if the optic vesicle is placed beneath ectoderm in the flank or abdomen of the same organism that ectoderm will not be able to respond. Only the head ectoderm is competent to respond to the signals from the optic vesicle by producing a lens.Therefore optical vesicle induces lens in anterior ectoderm but not in trunk region.
- 3. Competence This ability torespond to a specific inductive signal is called competence. Pax6 Proteins As Competence Factor For example, in the developing chick and mammalian eye the Pax6 protein is important in making the ectoderm competent to respond to the inductive signal from the optic vesicle. Pax6 expression is seen in the head ectoderm which can respond to the optic vesicle by forming lenses and it is not seen in other regions of the surface ectoderm. Paracrine Factors Paracrine signaling is a form of cell-to-cell communication in which a cell produces a signal to induce changes in nearby cells altering the behavior of those cells. Signaling molecules known as paracrine factors that are usually proteins. Paracrine factors diffuse over a relatively short distance to induce response in target cell.Paracrine factors are inducing proteins that are involved in the induction of numerous organs.Embryo use these proteins to construct heart,kidneys ,teeth,eyes and other organs.
- 4. Juxtacrine Signaling In juxtacrineinteractions proteins from the inducing cell interact with receptor proteins of adjacent responding cells. The inducer does not diffuse from the cell producing it. There are three types of juxtacrine interactions. A receptor on one cell binds to its ligand on the extracellular matrix secreted by another cell. In the third type the signal is transmitted directly from the cytoplasm of one cell through small into the cytoplasm of an adjacent cell. Signal Transduction Pathway Competent cells respond to paracrine factors through signal transduction pathways. Competence is the ability to bind and to respond to the inducers and it is often the result of a prior induction. Signal transduction pathways begin with the paracrine or juxtacrine factor causing a conformational change in its cell membrane receptor. The new shape results in enzymatic activity in the cytoplasmic domains of the receptor protein. This allows the receptor to phosphorylate other cytoplasmic proteins thereby activating a dormant kinase activity. Eventually, a transcription factor is activated that activates or represses specific gene activity.
- 5. Reciprocal and sequentialinductive events Reciprocal Events Lens induces formation of optic cup Another feature of induction is the reciprocal nature of many inductive interactions.Once the lens has formed it can then induce other tissues. One of these responding tissues is the optic vesicle itself. Now the inducer is being induced under the influence of factors secreted by the lens the optic vesicle becomes the optic cup and the wall of the optic cup differentiates into two layers the pigmented retina and the neural retina. Such interactions are called reciprocal inductions. Lens induces formation of Cornea At the same time, the lens is also inducing the ectoderm above it to become the cornea. Like the lens forming ectoderm the cornea forming ectoderm has achieved a particular competence to respond to inductive signals and in this case the signals is from the lens. Under the influence of the lens, the corneal ectodermal cells become columnar and secrete multiple layers of collagen. Sequantial Events Hyaluronidase Mesenchymal cells from the neural crest use this collagen matrix to enter the area and secrete a set of proteins including the enzyme hyaluronidase that further differentiate the cornea. Thyroxin Signal Another signal the hormone thyroxine dehydrates the tissue and makes it transparent. Thus, there are sequential inductive events and multiple causes for each induction.
- 6. Instructive interaction Howard Holtzer(1968) distinguished two major modes of inductive interaction.In instructive interaction a signal from the inducing cell is necessary for initiating new gene expression in the responding cell. Without the inducing cell the responding cell would not be capable of differentiating in that particular way.For example when the optic vesicle is experimentally placed under a new region of the head ectoderm and causes that region of the ectoderm to form a lens that is an instructive interaction. Permissive Interaction The second type of induction is permissive interaction. Here the responding tissue contains all the potentials that are to be expressed and needs only an environment that allows the expression of these traits. For instance, many tissues need a solid substrate containing fibronectin or laminin in order to develop. The fibronectin or laminin does not alter the type of cell. They help in the expresssion of traits. Epithelial-mesenchymal interactions Induction that are invovled in the interactions of sheets of epithelial cells with adjacent mesenchymal cells.These interactions are called epithelial mesenchymal interactions.Epithelia are sheets or tubes of connected cells they can originate from any germ layer. Mesenchyme refers to loosely packed unconnected cells. Mesenchymal cells are derived from the mesoderm or neural crest. All organs consist of an epithelium and an associated mesenchyme so epithelial-mesenchymal interactions are among the most important phenomena in nature.
- 7. Some Epithelia-MesenchymalInteraction Organs Epithelial components Mesenchymal components CutaneosStructure (Feathers, sweat glands and mammary glands) Epidermis (ectoderm) Dermis (mesoderm) Limb Epidermis (ectoderm) Mesenchyme (mesoderm) Gut organs (liver, pancreas, salivary glands) Epithelium (endoderm) Mesenchyme (mesoderm) Pharyngeal and respiratory associated organs (lungs, thymus, thyroid) Epithelium (endoderm) Mesenchyme Kidney Ureteric bud epithelim (mesoderm) Mesenchyme Tooth Jaw epithelium (ectoderm) Mesenchyme (neural crest)
- 8. Regionalspecificity of induction Thefirst property of epithelial-mesenchyma interaction is regional specificity of induction. Skin is composed of two main tissues an outer epidermis an epithelial tissue derived from ectoderm and a dermis a mesenchymal tissue derived from mesoderm. mesenchyme is responsible for the regional specificity of induction in the competent epidermal epithelium. The same type of epithelium develops cutaneous structures according to the region from which the mesenchyme was taken. Here, the mesenchyme plays an instructive role. Wing dermal mesenchyma and wing epidermal epithelium give rise to wing epithelium. Thigh dermal mesenchyma and wing epiderml epithelium give rise to thigh feather. Foot dermal mesenchyma and wing epidermal epithelium give rise to scales and claws. Regionally specific inductions can generate different structures from the same tissue. Conclusion Interaction among different cells occurs through paracrine and juxtacrine induction.Formation of heart and lens are secondary embryonic induction whereas tissue interaction that that generate neural tube is known as primary embryonic induction.
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