Organs are formed by interactions between cells and tissues. Most often, one group of cells or tissues causes another set of cells or tissues to change their fate, a process called induction. In each such interaction, one cell type or tissue is the inducer that produces a signal, and one is the responder to that signal. The capacity to respond to such a signal is called competence, and competence requires activation of the responding tissue by a competence factor. Many inductive interactions occur between epithelial and mesenchymal cells and are called epithelial—mesenchymal interactions (Fig. 1). Epithelial cells are joined together in tubes or sheets, whereas mesenchymal cells are fibroblastic in appearance and dispersed in extracellular matrices (Fig. 1). Examples of epithelial—mesenchymal interactions include the following: gut endoderm and surrounding mesenchyme to produce gut-derived organs, including the liver and pancreas; limb mesenchyme with overlying ectoderm (epithelium) to produce limb outgrowth and differentiation; and endoderm of the ureteric bud and mesenchyme from the metanephric blastema to produce nephrons in the kidney. Inductive interactions can also occur between two epithelial tissues, such as induction of the lens by epithelium of the optic cup. Although an initial signal by the inducer to the responder initiates the inductive event, crosstalk between the two tissues or cell types is essential for differentiation to continue (Fig. 1, arrows).

Fig1. Drawing illustrating an epithelial— mesencnymal interaction. Following an initial signal from one tissue, a second tissue is induced to differentiate into a specific structure. The first tis— sue constitutes the inducer, and the second is the responder. Once the induction process is initiated, signals [arrows] are transmitted in both directions to complete the differentiation process.

مواضيع ذات صلة

تركيب المشيمة Structure Of The Placenta

مشتقات طبقة الاكتوديرم

تشكل اللواحق الجنينية

تشكل الانثناءات التي تعزل جنين الدجاج عن المح

تشكل الكيس المحي وانعزال المعي الأولي

السلي والمشيماء

السقاء Allantois

الأغشية الجنينية عند الثدييات

المشيمة

الولادة