Cells that are not organized into an epithelium are called mesenchymal. A mesenchyme is a loose arrangement of cells (that may be individually mobile) surrounded on all sides by a matrix of extracellular material. Cells can switch from epithelial to mesenchymal, and vice versa.

What controls A/P Patterning of endodermal organs? Hox genes are expressed collinearly along the antero-posterior axis in the endoderm and mesoderm. Distinct Hox expression domains oft coincide with distinct intestinal domains. This is reminiscent of Hox gene expression in morphologically distinct rhombomere and somite units. However, Hox deletions generally cause malformations (instead of homeotic transformations) [...]

What Are Ectodermal Placodes? What Sensory Organs Do They Form? A placode is a thickened region of the ectoderm, and are induced at the anterior end of the neural plate where BMP levels are intermediate (this same region gives rise to neural crest cells). Ectoderm is epithelial, while the mesoderm contains both mesenchymal and epithelial [...]

How Does the Node Compare To the Spemann Organizer? What Is the Node? Hensen’s Node is present in birds and mammals. The Spemann Organizer is present in Xenopus. The Node establishes l What Different Types of Mesoderm Arise? Their Derivates? Inducing Signaling Molecules? For example, we learned that the mesoderm that comes to occupy the [...]

The mesodermal layer of the early embryo forms as a result of gastrulation. This mesodermal layer of cells initially constitutes an epithelium; after gastrulation the cells of this epithelium lose their close association with each other (undergo an epithelial-mesenchymal transformation). Mesoderm Position Overview Axial Mesoderm Most Dorsal Forms the notochord. Paraxial Mesoderm Along Dorsal Positioned [...]

Clarifying Note: Splanchnic mesoderm & somatic mesoderm are layers of lateral plate mesoderm adjacent to the endoderm & ectoderm, respectively. Layer Vertebrates Insects Overview Ectoderm Gut,Liver,Lungs Gut The ectoderm gives rise to the skin and its differentiated structures: hair, nails, feathers, scales, mammary glands and teeth. Ectodermal placodes give rise to the eye, ear and [...]

Development of endodermal organs is dependent on interaction between the endoderm and the surrounding mesoderm. Endodermal tissue cultured in vitro does not differentiate without its surrounding mesoderm; endodermal tissue co-cultured in vitro with splanchnic mesoderm undergoes organ-specific differentiation. Furthermore, organ development depended on the position of the mesoderm — not the endoderm — along the [...]

In birds and mammals, the endoderm is a disk that invaginates, starting at its anterior and posterior ends, to form a closed epithelial cylinder surrounded by a thin layer of splanchnic mesoderm. The splanchnic mesoderm later becomes smooth muscle. The endoderm gives rise to the pancreas, liver and lungs via branching of endodermal tubes. Regional [...]

Epidermal ectoderm gives rise to teeth, hair, nails, mammary glands, scales and feathers. Via epithelio-mesenchymal interaction, underlying mesenchyme determines which structures are formed from the epidermis. For example, if chicken thigh mesoderm is grafted beneath wing ectoderm, then the wing ectoderm will form thigh feathers rather than wing feathers. Epithelio-mesenchymal interaction occurs in three steps: [...]

Development of placodes in the epithelium involves multiple inductive interactions. In the case of the sensory placodes, the neural tissue induces placode formation. In the case of ectodermal appendages, the mesenchyme is a source of inductive signals. In all cases, there is reciprocal signaling between the epithelium and the mesenchyme. In addition to positive signals [...]