By Levi Clancy for Student Reader on
What Is the Node? How Does the Node Compare To the Spemann Organizer?
Hensen's Node is present in birds and mammals. The Spemann Organizer is present in Xenopus. The Node establishes
What Different Types of Mesoderm Arise? Their Derivates? Inducing Signaling Molecules?
For example, we learned that the mesoderm that comes to occupy the most dorsal position in the embryo, the dorsal mesoderm, will become the notochord. Mesoderm that occupies more ventral positions go on to become other derivatives. For example, we also saw that the endoderm becomes surrounded by mesoderm, and that mesoderm is a more ventral type (the splanchnic mesoderm).
|Axial||It gives rise to the notochordal process which later becomes the notochord.|
|Paraxial||On either side of the neural tube lie bands of paraxial mesoderm. Paraxial mesoderm gives rise to somites. Somites form the vertebral column, dermis and skeletal muscle. Paraxial mesoderm also gives rise to branchial arches, which develop into facial muscle and cartilage. Paraxial mesoderm is exposed to BMP antagonists, but at a lesser concentration than dorsal axial mesoderm. Posterior paraxial mesoderm expresses high levels of FGF, thus keeping it in a proliferative and undifferentiated state. As the primitive streak regresses posteriorly, cells further from the node are no longer under the influence of FGF. Outside the reach of FGF, the paraxial mesoderm cells begin to compartmentalize into somites. Cells within an individual somite become compacted (which involves an increase in cadherin expression). These changes in cell adhesion cause the newly forming somite to separate from the rest of the paraxial mesoderm.|
|Intermediate||Intermediate mesoderm is located between the paraxial mesoderm and the lateral plate. It develops into the part of the urogenital system (kidneys and gonads).|
|Lateral Plate||This is ventral mesoderm and gives rise to limbs.|
Describe the Notch Pathway in Somite Boundary Formation
|Transplant||Transplanting a small group of cells from a region that will eventually form a somite boundary, into a region of unsegmented paraxial mesoderm that normally would not be part of a boundary.|
|Result||Transplanted cells instruct the cells anterior to them to undergo a mesenchymal-epithelial transition and to separate from the unsegmented mesoderm.|
|Boundaries are signaling centers, and the somite boundary instructs neighboring cells to undergo a mesenchymal-epithelial transition. Nonboundary cells can acquire this ability to induce boundary formation if the Notch pathway is activated in them, for example, by introducing an activated Notch receptor.|
|Mutations in Notch signaling lead to defects in somite formation. For example, mice lacking the Notch ligand Delta-like 3 (Dll3) have serious vertebral and rib defects. As we will see below, these structures are derived from somites. In Dll3-/- mice, somite formation is irregular and delayed. As a result the structures that form from the somites are abnormal.|
Notch Signaling Mechanisms
Notch controls somite size and segmentation in a negative feedback pathway. The Notch pathway establishes an oscillating pattern in somites, and one cycle of the oscillation corresponds to the budding off of one somite from the unsegmented paraxial mesoderm. Notch signaling activates a transcription factor (RBJ) that activates the expression of Hes.
Hes is a transcriptional repressor that has two functions. First, it represses expression of itself. This limits the duration of the Notch response. Second, Hes represses expression of an inhibitor of the Notch receptor, lunatic fringe (Lfng). Thus, this activity of Hes would serve to activate the Notch pathway. The oscillations are created because Hes has a very short half-life, leading to transient repression of Notch.
Into what structures does the somite subdivide?
What are the derivatives of these structures?
What signals are involved in this subdivision?
When the somite first separates from the presomitic mesoderm, it can give rise to any somite-derived structure. As the somite matures, its various regions become committed to forming certain cell types.
|Sclerotome||Medial||Ventral-medial cells are farthest from the back but closest to the neural tube. These undergo mitosis and an epithelial-mesenchymal transition. They eventually become chondrocytes (cartilage cells) of the vertebrae and most (if not all) of each rib. The sclerotome is induced by paracrine factors, especially Shh, secreted from the notochord and neural tube floor plate. If any source of Shh is transplanted next to other regions of the somite, they too will become sclerotome cells. Sclerotome cells express Pax1, which induces them to differentiate into cartilage; also, pax1 is necessary for formation of the vertebrae. Sclerotome cells also express I-mf, an inhibitor of the myogenic bHLH family of transcription factors that initiate muscle formation.|
Cells in the two lateral portions of the epithelium (closest and farthest from the neural tube) give rise to dermamyotome, a double-layered structure composed of myotome in the lower layer and dermatome in in the upper layer.
Describe the Mesodermal Mesenchymal←→Epithelilal Transitions Involved in
Forming the Primitive Streak, Somite Formation and Somite Subdivision.
|Streak||Cells ingress into the streak via an epithelial→mesenchymal transition.|
|Somites||Somites go mesenchymal→epithelial to separate from the unsegmented mesoderm.|
|Sclerotome||Sclerotome goes epithelial→mesenchymal to generate chondrocytes of the vertebrae and most (if not all) of the ribs.|
|Dermatome||Dermatome goes epithelial→mesenchymal to generate mesenchymal connective tissue of the back dermis.|