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In the chick embryo, somites are formed every 90 minutes. In the mouse the interval is 2 hours. [7] For some species, the number of somites may be used to determine the stage of embryonic development more reliably than the number of hours post-fertilization because rate of development can be affected by temperature or other environmental factors.
Somites form with regularity every 90 minutes. Stage 10 embryos have 10 somites, and as a rule of thumb, the embryo gains 3 somites during each stage (i.e. Stage 11 embryos have 13 somites, Stage 12 embryos have 16, etc.). However, beyond 22 somites (HH14) it is better to rely on other markers.
During somitogenesis, somites form from the pre-somitic mesoderm, a region of mesoderm at the posterior of the developing embryo. This tissue undergoes convergent extension as the primitive streak regresses, or as the embryo gastrulates. The notochord extends from the base of the head to the tail; with it extend thick bands of paraxial mesoderm ...
The tissue undergoes convergent extension as the primitive streak regresses, or as the embryo gastrulates. The notochord extends from the base of the head to the tail; with it extend thick bands of paraxial mesoderm. [3] As the primitive streak continues to regress, somites form from the paraxial mesoderm by "budding off" rostrally.
The embryo during this process is called a gastrula. The germ layers are referred to as the ectoderm, mesoderm and endoderm. In diploblastic animals only the ectoderm and the endoderm are present. [8] * Among different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
Transverse section of a chick embryo of forty-five hours' incubation. * Axial mesoderm: yellow, at notochord. * Paraxial mesoderm: red, at somite. * Intermediate mesoderm: purple, near Wolffian duct. * Lateral plate mesoderm: purple, divided into "Somatic mesoderm" and "Splanchnic mesoderm".
This is controlled by different means in different species, such as through a simple negative feedback loop in zebrafish or in a complicated process in which FGF and Wnt clocks affect the Notch clock, as in chicks and mice. [2] [3] Generally speaking, the segmentation clock model is highly evolutionarily conserved. [4]
The paraxial mesoderm is arranged along the sides of the notochord and neural tube as repetitive structures called somites. In fact, somites are one of the first embryonic structures to display segmentation. Structures associated with the peripheral nervous system (PNS) such as dorsal root ganglia (DRG) develop a segmental alignment with the ...