University:
Kentucky State University
Course:
BIO 111 | Principles of Biology
Academic year:

2023
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294

Pages:

12
Author:

Turfmorrybk9

Chapter 17 Paraxial mesoderm: the somite and their derivatives Segmentation of the body plan • On either sides of the neural tube lie thick bands of mesodermal cells that are divided into paraxial mesoderm, intermediate, and lateral plate mesoderm. • The early paraxial mesoderm directly adjacent to the notochord lacks somite, it takes the form of bilateral streaks of continuous mesenchymal cells, referred to as the presomitic mesoderm (PSM), also known as the segmental plate. Notochord Presomitic mesoderm (PSM)/ segmental plate Segmentation of the body plan • The regions of trunk and head mesoderm and their derivatives are summarized as the following: 1- The central region of the trunk mesoderm is the chordamesoderm (axial mesoderm). • This tissue forms notochord: a transient tissue whose major functions include inducing and patterning the neural tube and establishing the anterior- posterior body axis. Segmentation of the body plan • The regions of trunk and head mesoderm and their derivatives are summarized as the following: 2- Flanking the notochord on both sides is the paraxial, or somitic mesoderm. • The cells of the paraxial mesoderm will form somites: transitory epithelial blocks of mesodermal cells on either sides of the neural tube which produce the muscle and many connective tissue of the back Segmentation of the body plan • The regions of trunk and head mesoderm and their derivatives are summarized as the following: 3- The intermediate mesoderm is positioned directly lateral to the paraxial mesoderm. • It forms the urogenital system, and the outer (cortical) portion of the adrenal gland. Segmentation of the body plan • The regions of trunk and head mesoderm and their derivatives are summarized as the following: 4- Farthest away from the notochord, the lateral plate mesoderm is positioned. • It gives rise to the components of the circulatory system, the lining of the body cavity, the pelvic and limb skeleton, and also helps forming a series of extraembryonic membranes. Segmentation of the body plan • The regions of trunk and head mesoderm and their derivatives are summarized as the following: 5- Anterior to the trunk mesoderm is the head mesoderm. • The head mesoderm consists of the unsegmented paraxial mesoderm and prechordal mesoderm. • This region provides the head mesenchyme which along with the neural crests forms the skeleton, muscles, and connective tissues of the face and skull. Cell types of the somite • The somite is shaped by an epithelial cell contacts that create a block- like mass with mesenchymal cells at its core. • As the somite matures, portions of it undergo epithelial- to- mesenchymal transition (EMT), and its various regions become committed to forming only certain cell types. Cell types of the somite • The sclerotome can be further subdivided into the progenitor zones for specific cell lineages. Cell types of the somite • The dermomyotome contains progenitor cells for making skeletal muscle and the dermis of the back. • The myotome forms the musculature of the back, rib cage, and ventral body wall. • Muscle progenitors are also provided by the cells that detach from the lateral edge of the dermomyotome. • The detached cells migrate into the limbs to generate the musculature of the forelimbs and hindlimbs. Somitogenesis How does the segmental plate become partitioned into correct number of appropriately sized and bilaterally symmetrical somites? • As the mesenchymal cells of the presomitic mesoderm mature, they become organized into “Whorls” of cells that constitute the somite precursors known as somitomeres. • Somitogenesis: the process of somite formation which involves the periodic creation of an epithelial fissure by the mesenchymal cells of the presomitic mesoderm. Somitogenesis • To accurately describe the somites formed and the position of somitomeres in the presegmental plate, a numbering scheme was established using roman numbers. • Because the embryos within a species can develop at a slightly different rates, the number of somites formed is the best indicator of how development has proceeded. • The number of somites in an adult individual is species specific. Numbered counting up Numbered counting backward

Chapter 17 Araxial Mesoderm The Somite and Their Derivatives

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