DevelopmentW2010.Lecture11notes - Development BIO 120...

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Development BIO 120 Winter 2010 Jeremy Lee Eva Murdock Patrick Yuh Lecture 11 Development of Selected Mesodermal Tissues: Muscle, Bone, Circulatory System I. Types of Vertebrate Mesoderm (Figs. #1, #2) a. Axial mesoderm (chordamesoderm): Composed of cells ingressing at Hensen's node. Forms the notochord = temporary cartilage-like rod that plays a role in neural tube formation. b. Intermediate mesoderm : forms urinary tract and reproductive system. c. Lateral plate mesoderm : differentiates into the heart, blood vessels, blood cells, lining of body cavity. d. Head mesenchyme : facial connective tissues (bones, cartilage) and muscle of the face. e. Paraxial (somitic dorsal) mesoderm : forms the somites (Fig. #3), which are temporary blocks of cells that produce bones, muscle, cartilage and dermis of the back. II. Formation and Differentiation of Somites along Neural Tube a. Paraxial mesoderm "buds off" groups of cells which form somites , epithelial cells that surround a cavity (Fig. #3.) 1. Somite formation is initiated, in part, by expression of N-cadherin; this converts cells from "loose" mesenchyme to a structured epithelium. 2. Somites form from anterior to posterior; stage of embryonic development often is referred to by the number of somites developed. 3. Quail and chick transplantation experiments suggested Notch-expressing cells are critical for formation of individual somites; Notch role confirmed in transgenic experiments (Fig. #4.1) and in mice carrying mutation in the Notch receptor Delta- like3 gene, which have defects in rib formation (Fig. #4.2) b. Somite differentiates into 2 different regions, based on cell-to-cell signaling . (Figs. #2, #5, #6). These regions of the somite are called the sclerotome and the dermamyotome . 1. Sclerotome A. Differentiation of sclerotome cells is induced by Sonic hedgehog protein secreted from notochord and ventral floor plate (Fig. #6.) Sonic hedgehog signal induces PAX-1 expression in sclerotome cells. B. Eventually, sclerotome cells migrate out from somite as mesenchyme cells C. Sclerotome cells migrate toward notochord, and become chondrocytes , which will form the axial skeleton (vertebrae, ribs, and associated cartilage and ligaments.) (Fig. #7) D. Where vertebrae form, most notochord cells die; some notochord cells survive to form the elastic intervertebral discs between vertebrae.
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2 2. Dermamyotome: Remaining epithelium of somite when sclerotome cells are migrating as mesenchyme. Two primary layers of dermamyotome form and are called the dermatome and myotome. A. Dermatome: outer layer of dermamyotome; it is induced by neurotrophin-3 and Wnt-1 secreted by dorsal neural tube (Fig. #6.) These cells eventually migrate as mesenchyme cells to become the dermis of the back. Dermis is the cell layer that underlies the epidermis. B. Myotome
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DevelopmentW2010.Lecture11notes - Development BIO 120...

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