Cl06 - MCDB 4650 DEVELOPMENTAL BIOLOGY CLASS NOTES Class 6...

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MCDB 4650 DEVELOPMENTAL BIOLOGY CLASS NOTES Class 6 Morphogenesis: Gastrulation and Neurulation Reading : Chp 10: 291-302, 337-342; Chp 12: 373-380 Learning goals Be able to: Define the terms blastula, blastocoel, blastopore, archenteron, gastrula. Explain the difference between epithelial and mesenchymal cells, and why transitions between the two cell types are a fundamental part of development. Explain what gastrulation, in general, accomplishes for an embryo. Predict what happens in the absence of gastrulation (with regard to cell type specification and differentiation). Compare gastrulation movements in different species and hypothesize why the differences (and similarities) exist. Compare the movements of gastrulation to the movements (and the purpose of) neurulation. Gastrulation reorganizes the blastula into a three-layered embryo with a recognizable body plan. In contrast to cleavage, the primary mechanisms of gastrulation involve cell movements; cell divisions occur but are less important to the morphogenetic process. In this class we will discuss aspects of gastrulation in some favorite model organisms; don’t worry about details, but rather focus on what gastrulation accomplishes for the embryo. Central to this topic are the characteristics of epithelia and mesenchyme , and the epithelial/mesenchymal transition , which we will encounter examples of throughout the course. After an embryo gastrulates, the next set of morphogenetic movements are called neurulation. This is the process of forming a hollow tube of neural cells, and segregating this tube of cells from the overlying epithelial cells. Obviously, both of these processes are crucial for setting up the normal structure of an embryo, and the two processes also share some features, discussed below. Epithelia and mesenchyme There are two basic states of cell organization and behavior termed epithelial and mesenchymal. Both are found during development in all three germ layers. Many developmental events involve conversion of one state to the other. Epithelial cells : Regular in shape, arranged in layer or sheet, often only one cell thick, called an epithelium. Major feature is polarity. Bounded on one side (basal surface) by a basal lamina, thin mat of ECM secreted by the cells. Further from cells may be a thicker basement membrane. Opposite side is the apical surface; differs for different epithelia. Often secretory; small protrusions called microvilli increase surface area. Different transport proteins in apical and basal plasma membranes allow transfer of specific molecules across an epithelium. Three general types of junctions hold epithelial cells together, allowing them to communicate: 1) Tight junctions prevent leakage between cells. 2) Gap junctions allow passage of small molecules (< 1 kD). 3) Other structural junctions include transmembrane linker proteins, e.g. cadherins. Mesenchymal cells:
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Cl06 - MCDB 4650 DEVELOPMENTAL BIOLOGY CLASS NOTES Class 6...

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