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Biology 211 Intro Molecular and Cell Biology Lectures 22 and 23 Animal Development Reading: Campbell et al., Chap. 47 to p. 946 Outline: 1. Fertilization 2. Cleavage and early development 3. Gastrula 4. Organogenesis Lecture: In these final series of lectures, we will examine how cells are organized into tissues and organs in the process of development. Development: Transformation from a single cell to a multicellular organism. Involves cell division and differentiation. 1. Fertilization: The union of egg and sperm to form a zygote, which goes on to form a new individual. Model organism: sea urchin (invertebrate animal) The "Acrosomal Reaction" is described in Fig. 47.2. This sequence of events ensures that only one sperm fertilizes a single egg and that the sperm is from the correct species. The "Cortical Reaction" is a series of changes in the outer zone of the egg cytoplasm. The fusion of sperm and egg triggers a pathway that leads to the egg's release of calcium ions from the endoplasmic reticulum into the cytosol. This reaction leads to changes in the membrane of the egg (vitelline membrane) to produce the fertilization membrane that resists entry of additional sperm. Activation of the egg. Calcium ion release and pH changes (increased intracellular pH) leads to metabolic changes in the egg. This enables the egg to begin making proteins. Fusion of the egg and sperm nuclei creates the diploid nucleus of the zygote. DNA synthesis begins, then the cell divides. 2. Cleavage and early development Fertilization is followed by 3 stages that build the animal's body: A. Cleavage creates a multicellular embryo (blastula) from the zygote B. Gastrulation produces a three-layered embryo C. During organogenesis, organs begin to develop Cleavage--Series of mitotic cell divisions that partition the zygote into many smaller cells. Most eggs (except mammalian eggs) have polarity. Animal pole--forms head Vegetal pole--where yolk is concentrated; forms tail Cell division is assymetrical Early events of development determined by asymmetric distribution of molecules in egg and early embryo. Figure 47.8 Cleavage in a frog embryo Cleavage stages: Zygote--fertilized egg 2-cell stage 4-cell stage 8-cell stage 16-cell stage (morula--solid ball of cells) 64-cell stage (morula) 128-cell stage (blastula--hollow ball of cells) 3. Gastrula Gastrulation--Rearrangement of cells of the blastula to form the three main cell layers. Three layers formed: Ectoderm: Forms outer layer of gastrula; become our nervous system and the skin. Endoderm: Lines the digestive tract; the lining of the digestive tract and associated organs such as liver and pancreas come from this layer. Mesoderm: Partly fills the space between the ectoderm and the endoderm; kidney, heart, muscles, skeleton, and the inner layer of skin (dermis) develop from this layer. Observe process of gastrulation in sea urchin (Fig. 47.9) and frog embryo (Fig. 47.10). Key elements of process: Inward migration (involution) of cells. Formation of primitive gut (archenteron), lined with endoderm). Outer cell layer forms ectoderm. Layer of cells between archenteron and ectoderm forms the mesoderm. 4. Organogenesis--formation of organs and organ systems. First organs formed in chordates: Neural tube--forms brain and spinal cord. Notochord--forms support structure for skeleton. Somites--repeating blocks of mesoderm that form muscles. Figure 47.11 Frog early organogenesis. How does organogenesis take place? Contact between cell layers "induces" structures to form. Molecules in these cells have been found to be the "inducers".