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Tissue level of organization.docx

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University of Toronto Scarborough
Biological Sciences
Kenneth Welch

The Tissue Level of Organization Tissues and tissue types • Tissues are: • Collections of specialized cells and cell products organized to perform a limited number of functions • Histology = study of tissues • The four tissue types are: • Epithelial • Connective • Muscular • Nervous Epithelial tissue • Includes glands and epithelium • Glands are secretory • Is avascular • Forms a protective barrier that regulates permeability • Cells may show polarity Functions of epithelium • Physical protection • Control permeability • Provide sensation • Produce specialized secretions Specializations of epithelium • Perform secretory functions • Perform transport functions • Maintain physical integrity • Ciliated epithelia move materials across their surface Maintaining the integrity of epithelium • Cells attach via cell adhesion molecules (CAM) • Cells attach at specialized cell junctions • Tight junctions • Desmosomes • Gap junctions Structure of typical epithelium • Basal lamina attaches to underlying surface • Lamina lucida • Lamina densa • Germinative cells replace short-lived epithelial cells Classification of epithelia • Number of cell layers • Simple • Stratified • Shape of apical surface cells • Squamous • Cuboidal • Columnar Glandular epithelia • Exocrine glands • Secrete through ducts onto the surface of the gland • Endocrine glands • Release hormones into surrounding fluid Glandular secretions can be: • Merocrine (product released through exocytosis) • Apocrine (involves the loss of both product and cytoplasm) • Holocrine (destroys the cell) Glands • Unicellular • Individual secretory cells • Multicellular • Organs containing glandular epithelium • Classified according to structure Connective Tissues Connective tissue functions: • Establishing a structural framework • Transporting fluids and dissolved materials • Protecting delicate organs • Supporting, surrounding and interconnecting tissues • Storing energy reserves • Defending the body from microorganisms Connective tissues contain • Specialized cells • Matrix • Composed of extracellular protein fibers and a ground substance Connective tissue proper • Contains varied cell populations • Contains various fiber types • A syrupy ground substance Fluid connective tissue • Contains a distinctive cell population • Watery ground substance with dissolved proteins • Two types • Blood • Lymph Supporting connective tissues • Less diverse cell population • Dense ground substance • Closely packed fibers • Two types • Cartilage • Bone Connective tissue proper • Contains fibers, a viscous ground substance, and a varied cell population • Fibroblasts • Macrophage • Adipocytes • Mesenchymal cells • Melanocytes • Mast cells • Lymphocytes • Microphages Connective tissue proper • Three types of fiber • Collagen fibers • Reticular fibers • Elastic fibers Connective tissue proper • Classified as loose or dense • Loose • Embryonic mesenchyme
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