tissues and histology.docx

Tissues/Histolgy Tissues • Groups of cells similar in structure and function • The four types of tissues • Epithelial • Connective • Muscle • Nerve Epithelial Tissue • Cellularity – composed almost entirely of cells • Special contacts – form continuous sheets held together by tight junctions and desmosomes • Polarity – apical and basal surfaces • Supported by connective tissue – reticular and basal laminae • Avascular but innervated – contains no blood vessels but supplied by nerve fibers • Regenerative – rapidly replaces lost cells by cell division Classification of Epithelia • Simple or stratified • Squamous, cuboidal, or columnar Epithelia: Simple Squamous • Single layer of flattened cells with disc-shaped nuclei and sparse cytoplasm • Functions • Diffusion and filtration • Provide a slick, friction-reducing lining in lymphatic and cardiovascular systems • Present in the kidney glomeruli, lining of heart, blood vessels, lymphatic vessels, and serosae Epithelia: Simple Cuboidal • Single layer of cubelike cells with large, spherical central nuclei • Function in secretion and absorption • Present in kidney tubules, ducts and secretory portions of small glands, and ovary surface Epithelia: Simple Columnar • Single layer of tall cells with oval nuclei; many contain cilia • Goblet cells are often found in this layer • Function in absorption and secretion • Nonciliated type line digestive tract and gallbladder • Ciliated type line small bronchi, uterine tubes, and some regions of the uterus Epithelia: Pseudostratified Columnar • Single layer of cells with different heights; some do not reach the free surface • Nuclei are seen at different layers • Function in secretion and propulsion of mucus • Present in the male sperm-carrying ducts (nonciliated) and trachea (ciliated) Epithelia: Stratified Squamous • Thick membrane composed of several layers of cells • Function in protection of underlying areas subjected to abrasion • Forms the external part of the skin’s epidermis (keratinized cells), and linings of the esophagus, mouth, and vagina (nonkeratinized cells). Epithelia: Stratified Columnar • Several cell layers with cuboidal basal cells and columnar superficial cells • Functions in protection and secretion • Present in large ducts of some glands, and in portions of the male urethra Epithelia: Transitional • Several cell layers, basal cells are cuboidal, surface cells are dome shaped • Stretches to permit the distension of the urinary bladder • Lines the urinary bladder, ureters, and part of the urethra Glandular Epithelia • A gland is one or more cells that makes and secretes an aqueous fluid • Classified by: • Site of product release – endocrine or exocrine • Relative number of cells forming the gland – unicellular or multicellular Endocrine Glands • Ductless glands that produce hormones • Secretions include amino acids, proteins, glycoproteins, and steroids Exocrine Glands • More numerous than endocrine glands • Secrete their products onto body surfaces (skin) or into body cavities • Examples include mucous, sweat, oil, and salivary glands • The only important unicellular gland is the goblet cell • Multicellular exocrine glands are composed of a duct and secretory unit Multicellular Exocrine Glands • Classified according to: • Simple or compound duct type • Structure of their secretory units Modes of Secretion • Merocrine – products are secreted by exocytosis (e.g., pancreas, sweat, and salivary glands) • Holocrine – products are secreted by the rupture of gland cells (e.g., sebaceous glands) Connective Tissue • Found throughout the body; most abundant and widely distributed in primary tissues • Connective tissue proper • Cartilage • Bone • Blood Functions of Connective Tissue • Binding and support • Protection • Insulation • Transportation Characteristics of Connective Tissue • Connective tissues have: • Mesenchyme as their common tissue of origin • Varying degrees of vascularity • Nonliving extracellular matrix, consisting of ground substance and fibers Structural Elements of Connective Tissue • Ground substance – unstructured material that fills the space between cells • Fibers – collagen, elastic, or reticular • Cells – fibroblasts, chondroblasts, osteoblasts, and hematopoietic stem cells Ground Substance • Interstitial (tissue) fluid • Adhesion proteins – fibronectin and laminin • Proteoglycans – glycosaminoglycans (GAGs) • Functions as a molecular sieve through which nutrients diffuse between blood capillaries and cells Fibers • Collagen – tough; provides high tensile strength • Elastic – long, thin fibers that allow for stretch • Reticular – branched collagenous fibers that form delicate networks Connective Tissue: Fundamental Cell Type • Fibroblasts – connective tissue p