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Tissues: Concept and Classification Tissues Concept and Classification

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Tissues: Concept and Classification Tissues Concept and Classification Tissues: Concept and Classification Tissues Concept and Classification *Tissues are aggregation or groups of cells organized in 3D, that function in a collective manner to perform one or more specific functions.. * Tissue classification based on structure of cells, composition of noncellular extracellular matrix, and cell function... * Major types of adult tissues - Epithelial (epithelial tissue) covers body surfaces, lines body cavities, and forms glands - Connective underlies or supports the other three basic tissues, both structurally and functionally. - Muscle is made up of contractile cells and is responsible for movement. - Nervous receives, transmits, and integrates information from outside and inside the body to control the activities of the body. Histogenesis of tissues Ectoderm Derivatives * Ectoderm is the outermost of the three germ layers. The derivatives of the ectoderm may be divided into two major classes: surface ectoderm and neuroectoderm. - Surface ectoderm gives rise to: • epidermis and its derivatives (hair, nails, sweat glands, sebaceous glands, and the parenchyma and ducts of the mammary glands), • cornea and lens epithelia of the eye, • enamel organ and enamel of the teeth, • components of the internal ear, • adenohypophysis (anterior lobe of pituitary gland), and • mucosa of the oral cavity and lower part of the anal canal. - Neuroectoderm gives rise to: • the neural tube and its derivatives, including components of the central nervous system, ependyma (epithelium lining the cavities of the brain and spinal cord), pineal body, posterior lobe of pituitary gland (neurohypophysis),and the sensory epithelium of the eye, ear, and nose; • the neural crest and its derivatives, including componentsof the peripheral nervous system (cranial, spinal, and autonomic ganglia, peripheral nerves, and Schwann cells); glialcells (oligodendrocytes and astrocytes); chromaffin (medullary) cells of the adrenal gland; enteroendocrine (APUD) cells of the diffuse neuroendocrine system; melanoblasts, the precursors of melanocytes; the mesenchyme of the head and its derivatives (such as pharyngeal arches that contain muscles, connective tissue, nerves, and vessels); odontoblasts; and corneal and vascular endothelium. Mesodermal Derivatives - Mesoderm is the middle of the three primary germ layers of an embryo. It gives rise to: • connective tissue, including embryonic connective tissue (mesenchyme), connective tissue proper (loose and dense connective tissue), and specialized connective tissues (cartilage, bone, adipose tissue, blood and hemopoietic tissue), and lymphatic tissue; • striated muscles and smooth muscles; • heart, blood vessels, and lymphatic vessels, including their endothelial lining; • spleen; • kidneys and the gonads (ovaries and testes) with genital ducts and their derivatives (ureters, uterine tubes, uterus, ductus deferens); • mesothelium, the epithelium lining the pericardial, pleural, and peritoneal cavities; and • the adrenal cortex. Endodermal Derivatives - Endoderm is the innermost layer of the three germ layers. In the early embryo it forms the wall of the primitive gut and gives rise to epithelial portions or linings of the organs arising from the primitive gut tube. Derivatives of the endoderm include: • alimentary canal epithelium (excluding the epithelium of the oral cavity and lower part of the anal canal, which are of ectodermal origin); • extramural digestive gland epithelium (e.g., the liver, pancreas, and gallbladder); • lining epithelium of the urinary bladder and most of the urethra; • respiratory system epithelium; • thyroid, parathyroid, and thymus gland epithelial components; • parenchyma of the tonsils; • lining epithelium of the tympanic cavity and auditory (Eustachian) tubes. Derivatives of the three germ layers Epithelium Tissue Learning objectives: - Understand the general features and classification of epithelial tissue. - Be able to correlate different types of epithelia to their functions. - Know the structure of apical , basal, and lateral specializations and their functions. - Understand the L. M structure, E. M structured features and functions of microvilli and cilia. - Understand the ultra-structure features and functions of varieties of intercellular conjunctions. - Understand the position, L. M structure, ultra-structure and function of basement membrane. - Know the conception of glandular cells, glandular epithelium, and the morphological classification of exocrine glands. Epithelial tissue; General Characteristics & Functions * Epithelium is an avascular tissue composed of cells that: - Covers exterior body surface and lines internal closed cavities body cavity and body tubes. - Forms the secretory portion (parenchyma) of glands and their ducts. - Specialized epithelial cells function as receptors for the special senses Special Characteristics of Epithelia (General structural features) * Cellularity - cells are in close contact and adhere to one another by means of specific cell-to-cell adhesion molecules with little or no intercellular space between them. * Specialized contacts - may have junctions for both attachment and communication * Polarity - epithelial tissues always have free surface or apical domain, a lateral domain, and a basal domain. * Support by connective tissue - at the basal surface, both the epithelial tissue and the connective tissue contribute to the basement membrane * Avascular - nutrients must diffuse * Innervated * Regenerative - epithelial tissues have a high capacity for regeneration Special Characteristics of Epithelia * In special situations, epithelial cells lack a free surface ( Epithelioid tissues). - Epithelioid organization is typical of most endocrine glands. - Epithelioid patterns are also formed by accumulations of connective tissue macrophages in response to certain types of injury and infections as well as by many tumors derived from epithelium. * Functions of epithelium Epithelium may serve one or more functions, depending on the activity of the cell types that are present: - Mechanical protection, - Absorption, - Secretion, - Transportation - Filtration (Selective barrier ), - Forms slippery surfaces and - Sensory reception * Basal cells are the stem cells that give rise to the mature functional cells of the epithelium, thus balancing cell turnover. Classifications of Epithelia * First name of tissue indicates number of layers - Simple – one layer of cells - Stratified – more than one layer of cells Classifications of Epithelia * Last name of tissue describes shape of cells - Squamous – cells wider than tall (plate or “scale” like) - Cuboidal – cells are as wide as tall, as in cubes - Columnar – cells are taller than they are wide, like columns Classification of epithelial tissue on the base of the function Naming Epithelia * Naming the epithelia includes both the layers (first) and the shape of the cells (second) - i.e. stratified cuboidal epithelium * The name may also include any accessory structures or specialization of the apical cell surface domain. - Mucous cells - Microvilli - Cilia - Keratin * Special epithelial tissues (don’t follow naming convention - Psuedostratified - Transitional (urothelium) is a stratified epithelium * The cells in some exocrine glands are more or less pyramidal. Simple Squamous Epithelium * Description - single layer of flat cells with disc-shaped nuclei * Special types - Endothelium (inner covering) . lining of the blood and lymphatic vessels. - Endocardium . lining of ventricles and atria of the heart. - Mesothelium (middle covering) . Lines peritoneal, pleural, and pericardial cavities . Covers visceral organs of those cavities Simple Squamous Epithelium * Function - Transepithelial transport by passive diffusion and filtration - Secretes lubricating substances in serous membranes * Location - Renal corpuscles - Alveoli of lungs - Lining of heart, blood and lymphatic vessels - Lining of ventral body cavity (serosae/serous memb.) Simple Squamous Epithelium Simple Squamous – Endothelium (En) Simple Squamous – Endothelium (Ed) by TEM Endothelium Simple squamous epithelial line loop of Henle Simple squamous (Surface of mesothelium) Cell margins nuclei Bio 348 Lapsansky - 2007 Mesothelium and endothelium Mesothelium Simple Squamous Epithelium (mesothelium) Surface view of lining of peritoneal cavity • Section of intestine showing serosa Simple Squamous (Mesothelium) by TEM Simple Cuboidal Epithelium * Description - single layer of cube-like cells with large, spherical central nuclei * Function - secretion and absorption * Location - kidney tubules, secretory portions of small glands, ovary surface Simple Cuboidal Epithelium Simple Cuboidal by TEM Simple Columnar Epithelium * Description - single layer of column-shaped (rectangular) cells with oval nuclei . Some bear cilia at their apical surface . Some bear microvilli ------------ . May contain goblet cells . Some consist of mucus cells * Function - Absorption; secretion of mucus, enzymes, and other substances (The height of the cells often reflects the level of secretory or absorptive activity). - Ciliated type propels mucus or reproductive cells by ciliary action Simple Columnar Epithelium * Location - Consist of mucus cells . Lines stomach - Bear microvilli . Lines digestive tract, gallbladder (striated border) , kidney tubule (brush border) and ducts of some gland (striated duct ). - Ciliated form . Uterine tubes, and uterus. Simple Columnar mucus secreting ( stomach) Simple Columnar Epithelium (Human duodenum.) Simple Columnar Epithelium
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