Basic Histology and Connective Tissue Chapter 5

Basic Histology and Connective Tissue Chapter 5 • Histology, the Study of Tissues • Tissue Types • Connective Tissues Histology is the Study of Tissues • 200 different types of cells in the human body. • A Tissue consist of two or more types of cells that function together. • Four basic types of tissues: – epithelial tissue – connective tissue – muscular tissue – nervous tissue • An Organ is a structure with discrete boundaries that is composed of 2 or more tissue types. • Example: skin is an organ composed of epidermal tissue and dermal tissue. Distinguishing Features of Tissue Types • Types of cells (shapes and functions) • Arrangement of cells • Characteristics of the Extracellular Matrix: – proportion of water – types of fibrous proteins – composition of the ground substance • ground substance is the gelatinous material between cells in addition to the water and fibrous proteins • ground substance consistency may be liquid (plasma), rubbery (cartilage), stony (bone), elastic (tendon) • Amount of space occupied by cells versus extracellular matrix distinguishes connective tissue from other tissues – cells of connective tissues are widely separated by a large amount of extracellular matrix – very little extracellular matrix between the cells of epithelia, nerve, and muscle tissue Embryonic Tissues • An embryo begins as a single cell that divides into many cells that eventually forms 3 Primary Layers: – ectoderm (outer layer) • forms epidermis and nervous system – endoderm (inner layer) • forms digestive glands and the mucous membrane lining digestive tract and respiratory system – mesoderm (middle layer) • Forms muscle, bone, blood and other organs. Histotechnology • Preparation of specimens for histology: – preserve tissue in a fixative to prevent decay (formalin) – dehydrate in solvents like alcohol and xylene – embed in wax or plastic – slice into very thin sections only 1 or 2 cells thick – float slices on water and mount on slides and then add color with stains • Sectioning an organ or tissue reduces a 3-dimensional structure to a 2- dimensional slice. Planes of Section Longitudinal section – tissue cut along the longest direction of a structure Cross section – tissue cut perpendicular to the length of a structure Oblique section – tissue cut at an angle between a cross section and a longitudinal section Two Dimensional Sections of Solid Three Dimensional Objects 1 2 3 4 5 • Slicing through a boiled egg is similar to sectioning a cell and its nucleus. 1 2 5 4• Slices 1 and 5 miss the yolk. • Yolk appears larger in section 3 than in sections 2 and 4. 3Sections of Complex Hollow Structures <ul style="display: flex;"><li style="flex:1">A</li><li style="flex:1">B</li></ul>• Image A is a cross section of a curved tubular structure like a blood vessel or a section of intestine. • Image B is a longitudinal section of a spiraling, tubular structure like a sweat gland. • Notice what a single slice could look like. Epithelial Tissue (Epithelia) • One or more layers of closely adhering cells. • Forms a flat sheet with an unattached free surface (may be exposed to the environment or an internal body cavity) and a basal surface attached to the basement membrane made of collagen. • Epithelia are avascular. Epithelial cells depend on diffusion of nutrients from capillaries in the underlying connective tissue or from the free surface. • Epithelia are innervated by sensory neurons. • Basement membrane is a is semi-permeable layer of collagen and adhesive proteins that anchors epithelial cells to underlying connective tissue. • The connective tissue under an epithelium is called the lamina propria. Free Surface Basal Surface Lamina Propria Naming Epithelia Epithelia are named for: • the number of layers of cells – simple epithelium = one layer of cells – stratified epithelium = more than one layer of cells – pseudostratified epithelium = simple that looks stratified • the shape of cells at the surface – squamous – cuboidal – columnar – transitional • surface modifications – cilia – microvilli – keratinization Simple Squamous Epithelium • Single row of squamous (flat) cells. • Can allow rapid diffusion of substances or secretion of fluid. • Example: lining of blood vessels or lining of lung alveoli Simple Cuboidal Epithelium • Single row of cube-shaped cells • Functions include absorption, secretion, conduction • Example: most kidney tubules Simple Columnar Epithelium Microvilli Absorptive Cell Mucus Goblet Cell Nucleus • Single row of tall, narrow cells • Free Surface may have microvilli or cilia • Layer of microvilli is called the brush border • Functions: absorption, secretion (of mucus) • Example: Lines the intestines Pseudostratified Epithelium Cilia Goblet Cells Basal Cells • Single row of cells all attached to basement membrane • Not all cells reach the free surface – nuclei of basal cells give a stratified appearance • Secretes and propels respiratory mucus • Example: lining of trachea Mucous Membranes • Consists of a mucous-producing epithelium and underlying layers of connective tissue (lamina propria) and smooth muscle (muscularis mucosae). • Lines passageways that open to the exterior: digestive, respiratory, urinary and reproductive tracts. • Mucous forms a barrier and traps foreign particles or pathogens. • Epithelia of upper respiratory tract and parts of the reproductive tract (oviducts) are ciliated to sweep the mucous out of the body. Stratified Epithelia • Composed of more than one layer of cells. • Always named for shape of surface cells. • Deepest cells sit on basement membrane and are the source of replacement cells for the epithelium. • Keratinization: – keratinized epithelium has surface layer of dead cells that contain abundant protein and are surrounded by lipids – nonkeratinized epithelium has living cells with nuclei in all layers Nonkeratinized Stratified Squamous • Stratified epithelium of living cells forms an abrasion-resistant, moist, slippery layer. • Examples: lining of the mouth, esophagus, vagina Keratinized Stratified Squamous Epithelium dead, keratinized epithelial cells living epithelial cells connective tissue • Surface layer of dead squamous cells surrounded by lipids and packed with granules of keratin protein. • Dead layer is “keratinized” or “cornified”. • Retards water loss and prevents penetration of microorganisms. • Example: skin Stratified Cuboidal and Columnar Epithelium <ul style="display: flex;"><li style="flex:1">sweat gland duct </li><li style="flex:1">kidney collecting duct </li></ul>• In certain ducts, stratified columnar and cuboidal epithelia can occur. As epithelial types, both are uncommon. Basal cells are typically cuboidal with surface cells either columnar or cuboidal. • Example: large ducts of salivary glands Stratified Columnar Epithelium Transitional Epithelium • Stratified epithelium with rounded (domed) surface cells. • Stretches to allow storage of urine. • Example: urinary bladder. Quiz is on material up to this point. Intercellular Junctions • All cells except blood cells are anchored to each other or to the matrix surrounding them by intercellular junctions. Tight Junctions • Tight junctions completely encircle the cell (like a sweat band around a person’s head) • Tight Junctions form a zipper-like pattern of complementary grooves and ridges that prevent substances and bacteria from passing between cells. Tight Junctions Desmosomes • Attachment between cells that holds them together against mechanical stress (shearing forces). • A mesh of protein filaments connects integral membrane proteins and cytoskeletal proteins. • Abundant in muscle and skin • Hemidesmosomes attach cells to the basement membrane. Desmosome Hemidesmosome Gap Junctions • Also called communicating junctions. • Cluster of tube-shaped transmembrane proteins that make channels between cells. • Small solutes and electrical signals pass directly from cell to cell and can synchronize the activity of groups of cells. • Found in embryos, cardiac muscle and smooth muscle. Gap Junction Glands • Glands secrete substances for elimination or for use elsewhere in the body • Glands are composed predominantly of epithelial tissue • Exocrine glands maintain connection to the surface through a duct (examples: sweat glands, salivary glands) • Endocrine glands have no ducts but secrete their products (hormones) onto capillaries for absorption directly into bloodstream (pituitary, adrenal) or into interstitial fluid • Mixed organs have both types of glands: – pancreas secretes digestive enzymes into ducts and hormones into blood – gonads release gametes into ducts and secrete hormones into blood Types of Glandular Secretions • Serous – thin, watery secretions such as sweat, milk, tears and digestive juices. • Mucus – the sticky secretion called mucus is a glycoprotein, mucin, that absorbs water • Mixed Glands secrete both serous fluid and mucus • Note: Mucus is a noun. Mucous is an adjective. “Mucus is secreted by mucous glands.” • Cellular mechanisms of glandular secretion include: 1) merocrine 2) apocrine 3) holocrine

Basic Histology and Connective Tissue Chapter 5

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