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 5 • Slices 1 and 5 miss the yolk. • Yolk appears larger 2 3 4 in section 3 than in sections 2 and 4. Sections of Complex Hollow Structures A B • 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 sweat gland duct kidney collecting duct • 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 Merocrine Secretion • Cells of Merocrine Glands release their product by exocytosis. • Clusters of secretroy Duct cells are called acini. • Products include tears, sweat, milk, pancreatic enzymes, gastric enzymes and acid Acinus Cellular Mechanisms of Glandular Secretion 1) Merocrine secretion is the most common type of glandular secretion. Secretory cells produce secretory granules from the Golgi. Secretory granules gather at the apical region of the cell. Then, the granule’s membrane fuses with the apical membrane of the cell and the contents of the granule are opened and released by the process of exocytosis. Cellular Mechanisms of Glandular Secretion