Chapter 4 Tissues and Histology• Tissues - collections of similar cells and the substances surrounding them• Tissue classification based on structure of cells, composition of noncellular extracellular matrix, and cell function• Major types of adult tissues – Epithelial – Connective – Muscle – Nervous• Histology: Microscopic Study of Tissues – Biopsy: removal of tissues for diagnostic purposes – Autopsy: examination of organs of a dead body to determine cause of death
Embryonic Tissue• 3 major germ layers that form the embryonic disc (source of stem cells) – Endoderm • Inner layer • Forms lining of digestive tract and derivatives – Mesoderm • Middle layer • Forms tissues as such muscle, bone, blood vessels – Ectoderm • Outer layer • Forms skin and neuroectoderm
I. Epithelial Tissue • Cellularity - Consists almost entirely of cells • Covers body surfaces, lines hollow organs, and forms glands – Outside surface of the body – Lining of digestive, respiratory and urogenital systems – Heart and blood vessels – Linings of many body cavities • Polarity - Has apical, basal, and lateral surfaces • Rests on a basement membrane • Specialized cell contacts bind adjacent cells together • Avascular - no blood vessels • Regenerative -Replaces lost cells by cell division
Functions of Epithelia• Protecting underlying structures; e.g., epithelium lining the mouth• Acting as barriers; e.g., skin• Permitting the passage of substances; e.g., cells lining air sacs in lungs and nephrons in kidney• Secreting substances; e.g., pancreatic cells• Absorbing substances; e.g., lining of stomach and small intestine
Special Characteristics of Epithelia Figure 4.1
Classification of Epithelium• Number of layers of cells – Simple- one layer of cells. Each extends from basement membrane to the free surface – Stratified- more than one layer. – Pseudostratified- tissue appears to be stratified, but all cells contact basement membrane so it is in fact simple• Shape of cells – Squamous- flat, scale-like – Cuboidal- about equal in height and width – Columnar- taller than wide
Stratified Epithelia• Contain two or more layers of cells• Regenerate from below• Major role is protection• Are named according to the shape of cells at apical layer
Stratified Squamous Epithelium• Description – Many layers of cells – squamous in shape – Deeper layers of cells appear cuboidal or columnar – Thickest epithelial tissue – adapted for protection
Stratified Squamous Epithelium• Specific types – Keratinized – contain the protective protein keratin • Surface cells are dead and full of keratin – Non-keratinized – forms moist lining of body openings
Stratified Squamous Epithelium• Function – Protects underlying tissues in areas subject to abrasion• Location – Keratinized – forms epidermis – Non-keratinized – forms lining of esophagus, mouth, and vagina
Epithelium: Glandular• A gland is one or more cells that makes and secretes an aqueous fluid• Two types of glands formed by infolding of epithelium: – Endocrine: no contact with exterior of body; ductless; produce hormones (pituitary, thyroid, adrenals, pancreas) – Exocrine: open to exterior of body via ducts (sweat, oil)• Exocrine glands classified either by structure or by the method of secretion• Classified by structure – Unicellular: goblet cells – Multicellular: sweat, oil, pituitary, adrenal
Multicellular Exocrine Glands• Classified on the basis of types of ducts or mode of secretion• Types of ducts – Simple: ducts with few branches – Compound: ducts with many branches • If ducts end in tubules or sac-like structures: acini. Pancreas • If ducts end in simple sacs: alveoli. Lungs
Lateral Surface Features• Tight junctions• Desmosomes• Gap junctions
Membrane Junctions: Tight Junction • Integral proteins of adjacent cells fuse together • Completely encircle the cell and form an adhesion belt. • Form an impermeable junction. • Common near apical region
Lateral Surface Features – Cell Junctions• Desmosomes – two disc-like plaques connected across intercellular space – Plaques of adjoining cells are joined by proteins called cadherins – Proteins interdigitate into extracellular space – Intermediate filaments insert into plaques from cytoplasmic side
Membrane Junctions: Desmosome Linker proteins extend from plaque like teeth of a zipper. Intermediate filaments extend across width of cell.• Common in superficial layers of skin; skin peels after a sunburn• Reduces chance of tearing, twisting, stretching Figure 3.5b
Membrane Junctions: Gap Junction• Connexon proteins are trans- membrane proteins.• Present in electrically excitable tissues (heart, smooth muscle)
Basal Feature: The Basal Lamina• Noncellular supporting sheet between the epithelium and the connective tissue deep to it• Consists of proteins secreted by the epithelial cells• Functions: • Acts as a selective filter, determining which molecules from capillaries enter the epithelium • Acts as scaffolding along which regenerating epithelial cells can migrate• Basal lamina and reticular layers of the underlying connective tissue deep to it form the basement membrane
Epithelial Surface Features• Apical surface features – Microvilli – finger-like extensions of plasma membrane • Abundant in epithelia of small intestine and kidney • Maximize surface area across which small molecules enter or leave – Cilia – whip-like, highly motile extensions of apical surface membranes • Movement of cilia – in coordinated waves
Connective Tissue• Most diverse and abundant tissue• Main classes – Connective tissue proper – Cartilage – Bone tissue – Blood• Characteristics – Mesenchyme as their common tissue of origin (mesenchyme derived from mesoderm) – Varying degrees of vascularity – Nonliving extracellular matrix, consisting of ground substance and fibers – Cells are not as abundant nor as tightly packed together as in epithelium
Functions of Connective Tissue• Enclose organs as a capsule and separate organs into layers. Areolar• Connect tissues to one another. Tendons and ligaments.• Support and movement. Bones.• Storage. Fat.• Insulation. Fat.• Transport. Blood.• Protection. Bone, cells of the immune system.
Structural Elements of Connective Tissue• Ground substance – unstructured material that fills the space between cells• Fibers – collagen, elastic, or reticular• Cells – fibroblasts, chondroblasts, osteoblasts, hematopoietic stem cells, and others
Connective Tissue Cells• Fibroblasts - secrete the proteins needed for fiber synthesis and components of the extracellular matrix• Adipose or fat cells (adipocytes). Common in some tissues (dermis of skin); rare in some (cartilage)• Mast cells. Common beneath membranes; along small blood vessels. Can release heparin, histamine, and proteolytic enzymes in response to injury.• Leukocytes (WBC’s). Respond to injury or infection• Macrophages. Derived from monocytes (a WBC). Phagocytic; provide protection• Chondroblasts - form cartilage• Osteoblasts - form bone• Hematopoietic stem cells - form blood cells• Undifferentiated mesenchyme (stem cells). Have potential to differentiate into adult cell types.
Extracellular Matrix - ECM• ECM has 3 major components 1. Protein fibers 2. Ground substance 3. Fluid• Protein fibers – Collagen fibers. Composed of the protein collagen. Strong, flexible, inelastic; great tensile strength (i.e. resist stretch). Perfect for tendons, ligaments – Elastic fibers. Contain molecules of protein elastin that resemble coiled springs. Returns to its original shape after stretching or compression. Perfect for lungs, large blood vessels – Reticular fibers. Formed from fine collagenous fibers; form branching networks (stroma). Fill spaces between tissues and organs.
Ground Substance• Interstitial (tissue) fluid within which are one or more of the molecules listed below: – Hyaluronic acid: a polysaccharide. Very slippery; serves as a good lubricant for joints. Common in most connective tissues. – Proteoglycans: protein and polysaccharide complex. Polysaccharides called glyocosaminoglycans (chondroitin sulfate, keratin sulfate). Protein part attaches to hyaluronic acid. Able to trap large amounts of water. – Adhesive molecules: hold proteoglycan aggregates together. Chondronectin in cartilage, osteonectin in bone, fibronectin in fibrous connective tissue.• Functions as a molecular sieve through which nutrients diffuse between blood capillaries and cells
Embryonic Connective Tissue • Mesenchyme: source of all adult connective tissue. – Derived from mesoderm – Delicate collagen fibers embedded in semifluid matrix • Mucus: found only in the umbilical cord. Wharton’s jelly.
Elastic Connective Tissue• Bundles and sheets of collagenous and elastic fibers oriented in multiple directions• In walls of elastic arteries (aorta), lungs, vocal ligaments• Strong, yet elastic; allows for recoil of tissue after being stretched
Connective Tissue: Cartilage• Composed of chondrocytes (cells) located in matrix- surrounded spaces called lacunae.• Type of cartilage determined by components of the matrix.• Firm consistency.• Ground substance: Proteoglycans and hyaluronic acid complexed together trap large amounts of water (microscopic sponges). Allows tissue to spring back after being compressed.• Avascular and no nerve supply. Heals slowly.• Perichondrium. Dense irregular connective tissue that surrounds cartilage. Fibroblasts of perichondrium can differentiate into chondroblasts (cartilage-forming cells)• Types of cartilage – Hyaline – Fibrocartilage – Elastic
Muscle Tissue• Characteristics – Cells are referred to as fibers – Contracts or shortens with force when stimulated – Moves entire body and pumps blood• Types – Skeletal:attached to bones – Cardiac: muscle of the heart. – Smooth: muscle associated with tubular structures and with the skin. Nonstriated and involuntary.
Tissues and Aging• Cells divide more slowly• Collagen fibers become more irregular in structure, though they may increase in number – Tendons and ligaments become less flexible and more fragile• Elastic fibers fragment, bind to calcium ions, and become less elastic – Arterial walls and elastic ligaments become less elastic• Changes in collagen and elastin result in – Atherosclerosis and reduced blood supply to tissues – Wrinkling of the skin – Increased tendency for bones to break• Rate of blood cell synthesis declines in the elderly• Injuries don’t heal as readily