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• Tissues are:– Collections of specialized cells and cell products
organized to perform a limited number of functions• Histology = study of tissues
• The four tissue types are:– Epithelial– Connective– Muscular– Nervous
Tissues and tissue types
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, cardiovascular and urogenital
systems - Lining of many body cavities
• Polarity - Has apical, basal, and lateral surfaces• Rests on a basement membrane• Avascular - no blood vessels• Regenerative -Replaces lost cells by cell division
Special Characteristics of Epithelia
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
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
Classifications of Epithelia
Simple Squamous Epithelium
Figure 4.3a
Simple Cuboidal Epithelium
Simple Columnar Epithelium
Figure 4.3c
Pseudostratified Ciliated Columnar Epithelium
Figure 4.3d
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
Stratified Squamous Epithelium
Figure 4.3e
Transitional Epithelium
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
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: DesmosomeLinker proteins extend from plaque like teeth of a zipper.Intermediate filaments extend across width of cell.
Membrane Junctions: Gap Junction
• Connexon proteins are trans- membrane proteins.
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
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
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
Connective Tissue: Embryonic Origin
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 – 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 in 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.
Areolar Connective Tissue
Figure 4.12b
Adipose Tissue
Figure 4.12c
Reticular Connective Tissue
Figure 4.12d
Dense Irregular Connective Tissue
Figure 4.12e
Dense Regular Connective Tissue
Figure 4.12f
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
Hyaline Cartilage
Figure 4.12g
Elastic Cartilage
Figure 4.12h
Fibrocartilage
Figure 4.12i
Bone Tissue
Figure 4.12j
Blood Tissue
Figure 4.12k
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.
Skeletal Muscle Tissue
Figure 4.14a
Cardiac Muscle Tissue
Figure 4.14b
Smooth Muscle Tissue
Nervous Tissue