• 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