Epithelium, cells,tissues & histology

Chapter 4Tissues 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

Classifications of Epithelia

Simple Squamous Epithelium

Figure 4.3a

Simple Cuboidal Epithelium

Figure 4.3b

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

Figure 4.3h

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

Figure 3.5b

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

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

Connective Tissue: Embryonic Origin

Figure 4.5

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.

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

Figure 4.14c

Nervous Tissue

Figure 4.15

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