Week 2 Notes: Tissues and Glands (Epithelium, Connective Tissue, Membranes, Inflammation, Aging)

Epithelium and Tissues: Week 2 Notes

• Histology and tissues

Histology is the study of tissue structure and organization

A tissue is a group of cells with similar structure and function plus extracellular substance surrounding them

Major tissue types: epithelial, connective, muscle, nervous

• Focus on epithelium

Epithelium covers and protects surfaces, both external and internal

Includes glands: exocrine glands (ducted) and endocrine glands (ductless)

• Key properties and features of epithelial tissues

Characteristics of epithelial tissue

Usually composed of cells with very little extracellular material

Covers body surfaces and lines interior surfaces

Free/apical surface faces the lumen or exterior

Basal surface attached to the basement membrane

Specialized cell connections and matrix attachments

Avascular (no blood vessels); relies on diffusion from underlying tissues

Capable of regeneration

Basement membrane connects epithelium to underlying connective tissue

Nucleus and DNA: nucleus houses the cell’s genetic material which directs protein synthesis and cell activity

Exposed/free surface (apical) and basal surface

Epithelium functions:

Protect underlying structures (e.g., skin epithelium shields tissues from physical injury, microbes, dehydration)

Act as a selective barrier: regulate entry/exit (e.g., skin, GI lining)

Permit passage of substances via diffusion, filtration, osmosis (e.g., O₂ diffusion from lungs into blood)

Secrete substances via glandular epithelium (enzymes, hormones, mucus; includes sweat, saliva, endocrine glands)

Absorb nutrients and substances (e.g., microvilli in the small intestine)

• Classification of epithelial tissues

Based on two criteria:

Number of cell layers: $ext{simple}$ (one layer), $ext{stratified}$ (two or more layers), $ext{pseudostratified}$ (appears multi-layered but is a single layer)

Shape of superficial cells: $ext{squamous}$ (flat), $ext{cuboidal}$ (cube-like), $ext{columnar}$ (tall rectangle); sometimes transitional shape

Summary: epithelial tissues are classified by (i) number of cell layers and (ii) cell shape

• Simple epithelia (one layer)

Simple squamous epithelium

Structure: single, thin, flat cells

Function: diffusion and filtration; some secretion; reduces friction

Location: lining of blood vessels and heart (endothelium), all over internal surfaces where rapid exchange occurs

Simple cuboidal epithelium

Structure: cube-like cells

Function: secretion and absorption; higher secretory capacity than simple squamous

Location: kidney tubules, ducts of glands, choroid plexus, terminal bronchioles with ciliated cells, surface of ovaries

Simple columnar epithelium

Structure: tall, slender cells; nuclei aligned near base

Function: secretion (enzymes, mucus) and absorption; supports movement of particles

Specialized features: microvilli increase surface area for absorption; cilia move mucus in some contexts

Location: small intestine (intestinal absorption with microvilli), stomach lining (secretion), uterus, fallopian tubes (movement of ovum with cilia)

Pseudostratified columnar epithelium

Structure: appears multi-layered due to varying cell heights but is a single layer; often has goblet cells and hair-like cilia

Function: secretion of mucus; movement of mucus by cilia

Location: usually in upper respiratory tract (trachea, bronchi)

• Stratified epithelia (two or more layers)

General: deepest layer is attached to basement membrane; surface layers are more layered and protective

Stratified squamous epithelium

Structure: several layers; surface cells may be cuboidal or flattened as they migrate outward

• Keratinized vs nonkeratinized

Keratinized: outermost cells dead and filled with keratin; forms waterproof, protective skin surface; examples: skin, palms, soles

Nonkeratinized: surface cells alive with nuclei; moist surfaces; examples: oral cavity, esophagus, vagina, conjunctiva

Functions: protection against abrasion and infection; reduces water loss (keratinized) or provides moisture (nonkeratinized)

Location: keratinized outer layer of skin; nonkeratinized in mouth, throat, esophagus, vagina, cornea

Transitional epithelium (urothelium)

Structure: stratified epithelium that can stretch; cells change shape when stretched

Function: accommodates fluctuations in fluid volume; protects underlying tissues from urine

Location: lining of urinary bladder, part of ureters and urethra

• Free surface modifications (apical surface adaptations)

Microvilli: increase surface area for absorption/secretion; example: small intestinal epithelium

Cilia: move mucus, debris along the tract; present in simple columnar and pseudostratified columnar epithelia

Free surface modifications enhance absorption, secretion, protection, and movement

• Cell connections in epithelia

Desmosomes: mechanical links that bind cells together

Hemidesmosomes: anchor basal cells to the basement membrane

Tight junctions (zonula occludens): seal the space between cells; prevent passage of substances between cells; regulate paracellular transport; found in intestinal lining

Adhesion belts (zonula adherens): link adjacent cells to each other

Gap junctions: allow small molecules/ions to pass between cells; may coordinate cell activities (communication)

• Glands (secretory glands)

Glands secrete substances into surfaces or into the bloodstream

Composition: primarily epithelium with a connective tissue framework

Exocrine glands: have ducts; secrete onto surfaces or into cavities (e.g., saliva, sweat, pancreatic ducts)

Endocrine glands: ductless; secrete hormones into blood

Multicellular exocrine glands classified by:

Duct structure: simple vs compound

Secretory region shape: tubular (tubular), acinar/alveolar (acinar), or tubuloacinar (both)

Unicellular glands: single cells (e.g., goblet cells) that secrete mucus into epithelium

Simple glands examples:

Simple tubular glands: straight tubular secretory portion (e.g., stomach and colon)

Simple branched tubular glands: multiple tubular secretory portions from a single duct (stomach)

Simple acinar (alveolar) glands: secretory sac-like portion (sebaceous glands)

Simple branched acinar glands: several acinar portions from a single duct (sebaceous)

Compound gland examples:

Compound tubular glands (e.g., mucous glands in the duodenum)

Compound tubuloacinar glands (pancreas, mucous glands with both tubular and acinar portions)

Compound acinar glands (mammary glands)

Modes of secretion (secretory mechanisms):

Merocrine (exocytosis): most common (e.g., sweat glands)

Apocrine: release of secretory products with part of the cell membrane/portion of the cell (e.g., mammary glands during milk production)

Holocrine: shedding of entire secretory cells (e.g., sebaceous glands)

• Connective tissue: overview

Connective tissues are diverse and constitute a major portion of most organs

They differ from other tissues by having cells separated by an abundant extracellular matrix

Components: cells, protein fibers, ground substance, and fluid

Primary functions: enclose and separate tissues, connect and support body parts, protect, cushion, and insulate, store energy, transport substances, and provide immune defense

Common connective tissue cells reflect function (e.g., osteoblasts/blasts/siteds; fibroblasts/ fibrocytes; chondroblasts/chondrocytes; immune cells like macrophages, mast cells)

Extracellular matrix components:

Protein fibers: collagen, reticular fibers, elastic fibers

Ground substance: proteoglycans; glycosaminoglycans; water retention enabling resilience

Fluid: interstitial fluid and plasma components

Three major components of ECM (in general): $ext{protein fibers}, ext{ground substance}, ext{fluid}$

The matrix and its components determine tissue properties (e.g., bones and cartilage bear weight; elasticity of tissues)

Cell types include:

Osteoblasts/osteocytes/osteoclasts (bone)

Fibroblasts/ fibrocytes (connective tissue proper)

Chondroblasts/ chondrocytes (cartilage)

Macrophages (phagocytosis)

Mast cells (histamine release in inflammation)

• Embryonic vs. adult connective tissue

Embryonic connective tissue predominates early in development; by about eight weeks most become specialized into adult types

Loose connective tissue: relatively few protein fibers in a loose network with abundant ground substance

Subtypes: areolar, adipose, reticular

Areolar connective tissue

Structure: loose network of collagen and elastic fibers with ample spaces; cells include fibroblasts, macrophages, lymphocytes

Function: cushioning and supporting structures; provides nourishment

Adipose tissue (fat): adipocytes with large lipid stores; energy storage; insulation and padding

Reticular tissue: forms supportive framework of lymphatic tissues (spleen, lymph nodes) and bone marrow

• Dense connective tissues

Dense layers have many protein fibers and less ECM; categorized by fiber orientation

Dense collagenous connective tissue

Dense regular: collagen fibers aligned in the same direction; tendons and ligaments

Dense irregular: collagen fibers arranged in multiple directions; dermis, organ capsules

Dense elastic connective tissue: abundant elastic fibers interspersed with collagen; allows stretching and recoil (e.g., vocal cords; can be involved in Marfan syndrome when elastic fiber maintenance is defective)

• Supporting connective tissue: cartilage

Cartilage structure: chondrocytes in lacunae; extensive ECM rich in proteoglycans and collagen; matrix traps water for resilience

Functions: support and flexible framework; resists compression and bending; returns to original shape after deformation

Types of cartilage:

Hyaline cartilage: most abundant; covers ends of bones in joints; forms cartilaginous parts of the respiratory tract; nasal cartilage; attaches to sternum via costal cartilage

Fibrocartilage: with more collagen; withstands compression; found in intervertebral discs, knee joints, TMJ

Elastic cartilage: contains elastic fibers; able to recoil; found in external ear (pinna), epiglottis, auditory tube

• Bone (osseous tissue)

Structure: hard connective tissue with cells (osteocytes) in lacunae; mineralized matrix provides strength and rigidity

Types: spongy (trabecular) bone and compact bone

Spongy bone: porous with trabeculae; spaces contain marrow

Compact bone: dense with mineralized matrix; provides strong outer shell and weight-bearing support

Functions: support, protection, leverage for movement, mineral storage, hematopoiesis (in bone marrow)

• Blood: a connective tissue

Composition: liquid ECM (plasma) with formed elements (erythrocytes, leukocytes, platelets)

Functions: transport of oxygen, carbon dioxide, nutrients, hormones; immune functions via leukocytes; clotting via platelets

• Muscle tissue overview

Main function: contraction and shortening for movement

Three types:

Skeletal muscle: voluntary movement; striated appearance

Cardiac muscle: heart muscle; pump blood; involuntary; striated and interconnected

Smooth muscle: walls of hollow organs; involuntary; non-striated

• Nervous tissue

Origin: brain, spinal cord, nerves

Function: coordinate and control body activities via electrical signals

Components: neurons (cell body, dendrites, axon) and glial (support) cells

Neurons transmit action potentials and integrate information

• Membranes (tissue membranes)

Thin layers that cover surfaces or line cavities

Three primary internal membranes: mucous, serous, synovial

Cutaneous or integumentary membrane = skin (external surface)

Mucous membranes

Line digestive, respiratory, and reproductive tracts

Components: epithelium, basement membrane, and loose connective tissue (lamina propria)

Many mucous membranes secrete mucus; functions include protection, absorption, and secretion

Serous membranes

Line cavities that do not open to the exterior (e.g., pleura, pericardium, peritoneum)

Composition: simple squamous epithelium, basement membrane, and delicate loose connective tissue

Synovial membranes

Composed entirely of connective tissue; line joints

Produce synovial fluid for joint lubrication; dysfunction contributes to arthritis

• Inflammation and healing

Inflammation occurs after tissue damage; classic signs: redness, heat, swelling, pain, and impaired function

Chemical mediators: histamine and prostaglandins released by injured tissue and surrounding vessels

Process overview: mediators increase vascular permeability; clotting proteins and leukocytes migrate to damaged tissue

Phases of inflammation and healing

Initial injury and clot formation (scab formation)

Inflammatory response with immune cell recruitment

Tissue repair: regeneration (repair by same cell type) or replacement (scar formation; loss of some function)

Wound contraction and restoration of tissue integrity

Treatments mentioned in the transcript

Antihistamines block histamine effects

Aspirin inhibits prostaglandin synthesis

Cortisone reduces release of inflammatory mediators

• Wound healing specifics (skin injury)

Regeneration: same cell type replaces destroyed cells, restoring function

Replacement: new tissue type forms, potentially forming a scar

Steps in skin repair: blood clot forms, inflammatory response expands, edges of wound migrate and proliferate, granulation tissue forms (fibroblasts, collagen, capillaries), granulation tissue matures into normal connective tissue, wound contracts as it heals

• Aging of tissues

Aging effects on tissues and cells

Cell division slows down with age

Healing rate and blood cell production decline

ECM composition changes: collagen becomes less elastic and more fragile

Arterial walls become less elastic, increasing fracture risk

Resulting changes: wrinkles, reduced tissue resilience, slower repair

• Summary and practical implications

Epithelium provides protection, barrier function, selective permeability, secretion, and absorption; diverse in structure to fit function

Glands integrate with epithelial tissue to provide diverse secretions (merocrine, apocrine, holocrine) for maintenance and signaling

Connective tissues form the structural framework, support, and protection of organs; ECM composition dictates tissue mechanics

Membranes separate internal environments and contribute to homeostasis; mucous/serous/synovial membranes each serve distinct roles

Inflammation is a protective mechanism but can cause tissue damage if excessive; understanding mediators helps in clinical management

Aging affects all tissue types, with implications for disease susceptibility and healing capacity

• Quick reference of key terms (definitions)

$ext{Simple}$: one cell layer

$ext{Stratified}$: two or more cell layers

$ext{Pseudostratified}$: appears multi-layered but is a single layer

$ext{Squamous}$: flat cells

$ext{Cuboidal}$: cube-shaped cells

$ext{Columnar}$: tall, column-like cells

$ext{Glandular epithelium}$: gland-forming epithelium producing secretions

$ext{Gobl ets}$: unicellular mucus-secreting cells (goblet cells)

$ext{Merocrine}$, $ext{Apocrine}$, $ext{Holocrine}$: secretion modes

$ext{Osteoblasts/osteocytes/osteoclasts}$: bone-forming, maintaining, and bone-resorbing cells

$ext{Chondroblasts/chondrocytes}$: cartilage-forming and maintaining cells

$ext{Macrophages, mast cells}$: immune and inflammatory roles

$ext{Proteoglycans}$: ECM components that trap water and provide resilience

Connections to foundational principles and relevance

• Structure–function relationships are central: tissue architecture mirrors physiological roles (e.g., diffusion in simple epithelia; protection in stratified epithelia)

• ECM composition determines tissue mechanics (collagen for strength, elastin for stretch)

• Homeostasis relies on intact membranes, proper absorption/secretion, and regulated inflammation

• Pathophysiology: inflammatory mediators and ECM degradation contribute to disease; aging alters tissue resilience and healing capacity

Notable examples highlighted in the material

• Simple squamous lining blood vessels (endothelium) and heart

• Simple cuboidal in kidney tubules and gland ducts

• Simple columnar in small intestine (microvilli for absorption; enzyme/mucus secretion)

• Pseudostratified columnar in respiratory tract (cilia move mucus via goblet cells)

• Keratinized stratified squamous in skin; nonkeratinized in oral cavity, esophagus, vagina

• Transitional epithelium in urinary bladder

• Gland examples: sebaceous glands (holocrine), sweat glands (merocrine), mammary glands (apocrine activity during milk production), pancreas (compound tubuloacinar)

• Dense regular vs dense irregular connective tissue; dense elastic tissue in vocal cords

• Hyaline, fibrocartilage, elastic cartilage and their locations/functions

• Bone: spongy and compact forms; osteocytes in lacunae; mineralized matrix

• Blood as a connective tissue: plasma matrix with formed elements

• Membranes: mucous, serous, synovial, cutaneous

Quick checklist for exam preparation

• Be able to classify epithelium by layer number and shape and name representative locations

• Describe differences between keratinized and nonkeratinized stratified squamous epithelia and their functions

• Explain the role of goblet cells and cilia in airway and GI epithelia

• List major cell junctions and their functions (tight junctions, desmosomes, gap junctions, hemidesmosomes, adhesion belts)

• Distinguish exocrine vs endocrine glands and unicellular vs multicellular glands; identify secretory modes

• Compare and contrast connective tissue types (loose vs dense; cartilage vs bone vs blood; embryonic vs adult)

• Explain extracellular matrix components and their role in tissue properties

• Summarize the inflammatory process and typical pharmacologic interventions

• Describe aging effects on tissues and their functional implications