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Tissues Overview

• There are four major tissue types: epithelial, connective, muscle, and nervous (neural) tissue. There are many subtypes within each, which will be explored in later sections.

Epithelial Tissue

• Epithelia vs glands (glandular tissue)
  • Epithelia: form coverings and linings on external surfaces and internal surfaces (e.g., skin surface, lining of stomach, intestines, oral cavity, nasal cavity).
  • Glands (glandular tissue): secretory tissues, can be unicellular or multicellular.

Characteristics of epithelia

• Highly cellular: a large number of cells with little extracellular space between them (very tight cell packing).
  • Extracellular space is minimal; the space between cells is not large.
• Polarity: distinct apical and basal surfaces.
  • Basal surface is attached to a basement membrane (basal lamina).
  • Basal membrane anchors epithelium to underlying connective tissue.
• Avascularity: epithelia lack blood vessels.
  • Nutrients diffuse from underlying connective tissue through diffusion to reach epithelial cells.
• Innervation: epithelia are innervated via nerve endings, enabling sensory functions (e.g., pain, touch).
• Not easily observed in isolation under light microscope beyond cellularity, polarity, and basement membrane connections.

Basement membrane

• Basement membrane is a two-layer structure:
  • Lamina lucida (clear layer): top portion, lacks fibers; mainly proteoglycans and glycoproteins secreted by epithelial cells.
  • Lamina densa: bottom portion, contains fibers; produced by the underlying connective tissue.
• Function: anchors epithelium to connective tissue; provides support and selective barrier.
• Anchoring structures: specialized junctions (e.g., hemidesmosomes) connect epithelial cells to the basement membrane.

Functions of epithelial tissues

• Protection: coverings protect underlying tissues from mechanical and chemical injury.
• Permeability control: regulate diffusion and transport of substances (selective permeability).
• Secretion: glandular epithelia produce secretions (various glands).
• Sensation: nerve endings in epithelia contribute to sensory perception.
• Mechanical stress resistance: stratified epithelia resist abrasion; simple epithelia enable diffusion and absorption where protection is less critical.

Classification by shape and layers

• Layering:
  • Simple epithelia: one cell layer thick; optimized for absorption, secretion, and diffusion.
  • Stratified epithelia: multiple cell layers; optimized for protection against abrasion and stress.
• Cell shape on the apical surface:
  • Squamous (flat), cuboidal (cube-like), columnar (tall, column-shaped).
• Examples and functions:
  • Simple squamous: thin for rapid diffusion; found in gas exchange sites like alveoli.
  • Stratified squamous: protective barrier; found in skin (epidermis) and oral mucosa.
  • Simple cuboidal/columnar: secretory and absorptive functions in glands and ducts.

Examples and clinical correlations

• Epidermis: has a protective protein coating; barrier against water loss. If epidermis is damaged, fluid loss increases.
• Abrasion that bleeds indicates dermis involvement (blood vessels in dermis) because the epidermis itself is avascular.
• Permeability and burns: burns disrupt the epidermis and can lead to significant fluid loss; important in clinical management and skin grafting.
• Capillaries: allow passage of small molecules but restrict larger molecules like proteins and cells; this controlled permeability is critical in tissues like capillary walls.
• Nerve endings in epithelia contribute to sensation (pressure, touch, pain).

Cell junctions and the glycocalyx

• Cells are held together by specialized cell junctions and the glycocalyx (a carbohydrate-rich coating on the cell surface):
  • Gap junctions: align between adjacent cell membranes to form channels that allow rapid movement of ions and small molecules between cells.
  • Desmosomes: strong, spot-like attachments that resist mechanical stress; important in tissues under stretch (e.g., skin, heart).
  • Hemidesmosomes: anchor epithelial cells to the basement membrane.
  • Glycocalyx: extracellular coating aiding cell adhesion and protection.
• Cells can have multiple junction types simultaneously; junctions are not mutually exclusive within a single cell.

Epithelial turnover and oncology relevance

• Epithelial cells turn over and regenerate regularly (e.g., skin shedding as dander).
• In oncology, rapidly dividing epithelial cells are targeted by chemotherapy agents.
  • Chemotherapy targets fast-dividing cells (both cancerous and normal rapidly dividing cells).
  • Side effects include digestive issues (diarrhea, vomiting, malabsorption) and hair loss due to effects on GI epithelium and hair follicles.

Glandular epithelium: classifications and secretions

• Gland types by cell number:
  • Unicellular glands (e.g., goblet cells).
  • Multicellular glands.
• Gland types by secretion route:
  • Exocrine glands: secrete onto surfaces (external or internal surfaces) via ducts; ducted glands.
  • Endocrine glands: secrete into surrounding tissues/blood (ductless glands); secretions are hormones.
• Secretions:
  • Endocrine: hormones distributed via blood to target cells.
  • Exocrine: secretions released onto surfaces; can be in vesicles and released by exocytosis (merocrine); apocrine (mud leaves part of the cytoplasm in secretions, e.g., mammary glands); holocrine (rupture of the whole cell releasing contents).
  • Secretory methods: merocrine (exocytosis is key), apocrine, holocrine.
• Types of secretions (by composition):
  • Serous: watery secretions containing enzymes.
  • Mucinous (mucin): thick, mucous secretions.
  • Mixed glands: produce both serous and mucous components (e.g., some salivary glands; sublingual glands often mixed).
• Multicellular exocrine glands: structure-based classification by ducts and secretion portions
  • Duct structure: simple (duct is unbranched) vs compound (duct branched).
  • Secretory portion shape: tubular (tubular), alveolar (acinar), or tubuloalveolar (mixed).
  • Resulting simple tubular, simple alveolar, simple tubuloalveolar; compound tubular, compound alveolar, compound tubuloalveolar, etc.

Connective Tissue

Overview

• Connective tissue is composed of cells and extracellular matrix (ECM).
  • Cells can include fibroblasts, fibrocytes, immune cells (e.g., leukocytes), and others depending on tissue type.
  • ECM is the non-cellular component (fibers + ground substance) that provides structure and support.

Extracellular matrix components

• Fibers:
  • Collagen: very strong, thick fibers; can be stronger than steel for comparably sized fibers; present where tissues must tolerate high stress.
  • Reticular fibers: thinner collagen fibers that form a mesh (reticulum) providing a net-like framework; important for filtration and supporting cellular architecture.
  • Elastic fibers: composed of elastin; provide elasticity and recoil (e.g., skin dermis, lungs, and the aorta).
• Ground substance: a largely-water-rich gel that fills the ECM; contains dissolved proteins and molecules.
  • Water constitutes >90% of the ground substance.
  • Adhesion proteins: fibronectin and laminin help bind ECM components to each other and to cell surfaces.
  • Proteoglycans: core proteins with glycosaminoglycan (GAG) chains; form a hydrated gel that resists compression and provides lubrication.
  • Examples of GAGs: chondroitin sulfate, hyaluronic acid.

Functions and tissue examples

• ECM composition dictates mechanical properties and resilience:
  • Cartilage: thick, gel-like matrix rich in proteoglycans (GAGs) provides semi-solid consistency.
  • Bone: calcified matrix; rigid and supportive.
  • Dermis: abundant elastic fibers for elasticity and recoil.
  • Blood vessel walls: elastic fibers allow distension and recoil, especially in elastic arteries like the aorta.
• Notable connective tissue types: cartilage, bone, blood, adipose tissue, and others.
• Vascularity varies: cartilage is avascular (healing is slow), bone is highly vascularized, tendons have limited vascularity.

Ground substance, residence, and clinical relevance

• Ground substance hydrating and resists compression; contains proteoglycans and GAGs that contribute to tissue viscosity and resilience.
• In joints, the cartilage matrix relies on proteoglycans to maintain lubrication and load distribution; degradation leads to arthritis.
• Joint health supplements often include components like chondroitin sulfate and hyaluronic acid to support cartilage matrix, though absorption and efficacy depend on multiple factors.

Coagulation and connective tissue context

• Blood contains fibrinogen; coagulation converts fibrinogen to a fibrous mesh that forms a clot:
  • ext{Fibrinogen}
    ightarrow ext{Fibrin} ext{ via thrombin}
• Blood can form fibers (fibrin) but keeps them in soluble form as needed until clotting occurs.

Osseous tissue and membranes (brief mentions)

• Osseous tissue (bone) has a calcified matrix for rigidity.
• Cartilage is avascular, explaining slower healing.
• Connective tissues contribute to the structure of membranes and joint linings discussed later.

Membranes

Four main membrane types

1) Mucous membranes (mucosa)

• Line cavities that open to the exterior (e.g., oral cavity, nasal cavity, urinary tract, digestive tract).
• Epithelium varies by location and function:
  • Oral cavity: stratified squamous epithelium (tough to resist abrasion); overlying loose connective tissue called lamina propria.
  • Nasal cavity: pseudostratified columnar ciliated epithelium; supports mucus movement.
• Lamina propria: loose areolar connective tissue beneath the epithelium.
• Key concept: mucosa lines openings to the exterior; epithelium composition adapts to function.

2) Serous membranes (serosa)

• Line cavities that do not open to the exterior; surface is smooth and produces serous fluid to reduce friction.
• Composition: simple squamous epithelium (mesothelium) resting on loose areolar tissue.
• Examples and layout:
  • Pleura: serous membrane around the lungs; visceral pleura covers the lungs, parietal pleura lines the chest cavity; pleural fluid reduces friction during breathing.
  • Pericardium: around the heart; visceral and parietal layers with pericardial fluid.
  • Peritoneum: around abdominal organs; visceral and parietal layers with peritoneal fluid.
• Serous fluid (transudate) fills the potential space between the two layers to facilitate smooth sliding of surfaces during movement.

3) Cutaneous membrane (skin)

• The skin is a cutaneous membrane composed of epidermis (epithelial) and dermis (connective tissue).
• Details to be covered in Chapter 5.

4) Synovial membranes

• Line the cavities of synovial joints (e.g., knee, shoulder, elbow, wrist, hip).
• Do not contain epithelium like other membranes; instead, synovial membranes are connective tissue-based and secrete synovial fluid.
• Synovial fluid is very viscous and lubricates joints, supplies nutrients to cartilage, and reduces friction.
• Cartilage within joints is avascular, so the synovial fluid helps in nourishment.

Clinical and cross-disciplinary connections

• Epithelial health and oncology: rapidly dividing epithelium (e.g., GI tract, skin) is a common target of chemotherapy, leading to side effects like diarrhea, vomiting, malabsorption, and alopecia.
• Joint health and aging: proteoglycans and GAGs in cartilage (e.g., chondroitin sulfate, hyaluronic acid) contribute to cartilage resilience and lubrication; supplements target these components, though efficacy varies.
• Membrane function and disease: serous membranes maintain frictionless movement of internal organs; inflammation of serous membranes (serositis) can complicate conditions like pericarditis or pleuritis.
• Structural integrity and injury: desmosomes and hemidesmosomes contribute to tissue cohesion under mechanical stress; their disruption can lead to blistering diseases (e.g., pemphigus vulgaris involves desmosomes).

Quick recap: key terms to remember

• Epithelial types: epithelia vs glands; simple vs stratified; squamous, cuboidal, columnar.
• Basement membrane layers: $ext{Lamina Lucida}$ (clear, no fibers) and $ext{Lamina Densa}$ (fibrous).
• Cell junctions: gap junctions, desmosomes, hemidesmosomes; glycocalyx.
• Gland classifications: unicellular vs multicellular; exocrine (ducted) vs endocrine (ductless); merocrine vs apocrine vs holocrine secretion.
• Connective tissue ECM: collagen, reticular, elastic fibers; ground substance with proteoglycans and GAGs (e.g., $ext{chondroitin sulfate}, ext{hyaluronic acid}$).
• Membranes: mucous (open to exterior), serous (closed cavities; serous fluid), cutaneous (skin), synovial (joints).
• Clinical ties: chemotherapy targets rapidly dividing cells; cartilage is avascular; serous fluid reduces friction in joints and cavities.

Notes for exam prep

• Be able to distinguish simple vs stratified epithelia and name likely locations (e.g., alveoli -> simple squamous; skin -> stratified squamous).
• Explain the function of the basement membrane and differentiate lamina lucida vs lamina densa.
• Describe the types of gland secretions and their modes of release (merocrine, apocrine, holocrine).
• Identify the three main fiber types in connective tissue and give a functional example for each (collagen strongest, reticular mesh, elastic recoil in skin/arteries/lungs).
• Explain why cartilage heals slowly and the role of synovial fluid in joint health.
• List the four membranes and one example each (mucous = oral mucosa, serous = pleura, cutaneous = skin, synovial = joint capsule).

Key equations and LaTeX notes

• Tissue count: There are $4$ major tissue types.
• Fibrin formation in coagulation:
  • ext{Fibrinogen}
    ightarrow ext{Fibrin} ext{ (via thrombin)}
• Basement membrane components can be denoted as layers:
  • $ext{Lamina ext{a} lucida}, ext{Lamina ext{a} densa}$
• GAG example: $ext{Chondroitin sulfate}, ext{Hyaluronic acid}$
• Serous fluid function: reduces friction between opposing serous membranes.