Anatomy and Physiology: An Integrative Approach - Chapter 5: Tissue Level of Organization

Introduction to Histology and the Four Primary Tissue Types

• Foundational Overview of Tissues:

  • Tissues are defined as groups of similar cells and extracellular material, known as the extracellular matrix ($\text{ECM}$).

  • These components work together to perform a common function, such as providing protection to the body.

  • Histology: This is the specialized study of tissues.

• The Four Major Types of Tissues:

  • The human body contains four primary tissue types that vary in structure and function:

    • Epithelial Tissue: Involved in covering and lining surfaces.

    • Connective Tissue: Provides support and binds structures.

    • Muscle Tissue: Specialized for movement.

    • Nervous Tissue: Facilitates information transfer and integration.

Characteristics and Structural Polarity of Epithelial Tissue

• General Characteristics of Epithelium:

  • Composition: Consists of one or more layers of closely packed cells with little to no extracellular matrix.

  • Locations:

    • Covers all body surfaces.

    • Lines internal body cavities.

    • Forms the majority of glands.

  • Cellularity: Epithelium is composed almost entirely of tightly packed cells.

  • Avascularity: Epithelia lack blood vessels. They obtain nutrients across the apical surface or via diffusion from the basal surface.

  • Extensive Innervation: These tissues are richly innervated, allowing them to detect environmental changes.

  • High Regeneration Capacity: Epithelial tissue can continually replace lost or damaged cells.

• Polarity of Epithelial Cells:

  • Apical Surface: The surface exposed to the external environment or an internal body space. It may contain specialized structures:

    • Microvilli: Small projections that increase surface area.

    • Cilia: Hair-like projections that move substances across the surface.

  • Lateral Surface: Contains intercellular junctions that connect neighboring cells.

  • Basal Surface: The fixed side where the epithelium is attached to the underlying connective tissue.

  • Basement Membrane: Acts as a barrier between the epithelium and the connective tissue, providing attachment and support.

Functions and Classification Systems for Epithelia

• Primary Functions of Epithelial Tissue:

  • Physical Protection: Shields the body from dehydration, abrasion, and destruction.

  • Selective Permeability: Regulates the passage of substances, allowing some to cross while preventing others.

  • Secretions: Some epithelial cells are specialized to produce and release secretions.

  • Sensations: These tissues supply information to the nervous system through sensory nerve endings.

• Classification by Number of Cell Layers:

  • Simple Epithelium: Consists of a single cell layer ($1$) thick. Every cell is in direct contact with the basement membrane. Its primary functions are filtration, absorption, or secretion.

  • Stratified Epithelium: Consists of two ($2$) or more layers of cells. Only the basal layer is in contact with the basement membrane. These are found in areas subject to mechanical stress.

  • Pseudostratified Epithelium: Appears layered because nuclei are at different levels, but it is actually a single layer because all cells contact the basement membrane. However, not all cells reach the apical surface.

• Classification by Cell Shape:

  • Squamous Cells: Flat, wide, and irregular in shape with flat nuclei.

  • Cuboidal Cells: Approximately as tall as they are wide with spherical, centrally located nuclei.

  • Columnar Cells: Slender and taller than they are wide with oval nuclei oriented lengthwise in the basal region.

  • Transitional Cells: Cells that can change shape depending on the degree of stretch in the epithelium.

Specific Types of Epithelial Tissues and Their Locations

• Simple Squamous Epithelium:

  • A single layer of flat cells with spherical to oval nuclei.

  • Forms the thinnest barrier, allowing for the rapid movement of molecules across the surface.

  • Locations: Lines air sacs of lungs (alveoli), vessel walls (endothelium), and serous membranes (mesothelium).

• Simple Cuboidal Epithelium:

  • A single layer of uniformly shaped cells as tall as they are wide with centrally located spherical nuclei.

  • Designed for absorption and secretion; ideal for forming the structural components of glands.

• Simple Columnar Epithelium:

  • A single layer of cells taller than they are wide with oval nuclei in the basal region.

  • Nonciliated Form: Contains microvilli that form a "brush border." Includes unicellular glands called goblet cells which secrete mucin (forms mucus with water). Lines most of the digestive tract from the stomach to the anal canal.

  • Ciliated Form: Cilia project from the apical surface to move mucus. Goblet cells are interspersed. Lines the bronchioles and uterine tubes (where cilia help move the oocyte).

• Pseudostratified Columnar Epithelium:

  • Ciliated Form: Contains cilia and goblet cells that move trapped foreign particles. Located in large respiratory passageways.

  • Nonciliated Form: Rare; lacks cilia and goblet cells. Found mainly in the male urethra and epididymis.

• Stratified Squamous Epithelium:

  • Multiple layers; only the deepest are in contact with the basement membrane. Basal cells are cuboidal, while apical cells are squamous. Protects against abrasion/friction.

  • Keratinized: Superficial layers of dead cells lack nuclei and are filled with keratin (protective protein). Found in the epidermis.

  • Nonkeratinized: All cells remain alive and moist; nuclei are visible. Lines the oral cavity, pharynx, esophagus, vagina, and anus.

• Additional Stratified and Transitional Types:

  • Stratified Cuboidal: $2$ or more layers; superficial cells are cuboidal. Forms walls of exocrine gland ducts (sweat glands), parts of the male urethra, and periphery of ovarian follicles.

  • Stratified Columnar: Rare; $2$ or more layers with columnar apical cells. Found in large salivary gland ducts and parts of the male urethra.

  • Transitional Epithelium: Found in the urinary tract. In a relaxed state, basal cells are cuboidal/polyhedral and apical cells are large/rounded. In a stretched state, apical cells flatten. Some cells are binucleated (containing $2$ nuclei).

Glandular Epithelium: Endocrine and Exocrine Classification

• Definition of Glands: Individual cells or multicellular organs composed of epithelial tissue specialized for secretion.

• Endocrine vs. Exocrine:

  • Endocrine Glands: Lack ducts and secrete hormones directly into the blood.

  • Exocrine Glands: Maintain a connection to the epithelial surface via a duct (epithelium-lined tube). Examples include sweat, mammary, and salivary glands.

• Structural Classification of Exocrine Glands:

  • Unicellular: Single-celled; do not contain a duct (e.g., goblet cells).

  • Multicellular: Composed of numerous cells.

    • Acini: Cell clusters that produce secretions.

    • Ducts: Transport secretions.

    • Fibrous Capsule: Surrounds the gland; extensions called septa partition the gland into lobes.

  • Anatomic Forms:

    • Simple: Single, unbranched duct.

    • Compound: Branched ducts.

    • Tubular: Secretory portion and duct have the same diameter.

    • Acinar: Secretory portion forms an expanded sac.

    • Tubuloacinar: Contains both tubules and acini.

• Glandular Secretion Methods:

  • Merocrine Glands: Secretions packaged into vesicles and released via exocytosis (e.g., lacrimal and salivary glands).

  • Apocrine Glands: The apical membrane pinches off to become the secretion (e.g., mammary and ceruminous glands).

  • Holocrine Glands: The entire ruptured cell becomes the secretion (e.g., sebaceous/oil glands).

Connective Tissue: Components and General Functions

• General Characteristics of Connective Tissue ($\text{CT}$):

  • The most diverse, abundant, and widely distributed tissue type.

  • All types originate from mesenchyme.

  • Vascularity varies: bone and blood are highly vascular, while cartilage is avascular.

  • Regeneration varies: bone/blood have high capacity, while cartilage cannot regenerate.

• The Three Basic Components of Connective Tissue:

  1. Cells: Not in direct contact with each other.

  2. Protein Fibers: Part of the extracellular matrix.

  3. Ground Substance: A molecular material produced by the cells that, together with fibers, forms the $\text{ECM}$.

• Resident Cells (Stationary):

  • Fibroblasts: Most abundant; flat cells with tapered ends that produce the fibers and ground substance.

  • Adipocytes: Fat cells appearing in clusters.

  • Mesenchymal Cells: Embryonic stem cells that divide to replace damaged cells.

  • Fixed Macrophage: Large cells derived from monocytes; they phagocytize damaged cells/pathogens and stimulate the immune system.

• Wandering Cells (Mobile):

  • Continuously move through through the $\text{CT}$; these are primarily leukocytes (white blood cells) that protect the body and repair the $\text{ECM}$.

Fibers, Ground Substance, and Clinical Conditions of Connective Tissue

• Connective Tissue Protein Fibers:

  • Collagen Fibers: Long, unbranched, "cable-like" fibers; abundant in tendons and ligaments.

  • Reticular Fibers: Thinner than collagen; form a meshwork (stroma) in organs like lymph nodes.

  • Elastic Fibers: Contain the protein elastin; allow for stretch and recoil. Found in skin and arterial walls.

• Ground Substance Characteristics:

  • Consistency ranges from viscous (blood) to semisolid (cartilage) to solid (bone).

  • Glycosaminoglycans (GAGs): Large molecules with charges that attract cations and water.

  • Proteoglycans: Formed when a $\text{GAG}$ links to a protein.

  • Glycoproteins: Proteins with carbohydrates that bond cells and fibers to the ground substance.

• Clinical Views:

  • Scurvy: Caused by Vitamin C deficiency; Vitamin C is essential for healthy collagen fibers. Symptoms include gum ulceration, weakness, and hemorrhages.

  • Marfan Syndrome: A rare genetic disease involving an abnormal chromosome $15$. Causes skeletal, cardiovascular (aorta weakness), and visual (slipped lens) abnormalities. Individuals often die before age $50$ without medical management.

Classification of Connective Tissue Proper

• Loose Connective Tissue (Body's "Packing Material"):

  • Areolar CT: Loose organization of collagen and elastic fibers; highly vascular with viscous ground substance. Located in the papillary layer of the dermis and surrounding organs.

  • Adipose CT (Fat): Predominantly adipocytes. White adipose stores energy and cushions; Brown adipose (in newborns) generates heat.

  • Reticular CT: Meshwork of fibers and cells providing the stroma for lymphatic organs (spleen, thymus, lymph nodes, bone marrow).

• Dense Connective Tissue (Predominantly Collagen Fibers):

  • Dense Regular CT: Parallel collagen fibers (resembling lasagna noodles); found in tendons and ligaments. Stress is applied in one direction; few blood vessels lead to long healing times.

  • Dense Irregular CT: Collagen fibers extend in multiple directions to resist stress. Found in the dermis, periosteum, and organ capsules.

  • Elastic CT: Branching elastic fibers allowing for stretch/recoil. Found in large arteries, the trachea, and vocal cords.

Supporting and Fluid Connective Tissues

• Cartilage:

  • Semisolid $\text{ECM}$ with chondrocytes residing in lacunae. Avascular in mature state and surrounded by perichondrium (except fibrocartilage).

  • Hyaline Cartilage: Most common; glassy appearance. Located in the nose, trachea, larynx, costal cartilage, and fetal skeleton.

  • Fibrocartilage: Weight-bearing and resists compression. Located in intervertebral discs, pubic symphysis, and menisci of the knee.

  • Elastic Cartilage: Flexible and resilient with densely packed elastic fibers. Located in the external ear and epiglottis.

• Bone (Osseous Tissue):

  • Solid matrix containing organic (collagen/glycoproteins) and inorganic (calcium salts) components. Osteocytes reside in lacunae.

  • Compact Bone: Organized into cylindrical osteons with concentric lamellae around a central canal.

  • Spongy Bone: Latticework structure in the interior of bone.

  • Functions include support, protection, mineral storage, and housing hematopoietic cells.

• Fluid Connective Tissue:

  • Blood: Contains formed elements (erythrocytes, leukocytes, and platelets) in a liquid ground substance called plasma.

  • Lymph: Derived from blood plasma but lacks cellular fragments; eventually returns to the bloodstream.

Muscle Tissue and Nervous Tissue

• Muscle Tissue (Contractile):

  • Skeletal Muscle: Striated and voluntary; cells are long, cylindrical, multinucleated fibers arranged in parallel. Moves the skeleton.

  • Cardiac Muscle: Found in the myocardium (heart wall); cells are short, bifurcating (branching), and connected by intercalated discs. Contains $1$ or $2$ nuclei. Involuntary and controlled by pacemaker cells.

  • Smooth Muscle: Non-striated and involuntary; cells are spindle-shaped with one central nucleus. Found in walls of visceral organs (stomach, bladder, blood vessels).

• Nervous Tissue (Information Transfer):

  • Located in the brain, spinal cord, and nerves.

  • Neurons: Specialized to receive and transmit nerve impulses.

    • Cell Body: Houses organelles.

    • Dendrites: Short processes that receive incoming signals.

    • Axon: A single long process that carries outgoing signals.

  • Glial Cells: Do not transmit impulses; provide support, protection, and nourishment to neurons.

Organs, Membranes, and Tissue Development

• Organs: Structures composed of two ($2$) or more tissue types working together. For example, the stomach contains epithelium (secretion), connective tissue (support), smooth muscle (mixing), and nervous tissue (regulation).

• Body Membranes:

  • Mucous Membrane (Mucosa): Lines compartments opening to the exterior (digestive, respiratory tracts). Contains a $\text{CT}$ layer called the lamina propria.

  • Serous Membrane: Lines closed body cavities. Consists of a simple squamous mesothelium that produces serous fluid. Forms parietal (outer) and visceral (inner) layers.

  • Cutaneous Membrane (Skin): External surface; composed of keratinized stratified squamous epithelium and $\text{CT}$.

  • Synovial Membrane: Lines joints; composed of areolar $\text{CT}$ and squamous cells lacking a basement membrane. Secretes synovial fluid.

• Tissue Development:

  • A zygote (diploid cell) divides into a blastocyst.

  • By the $3^{rd}$ week, three primary germ layers form: Ectoderm, Mesoderm, and Endoderm.

Tissue Modification, Clinical Pathologies, and Aging

• Forms of Tissue Modification:

  • Hypertrophy: Increase in size of existing cells.

  • Hyperplasia: Increase in the number of cells.

  • Atrophy: Shrinkage due to decrease in cell size or number (e.g., from disuse or aging).

  • Metaplasia: Change of mature epithelium to a different form (e.g., in smokers, respiratory epithelium changes to nonkeratinized stratified squamous).

  • Dysplasia: Abnormal development; potentially precancerous.

  • Neoplasia: Out-of-control growth forming a neoplasm (tumor). Can be benign (localized) or malignant (metastasizes/cancer).

  • Necrosis: Tissue death due to irreversible damage (e.g., gangrene).

• Clinical View: Gangrene:

  • Dry Gangrene: Desiccated tissue, usually from extreme cold.

  • Wet Gangrene: Bacterial infection of tissues with lost blood supply.

  • Gas Gangrene: Bacteria invade necrotic tissue and produce gas bubbles.

• Aging of Tissues:

  • Maintenance and replacement become less efficient after middle age.

  • Epithelia thin, $\text{CT}$ loses resiliency, collagen declines, bones become brittle, and muscles undergo atrophy.