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Anatomy & Physiology Lecture Notes

Introduction to Tissues

  • The human body contains trillions of cells organized into more complex units known as tissues.

  • Definition of Tissues: Groups of similar cells and extracellular material (extracellular matrix) that perform a common function, such as providing protection or facilitating body movement.

  • Four Types of Tissues:

    • Epithelial

    • Connective

    • Muscle

    • Nervous

  • Tissues possess varied structures and functions.

5.1 Epithelial Tissue

5.1a Characteristics of Epithelial Tissue

  • Definition of Epithelium: Also referred to as epithelial tissue, characterized by the following:

    • Cellularity: Composed of one or more layers of tightly packed cells with minimal extracellular matrix between them.

    • Locations: You can find epithelial tissue covering body surfaces, lining body cavities, and forming the majority of glands.

5.1a Characteristics Continued

  • Polarity:

    • Apical Surface: Exposed to the external environment or internal body space, can be suspended with microvilli or cilia.

    • Lateral Surface: Features intercellular junctions.

    • Basal Surface: The part of the epithelium that is attached to connective tissue.

5.1b Functions of Epithelial Tissue

  • Physical Protection: Protects from dehydration, abrasion, and destruction.

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

  • Secretions: Specialized epithelial cells are capable of secreting substances.

  • Sensations: Epithelial tissue provides sensory information to the nervous system.

5.1c Classification of Epithelial Tissue

  • Classification by Number of Cell Layers:

    • Simple Epithelium: One cell layer thick, all cells contact the basement membrane, mainly functions in filtration, absorption, or secretion.

    • Stratified Epithelium: Two or more layers of cells, typical in areas subject to mechanical stress.

    • Pseudostratified Epithelium: Appears layered but is actually a single layer.

  • Classification by Cell Shape:

    • Squamous Cells: Flat, wide, irregularly shaped, with a flattened nucleus.

    • Cuboidal Cells: Equal height and width, with a spherical and central nucleus.

    • Columnar Cells: Taller than they are wide, with an oval nucleus oriented lengthwise in the basal region.

    • Transitional Cells: Capable of changing shape based on the stretch of the epithelium.

5.1d Types of Epithelial Tissue

  • Simple Squamous Epithelium: A single layer of flat cells with a spherical to oval nucleus; this type forms the thinnest barrier, allowing rapid movement of molecules across surfaces.

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

  • Simple Cuboidal Epithelium: A single layer of uniformly shaped cells, about as tall as they are wide, with a centrally located spherical nucleus. This type is ideal for absorption and secretion.

  • Simple Columnar Epithelium: A single layer of cells taller than wide, with an oval nucleus located in the basal region and specializes in secretory and absorptive functions; can be nonciliated (with microvilli) or ciliated.

  • Pseudostratified Columnar Epithelium: Appears to have multiple layers due to the varied positioning of nuclei but is a single layer; can be ciliated (which helps move mucus) or nonciliated.

  • Stratified Squamous Epithelium: Comprising multiple layers with basal cells being cuboidal, while apical cells are squamous; functions to protect underlying tissues from damage through abrasion and friction. May be keratinized (dead cells with keratin in the epidermis) or nonkeratinized (living cells, moist surfaces).

  • Stratified Cuboidal Epithelium: Two or more layers of cuboidal-shaped cells, functions primarily in protection and secretion, found in ducts of glands.

  • Stratified Columnar Epithelium: Rare tissue with two or more layers, columnar cells at the apical surface, serving protective and secretory purposes, found in areas like salivary glands.

  • Transitional Epithelium: Unique to the urinary tract, allowing appearance variation depending on the state (relaxed vs. stretched), consists of binucleated (two nuclei) cells, enabling stretching as the bladder fills.

5.1d Glands

  • Definition of Glands: Can be single cells or multicellular organs made up of epithelial tissue that secretes substances.

    • Endocrine Glands: Lack ducts, release hormones into the bloodstream for distribution.

    • Exocrine Glands: Connected to epithelial surface via duct, includes sweat, mammary, and salivary glands.

  • Unicellular Exocrine Glands: Examples include goblet cells, which do not have ducts and are situated close to the epithelial surface.

  • Multicellular Exocrine Glands: Composed of numerous cells, with acini producing secretions while ducts carry these to the surface.

    • Classification Based on Anatomic Form:

    • Simple glands: unbranched duct.

    • Compound glands: branched duct.

    • Tubular glands: secretory portion and duct share the same diameter.

    • Acinar glands: secretory portion forms an expanded sac.

    • Tubuloacinar gland: includes both tubular and acinar structures.

  • Classification Based on Method of Secretion:

    • Merocrine Glands: Release secretions via exocytosis (e.g., salivary glands).

    • Apocrine Glands: Pinch off the apical membrane for secretion (e.g., mammary glands).

    • Holocrine Glands: Entire cell ruptures for secretion (e.g., sebaceous glands).

5.2 Connective Tissue: Cells in a Supportive Matrix

  • Definition of Connective Tissue (CT): The most diverse, abundant, widely distributed, and varied in structure and function.

  • Characteristics of CT:

    • Originates from mesenchyme (embryonic connective tissue).

    • Vascular, with varying degrees of vascularity (high regenerative capacity of bone and blood; low in cartilage).

5.2a Characteristics of Connective Tissue

  • Three Basic Components:

    • Cells: Specific types for each kind of CT, primarily two classes: resident and wandering.

    • Resident Cells: Stationary and maintain or repair ECM (e.g., fibroblasts, adipocytes).

    • Wandering Cells: Move continuously, are immune components, primarily leukocytes.

    • Protein Fibers: Strengthen and support CT (e.g., collagen fibers, elastic fibers).

    • Ground Substance: Molecular material supporting CT cells and fibers; varies in consistency (viscous, semisolid, or solid), together with protein fibers forms extracellular matrix.

5.2b Functions of Connective Tissue

  • Functions include:

    • Physical protection,

    • Support and structure,

    • Binding structures,

    • Storage (nutrients, energy),

    • Transport (blood),

    • Immune protection.

5.2c Embryonic Connective Tissue

  • Types:

    • Mesenchyme: First CT formed in developing embryo; differentiates into other CT.

    • Mucous CT: Found only in the umbilical cord.

5.2d Classification of Connective Tissue

  • Connective Tissue Proper: Classified into loose and dense CT based on cell, fiber, and ground substance proportions.

    • Loose CT: Fewer cells and fibers, acts as packing material (e.g., areolar, adipose, reticular).

    • Dense CT: Composed primarily of collagen; includes dense regular, dense irregular, and elastic.

Types of Loose CT:

  • Areolar CT: Loose organization, highly vascularized, found in various tissues (e.g., dermis).

  • Adipose CT: Fat storage; types include white and brown adipose tissues.

  • Reticular CT: Protective framework (stroma) for lymphatic organs.

Dense CT Types:

  • Dense Regular CT: Fibers tightly packed and aligned, found in tendons, and ligaments.

  • Dense Irregular CT: Fibers extend in all directions, found in dermis and around organs.

  • Elastic CT: Contains densely packed elastic fibers allowing stretch and recoil, found in arterial walls.

Supporting CT Types:

  • Cartilage: Strong but flexible; includes hyaline, fibrocartilage, and elastic cartilage.

  • Bone: Osseous connective tissue; more solid than cartilage and contains both organic and inorganic components.

    • Types of Bone:

    • Compact Bone: Solid structure with canals for vasculature.

    • Spongy Bone: Lattice-like structure providing strength without excess weight.

Functions of Bone:

  • Provides levers for muscles, supports tissue, protects organs, stores minerals, and houses blood cell-producing cells.

Fluid CT:

  • Blood: Fluid with formed elements; transports gases, nutrients, and waste.

  • Lymph: Derived from blood plasma, returns to the bloodstream without cellular components.

Section Knowledge Checks

  1. Why does an epithelium need to be highly regenerative?

  2. What makes epithelium selectively permeable?

  3. Distinguish between simple and stratified epithelium.

  4. Identify the type of epithelial tissue lining air sacs in the lungs.

  5. Make a table comparing transitional epithelium and keratinized stratified squamous epithelium based on structure, function, and location.

  6. Describe the two basic parts of a multicellular exocrine gland.

  7. Compare holocrine and merocrine glands with examples.

  8. Discuss the functional differences between resident and wandering cells in connective tissue.

  9. What is the function of glycosaminoglycans (GAGs) in the ground substance?

  10. Outline the general functions of connective tissue.

  11. Describe mesenchyme's composition and function.

  12. Create a flow chart classifying all connective tissue types starting from a common origin.

  13. Detail the composition and location of fibrocartilage.

  14. Explain why blood is classified as connective tissue.