Topic #2, Part 1: Tissues

Introduction to Tissues

Tissues are groups of structurally similar cells that perform a common or related function. The organization of cells into tissues is essential for the division of labor and cooperation at the organ level, contributing to overall homeostasis in the human body.

• Definition: Tissue refers to a group of cells with similar structure and function.

• Histology: The study of tissues and their cellular organization.

• Why different tissues? Individual body cells are specialized for specific functions, allowing organs to work as integrated units.

Primary Types of Tissues

There are four primary tissue types in the human body, each with distinct roles:

• Epithelial tissue: Covers body surfaces and lines cavities; involved in protection, absorption, filtration, secretion, and sensory reception.

• Connective tissue: Provides support and structure; includes bone, cartilage, blood, and connective tissue proper.

• Muscle tissue: Responsible for movement; includes skeletal, cardiac, and smooth muscle.

• Nervous tissue: Controls and regulates body functions; consists of neurons and supporting cells.

Epithelial Tissue

Structure and Functions of Epithelial Tissue

Epithelial tissue consists of sheets of cells that cover body surfaces or line body cavities, creating boundaries between different environments.

• Covering and lining epithelium: Forms the outer layer of the skin and lines open cavities of the digestive and respiratory systems.

• Glandular epithelium: Forms glands that secrete products such as hormones, mucus, and enzymes.

Six Functions of Epithelial Tissue:

• Protection: Shields underlying tissues from mechanical, chemical, and infectious damage (e.g., skin).

• Absorption: Uptake of substances (e.g., nutrients in the GI tract).

• Filtration: Selective movement of substances (e.g., kidney filtration).

• Excretion: Removal of waste products (e.g., kidney).

• Secretion: Production and release of substances (e.g., glands, GI tract).

• Sensory reception: Detection of stimuli (e.g., taste buds, sensory membranes).

Special Structural Characteristics of Epithelial Tissue

• Polarity: Epithelial cells have apical (top) and basal (bottom) surfaces; the apical surface may have specializations such as cilia or microvilli.

• Specialized contacts: Cells are tightly joined by junctions (tight junctions, desmosomes) to maintain integrity and control permeability.

• Supported by connective tissue: The basement membrane consists of the basal lamina (glycoproteins) and reticular lamina (collagen fibers), providing support and regulating exchange.

• Avascular but innervated: Epithelial tissues lack blood vessels but are supplied by nerves; nourishment comes from underlying connective tissue.

• Regeneration: High capacity for renewal due to frequent cell division, necessary for maintaining tissue integrity.

Classification of Epithelial Cells

Epithelial tissues are classified based on two criteria:

• Cell shape: Squamous (flat), cuboidal (cube-shaped), columnar (tall).

• Number of layers: Simple (one layer), stratified (multiple layers).

Types of Simple Epithelia

• Simple squamous epithelium: Thin and permeable; functions in filtration and diffusion. Locations: Endothelium (lining of blood vessels), kidney, lung alveoli.

• Simple cuboidal epithelium: Secretion and absorption. Locations: Kidney tubules, small glands.

• Simple columnar epithelium: Secretion and absorption. Locations: GI tract lining.

• Pseudostratified (ciliated) columnar epithelium: Appears stratified but is a single layer; cilia and mucus secretion. Locations: Respiratory tract.

Types of Stratified Epithelia

• Stratified squamous epithelium: Protective role; basal cells are cuboidal and undergo mitosis to regenerate layers. Locations: Skin (most widespread), lining of mouth. Note: Apical layers become flattened and may be keratinized.

• Transitional epithelium: Stratified epithelium with cuboidal basal cells and apical cells that flatten as the organ fills. Locations: Urinary bladder.

Glandular Epithelium

• Gland: One or more cells that make and secrete a particular product.

• Endocrine glands: Ductless; secrete hormones directly into the bloodstream.

• Exocrine glands: Secrete products into ducts (e.g., mucus, sweat, oil, salivary glands, liver, pancreas).

• Unicellular exocrine glands: Single cells (e.g., goblet cells) that secrete mucus in digestive and respiratory tracts.

• Multicellular exocrine glands: Composed of duct and secretory cells, supported by connective tissue.

Types of Multicellular Exocrine Glands (by duct structure)

• Simple glands: Unbranched duct.

• Compound glands: Branched duct.

Modes of Secretion

• Merocrine: Secrete products by exocytosis (most common; e.g., salivary glands, most sweat glands).

• Holocrine: Secrete products by cell rupture (e.g., sebaceous glands).

• Apocrine: Cell apex pinches off with secretory product; cell repairs itself (e.g., mammary glands).

Connective Tissue (CT)

Types and Functions of Connective Tissue

Connective tissue is the most abundant and widely distributed tissue type, providing support, protection, insulation, and transportation.

• Types: Mesenchyme, CT proper, cartilage, bone, blood.

• Main functions:

  1. Binding and supporting (e.g., tendons, ligaments)

  2. Protecting (e.g., bone, cartilage)

  3. Insulating (e.g., adipose tissue)

  4. Transporting (e.g., blood)

Structural Organization of Connective Tissue

• Ground substance (matrix): Composed of interstitial fluid, cell adhesion proteins (fibronectin, laminin), and proteoglycans (act as molecular sieves).

• Fibers:

  • Collagen fibers: Provide high tensile strength.

  • Elastic fibers: Contain elastin; allow stretch and recoil.

  • Reticular fibers: Thin, fine collagen; form supportive networks.

• Cells:

  • Immature ('blast') cells: Actively divide and synthesize matrix during growth and repair (e.g., fibroblast, chondroblast).

  • Mature ('cyte') cells: Maintain tissue (e.g., fibrocyte, chondrocyte).

Table: Connective Tissue Cell Types

Types of Connective Tissue Proper

Loose Connective Tissue

• Areolar CT: Gel-like matrix with all three fiber types; contains fibroblasts, macrophages, mast cells, and some WBCs. Functions: Cushions organs, immunity, inflammation, fluid reservoir. Locations: Widely distributed under epithelia.

• Adipose tissue: Modified areolar CT for nutrient storage; consists of fat-filled adipocytes. Functions: Fuel reservoir, insulation, supports and protects organs. Locations: Under skin, around kidneys and eyeballs, in bones, within abdomen and breasts.

• Reticular CT: Like areolar CT but only reticular fibers. Functions: Forms soft internal skeleton supporting free blood cells. Locations: Lymphoid organs (lymph nodes, bone marrow, spleen).

Dense Connective Tissue

• Dense regular CT: Bundles of collagen fibers running parallel; great resistance to tension. Locations: Tendons, ligaments, aponeuroses.

• Dense irregular CT: Collagen bundles thicker and arranged irregularly. Locations: Dermis, submucosa of GI tract, fibrous capsules of organs.

• Elastic CT: Like dense regular CT but with high elastic fiber content. Locations: Some elastic ligaments.

Clinical Note: Connective Tissue Disease

• Rheumatoid arthritis: A common autoimmune disease where the immune system attacks synovial membranes of joints, leading to inflammation and joint damage.

Review Questions and Applications

• Which fibrous element gives CT high tensile strength? Answer: Collagen fibers.

• What type of CT can increase in volume without making more cells? Answer: Areolar CT (acts as a sponge, soaking up fluid during edema).

• Which kind of CT acts as a sponge, soaking up fluid when edema occurs? Answer: Areolar CT.

Additional info: Cartilage, bone, and blood are specialized connective tissues that will be covered in subsequent lessons.