Topic #2: 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: A tissue is a group of cells with similar structure and function.

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

• Why different tissues? Specialized cells allow for division of labor and organ-level cooperation.

The Four Primary Types of Tissues

The human body contains four main types of tissues, each with distinct roles:

• Epithelial tissue: Covers body surfaces and lines cavities; forms boundaries.

• Connective tissue: Supports, binds, and protects other tissues and organs.

• Muscle tissue: Responsible for movement.

• Nervous tissue: Controls and regulates body functions.

Epithelial Tissue

Functions and Characteristics of Epithelial Tissue

Epithelial tissue consists of sheets of cells that cover body surfaces or line body cavities, creating boundaries between different environments. It is highly specialized for various functions.

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

• Absorption: Uptake of substances (e.g., gastrointestinal tract).

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

• 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).

Example: The skin's epithelium protects against pathogens, while the lining of the small intestine absorbs nutrients.

Special Structural Characteristics of Epithelial Tissue

Epithelial tissues possess several unique structural features that support their functions:

• Polarity: Distinct apical (top) and basal (bottom) surfaces.

• Specialized contacts: Tight junctions and desmosomes bind cells together, maintaining tissue integrity.

• Supported by connective tissue: The basement membrane (composed of basal lamina and reticular lamina) anchors the epithelium and regulates exchange.

• Avascular but innervated: Contains nerves but lacks blood vessels; nutrients diffuse from underlying connective tissue.

• Regeneration: High capacity for renewal, essential for tissues subject to abrasion.

Additional info: Cancerous epithelial cells often lose respect for the basement membrane boundary, contributing to metastasis.

Surface Specializations of Epithelial Cells

Epithelial cells may have specialized surface structures to enhance their function:

• Cilia: Hair-like projections that move substances across the cell surface (e.g., respiratory tract).

• Microvilli: Finger-like extensions that increase surface area for absorption (e.g., intestinal lining).

Classification of Epithelial Tissue

Epithelial tissues are classified based on two criteria: cell shape and number of layers.

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

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

Types of Simple Epithelia

Simple epithelia consist of a single cell layer and are specialized for absorption, secretion, and filtration.

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

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

• Simple columnar epithelium: Secretion and absorption. Location: Most of the digestive tract.

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

Stratified Epithelia

Stratified epithelia contain two or more layers, providing protection in areas subject to abrasion.

• Stratified squamous epithelium: Basal cells are cuboidal and undergo mitosis; apical layers are flat and may be keratinized. Location: Skin (most widespread), lining of mouth.

• Transitional epithelium: Stratified epithelium with a mix of columnar and squamous cells; apical cells flatten as the organ fills. Location: Urinary bladder.

Glandular Epithelia

Glands are epithelial structures that produce and secrete products.

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

• Exocrine glands: Secrete products into ducts; can be unicellular (e.g., goblet cells) or multicellular (e.g., sweat, oil, salivary glands).

Types of Multicellular Exocrine Glands (by secretion method)

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

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

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

Matching Epithelial Types to Locations

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.

• 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)

Clinical Note: Rheumatoid arthritis is an autoimmune CT disease where the immune system attacks synovial membranes of joints.

Structural Organization of Connective Tissue

Connective tissue consists of cells embedded in an extracellular matrix composed of ground substance and fibers.

• Ground substance: Interstitial fluid, cell adhesion proteins (e.g., fibronectin, laminin), and proteoglycans (act as molecular sieves).

• Fibers:

  • Collagen fibers: Provide high tensile strength.

  • Elastic fibers: Allow stretch and recoil.

  • Reticular fibers: Form fine networks supporting soft tissues and blood vessels.

• Cells: Immature ('blast') cells are active in growth and repair; mature ('cyte') cells maintain tissue.

Types of Connective Tissue Proper

Loose Connective Tissue

• Areolar CT: Gel-like matrix with all three fiber types; widely distributed under epithelia. Functions: Cushions organs, immunity, inflammation, fluid reservoir. Location: Under skin, around organs.

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

• Reticular CT: Only reticular fibers; forms soft internal skeleton supporting free blood cells. Location: Lymphoid organs (lymph nodes, bone marrow, spleen).

Dense Connective Tissue

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

• Dense irregular CT: Collagen bundles thicker and arranged irregularly; withstands tension in many directions. Location: Dermis, submucosa of GI tract, fibrous capsules of organs.

• Elastic CT: High content of elastic fibers; found in elastic ligaments.

Other Connective Tissues

Other types such as cartilage, bone, and blood will be covered in subsequent lessons.

Review Questions

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

• Which type of CT can increase in volume without making more cells? Areolar CT.

• Which kind of CT acts as a sponge, soaking up fluid during edema? Areolar CT.