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 fundamental to the structure and function of organs, contributing to the overall homeostasis of the human body. Understanding tissue types and their properties is essential for studying anatomy and physiology.

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

• Why different tissues? Individual body cells are specialized for division of labor. At the organ level, different tissues cooperate for the organ to function as a whole.

The Four Primary Tissue Types

• 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 by transmitting electrical impulses.

Epithelial Tissue

Definition and General Characteristics

Epithelial tissue is a sheet of cells that covers a body surface or lines a body cavity. It forms boundaries between different environments and is involved in protection, absorption, filtration, excretion, secretion, and sensory reception.

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

• Glandular epithelium: Forms the glands of the body.

Functions of Epithelial Tissue

• Protection: Against mechanical, chemical, and infectious agents (e.g., skin).

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

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

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

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

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

Special Structural Characteristics of Epithelial Tissue

• Polarity: Epithelial cells have an apical (top) surface and a basal (bottom) surface, each with different structures and functions.

• Specialized contacts: Cells are closely joined by tight junctions and desmosomes to form continuous sheets and prevent leakage.

• Supported by connective tissue: The basement membrane, composed of the basal lamina and reticular lamina, provides support and anchorage.

• Avascular but innervated: Epithelial tissues lack blood vessels but are supplied by nerve fibers.

• High regenerative capacity: Epithelial cells rapidly divide to replace lost or damaged cells, which is essential due to their protective and absorptive roles.

Additional info: The basement membrane acts as a selective filter and provides structural support. Cancerous epithelial cells often lose their ability to respect the boundaries imposed by the basement membrane, contributing to metastasis.

Classification of Epithelial Tissue

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

• Number of layers:

  • Simple epithelium: Single layer of cells.

  • Stratified epithelium: Multiple layers of cells.

• Cell shape:

  • Squamous: Flat and scale-like.

  • Cuboidal: Cube-shaped.

  • Columnar: Tall and column-like.

Types of Simple Epithelia

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

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

• Simple columnar epithelium: Secretion and absorption. Locations: Digestive tract (stomach to rectum).

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

Types of Stratified Epithelia

• Stratified squamous epithelium: Protective role; basal cells are cuboidal and undergo mitosis. Location: Epidermis of skin, lining of mouth.

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

Glandular Epithelia

• 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.

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

Modes of Secretion in Exocrine Glands

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

• Holocrine: Entire cell ruptures to release product (e.g., sebaceous glands).

• Apocrine: Apex of cell pinches off with secretory product (e.g., mammary glands; debated if truly apocrine in humans).

Matching Epithelial Types to Locations

Connective Tissue (CT)

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue type in the body. It supports, binds, and protects other tissues and organs. CT is classified into several types based on structure and function.

• Main functions:

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

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

  • Insulating (e.g., adipose tissue)

  • Storing reserve fuel (e.g., adipose tissue)

  • Transporting substances (e.g., blood)

Major Types of Connective Tissue

• Mesenchyme: Embryonic connective tissue; gives rise to all other CT types.

• Connective tissue proper: Includes loose and dense CT.

• Cartilage

• Bone

• Blood

Structural Elements of Connective Tissue

• Ground substance: Unstructured material that fills the space between cells; composed of interstitial fluid, cell adhesion proteins (e.g., 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 collagen fibers; form supportive networks.

• Cells:

  • "Blast" cells: Immature, actively dividing cells (e.g., fibroblast, chondroblast, osteoblast).

  • "Cyte" cells: Mature, less active cells (e.g., fibrocyte, chondrocyte, osteocyte).

  • Other cells: Adipocytes (fat storage), white blood cells (immunity), mast cells (inflammation), macrophages (phagocytosis).

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 white blood cells. Functions: Cushions organs, holds tissue fluid, involved in inflammation. Location: Widely distributed under epithelia.

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

• Reticular CT: Like areolar CT but only reticular fibers. Functions: Forms soft internal skeleton (stroma) that supports other cell types. Location: Lymphoid organs (lymph nodes, bone marrow, spleen).

Dense Connective Tissue

• Dense regular CT: Parallel collagen fibers; great resistance to tension. Location: Tendons, ligaments, aponeuroses.

• Dense irregular CT: Thick collagen bundles arranged irregularly. Location: Dermis of skin, submucosa of GI tract, fibrous capsules of organs.

• Elastic CT: High content of elastic fibers. Location: Some ligaments, large arteries.

Clinical Note: Connective Tissue Disease

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

Review Questions and Key Points

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

• What type of CT can increase in volume without making more cells? Adipose tissue.

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

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