Tissues of the Human Body: Structure, Function, and Classification

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: Histology is 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; forms boundaries.

• Connective tissue: Provides support and structure.

• Muscle tissue: Responsible for movement.

• Nervous tissue: Controls and regulates body functions.

Epithelial Tissue

Definition and Functions

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

• Covering and lining epithelium: Found on external and internal surfaces (e.g., skin, lining of GI tract).

• Glandular epithelium: Forms glands that secrete products (e.g., sweat glands, salivary glands).

Six major functions of epithelial tissue include:

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

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

• Filtration: Selective passage 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).

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 tight junctions and 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 occurs via diffusion from underlying connective tissue.

• Regeneration: High capacity for renewal due to frequent cell loss and damage.

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 epithelia consist of a single cell layer and are primarily involved in absorption, secretion, and filtration.

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

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

• Simple columnar epithelium: Secretion and absorption. Locations: Lining of stomach and intestines.

• Pseudostratified (ciliated) columnar epithelium: Appears stratified but is a single layer; often contains cilia and goblet cells. Locations: Respiratory tract.

Stratified Epithelia

• Stratified squamous epithelium: Multiple layers; basal cells are cuboidal and undergo mitosis, apical cells are squamous. Function: Protection against abrasion. Location: Skin (epidermis), lining of mouth.

• Transitional epithelium: Stratified; basal cells are cuboidal, apical cells flatten as the organ fills. Location: Urinary bladder.

Glandular Epithelium

Glands are formed by epithelial cells that produce and secrete products.

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

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

• Unicellular exocrine glands: Single cells, such as goblet cells, secrete mucus.

• Multicellular exocrine glands: Composed of a duct and secretory unit; classified by duct structure (simple or compound) and secretory shape (tubular, alveolar).

Methods of Secretion

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

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

• Apocrine: Apex of cell pinches off with product (e.g., mammary glands; debated classification).

Connective Tissue (CT)

Types and Functions

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

  2. Protecting

  3. Insulating

  4. Transporting substances

Structural Organization of CT

• Ground Substance: Composed of interstitial fluid, cell adhesion proteins (e.g., fibronectin, laminin), and proteoglycans; acts as a molecular sieve for nutrient and waste exchange.

• Fibers:

  • Collagen fibers: Provide high tensile strength.

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

  • Reticular fibers: Thin, form supportive networks in soft tissues.

• Cells: Immature 'blast' cells (e.g., fibroblast, chondroblast) are active in growth and repair; mature 'cyte' cells (e.g., fibrocyte) maintain tissue.

Table: CT Cell Types

Tissue Type	Immature ('blast')	Mature ('cyte')
Connective Tissue Proper	Fibroblast	Fibrocyte
Cartilage	Chondroblast	Chondrocyte
Bone	Osteoblast	Osteocyte
Blood	Hematopoietic stem cell	RBCs, WBCs

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.

• Adipose CT: Modified areolar CT with fat-filled adipocytes. Functions: Energy storage, insulation, supports and protects organs. Locations: Under skin, around kidneys and eyeballs, within abdomen and breasts.

• Reticular CT: Contains only reticular fibers. Functions: Forms soft internal skeleton supporting free blood cells. Locations: Lymph nodes, bone marrow, spleen.

Dense Connective Tissue

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

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

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

Clinical Note: Connective Tissue Disease

• Rheumatoid arthritis: An 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 (due to its loose matrix).

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

Additional info:

• Other types of connective tissue (cartilage, bone, blood) will be covered in subsequent lessons.