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 or lines body cavities. Example: skin, lining of GI tract

• Connective tissue: Supports, binds, and protects other tissues. Example: bone, blood, cartilage

• Muscle tissue: Responsible for movement. Example: skeletal muscle, cardiac muscle

• Nervous tissue: Controls and regulates body functions. Example: brain, spinal cord, nerves

Epithelial Tissue

Definition and General Characteristics

Epithelial tissue is a sheet of cells that covers a body surface or lines a body cavity, creating boundaries between different environments. It is classified as either covering/lining epithelium or glandular epithelium.

• 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, small intestine).

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

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

• 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 distinct structures and functions.

• Specialized contacts: Cells are closely joined by tight junctions and desmosomes, which help maintain tissue integrity and control permeability.

• Supported by connective tissue: The basement membrane, composed of the basal lamina and reticular lamina, anchors the epithelium to underlying connective tissue.

• 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.

Surface Specializations

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

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

Classification of Epithelial Tissue

Epithelial tissues are classified based on two criteria:

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

• 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 glomeruli, lung alveoli.

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

• Simple columnar epithelium: Secretion and absorption. Locations: digestive tract, gallbladder.

• Pseudostratified (ciliated) columnar epithelium: Appears stratified but is a single layer; often ciliated and involved in secretion and movement of mucus. Location: respiratory tract.

Types of Stratified Epithelia

• Stratified squamous epithelium: Several layers; basal cells are cuboidal and undergo mitosis. Main function: protection. Location: epidermis of skin, lining of mouth.

• Transitional epithelium: Stratified epithelium with a mix of cuboidal 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 that open onto body surfaces or cavities (e.g., mucus, sweat, oil, salivary glands).

• 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.

Types of Multicellular Exocrine Glands by Secretion Method

• 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).

Table: Epithelial Types and 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.

• 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)

• Connective tissue proper (loose and dense types)

• Cartilage

• Bone

• Blood

Structural Organization of Connective Tissue

• Ground substance (matrix): 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: Macrophages, mast cells, white blood cells.

Types of Connective Tissue Proper

Loose Connective Tissue

• Areolar CT: Gel-like matrix with all three fiber types; supports and binds other tissues, holds body fluids, defends against infection. Location: under epithelia, around organs.

• Adipose CT: Modified areolar CT for nutrient storage; contains fat-filled adipocytes. Location: under skin, around kidneys and eyeballs, within abdomen and breasts.

• Reticular CT: Like areolar but only reticular fibers; forms a soft internal skeleton supporting free blood cells. Location: 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, irregularly arranged collagen fibers; withstands tension in many directions. Location: dermis, submucosa of GI tract, fibrous capsules of organs.

• Elastic CT: High content of elastic fibers; allows recoil. Location: certain 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.

Table: Connective Tissue Cell Types

Sample Questions and Key Facts

• 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 more detail in subsequent lessons.