Chapter 4 Notes: Epithelial, Connective Tissues and Membranes

Epithelial Tissue: Structure and Function

  • Structural characteristics
    • 22 surfaces: apical (top, exposed to exterior or lumen) and basal (bottom, attached to connective tissue).
    • Cells fit closely together with tight junctions and desmosomes.
    • Always rests on connective tissue (provides blood supply indirectly).
    • Avascular but innervated (no blood vessels; nutrients diffuse from connective tissue; has nerve supply).
    • High regeneration capacity: basal stem cells divide quickly to replace damaged/lost cells.
  • Functional characteristics
    • Protection of underlying tissues.
    • Absorption (nutrients in gut).
    • Secretion (glands, mucus).
    • Diffusion/filtration (lungs, kidneys).

Epithelial Tissue Types: Classification, Functions, and Locations

  • By layers
    • Simple = 11 layer (absorption, secretion, filtration).
    • Stratified = multiple layers (protection).
  • By shape
    • Squamous = flat.
    • Cuboidal = cube-shaped.
    • Columnar = tall, rectangular.
  • Types, chief functions, and locations
    • Simple Squamous: flat, single layer → rapid diffusion/filtration → alveoli, kidney glomeruli, serosa.
    • Simple Cuboidal: cube-shaped → secretion/absorption → kidney tubules, ducts of glands.
    • Simple Columnar: tall cells, often with microvilli/cilia, goblet cells → absorption/secretion → digestive tract, bronchi.
    • Pseudostratified Columnar: appears layered, but all cells touch basement membrane → secretion, mucus movement → trachea, respiratory tract.
    • Stratified Squamous: multiple layers, apical squamous → protection from abrasion → skin (keratinized), mouth/esophagus/vagina (nonkeratinized).
    • Transitional Epithelium: basal cuboidal/columnar, apical shape varies → stretches → ureters, bladder, urethra.

Gland

  • A gland is one or more cells that make and secrete an aqueous fluid called a secretion.
  • 11 or more cells: unicellular glands (e.g., goblet cells) and multicellular glands (e.g., sweat glands, salivary glands).

Endocrine vs Exocrine Glands; Unicellular vs Multicellular

  • Endocrine glands: ductless, secrete hormones into extracellular fluid → hormones travel via blood/lymph → regulate target organs.
  • Exocrine glands: secrete products onto body surfaces or into cavities via ducts (e.g., sweat, saliva, mucus).
  • Unicellular glands: single cells, e.g., goblet cells (secrete mucus).
  • Multicellular glands: consist of many cells, often with ducts (e.g., sweat glands, salivary glands).

Connective Tissue: Characteristics and Structural Elements

  • Characteristics
    • Most abundant and widely distributed tissue.
    • Varying vascularization (some vascular, some avascular).
    • Mostly made of extracellular matrix (fibers + ground substance).
  • Structural elements
    • Ground substance: fills spaces; supports tissue; medium for exchange.
    • Fibers:
    • Collagen = strength.
    • Elastic = stretch and recoil.
    • Reticular = supportive framework.
    • Cells: fibroblasts, adipocytes, chondrocytes, osteocytes, blood cells depending on type.

Connective Tissue: Types and Functions

  • Loose Connective Tissue
    • Areolar CT: wraps/cushions organs, holds fluids; under epithelia.
    • Adipose CT: fat storage, insulation, shock absorption; under skin, around kidneys/eyes.
    • Reticular CT: internal framework for lymphoid organs; lymph nodes, spleen, bone marrow.
  • Dense Connective Tissue
    • Dense Regular: parallel collagen fibers → tensile strength in one direction → tendons/ligaments.
    • Dense Irregular: irregular collagen fibers → tension in many directions → dermis, joint capsules.
    • Elastic CT: elastic fibers → stretch/recoil → arteries, vocal cords, bronchial tubes.
  • Cartilage (avascular, chondrocytes in lacunae)
    • Hyaline: firm, supports/cushions → ribs, nose, trachea, ends of long bones.
    • Elastic: flexible, strong → ear, epiglottis.
    • Fibrocartilage: tough, resists compression → intervertebral discs, pubic symphysis, menisci.
  • Bone: calcified matrix with collagen; support, protection, stores minerals, blood cell production.
  • Blood: cells (RBCs, WBCs, platelets) in plasma; transports gases, nutrients, wastes; immune defense.

Membranes: Structure, Function and Location

  • Cutaneous membrane (skin)
    • Structure: keratinized stratified squamous epithelium + dense connective tissue.
    • Function: protection, barrier, vitamin D support.
    • Location: covers body surface.
  • Mucous membranes (mucosae)
    • Structure: epithelium + lamina propria (areolar CT).
    • Function: secretion, absorption, protection.
    • Location: lines cavities open to exterior (respiratory, digestive, urinary, reproductive tracts).
  • Serous membranes (serosae)
    • Structure: simple squamous epithelium + areolar CT, double-layered with serous fluid in between.
    • Function: reduces friction between moving organs.
    • Location: closed ventral cavities (pleura = lungs, pericardium = heart, peritoneum = abdominal organs).