Tissues: Structure, Function, and Repair

4.1 Tissue Samples are Fixed, Sliced, and Stained for Microscopy

Microscopic examination of tissues requires careful preparation to preserve cellular structures and allow for clear visualization. This process involves fixation, slicing, and staining.

• Fixation: Tissue specimens are preserved to prevent decay and maintain structural integrity. Fixatives such as formalin are commonly used.

• Slicing: Tissues are sliced thinly enough to allow light transmission under a microscope.

• Staining: Different stains bind to various cell components, highlighting anatomical structures and allowing differentiation between cell types.

Example: Hematoxylin and eosin (H&E) staining is widely used to distinguish nuclei (blue/purple) from cytoplasm (pink).

4.2 Epithelial Tissue Covers Body Surfaces, Lines Cavities, and Forms Glands

Features of Epithelia

Epithelial tissue forms the covering or lining of body surfaces and cavities, and also constitutes glandular structures.

• Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

• Locations: Skin, lining of digestive tract, respiratory tract, and glands.

Special Characteristics of Epithelial Tissue

• Polarity: Epithelial cells have an apical (top) surface and a basal (bottom) surface attached to a basement membrane.

• Specialized Contacts: Cells are joined by tight junctions and desmosomes, forming continuous sheets.

• Supported by Connective Tissue: The basement membrane provides structural support.

• Avascular but Innervated: Epithelia lack blood vessels but are supplied by nerves.

• High Regeneration Capacity: Epithelial cells readily divide to replace lost or damaged cells.

Classification of Epithelia

Epithelia are classified by the number of cell layers and the shape of cells.

• Layers:

  • Simple epithelium: Single layer of cells.

  • Stratified epithelium: Multiple layers of cells.

• Cell Shapes:

  • Squamous: Flat and scale-like.

  • Cuboidal: Box-like.

  • Columnar: Tall and column-shaped.

Example: Simple squamous epithelium lines blood vessels and alveoli; stratified squamous epithelium forms the skin's outer layer.

Glandular Epithelium

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

• Exocrine Glands: Glands that secrete products into ducts leading to body surfaces or cavities.

• Unicellular Exocrine Glands: Single cells, such as goblet cells, that secrete mucus.

• Multicellular Exocrine Glands: Composed of multiple cells, classified by structure and secretion mechanism.

4.3 Connective Tissue: Most Abundant and Widely Distributed Tissue

Common Characteristics of Connective Tissue

Connective tissue supports, protects, and binds other tissues together. It is derived from mesenchyme and varies in vascularity.

• Extracellular Matrix: Composed of ground substance and fibers, separating cells.

• Fibers: Collagen (strength), elastic (stretch/recoil), and reticular (support).

• Cells: Each connective tissue type has a fundamental cell type in immature and mature forms.

Types of Connective Tissue

4.4 Muscle Tissue is Responsible for Body Movement

Muscle tissue consists of highly cellular, well-vascularized tissues that contract to produce movement.

• Skeletal Muscle: Attached to bones, voluntary control, long cylindrical cells, striated.

• Cardiac Muscle: Found in heart, involuntary, striated, branched cells with intercalated discs.

• Smooth Muscle: Found in walls of hollow organs, involuntary, non-striated, spindle-shaped cells.

Example: Skeletal muscle enables locomotion; cardiac muscle pumps blood; smooth muscle moves food through the digestive tract.

4.5 Nervous Tissue: Specialized for Communication

Nervous tissue is the main component of the nervous system, responsible for regulating and controlling body functions.

• Neurons: Specialized cells that generate and conduct electrical impulses.

• Neuroglia: Supporting cells that nourish, protect, and insulate neurons.

Example: Neurons transmit signals from the brain to muscles; neuroglia maintain the environment for neuronal function.

4.6 Membranes: Cutaneous, Mucous, and Serous

Body membranes are formed by epithelial and connective tissues, serving protective and functional roles.

• Cutaneous Membrane: The skin; dry, keratinized stratified squamous epithelium over dense connective tissue.

• Mucous Membranes: Line body cavities open to the exterior; moist, secrete mucus.

• Serous Membranes: Line closed body cavities; secrete serous fluid to reduce friction.

Example: The peritoneum is a serous membrane lining the abdominal cavity.

4.7 Tissue Repair: Inflammation, Organization, and Regeneration

Tissue repair restores tissue integrity after injury through a sequence of events: inflammation, organization, and regeneration.

• Inflammation: Damaged cells release chemicals that trigger an inflammatory response, increasing blood flow and attracting immune cells.

• Organization: Granulation tissue forms, restoring blood supply and supporting tissue repair.

• Regeneration: Replacement of damaged cells with the same cell type; fibrosis replaces damaged tissue with scar tissue if regeneration is not possible.

Example: Skin wounds heal by regeneration if the basal layer is intact; deeper injuries may result in scar formation.

Developmental Aspects of Tissues

• Primary Germ Layers: Ectoderm, mesoderm, and endoderm form during embryonic development and give rise to all tissues.

• Stem Cells: Some tissues retain populations of stem cells for repair and regeneration throughout life.

Additional info: In adults, most epithelial and blood-forming tissues retain high regenerative capacity, while other tissues such as cardiac muscle have limited ability to regenerate.