Animal Tissues Notes

Introduction

• Life evolved from unicellular organisms capable of basic functions.
• Multicellular organisms developed specialized cells for greater efficiency.
• Tissues are groups of similar cells performing specific functions.
• Animals have four basic tissue types organized into organs and systems, exhibiting division of labor.

Animal Tissues

• Tissue: group of similar cells with common origin and function.
• Study of tissues: histology.
• Four types: epithelial, connective, muscular, and neural.

Epithelial Tissue

• Covers/lines external and internal surfaces; forms glands.
• Structure: closely arranged cells with little intercellular material; basal surface on basement membrane; free surface.
• Types: simple and compound.

Simple Epithelium

• Single layer of cells on basement membrane; lines cavities, ducts, tubes.
• Types:
  • Squamous: flattened cells; Locations: blood vessels (endothelium), lung alveoli.
  • Function: Filtration, diffusion, and gas exchange.
  • Cuboidal: cube-like cells with central nuclei; Locations: kidney tubules, gland ducts, thyroid follicles.
  • Function: Absorption, secretion, and excretion.
  • Brush-bordered cuboidal epithelium (microvilli): reabsorption in kidney tubules.
  • Columnar: tall cells with elongated nuclei near base; Locations: lining of stomach and intestine.
  • Function: Absorption and secretion.
  • Goblet cells: mucus-secreting.
  • Ciliated: cuboidal or columnar cells with cilia; Locations: bronchioles, fallopian tubes, respiratory tract.
  • Function: Moves particles or mucus.
  • Pseudostratified: single layer of cells with nuclei at different levels; Locations: trachea.
• Glandular: specialized for secretion.
  • Unicellular: isolated single cells (e.g., goblet cells).
  • Multicellular: cluster of cells with ducts and secretory parts (e.g., salivary glands).
  • Exocrine: secrete via ducts (e.g., salivary, gastric glands).
  • Endocrine: ductless, secrete hormones directly into blood/lymph (e.g., thyroid, pituitary).
  • Heterocrine (Mixed): both exocrine and endocrine, e.g., pancreas and gonads.

Compound Epithelium

• More than one layer of cells; protection against mechanical/chemical stress.
• Stratified: many layers; Locations: skin (keratinized), buccal cavity (non-keratinized).
• Transitional: stretchable; Locations: urinary bladder, ureters.

Cell Junctions

• Tight junctions: prevent leakage across tissues.
• Gap junctions: facilitate communication between cells.
• Adhering junctions (desmosomes): cement neighboring cells together.

Connective Tissue

• Abundant tissue connecting different tissues/organs.
• Structure: matrix, cells, and fibers (collagen or elastin).
• Types: loose, dense, and specialized.

Loose Connective Tissues

• Cells and fibers loosely arranged in semi-fluid matrix.
  • Areolar Tissue: widespread; supports epithelium; contains fibroblasts, macrophages, mast cells.
  • Matrix contains collagen and elastic fibers.
  • Adipose Tissue: fat-storing; beneath skin; insulates and cushions organs.
  • Adipocytes contain large fat droplets.

Dense Connective Tissues

• Fibers and fibroblasts compactly packed.
  • Dense Regular: collagen fibers in rows; tendons and ligaments.
  • Tendons: muscle to bone, inelastic.
  • Ligaments: bone to bone, elastic.
  • Dense Irregular: fibers oriented differently; dermis of skin.

Specialized Connective Tissues

• Skeletal: cartilage and bone; solid extracellular matrix.
  • Cartilage: elastic, Locations: nose, ear, joints.
  • Matrix contains chondrocytes in lacunae.
  • Bone: hard, non-flexible; endoskeleton.
  • Matrix contains calcium salts and collagen fibers; Haversian canals.
• Fluid: blood and lymph; fibre-free fluid matrix.
  • Blood: plasma, RBCs, WBCs, platelets; transports materials.

Muscular Tissue

• Movement via contraction and relaxation.
• Muscle fibers contain myofibrils.
• Types: skeletal, smooth, and cardiac.

Skeletal Muscle

• Attached to bones; voluntary.
• Striated (striped) appearance.
• Multinucleated fibers with peripheral nuclei.

Smooth Muscle

• Walls of hollow organs; involuntary.
• Non-striated (unstriped).
• Spindle-shaped cells with central nucleus.

Cardiac Muscle

• Heart; involuntary.
• Striated and branched fibers with intercalated discs.
• Uninucleated cells.

Nervous (Neural) Tissue

• Controls body's responsiveness to changing conditions; transmits messages.
• Brain, spinal cord, and nerves.
• Neurons: structural and functional units; excitable cells.
• Neuroglia: support and protect neurons.

Neuron Structure

• Cyton (cell body): nucleus and cytoplasm with Nissl's granules.
• Dendrites: carry impulses toward cell body.
• Axon: carries messages away from cell body; may have myelin sheath and Schwann cells.
• Synapse: junction between neurons for impulse transmission via neurotransmitters.