Biology: Tissues of the Body

Tissues of the Body

• A tissue is a group of cells with similar morphological characteristics specialized to perform specific functions.
• There are 4 basic tissues in the body: epithelial, connective, muscular, and nervous tissue.

Epithelial Tissue

• Forms continuous sheets of cells that line internal cavities and cover external surfaces (skin) of the body.
• General characteristics:
  • Cells are tightly joined together.
  • Cell boundaries are ill-defined.
  • Cells rest on a basement membrane.
  • Originate from the three germ layers: ectoderm, mesoderm, and endoderm.
  • Avascular but highly innervated.
  • Cells can regenerate.
  • Can modify to form other types (glandular epithelium, neuro-epithelium, and myo-epithelium).

Classification of Epithelium

• Surface Epithelium:
  • Simple Epithelium:
    • One layer of cells.
    • Types: squamous, cuboidal, columnar, and pseudo-stratified columnar.
  • Stratified Epithelium:
    • More than one layer of cells.
    • Types: stratified squamous (keratinized and non-keratinized), transitional, and stratified columnar.

Simple Epithelium

• Simple Squamous Epithelium:
  • Lines blood vessels.
  • Lines upper respiratory tract (nasal cavity, trachea, and bronchi).
• Simple Columnar Epithelium:
  • Non-ciliated: lines the gastrointestinal tract (GIT).
  • Ciliated: lines the fallopian tube.
• Simple Cubical Epithelium:
  • Lines the gastrointestinal tract (GIT).
  • Found in thyroid follicles and fallopian tube.

Stratified Epithelium

• Mainly protective function.
• Types:
  • Stratified Squamous Epithelium:
    • Non-keratinized: esophagus.
    • Keratinized: skin.
  • Stratified Columnar Epithelium:
    • Urinary passages (urinary bladder and ureters).

Glandular Epithelium

• Classified according to:
  • Presence or absence of ducts:
    • Endocrine glands (no ducts): pituitary gland, thyroid gland.
    • Exocrine glands (with ducts): salivary gland.
  • Number of cells:
    • Multicellular glands: salivary glands.
    • Unicellular glands: goblet cells.
• Modes of secretion:
  • Merocrine secretion: exocytosis of secretory products without loss of cell membrane (e.g., pancreatic acinar cells).
  • Apocrine secretion: release of secretory products with loss of apical cytoplasm and cell membrane (e.g., lactating mammary glands).
  • Holocrine secretion: destruction of secretory cells with secretion (e.g., sebum from sebaceous glands).
• Nature of secretion:
  • Serous (watery) secretion: containing enzymes (e.g., pancreas and parotid gland).
  • Mucous secretion (e.g., goblet cells).
  • Mixed (muco-serous) secretion (e.g., salivary glands).
  • Waxy secretion (e.g., ceruminous glands in the external ear).
  • Creamy secretion (e.g., sebum from sebaceous glands).

Neuro-Epithelium

• Epithelial cells modified and specialized to receive sensory stimuli.
• Epithelial cells act as sensory receptors for special senses (e.g., olfactory epithelium for olfaction sensation and taste buds for taste sensation).