Introduction to Tissues and Epithelial Tissue

Introduction to Tissues and Epithelial Tissue

• Tissues are groups of cells similar in structure and function
• Epithelial tissue forms coverings and linings of surfaces
• Connective tissue provides support (e.g., bone, ligaments, fat)
• Muscle tissue facilitates movement
• Nervous tissue enables control (e.g., brain, nerves, spinal cord)
• As the human body develops from single to multicellular, cells specialize.
• The body is an interdependent system; dysfunction in one cell group can be catastrophic.

Function of Epithelial Tissue

• Protection: Skin protects from sunlight, bacteria, and physical damage.
• Absorption: Lining of the small intestine absorbs nutrients into the blood.
• Filtration: Lining of kidney tubules filters wastes from blood plasma.
• Secretion: Various glands produce perspiration, oil, digestive enzymes, and mucus.

Characteristics of Epithelial Tissue

• Epithelial cells form continuous sheets, fitting together like tiles.
• Apical Surface: The top surface of epithelial cells that borders an open space is called a lumen.
• Basement Membrane: The underside of epithelial cells anchors them to connective tissue, acting as a foundation.
• Avascularity: Epithelial tissue lacks blood vessels.
• Nourishment: Epithelial tissue is nourished by connective tissue.
• Regeneration: Epithelial tissue regenerates and repairs quickly.

Classification of Epithelial Tissue

• Cell Shape: Epithelial cells can be squamous (flattened), cuboidal (cube-shaped), or columnar (column-shaped).
• Cell Layers: Simple epithelium is one layer thick; stratified epithelium has multiple layers.
• Classification is based on the cell type and layer number at the apical surface.

Simple Squamous Epithelium

• Structure: Single layer of flattened cells.
• Function: Absorption and filtration; not highly protective.
• Location: Walls of capillaries, air sacs in lungs, serous membranes.

Simple Cuboidal Epithelium

• Structure: Single layer of cube-shaped cells.
• Function: Secretion and transportation in glands, filtration in kidneys.
• Location: Glands and ducts, kidney tubules, surface of ovaries.

Simple Columnar Epithelium

• Structure: Elongated layer of cells; nuclei are at the same level.
• Function: Absorption, protection, secretion.
• Location: Linings of the entire digestive tract; includes microvilli (increasing surface area for absorption) and goblet cells (producing protective mucus).

Pseudostratified Columnar Epithelium

• Structure: Irregularly shaped cells with nuclei at different levels; all cells reach the basement membrane.
• Function: Absorption and secretion; goblet cells produce mucus, cilia sweep substances.
• Location: Respiratory tract linings & reproductive tract.

Stratified Squamous Epithelium

• Structure: Many layers, usually cuboidal/columnar at the base and squamous at the top.
• Function: Protection; keratin (a protein) fills in older cells, providing waterproofing and toughness to the skin.
• Location: Keratinized skin, mouth, throat; non-keratinized in areas where a moist surface is needed.

Transitional Epithelium

• Structure: Many layers of varied cells; cells at the base can be cuboidal or columnar, but those on the surface can vary.
• Function: Allows for stretching (e.g., urinary bladder changing size).
• Location: Urinary bladder, ureters, urethra.

Glands

• Definition: One or more cells that make and secrete a product.
• Secretion: Protein in aqueous solution (e.g., hormones, acids, oils).
• Endocrine Glands: Ductless; release hormones directly into blood vessels.
• (e.g., thyroid, adrenal, pituitary).
• Exocrine Glands: Produce secretions that empty onto epithelial surfaces via ducts; (e.g., sweat, oil, salivary and mammary).

Shapes of Exocrine Glands

• Branching: Simple (unbranched duct) or compound (branched duct).
• Shape: Tubular (tube-like) or alveolar (flask- or sac-like), as well as tubuloalveolar (combination).

Modes of Secretion

• Merocrine: Cells release products by exocytosis without damage to cells, (e.g., sweat, salivary).
• Holocrine: Cells rupture and release their products along with the cell debris, (e.g., sebaceous oil glands).