Animal Tissue: Comprehensive Notes

1. What is Tissue?

A group of cells having a similar origin, similar function, and may or may not have a similar structure.

• Term coined by: Bichat (already coined by N. Grew in plants).
• Study of Tissues: Histology.
• First evolution: Tissue first evolved in Coelenterata for division of labour.

Types of Animal Tissues:

1. Epithelial Tissue
2. Connective Tissue
3. Muscular Tissue
4. Nervous Tissue

A. Epithelial Tissue

Covers or lines body surfaces, organs, and body cavities. Contains very little intercellular matrix and cells are compactly packed.

• High wear & tear capacity with high regeneration speed.
• Has a free surface which faces either external environment or body fluids.
• Rests on a non-cellular Basement Membrane (made of Basal Lamina + Reticular Lamina).

3. Types of Simple Epithelium

Single layer of cells resting on a basement membrane.

a. Simple Squamous Tissue

Large, flattened cells fitting like tiles on a floor (also known as Pavement Epithelium).

• Locations:
  • Walls of blood vessels and lymph vessels (Endothelium).
  • Lining of body cavity walls / Coelem (Mesothelium — cells are wavy, called Tesselated Epithelium).
  • Lining of lung alveoli (Pneumocytes).
  • Bowman’s capsule and Loop of Henle in Nephron.
• Functions: Exchange of materials between blood capillary and tissue fluid, gas exchange, and ultra-filtration.

b. Simple Cuboidal Epithelium

Single layer of cube-like cells with round central nucleus.

• Locations: PCT of nephron, ducts of glands, thyroid follicles, ovaries & testes (Germinal epithelium).
• Special Surface: Surface may have microvilli (Brush Border) to increase surface area for reabsorption.
• Functions: Reabsorption, secretion, and excretion.

c. Simple Columnar Epithelium

Tall, pillar-like cells with elongated nuclei located at the base.

• Locations: Lining of Stomach and Intestine.
• Special Cells: Contains Goblet cells (Granular cells) that secrete mucus in the alimentary canal.
• Functions: Secretion and absorption.

4. Ciliated Epithelium

Cuboidal or Columnar cells bearing hair-like cilia on their free surface.

• Cuboidal Ciliated: Found in Bronchioles (moves trapped mucus particles out of pharynx).
• Columnar Ciliated: Found in Fallopian tubes (facilitates movement of ova).
• Function: Transportation/movement of particles in a specific direction.

5. Pseudostratified Epithelium

Single layer of cells that appears multi-layered (stratified) because nuclei are located at different levels (contains long cells + short round-nucleated cells).

• Locations: Male urethra, Parotid gland duct, Trachea, and Bronchi.

6. Glandular Epithelium

Specialized cuboidal or columnar cells adapted primarily for secretion.

Classification by Cell Count:

• Unicellular Glands: Single isolated secretory cells. Example: Goblet cell (Mucocyte).
• Multicellular Glands: Clusters of cells forming duct and secretory units. Example: Salivary, Sweat, Gastric, Sebaceous glands.

Classification by Mode of Secretion:

• Merocrine (Eccrine) Glands: Secretion released via exocytosis without cell damage (e.g., Pancreas, Salivary glands).
• Apocrine Glands: Apical portion of the cell is discharged along with secretion (e.g., Mammary glands, Axillary sweat glands).
• Holocrine Glands: Entire cell ruptures and dies to release secretory product (e.g., Sebaceous oil glands).

7. Compound Epithelium

Made of more than one layer of cells. Main function is protection against mechanical and chemical stress (plays little role in secretion or absorption).

a. Stratified Epithelium Types:

• Keratinized Stratified Squamous: Covered with dead keratin protein layer. Impervious to water and resistant to friction. Example: Epidermis of skin.
• Non-Keratinized Stratified Squamous: Provides wet protection. Examples: Buccal cavity, Eye cornea, Pharynx, Oesophagus, Vagina.
• Stratified Cuboidal: Examples: Large ducts of salivary and pancreatic glands.

b. Transitional Epithelium

Highly stretchable, multi-layered epithelium. Lacks a distinct basement membrane (LBA absent).

• Locations: Urinary Bladder and Ureters.

7. Cellular Junctions

Structures that connect adjacent cells or connect cells to the extracellular matrix.

• Tight Junction (Zona Occludens): Prevents leakage of substances across tissues.
• Desmosome (Macula Adherens): Acts as anchoring junction using tonofibrils (keratin filaments).
• Gap Junction: Facilitates rapid cellular communication and transfer of ions/molecules between cells.
• Hemidesmosome: Anchors epithelial cells to the underlying Basement Membrane.
• Interdigitation: Interlocking finger-like processes for exchange (e.g., Placenta).

B. Connective Tissue

Most abundant and widely distributed tissue in the animal body. Connects different organs and forms packaging around organs.

• Matrix: Non-living ground substance made of polysaccharide-protein complexes secreted by cells.
• 3 Components: Cells + Matrix + Fibers.

Types of Cells in Connective Tissue:

• Fibroblasts: Most abundant cells; secretes fibers:
  • Collagen Fibers: White, strong, unbranched, inelastic.
  • Elastin Fibers: Yellow, flexible, branched, elastic.
• Macrophages (Histiocytes / Clasmatocytes): Derived from Monocytes; perform phagocytosis.
• Plasma Cells: Derived from B-lymphocytes; “Cart-wheel” shaped nucleus; secretes antibodies during infection.
• Mast Cells: Similar to Basophils; secretes three important substances:
  • Histamine: Vasodilator (increases blood vessel diameter); Bronchioconstrictor (active in asthma).
  • Serotonin: Vasoconstrictor (decreases blood vessel diameter).
  • Heparin: Natural anticoagulant (maintains liquid nature of blood).

Types of Connective Tissue:

1) Loose Connective Tissue:

Matrix more

cells + Fibres – less

cells & Fibres – lossely

a. Areolar Tissue

Most widely distributed connective tissue present beneath the skin. Serves as a support framework for epithelium.

• Joins skin to muscle and fills spaces inside organs around blood vessels and nerves.
• Contains Macrophages, Fibroblasts, and Mast cells.
• Function: Support, packaging, repair, and diffusion of materials.

b. Adipose Tissue

Fat-storing tissue composed of specialized cells called Adipocytes (contains fat globules).

• Locations: Subcutaneous skin layer, around heart, kidney, and eyeball.
• Functions: Acts as a shock absorber, fat reservoir, and insulator.
• Adipose tissue : Fat cells

Adipocytes:

 Types of Adipocytes (White vs. Brown Fat)

a. White Adipose Tissue

• Monocular (single large central fat droplet).
• Contains fewer mitochondria.
• Functions as subcutaneous insulation, shock absorber, and reserve food storage.
• Cannot be easily removed from the body after exercise.

b. Brown Adipose Tissue

• Multilocular (several small fat droplets).
• Contains abundant mitochondria and cytochrome pigments giving it a brown color.
• Found in hibernating mammals and newborn babies.
• Generates heat to prevent shivering in newborns (never used as a food substrate).

2) Dense Connective Tissue

a. Dense Regular Connective Tissue

Collagen fibers are arranged in parallel rows between many parallel bundles.

• Tendon: Connects Bone to Muscle. Made of strong, inelastic white collagen fibers.
• Ligament: Connects Bone to Bone. Made of elastic yellow fibers with few collagen fibers.
  *Sprain is caused due to excessive pulling/stretching of ligaments.

b. Dense Irregular Connective Tissue

Fibers and fibroblasts are oriented differently in various directions. Example: Skin dermis.

3) Specialized Connective Tissue: Cartilage

Harder than dense connective tissue but softer than bone. Matrix is solid, pliable, and resists compression.

• Cartilage Cells: Chondrocytes (enclosed in fluid-filled cavities called Lacunae, 2-4 cells per lacuna).
• Precursor Cells: Chondroblasts. Cartilage-destroying cells: Chondroclasts.
• Protein