Epithelium (2)

📌 Definition & General Concepts

• Tissue = group of cells similar in structure and function.
• Four main tissue types: epithelial, connective, muscular, and nervous.

🖇 Epithelial Tissue (Epithelium)
Also called lining tissue (النسيج الطلائي).

📌 Functions & Locations

• Covers external surfaces (skin) and lines internal cavities or lumens.
• Found in mouth, tongue, pharynx, esophagus, arteries, and peritoneum.

📌 Classification (General Types)

1. Covering Epithelium (Membranous or Lining Epithelium) (Exam Point).
2. Glandular Epithelium – forms glands (thyroid, pituitary).
3. Neuroepithelium – specialized for sensory function.

🟠 General Characteristics

• Highly cellular with minimal intercellular space.
• Rests on basement membrane (BM).
• Avascular, nourished by diffusion from connective tissue.
• Polarized cells with distinct surfaces:
• • Basal surface → attached to BM.
  • Apical surface → faces lumen/exterior.
  • Lateral surface → between adjacent cells.

🟣 Classification of Covering/Lining Epithelium

📌 A. According to Number of Layers

• Simple Epithelium: One layer; all cells touch BM.
• Stratified Epithelium: Two or more layers; only basal layer touches BM.
• Pseudo-stratified Epithelium: Appears multilayered, but all cells touch BM (false stratification).

📌 B. According to Shape of Surface (Apical) Cells

• In simple epithelium: may be squamous, cuboidal, or columnar.
• In stratified epithelium: named by apical cell shape (squamous, cuboidal, or columnar).
• Special Type: Transitional epithelium.

🚨 Condition (Exam Point): In stratified epithelium, only the basal layer contacts the BM.

🟠 Detailed Types of Stratified Epithelium

📌 1. Stratified Squamous Epithelium

• Many layers; apical cells are flat.

A. Keratinized Type (Exam Point)

• Outermost layer: dead, flat, anuclear cells packed with keratin.
• Location: entire skin (thick skin = palms, soles; thin skin = rest of body).
• Function: protection from abrasion, water loss, infection.

B. Non-Keratinized Type

• Outermost layer: flat nucleated cells, no keratin.
• Location (Exam Point): oral cavity, tongue, pharynx, esophagus, anal canal, vagina.
• Function: protection against friction and mechanical stress.

📌 2. Stratified Cuboidal Epithelium

• Two or more layers; surface cells cuboidal.
• Location: ducts of sweat glands, ovarian follicles.
• Function: secretion (sweat, ovarian hormones).

📌 3. Stratified Columnar Epithelium (Rare)

• Two or more layers; surface cells columnar; not all cells touch BM.
• Non-ciliated type: found in parts of anal canal, pharynx, and epiglottis.
• Ciliated type: found in fetal esophagus and some epiglottis areas.
• Function: protection and secretion.

🟣 Exceptions to Standard Classification

📌 1. Pseudo-stratified Columnar Epithelium (PCCE)

• “Pseudo” means false; appears stratified due to nuclei at different levels.
• (Exam Point) All cells touch BM → not truly stratified.
• Not all cells reach the apical surface.

Types and Locations:

🟠 A. Ciliated PCCE (with Goblet Cells)

• Has cilia and mucus-secreting Goblet Cells.
• Location (Exam Point): respiratory tract (nasal cavity, bronchi).
• Function: mucus traps particles; cilia move mucus and debris out (protective clearance).

🟠 B. Non-Ciliated PCCE

• Found in male reproductive tract: vas deferens and membranous urethra.
• Clinical Note: absence of cilia and Goblet Cells prevents mucus obstruction of urinary flow.

📌 2. Transitional Epithelium (Urothelium)

• Variable number of layers depending on bladder distension or contraction.
• Location (Exam Point): urinary system—renal pelvis, ureter, urinary bladder.

🟠 Structure and Mechanism

• Contracted state (empty bladder): thick epithelium with 5–7 (or 6–8) layers; apical cells dome-shaped.
• Distended state (full bladder): layers reduce to 2–3; cells flatten and slide over each other.
• Apical cells are often binucleated.

📌 Specialized Apical Membrane (Exam Point)

• Thicker than normal (~12 nm vs 7–8 nm).
• Contains thickened regions called plaques (bileaves).
• Supported by actin filaments to resist overstretching.
• During contraction, folds appear at thin inter-plaque areas forming fusiform vesicles (seen in EM).

📌 Key Exam Points Summary

• Covering Epithelium = Membranous Epithelium.
• Stratified epithelium named by apical layer shape.
• Only basal layer touches BM in stratified types.
• PCCE: all cells reach BM, not all reach surface.
• Transitional epithelium: lines urinary organs; dome-shaped surface cells when relaxed.
• Keratinized vs Non-Keratinized → presence or absence of keratin layer.