Epithelium (1)

🖇 TISSUES OF THE BODY

📌 Definition of Tissue
A tissue is a group of cells that have a similar structure and act together to perform a specific function.

📌 Four Basic Tissues of the Body (Exam Point)

1. Epithelial tissue
2. Connective tissue
3. Muscle tissue
4. Nervous tissue

🟠 EPITHELIAL TISSUE

📌 General Characteristics of Epithelial Tissue (Exam Point)

1. Composed of single or multiple layers of close-packed cells with very little extracellular material.
2. Cells are held together by intercellular junctions.
3. Lies on a basement membrane.
4. Avascular (lacks blood vessels); nourished by diffusion from underlying connective tissue.
5. Rich in sensory nerve endings.
6. Has the capacity to regenerate.
7. Derived from all three germ layers: ectoderm, mesoderm, endoderm.
8. Covers the body surface, lines body cavities, forms glandular linings, and constitutes the bulk of most glands. (Exam Point)

🟣 Classification of Epithelium

Epithelium is classified into three main types:

1. Covering / Lining Epithelium
2. Glandular Epithelium
3. Special Types

📌 Covering / Lining Epithelium

🖇 Classification Criteria

According to Number of Cell Layers:

• Simple (one layer)
• Stratified (multiple layers)
• Pseudostratified (appears multilayered but all cells rest on the basement membrane)

According to Shape of Cells:

• Squamous (flattened)
• Cuboidal (cube-shaped)
• Columnar (tall and column-shaped)

By combining the two criteria, the main types are:

• Simple squamous epithelium
• Simple cuboidal epithelium
• Simple columnar epithelium
• Stratified squamous epithelium
• Stratified cuboidal epithelium
• Stratified columnar epithelium

🟠 Types of Simple Epithelium (Exam Point)

1. Simple Squamous Epithelium
2. Simple Cuboidal Epithelium
3. Simple Columnar Epithelium
4. Pseudostratified Columnar Epithelium

📌 1. Simple Squamous Epithelium

🖇 Microscopic Appearance (Exam Point)

• Single layer of flattened cells.
• Flattened nucleus in plane of the cell.
• Cytoplasm very thin and difficult to see under microscope.

🖇 Locations (Exam Point)

• Alveoli of lungs (function: diffusion).
• Bowman’s capsule of kidneys (function: filtration).
• Lining of heart and all blood vessels (called Endothelium) for smooth passage.
• External lining of visceral organs (called Mesothelium) allowing free mobility. (Exam Point)

🖇 Functions (Exam Point)

• Allows rapid diffusion and transport due to minimal thickness.
  Example: Gas exchange in lungs and blood filtration in kidneys.
• Secretes lubricating fluid in serous membranes (mesothelium).
• Allows free mobility of organs covered by mesothelium.

📌 2. Simple Cuboidal Epithelium

🖇 Microscopic Appearance (Exam Point)

• Single layer of cube-shaped cells.
• Centrally placed, round nuclei.

🖇 Locations (Exam Point)

• Liver, pancreas, and salivary glands.
• Thyroid follicles and most kidney tubules.

🖇 Functions (Exam Point)

• Secretion in organs like liver, pancreas, and salivary glands.
• Ion exchange in kidney tubules and thyroid follicles.

📌 3. Simple Columnar Epithelium

🖇 Microscopic Appearance (Exam Point)

• Single layer of tall, narrow columnar cells.
• Basally located oval nuclei.
• Secretory vesicles may appear in the apical region.
• Microvilli or cilia may be present on the apical surface.
• Goblet cells (mucus-secreting) are often present between cells.

🖇 Locations (Exam Point)
Non-ciliated type:

• Stomach, small intestine, large intestine, and gallbladder.
  Ciliated type:
• Uterus and uterine tubes.

🖇 Functions (Exam Point)

• Absorption in small intestine due to presence of microvilli that increase surface area.
• Secretion of mucus in large intestine to lubricate and ease passage of feces.
• Movement of ovum and embryo by ciliary action in uterus and uterine tubes. (Exam Point)