Chapter 4 – Tissues

Tissues are groups of similar cells performing a common function. In the human body, there are four primary tissue types: epithelial, connective, muscle, and nervous tissue. This section focuses on epithelial tissue, its characteristics, roles, and classification.

Epithelial Tissue

Epithelial tissue is a sheet of cells that covers body surfaces, lines internal cavities and organs, and forms glands. It serves as a protective barrier and is involved in absorption, secretion, and filtration.

Location

• Coverings – Forms the outer layer of the skin and covers body organs.

• Linings – Lines internal cavities (such as the thoracic and abdominal cavities) and hollow organs (such as the stomach and intestines).

• Glands – Forms the secretory tissue in glands.

Roles of Epithelial Tissue

• Protection – Shields underlying tissues from mechanical and chemical injury, pathogens, and dehydration.

• Absorption – Specialized for uptake of substances (e.g., nutrients in the intestines).

• Filtration – Allows selective movement of substances (e.g., in kidney tubules).

• Secretion – Produces and releases substances such as enzymes, hormones, and mucus.

• Sensation – Contains sensory nerve endings (e.g., in skin and taste buds).

Characteristics of Epithelial Tissue

• Cells fit closely together – Epithelial cells are tightly packed with minimal extracellular material, forming continuous sheets.

• Tissue always has one free edge (apical surface) – This surface is exposed to the body’s exterior or to the cavity of an internal organ. The opposite side is attached to a basement membrane.

• Avascular – Epithelial tissue lacks blood vessels and receives nutrients by diffusion from underlying connective tissue. Additional info: This point is standard for epithelial tissue but not explicitly shown in the images.

• Regeneration – Epithelial cells have a high capacity for renewal, which is important for tissues exposed to friction or harsh environments.

Glandular Epithelium

Glandular epithelium consists of one or more cells that produce and secrete a particular product (secretion). Glands are classified based on how they release their products.

• Endocrine glands – Ductless glands that secrete hormones directly into the bloodstream. Examples include the thyroid and pituitary glands.

• Exocrine glands – Glands that secrete their products through ducts to the epithelial surface. Examples include sweat glands, salivary glands, and oil glands.

Comparison of Endocrine and Exocrine Glands

Summary Table: Key Features of Epithelial Tissue

Example: Epithelial Tissue in the Small Intestine

• The lining of the small intestine is composed of simple columnar epithelium, which is specialized for absorption and secretion.

• The apical surface faces the intestinal lumen and may have microvilli to increase surface area.

Additional info: The notes reference images of epithelial tissue and the small intestine, which are standard examples in anatomy and physiology.