Tissues and Early Embryology

Introduction to Tissues

The human body is organized into levels of increasing complexity, from molecules and cells to tissues, organs, and organ systems. Tissues are groups of similar cells that perform specific functions. There are four primary tissue types in the human body: epithelial, connective, muscle, and neural tissues.

• Epithelial Tissue: Covers exposed surfaces and lines internal cavities and passageways. It also forms secretory glands.

• Connective Tissue: Fills internal spaces, provides structural support, stores energy, and transports materials.

• Muscle Tissue: Contracts to produce movement and includes skeletal, cardiac, and smooth muscle types.

• Neural Tissue: Conducts electrical impulses and carries information throughout the body.

Epithelial Tissue

Epithelial tissue consists of sheets of closely packed cells that cover body surfaces and line internal organs and cavities.

• Functions:

  • Physical protection: Shields underlying tissues from mechanical and chemical injury.

  • Control permeability: Regulates the movement of substances in and out of the body or organs.

  • Provide sensation: Contains sensory nerve endings for detecting environmental changes.

  • Produce secretions: Forms glands that secrete substances such as enzymes, hormones, and mucus.

• Specializations of Epithelial Cells:

  • Microvilli: Finger-like projections that increase surface area for absorption (e.g., in the intestines).

  • Stereocilia: Long, non-motile microvilli found in the male reproductive tract and inner ear.

  • Ciliated epithelium: Contains cilia that move substances across the epithelial surface (e.g., respiratory tract).

Definition: An epithelium is a sheet of cells that covers exposed surfaces and lines internal cavities and passageways.

Examples and Applications

• Skin epithelium protects against pathogens and dehydration.

• Intestinal epithelium absorbs nutrients and secretes digestive enzymes.

• Respiratory epithelium uses cilia to move mucus and trapped particles out of the airways.

Additional info: Epithelial tissues are avascular (lack blood vessels) and rely on diffusion from underlying connective tissues for nutrients. They have a high rate of cell division, which aids in rapid repair and regeneration.