Tissues: Introduction

Definition and Overview

Tissues are groups of cells that are similar in structure and function. In the human body, tissues are fundamental units that combine to form organs and organ systems, each specialized for particular physiological roles.

• Epithelial Tissue

• Connective Tissue

• Muscle Tissue

• Nervous Tissue

Primary Tissues

Types and Characteristics

The four general types of tissues each have distinct structures and functions:

• Epithelial Tissue:

  • Avascular (lacks blood vessels), but innervated (has nerve supply).

  • Attached to a basement membrane.

  • Composed of layers of closely spaced, highly mitotic cells.

  • Covers organ surfaces and lines cavities; functions in protection, secretion, and absorption.

  • Examples: epidermis, lining of organs, glands.

• Connective Tissue:

  • Contains more extracellular matrix than cell volume.

  • Specialized to support, bind together, and protect organs.

  • May be vascularized (e.g., bone) or avascular (e.g., cartilage).

  • Examples: tendons, ligaments, bone, cartilage, blood, lymph.

• Nervous Tissue:

  • Contains excitable cells specialized for rapid transmission of coded information (action potentials).

  • Examples: brain, spinal cord, nerves.

• Muscular Tissue:

  • Composed of elongated, excitable cells specialized for contraction.

  • Examples: skeletal muscle (striated), cardiac muscle, smooth muscle.

Primary Germ Layers

Embryonic Origins of Tissues

All mature tissues in the body arise from three primary germ layers formed during embryonic development:

• Ectoderm:

  • Outer layer; gives rise to epidermis and nervous system.

• Mesoderm:

  • Middle layer; gives rise to connective tissues and most muscle tissue.

  • Connective tissue types: areolar, adipose, regular, irregular, elastic, hyaline cartilage, elastic cartilage, compact and spongy bone, blood, lymph.

• Endoderm:

  • Inner layer; gives rise to epithelial linings of the digestive and respiratory tracts, and inner linings of the heart and blood vessels.

Epithelial Cells and Basement Membrane

Cell Surfaces and Connections

Epithelial cells are polarized, with distinct surfaces that contribute to their function:

• Apical Surface:

  • Faces the lumen or external environment.

  • May contain microvilli (increase surface area for absorption) or cilia (motile, move substances), but not both simultaneously.

• Lateral Surface:

  • Sides of cells; may contain cellular junctions (tight junctions, adherens junctions, desmosomes).

• Basal Surface:

  • Adheres epithelium to underlying extracellular materials (basement membrane).

  • Connection via hemidesmosomes.

Aspects of Epithelial Tissue

Structural and Functional Features

• Cellularity: Composed almost entirely of cells with very little extracellular fluid (ECF) or matrix.

• Special Contacts: Cells form continuous sheets held together by tight junctions and desmosomes.

• Polarity: Distinct apical and basal surfaces.

• Avascular but Innervated: No blood vessels, but supplied by nerve fibers.

• Regenerative: Rapidly replaces lost cells by cell division (mitosis).

• Supported by Connective Tissue: The basement membrane anchors epithelium to underlying connective tissue.

Key Terms and Definitions

Important Structures in Epithelial Tissue

• Microvilli: Minute outgrowths of the plasma membrane that increase surface area for absorption; form the brush border.

• Cilia: Motile extensions of the cell surface, commonly found lining the respiratory tract.

• Goblet Cells: Unicellular mucus-secreting exocrine gland cells, shaped like a wineglass; found in many mucous membranes.

• Basement Membrane: Anchors epithelium to connective tissue.

• Keratin: Protein found in epithelial tissue exposed to friction (e.g., dry skin).

Additional info:

• Images in the original file illustrate the structure of germ layers, epithelial cell polarity, and tissue types, supporting the textual explanations.