Module 4.1 Introduction to Tissues

Definition and General Features of Tissues

Tissues are groups of structurally and functionally related cells and their external environment that together perform common functions. All tissues share two basic components: cells and extracellular matrix (ECM).

• Histology: The study of the normal structures of tissues.

• Cells: Consist of discrete populations of cells related in structure and function.

• Extracellular Matrix (ECM): Surrounding material composed of ground substance and protein fibers.

Types of Tissues

There are four primary tissue types, each defined by the kind and number of cells, the amount and composition of ECM, and their specific functions:

1. Epithelial tissues: Tightly packed sheets of cells with little ECM that cover and line all body surfaces and cavities. Specialized epithelial form glands that manufacture secretions such as sweat, saliva, or chemical messengers called hormones.

2. Connective tissues: Connect all other tissues in the body to one another. The ECM is a prominent feature for most connective tissue types, with cells scattered throughout. These bind, support, protect, and allow for transportation of substances.

3. Muscle tissues: Capable of generating force by contracting, have little ECM between cells.

4. Nervous tissues: Consist of cells capable of generating, sending, and receiving messages, and cells that support this activity with a unique ECM.

The Extracellular Matrix (ECM)

The ECM is composed of substances in a liquid, thick gel, or solid that surround the cells of a tissue. It consists of two main components:

• Ground substance: Makes up most of the ECM and consists of interstitial or extracellular fluid (ECF) and their components: water, nutrients, and ions. Three families of macromolecules are found in ground substance:

  • Glycosaminoglycans (GAGs)

  • Proteoglycans

  • Cell-adhesion molecules (CAMs)

• Protein fibers: Embedded within the ground substance, these are large molecules composed of multiple fibrous subunits with a rope-like structure. Types include:

  • Collagen fibers: Provide tensile strength and resist stretching.

  • Elastic fibers: Allow tissues to stretch and return to original length.

  • Reticular fibers: Form a supportive meshwork or scaffold.

Functions of the ECM:

• Provides strength to resist tension and compressive forces.

• Directs cells to proper places within a tissue.

• Regulates development, mitotic activity, and survival of cells.

Cell Junctions

Cell junctions are ways by which neighboring cells' plasma membranes are linked by integral proteins. The three major types are:

• Tight junctions: Hold cells closely together, making the space between them impermeable to movement of macromolecules.

• Desmosomes: Integral proteins that link cells, increasing the strength of a tissue by holding cells together at a single point.

• Gap junctions: Small pores formed by protein channels that allow substances to flow freely between adjacent cells' cytoplasm; important for communication (e.g., cardiac muscle cells).

Module 4.2 Epithelial Tissues

Functions of Epithelial Tissue

Epithelial tissues are found on every internal and external body surface and serve as barriers between the body and external environment or between organs and fluid-filled cavities. Their functions include:

• Protection: Shields underlying tissues from mechanical and thermal injury.

• Immune defenses: Prevents invasion by microorganisms and enhances immune function.

• Secretion: Forms glands that produce substances like hormones and oils.

• Transport into other tissues: Selectively permeable membranes allow certain substances to cross.

• Sensation: Specialized epithelial cells detect changes in internal and external environments.

Components and Classification of Epithelia

• Cellularity: Epithelial tissues are composed of tightly packed cells linked by tight junctions and desmosomes.

• Avascularity: Epithelial tissues lack blood vessels and must obtain nutrients by diffusion from deeper tissues.

• Basement membrane: Epithelia are attached to underlying connective tissue by a structure called the basement membrane, which consists of the basal lamina and reticular lamina.

Classification Criteria

• Number of cell layers:

  • Simple epithelia: Single cell layer.

  • Stratified epithelia: More than one cell layer.

• Shape of cells:

  • Squamous: Flattened cells.

  • Cuboidal: Cube-shaped cells.

  • Columnar: Tall and elongated cells.

Types and Functions of Epithelia

• Simple squamous epithelium: Thin, single layer; adapted for rapid diffusion (e.g., air sacs of lungs, lining blood vessels).

• Simple cuboidal epithelium: Single layer of cube-shaped cells; found in kidney tubules and glands.

• Simple columnar epithelium: Single layer of tall cells; may have microvilli or cilia for absorption or movement (e.g., digestive tract, uterine tubes).

• Pseudostratified columnar epithelium: Appears layered but is a single layer with nuclei at various heights; found in respiratory tract.

• Stratified squamous epithelium: Multiple layers; protects against abrasion. Can be keratinized (skin) or nonkeratinized (mouth, esophagus).

• Stratified cuboidal/columnar epithelium: Rare; found in ducts of sweat glands and salivary glands.

• Transitional epithelium: Found in urinary tract; cells can change shape to allow stretching.

Glandular Epithelium

Glands are structures of epithelial origin that synthesize and secrete products. They are classified as:

• Endocrine glands: Secrete hormones directly into the bloodstream.

• Exocrine glands: Release products onto body surfaces or into ducts. Methods of secretion include:

  • Merocrine secretion: Products released by exocytosis (e.g., sweat glands).

  • Holocrine secretion: Secretory cells rupture and die to release product (e.g., sebaceous glands).

Module 4.3 Connective Tissues

General Features and Functions

Connective tissues are divided into two groups: connective tissue proper and specialized connective tissue. They perform the following functions:

• Connecting and binding: Link organs and anchor tissues.

• Support: Provide structural support (e.g., bone, cartilage).

• Protection: Immune defense, shock absorption, and physical protection.

• Transport: Blood is a fluid connective tissue that transports substances.

Connective Tissue Proper

Also known as general connective tissue, it is widely distributed and connects tissues and organs. It is composed of:

• Cells: Resident cells (e.g., fibroblasts) and migrant cells (e.g., immune cells).

• ECM: Contains ground substance and protein fibers.

Fibroblasts are the most common resident cells, responsible for producing protein fibers and ground substance.

Summary Table: Major Tissue Types and Key Features

Additional info:

• Keratin is a tough, fibrous protein that makes the epithelium resistant to friction and water loss.

• Glycosaminoglycans (GAGs) in the ECM help retain water and maintain tissue hydration.

• Desmosomes are especially important in tissues subject to mechanical stress, such as skin and heart muscle.