Connective Tissues

Functions of Connective Tissue

Connective tissues are essential components of the human body, serving multiple structural and physiological roles. Their functions include:

• Connecting and Binding: Bind tissue layers together in organs and anchor organs in place and to one another.

• Support: Certain connective tissues, such as bone and cartilage, support the weight of the body.

• Protection: Bone protects internal organs; cartilage and fat tissue provide shock absorption. Elements of the immune system are found within connective tissues.

• Transport: Blood, which is the main transport medium in the body, is a type of connective tissue.

Common Characteristics of Connective Tissue

• Common Origin: All connective tissues arise from mesenchyme, an embryonic tissue.

• Degrees of Vascularity: Vascularity varies among connective tissues.

  • Cartilage is avascular (lacks blood vessels).

  • Dense connective tissue is slightly vascular.

  • Other connective tissues are highly vascular.

• Matrix: All connective tissues have a nonliving extracellular matrix consisting of ground substance and fibers.

Classification of Connective Tissue

Connective Tissue Proper

This class, also known as General Connective Tissue, is widely distributed in the body. It connects tissues and organs to one another and forms part of the internal architecture. It includes:

• Loose connective tissue

• Dense connective tissue

• Reticular tissue

• Adipose tissue

Specialized Connective Tissues

This class includes cartilage, bone (osseous tissue), and blood. Bone and blood are discussed in detail in dedicated chapters.

General Role of Connective Tissues

• Connect all other tissues together

• Cells are scattered through the ECM

• Bind, support, protect, and allow transport of substances

Extracellular Matrix (ECM)

Definition and Components

The extracellular matrix is composed of substances surrounding the cells within a tissue. It has two main components:

• Ground Substance

• Protein Fibers

Ground Substance

Ground substance is a gel-like material containing extracellular fluid (ECF) with water, ions, nutrients, and other solutes. It contains three types of macromolecules:

• Glycosaminoglycans (GAGs): Negatively charged polysaccharide chains, including chondroitin sulfate and hyaluronic acid. They attract positively charged ions, which draws water into the ECM.

• Proteoglycans: GAGs bonded to a protein core; thousands bind to other GAGs to form aggregates, making the ECM firmer and resistant to compression. They also act as a barrier to diffusion of substances through the ECM.

• Glycoproteins: Collectively called Cell-Adhesion Molecules (CAMs), these bind cell surface proteins and protein fibers, maintaining the architecture of the tissue.

Protein Fibers in the ECM

Protein fibers are entwined fibrous protein subunits that provide strength and support. Three types are found in the ECM:

• Collagen Fibers: At least 20 different types exist in the body, making up 20-25% of all protein. They resemble entwined steel cables and are very resistant to tension and pressure.

• Elastic Fibers: Made of elastin protein surrounded by glycoproteins. They may stretch to 1.5 times their resting length (distensibility) and then return to their original length (elasticity).

• Reticular Fibers: A type of collagen fiber that is thinner and shorter than regular collagen. They interweave to form scaffolds or webs in some lymphatic organs to trap foreign cells.

Summary Table: Types of Protein Fibers in ECM

Key Terms and Definitions

• Mesenchyme: Embryonic connective tissue from which all connective tissues derive.

• Extracellular Matrix (ECM): Nonliving material between cells, composed of ground substance and fibers.

• Glycosaminoglycans (GAGs): Long, unbranched polysaccharides involved in water retention and ECM structure.

• Proteoglycans: Large molecules formed by GAGs attached to a protein core, contributing to ECM firmness.

• Cell-Adhesion Molecules (CAMs): Glycoproteins that help cells stick to each other and to the ECM.

Example: Importance of ECM in Tissue Function

The ECM in cartilage contains abundant GAGs and proteoglycans, which attract water and provide resistance to compression, making cartilage ideal for shock absorption in joints.

Additional info: The ECM is also involved in cell signaling, tissue repair, and regulation of cell behavior.