Connective Tissue: Structure, Types, and Functions

Connective Tissue

Introduction

Connective tissue is one of the four primary tissue types in the human body. It plays a crucial role in supporting, binding, and protecting other tissues and organs. This study guide summarizes the main characteristics, components, subtypes, and functions of connective tissue, as well as the specialized cells and extracellular matrix that define its structure and function.

Common Characteristics of Connective Tissue

Origin and General Features

• Embryonic Origin: All connective tissues are derived from mesenchyme, an embryonic connective tissue.

• Vascularity: Connective tissues exhibit varied vascularity, ranging from highly vascular (e.g., bone) to avascular (e.g., cartilage).

• Composition: All connective tissues consist of three main components:

  • Cells

  • Extracellular matrix (composed of ground substance and fibers)

• Variation: There is significant diversity in structure and function among connective tissue types.

Structural Components of Connective Tissue

1. Cells

• Mitotic Activity: Most connective tissue cells are capable of division (mitotic).

• Matrix Secretion: Cells secrete the ground substance and fibers that form the extracellular matrix.

• Cell Types:

  • -blasts: Immature, highly active cells that build the matrix (e.g., fibroblast, chondroblast, osteoblast).

  • -cytes: Mature, less active cells that maintain the matrix (e.g., fibrocyte, chondrocyte, osteocyte).

  • Adipocytes: Specialized for lipid storage.

  • Mast cells: Release histamine and other chemicals during inflammation.

  • Leukocytes (white blood cells): Migrate into connective tissue during infection or injury (e.g., lymphocytes, neutrophils).

  • Macrophages: Phagocytic cells that engulf pathogens and debris.

2. Extracellular Matrix

• Ground Substance: The unstructured material that fills the space between cells and fibers. It consists of:

  • Interstitial fluid: Allows diffusion of nutrients and wastes.

  • Cell adhesion proteins: Act as connective tissue "glue".

  • Proteoglycans: Proteins with glycosaminoglycans (GAGs) that trap water, affecting tissue stiffness and resistance to compression.

• Fibers: Provide support and structure. Three main types:

  • Collagen fibers: Thick, strong, and provide high tensile strength.

  • Elastic fibers: Contain elastin; allow stretch and recoil.

  • Reticular fibers: Fine, branching collagenous fibers that form delicate networks.

Functions of Connective Tissue

• Binding and Support: Connects and supports other tissues and organs.

• Protection: Shields delicate organs (e.g., bone protects the brain).

• Insulation: Adipose tissue insulates the body.

• Storage: Stores energy (fat), minerals (bone), and water.

• Transportation: Blood transports gases, nutrients, wastes, and hormones.

Classes and Subtypes of Connective Tissue

Overview

Connective tissue is classified into four main types, each with distinct subtypes, cells, matrix composition, and functions.

Class	Subclasses	Cells	Matrix	General Features
Connective tissue proper	Loose connective tissue (areolar, adipose, reticular) Dense connective tissue (regular, irregular, elastic)	Fibroblasts, fibrocytes, defense cells, adipocytes	Gel-like ground substance; all three fiber types (collagen, reticular, elastic)	Varies in density and fiber types Functions as binding tissue Resists mechanical stress Reservoir for water and salts Energy (fat) storage
Cartilage	Hyaline cartilage Elastic cartilage Fibrocartilage	Chondroblasts (in growing cartilage), chondrocytes	Gel-like ground substance; fibers: collagen, elastic (in some)	Resists compression due to large amounts of water in the matrix Functions to cushion and support body structures
Bone	Compact bone Spongy bone	Osteoblasts, osteocytes	Hard, calcified matrix with collagen fibers	Hard tissue that resists compression and tension Functions in support and protection
Blood	Blood cells (RBCs, WBCs, platelets)	Red blood cells, white blood cells, platelets	Plasma (fluid matrix); no fibers	Fluid tissue Transports gases, nutrients, wastes, and other substances

Key Terms and Definitions

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

• Fibroblast: Cell that produces fibers and ground substance in connective tissue proper.

• Chondroblast: Cell that produces the matrix in cartilage.

• Osteoblast: Cell that produces the matrix in bone.

• Adipocyte: Fat cell specialized for storing energy as fat.

• Proteoglycan: Large molecule consisting of a protein core with attached glycosaminoglycans (GAGs); important for trapping water and providing resistance to compression.

• Collagen fiber: The strongest and most abundant fiber type, providing tensile strength.

Examples and Applications

• Example: Tendons are composed of dense regular connective tissue, providing strong attachment between muscles and bones.

• Application: Cartilage in joints (hyaline cartilage) cushions bones and resists compressive forces.

Summary Table: Major Connective Tissue Types

Type	Main Cell	Matrix	Main Function
Connective tissue proper	Fibroblast	Gel-like, all fiber types	Binding, support, storage
Cartilage	Chondroblast/chondrocyte	Gel-like, collagen/elastic fibers	Cushioning, support
Bone	Osteoblast/osteocyte	Hard, calcified, collagen	Support, protection, storage
Blood	Hematopoietic cells	Fluid (plasma)	Transport

Additional info:

• Glycosaminoglycans (GAGs) such as hyaluronic acid and chondroitin sulfate are key components of proteoglycans, contributing to the viscosity and compressive strength of connective tissue ground substance.

• Connective tissue is essential for immune defense, as it houses various immune cells (e.g., macrophages, lymphocytes) that respond to pathogens and injury.