Unit 2 – Introduction to Connective Tissues
Introduction to Connective Tissues
Connective tissues are a diverse group that differ greatly in structure, function, and appearance inside the human body. Although some connective tissues do connect or attach other tissues together, the term connective tissue is somewhat of a misnomer since not all connect tissues serve to connect tissues together. Instead, what all connective tissues have in common is the fact that they are composed of various types of cells that produce, maintain, and/or exist in an extracellular matrix. The extracellular matrix (a.k.a. interstitial matrix) which fills the open space between cells can be composed of a variety of protein fibers and various types of fluids (known as ground substance).
The chapters in this unit will focus on the characteristics of different connective tissues by discussing the types of cells and the physical properties of the extracellular matrix found in each connective tissue. There are three subcategories of connective tissues found in the human body which are distinct in their physical characteristics. These three subcategories are connective tissue proper, supporting connective tissue, and fluid connective tissue.
Connective tissue proper include areolar tissue, adipose (fat) tissue, reticular tissue, dense regular connective tissue, dense irregular connective tissue, and elastic tissue. These tissues are characterized by the presence of a diverse group of cells which include fibroblasts, fibrocytes, adipocytes, and mesenchymal cells. These cells produce and/or maintain an extracellular matrix containing a viscous (thick) ground substance and three potential types of protein fibers (collagen, reticular, or elastic). Chapters in this unit that cover connective tissue proper will focus on the types of cells and the characteristics of protein fibers found in each tissue.
Supporting connective tissues include hyaline cartilage, elastic cartilage, fibrocartilage, dense (compact) bone, and spongy (cancellous) bone. These various types of cartilage and bone provide a strong structural framework for other body tissues. Cartilaginous tissues are composed of cells known as chondroblasts (cells that create new cartilage matrix) and chondrocytes (mature cartilage cells that maintain cartilage matrix) which produce and maintain a dense gel-like matrix. Chapters that cover cartilage will focus on the composition of the ground substance and the type of protein fibers in the matrix which determine the physical characteristics of each type of cartilage (hyaline cartilage, elastic cartilage, or fibrocartilage). Bone tissue is composed of cells known as osteoblasts (cells that create new bone matrix) and osteocytes (mature bone cells that maintain bone matrix) which produce and maintain a matrix of collagen fibers and hardened calcium salts. The hardened (ossified) matrix of bone tissue provides the ridged structural framework of the human body.
Fluid connective tissues include blood and lymphatic fluid which serve to transport cells and dissolved substances through the body. Blood is composed of red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes) suspended in a liquid matrix known as plasma. Blood is contained within blood vessels of various sizes (arteries, arterioles, capillaries, venules, and veins) and circulates to all regions of the body due to the pumping action of the heart. Lymphatic fluid or lymph is essentially interstitial fluid that enters tiny lymphatic vessels at tissues and drains along increasingly larger lymphatic vessels on its way to entering the bloodstream at the subclavian veins. Fluid connective tissues will not be included in this unit but covered instead as part of the cardiovascular and lymphatic system units.
