Connective Tissue

I. Definition:

• Connective tissue is a type of tissue that supports, connects, and protects other tissues and organs in the body. It is avascular, meaning it does not have its own blood supply.

• Connective tissue contains blood vessels, supplying oxygen and nutrients to itself and the surrounding tissues.

• Connective tissue is not a single tissue but a diverse group of tissues with varied functions.

II. Origin:

• Connective tissue originates from the mesoderm, except for some connective tissue in the head region which may arise from neural crest cells.

III. Structure:

• Connective tissue consists of three main components:

• Connective tissue cells: These include fixed cells and wandering cells.

• Connective tissue fibers: These include collagen fibers, elastic fibers, and reticular fibers.

• Ground substance: This is the extracellular matrix, composed of water, glycosaminoglycans (GAGs), proteoglycans, and adhesive glycoproteins.

IV. Extracellular matrix:

• The extracellular matrix is the combination of connective tissue fibers and ground substance. It provides a supportive and surrounding environment for the cells within connective tissue.

V. Specialized Connective Tissues:

• Specialized connective tissues are types of connective tissue that are specialized to perform specific functions. These include:

• Cartilage: Provides support for joints, protects cartilage and bone.

• Bone: Provides a framework for the body, protects internal organs.

• Adipose tissue: Stores energy, provides insulation and cushioning for organs.

• Reticular tissue: Provides a framework for hematopoietic organs like lymph nodes, spleen, and bone marrow.

VI. Components of Connective Tissue:

1. Connective Tissue Cells:

• Fixed cells:

• Mesenchymal cells: These are multipotent stem cells that can differentiate into other connective tissue cells.

• Fibroblasts/Fibrocytes: Responsible for producing collagen and elastic fibers.

• Pericytes: Regulate blood vessel size.

• Endothelial cells: Line blood and lymph capillaries.

• Wandering cells:

• Macrophages: Phagocytize bacteria, dead cells, and debris.

• Plasma cells: Produce antibodies.

• Mast cells: Contain basophilic granules that release histamine and heparin, participating in inflammatory responses.

2. Connective Tissue Fibers:

• Collagen fibers:

• Structure: Composed of tropocollagen.

• Classification:

• Fibril-forming collagen: Type I, II, III (form thick, strong fibers)

• Sheet-forming collagen: Type IV (forms basement membranes)

• Linking collagen: Type VII (links other collagen fibers)

• Function: Provides tensile strength and support to connective tissue.

• Elastic fibers:

• Structure: Composed of elastin and fibrillin.

• Function: Allows connective tissue to stretch and recoil to its original shape.

• Detection: Stains purple with aldehyde fuchsin.

• Reticular fibers:

• Structure: Composed of type III collagen.

• Function: Provides a framework for hematopoietic organs.

3. Ground Substance:

• Water: The main component of ground substance.

• GAGs: Long polysaccharide chains that are negatively charged, attracting water and giving ground substance its viscosity.

• Proteoglycans: Complexes of GAGs bound to proteins.

• Adhesive glycoproteins: Proteins that bind to GAGs and collagen, helping to organize ground substance.

VII. Official Connective Tissue Classification:

• Mucous connective tissue:

• Characteristics: Ground substance predominates, with few fibers and cells.

• Location: Umbilical cord, embryonic tissues.

• Function: Supports and protects organs in embryos.

• Loose connective tissue:

• Characteristics: Cells predominate, with many blood vessels.

• Location: Under the skin, surrounding internal organs.

• Function: Provides support, protection, and nutrient supply to organs.

• Dense connective tissue:

• Characteristics: Collagen fibers predominate, with few cells and blood vessels.

• Classification:

• Dense irregular: Collagen fibers are arranged randomly, forming protective layers.

• Dense regular: Collagen fibers are arranged in a parallel manner, forming tendons and ligaments.

• Function: Provides tensile strength and support to connective tissue.

• Elastic connective tissue:

• Characteristics: Contains many elastic fibers, highly stretchable.

• Location: Ligamenta flava of the vertebral column, vocal cords, large arteries.

• Function: Allows organs to stretch and recoil to their original shape.

• Reticular connective tissue:

• Characteristics: Contains many reticular cells and reticular fibers, with no ground substance.

• Location: Lymph nodes, spleen, bone marrow.

• Function: Provides a framework for hematopoietic organs.

VIII. Functions of Connective Tissue:

• Support and protection: Provides a framework for the body, protects internal organs.

• Connects tissues and organs: Helps tissues and organs to connect with each other, forming a unified structure.

• Exchange of substances: Supplies nutrients and oxygen to tissues and organs, transports waste products.

• Body defense: Protects against pathogens, participates in immune responses.

• Energy storage: Adipose tissue stores energy reserves for the body.

• Insulation: Adipose tissue helps to insulate the body.

IX. Notes:

• Mesentery: Connective tissue in the embryo that can differentiate into other connective tissue types.

• Phagocytic system: Includes cells with phagocytic capabilities, including microglia, Kupffer cells, monocytes, and immune accessory cells.

• Inflammatory macrophages: Originate from monocytes.

• Kupffer cells: Resident macrophages in the liver.

• Pericytes: Regulate blood vessel size, located near capillaries.

• Adipose tissue: Contains type III collagen.

X. Adipose Tissue Activity:

• Brown adipose tissue: More active than white adipose tissue, involved in heat production.

• White adipose tissue: Stores energy reserves.

XI. Relevance to Disease:

• Diseases related to connective tissue: Arthritis, lupus, scleroderma, systemic sclerosis, connective tissue cancers…

XII. Summary:

Connective tissue is a very diverse type of tissue that supports, connects, and protects other tissues and organs in the body. It is composed of connective tissue cells, connective tissue fibers, and ground substance. Connective tissue is classified into various types, each with a distinct structure and function. Understanding connective tissue is crucial for understanding the human body and diseases related to connective tissue.