Lesson 2: Connective Tissue

I. Introduction:

• Connective tissue is a very diverse type of tissue found in almost all organs and organ systems in the body.

• It plays a vital role in supporting, protecting, connecting tissues and organs, and is also involved in many other important biological processes.

II. Origin:

• Connective tissue is derived from mesenchymal cells, a type of multipotent stem cells that have the ability to differentiate into various cell types.

• During embryonic development, mesenchymal cells can also arise from neural crest cells.

III. Structure:

• Connective tissue is composed of three main components:

1. Connective tissue cells:

• There are two main groups:

• Fixed cells: including mesenchymal cells, fibroblasts, fibrocytes, pericytes, and endothelial cells.

• Wandering cells: including macrophages, plasma cells, and mast cells.

2. Connective tissue fibers:

• There are two main types:

• Collagen fibers: the most common type, highly durable, forming large bundles, providing a supporting framework for the tissue.

• Elastic fibers: have good elasticity, allowing connective tissue to stretch and return to its original shape.

3. Ground substance:

• A gel-like, water-loving substance that connects cells and connective tissue fibers, creating an environment for material exchange.

• It is composed of glycosaminoglycans (GAGs), proteoglycans, and fibronectin.

IV. Function:

• Support, protection:

• Provides a framework for the body, connecting tissues and organs.

• Protects organs from mechanical damage and external environmental influences.

• Material exchange:

• Ground substance acts as a mediator in the exchange of materials between blood and tissues.

• Synthesis, repair:

• Fibroblasts play a crucial role in the production and maintenance of connective tissue fibers, and also participate in the repair of injuries.

• Fat storage:

• Adipose tissue is a specialized type of connective tissue that functions in energy storage, insulation, and organ protection.

V. Types of connective tissue cells:

1. Fixed cells:

• Mesenchymal cells:

• Small, star-shaped or elongated nucleus.

• Nucleus located in the middle of the cell.

• Capable of differentiating into many other types of connective tissue cells.

• Considered multipotent stem cells, abundant during embryonic development.

• Fibroblasts:

• Synthesize and secrete connective tissue fibers and ground substance.

• Produce enzymes that break down the extracellular matrix, helping to reabsorb and constantly renew the extracellular matrix.

• Fibrocytes:

• Mature form of fibroblasts.

• Responsible for maintaining and protecting the structure of connective tissue.

• Pericytes:

• Located near blood vessels, separated from endothelial cells by the basement membrane.

• Involved in regulating the lumen of blood vessels.

• Endothelial cells:

• Line the inner surface of blood vessels.

• Large, thin, with many vacuoles and pinocytic vesicles.

• Protect and form a biological barrier, facilitating the exchange of substances, gases between blood and tissues.

2. Wandering cells:

• Macrophages:

• Derived from monocytes.

• There are three types:

• Resident macrophages: permanently reside in a specific location.

• Inflammatory macrophages: migrate to the site of injury to phagocytose pathogens and dead cells.

• Immunostimulatory macrophages: participate in the body’s immune system.

• Plasma cells:

• Derived from B lymphocytes.

• Round nucleus, located eccentrically, with condensed chromatin adhering to the nuclear membrane.

• Synthesize and store immunoglobulins.

• Mast cells:

• Derived from bone marrow stem cells.

• Located around blood vessels, with a round nucleus.

• Contain many basophilic granules containing histamine, proteases, and ECF-A.

• Cause skin rashes, hives when exposed to foreign antigens, releasing basophilic granules.

VI. Types of connective tissue fibers:

• Collagen fibers:

• The most abundant type of fiber in connective tissue.

• Composed of microfibrils arranged in parallel, each microfibril is formed from tropocollagen molecules.

• Tropocollagen is a protein consisting of three alpha helical chains, each chain is a helical polypeptide.

• There are over 12 types of collagen, differing in the amino acid sequence in the alpha helical chain.

• 6 common types of collagen:

• Type 1: most common, accounting for over 80% of collagen in the body, found in the dermis of the skin, tendons, ligaments, bones.

• Type 2: characteristic of hyaline cartilage, elastic cartilage.

• Type 3: branched, linked into networks, forming reticular fibers, surrounding adipose tissue and smooth muscle.

• Type 4: only found in the basement membrane of the renal glomerulus.

• Type 5: characteristic of placenta, skin, bones.

• Type 7: synthesized by keratinocytes.

• Elastic fibers:

• Have good elasticity but are less common than collagen fibers.

• Abundant in organs that frequently change volume (lungs, aorta, pulmonary artery).

• Composed of the protein elastin, combined with collagen fibers.

• Produced by fibroblasts and fibrocytes.

VII. Ground substance:

• A gel-like, water-loving substance that connects cells and connective tissue fibers, creating an environment for material exchange.

• Composed of:

• Glycosaminoglycans (GAGs): long, water-loving polysaccharide chains, negatively charged.

• Proteoglycans: large molecules, formed by GAGs combined with proteins.

• Fibronectin: an adhesive protein, helps connect cells to collagen fibers and ground substance.

VIII. Types of connective tissue:

• Mucous connective tissue:

• Ground substance is the dominant component.

• Most abundant in embryos, in adults it is only found in some glandular tissues and blood vessels.

• Loose connective tissue:

• Cells are the dominant component, with very little ground substance.

• Examples: subcutaneous connective tissue, tissue around internal organs.

• Dense connective tissue:

• Has many collagen fibers arranged in large bundles, with less ground substance and fibroblasts.

• Divided into two types:

• Dense regular connective tissue: fiber bundles arranged in parallel, commonly found in tendons, ligaments.

• Dense irregular connective tissue: the most common type, fiber bundles arranged without a fixed direction, e.g. dermis of the skin.

• Elastic connective tissue:

• Elastic fibers are dominant.

• Often found in the yellow ligaments of the spine, aorta, pulmonary artery.

• Reticular connective tissue:

• Consists of two components: reticular cells and reticular fibers (collagen type 3).

• Provides a supporting framework for hematopoietic organs (spleen, bone marrow, lymph nodes).

• Adipose tissue:

• Adipose cells surrounded by a network of collagen type 3 fibers.

• Stores energy in the body, functions in protection, shaping, and insulation.

• There are two types:

• White fat: large lipid droplets, pushing the nucleus close to the cell membrane.

• Brown fat: small lipid droplets, rich in capillaries, containing many mitochondria.

IX. Notes:

• The structure and function of connective tissue can vary depending on the location and type of tissue.

• Connective tissue is a very dynamic type of tissue, constantly being regenerated and renewed.

• Connective tissue can be damaged by factors such as trauma, infection, and aging.

• Understanding the structure and function of connective tissue helps us understand the body better and how organs work.

X. Conclusion:

Connective tissue is a very diverse and important type of tissue, playing a key role in supporting, protecting, and connecting organs in the body. It is involved in many important biological processes, contributing to the maintenance of life and the normal functioning of the body.