Connective Tissue and ECM I and II PDF - Fall 2024

Connective Tissue and ECM MED-202 Histology I Fall 2024 Annita Achilleos, PhD  Reading Material Wheater’s Functional Histology - Chapter 4 Histology: A Text and Atlas, Wojech Pawlina: Chapter 6 Lecture Material  Learning Objectives...

Connective Tissue and ECM MED-202 Histology I Fall 2024 Annita Achilleos, PhD  Reading Material Wheater’s Functional Histology - Chapter 4 Histology: A Text and Atlas, Wojech Pawlina: Chapter 6 Lecture Material  Learning Objectives List the different types of connective tissue and their major histologic features Describe the two major classes of connective tissue cells Describe the different components of the ECM and their microscopic features Briefly discuss the characteristics of special connective tissue Tissue types of the human body Structural Elements of Connective Tissue Two main components: 1. Cells 2. Extracellular matrix (ECM) a. Protein fibers (collagen, elastic, reticular) b. Ground substance (amorphous component with specialized molecules) Cells of connective tissue are more widely dispersed within the ECM – as opposed to epithelial cells Dense irregular connective tissue  Structural Elements of Connective Tissue Different types of connective tissue are responsible for a variety of functions - based on the types of cells and ECM (fibers and ground substance) ECM - determines the physical properties of the tissue - provides structural and biochemical support to surrounding cells - is produced by the cells in the connective tissue - most commonly fibroblasts - osteocytes, endothelial cells etc. - plays a major role in the function of the tissue - its major component is ground substance usually a fluid, but it can also be mineralized and solid (e.g. bones) - The amount and structure of each component correlates with the function of the tissue. Structural Elements of Connective Tissue Classification of Connective Tissue Embryonic Connective Tissue Mesenchyme Mucus connective tissue (found in the umbilical cord) Connective Tissue Proper Loose connective tissue Dense connective tissue Regular Irregular Specialized Connective Tissue Cartilage Blood Bone Hemopoietic tissue Adipose tissue Lymphatic tissue  General Functional Features of Connective Tissue Functions: Support and connect other tissues: - connective tissue sheath that surrounds a muscle - the tendons that attach muscles to bones - the skeleton that supports the positions of the body Protection: - Fibrous capsules and bones that protect delicate organs Defense: - Specialized cells in the blood defend the body from microorganisms that enter the body Nutrition: - Transport of gases, nutrients, waste, and chemical messengers by blood and lymph Insulation: - Energy storage in the form of fat and contribution to thermal insulation of the body by adipose cells  Connective Tissue Cells Cells in connective tissue can be found in: a. an active form (–blast) dividing and secreting the components of ground substance b. an inactive form (–cyte) - Derive from the mesenchyme (fetal supporting tissue) - Their common function is synthesis, maintenance and recycling of ECM material  Connective Tissue Cells 1. Fibroblasts: secrete, maintain and recycle the matrix in most tissues 2. Adipocytes: modified support cells specialized in the storage and metabolism of fat - collectively they form adipose tissue 3. Chondrocytes, osteoblasts and osteocytes: responsible for secreting and maintaining the matrix in cartilage and bone, respectively 4. Mast cells and tissue macrophages: cells of the innate immune system (activated by chemicals of the antigen) 5. Lymphocytes: cells of the adaptive immune system (activated by antigen)  Cell Types - Fibroblasts Fibroblast - the principal cell of connective tissue - synthesizes: Collagen Elastic fibers Reticular fibers  Cell Types - Mast cells Mast cells: - have many cytoplasmic granules that contain the chemical signals histamine and heparin - when irritated or damaged, mast cells initiate an allergic response by the release of histamine - derive from hematopoietic stem cells and are part of the immune system Cell Types - Mast cells  Cell Types - Macrophages Macrophages - large cells that derive from a monocyte, essential components of the immune system Monocyte - when stimulated, they release cytokines, small proteins that act as chemical messengers. Cytokines recruit other cells of the immune system to infected sites and stimulate their activities Osteoclast - permanent residents of their tissues (other than blood) Macrophage Cell Types - Macrophages  Connective Tissue Fibers Three main types of fibers are produced by fibroblasts: 1. Collagen fibers 2. Elastic fibers 3. Reticular fibers Elastic fibers Collagen fibers Fibroblast nuclei Reticular fibers  Collagen fiber Collagen - the most abundant structural component of the connective tissue - the most abundant protein in mammals (25% to 35% of the whole-body protein content) - one fiber is made of collagen fibrils - secreted into the ECM by fibroblasts in the form of a collagen molecule TEM – collagen fibers  Collagen fiber Collagen molecule α chain Procollagen (triple helix Collagen fibril Collagen fibre with loose 10-500nm 1-500µm ends)  Collagen fiber Collagen fiber - flexible but also have great tensile strength resist stretching give ligaments and tendons their characteristic resilience - at least 28 different types of collagen (I to XXVIII) have now been delineated Collagen fibers  Elastic fiber Elastic fiber - contains the protein elastin along with lesser amounts of other proteins (fibrillin) and glycoproteins elastin will return to its original shape after being stretched or compressed allows tissues to respond to stretching and distension - synthesized by fibroblasts and vascular smooth muscle cells - are prominent in elastic tissues found in: - skin Elastic fibers - the walls of large blood vessels - a few ligaments that support the spine  Reticular fiber Reticular fiber - is comprised predominantly of type III collagen - synthesized by: a. fibroblasts b. reticular cells (hemopoietic and lymphatic tissues) - found throughout the body, but are most abundant in the reticular tissue of soft organs, such as the liver and spleen they anchor and provide structural support to the parenchyma (the functional cells, blood vessels, and Reticular fibers nerves of the organ) – found in embryonic tissue and during wound healing an indicator of tissue maturation  Ground Substance Ground substance - an amorphous gel-like substance in the extracellular space that contains all components of the ECM, except the fibers occupies the spaces between the cells and fibers Ground - high water content substance - provides lubrication for collagen fibers - is secreted by fibroblasts Consist predominately of: a. Proteoglycans b. Glycosaminoglycan molecules (GAGs) c. Multiadhesive glycoproteins Extracellular Matrix  Structural Elements of Connective Tissue Extracellular Matrix Cells Fibers Ground Substance Collagen Proteoglycans Fibroblast Elastic GAGs Macrophage Reticular Multiadhesive glycoproteins Mast cell Plasma cell Lymphocytes Leukocytes Adipose cell  Classification of Connective Tissue Common origin (mesenchyme) Connective tissue proper Supporting connective tissue Fluid connective tissue Loose Dense connective connective Cartilage tissue (fewer tissue (more Bone (semisolid Blood fibers, more fibers, less (solid matrix) matrix) ground ground substance) substance) 1. Areolar 1. Regular 1. Hyaline 1. Compact 2. Adipose 2. Irregular 2. Fibrocartilage 2. Spongy 3. Reticular 3. Elastic 3. Elastic Classification of Connective tissue Connective tissue proper: Loose Connective Tissue General features − loosely arranged fibers and abundant cells of various types − primarily located beneath epithelia that cover the body surfaces and line internal surfaces of the body − most cell types are transient (respond to stimuli) − site of inflammatory and immune reactions 1. Areolar tissue − holds organs in place − attaches epithelial tissue to other underlying tissues 2. Reticular tissue − dominant in locations with high cellular content 3. Adipose tissue − the most common type of connective tissue in humans Connective tissue proper: Loose Connective Tissue Connective tissue proper: Dense Connective Tissue Dense connective tissue: - fibers are the main matrix element (fewer cells) fibers are mainly composed of type I collagen - between the collagen fibers are rows of fibroblasts - forms strong, rope-like structures such as tendons and ligaments tendons attach skeletal muscles to bones ligaments connect bones to bones at joints - make up the lower layers of the skin (dermis), where it is arranged in sheets Tendon – dense regular connective tissue Thick skin – dense irregular connective tissue Supportive Connective tissue - Cartilage Cartilage - composed of chondrocytes produce ECM abundant in ground substance embedded within the cartilage matrix the space they occupy are called lacunae - is avascular all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues. Supportive Connective tissue - Cartilage Three main types of cartilage tissue: 1. Hyaline cartilage - the most common type of cartilage in the body - consists of short and dispersed collagen fibers - contains large amounts of proteoglycans - found in: - rib cage - nose - covers bones where they meet to form moveable joints 2. Fibrocartilage - tough due to thick bundles of collagen fibers dispersed through its matrix - the intervertebral discs are examples of fibrocartilage 3. Elastic cartilage - contains elastic fibers, collagen and proteoglycans - provides support and elasticity (e.g. external ear)  Supportive Connective tissue - Bone Bone - the hardest connective tissue - provides protection to internal organs and supports the body - its ECM contains mostly collagen fibers embedded in a mineralized ground substance calcium phosphate - Osteoblasts: the active bone forming cells, producing the organic part of the extracellular matrix Osteocytes: the mature bone cells, located within lacunae - bone is a highly vascularized tissue can recover from injuries in a relatively short time Compact bone – Haversian lamellae Spongy bone  Fluid Connective Tissue Blood and lymph are fluid connective tissues - cells circulate in a liquid extracellular matrix, the plasma Blood contains: 1. Erythrocytes - transport oxygen and carbon dioxide 2. Leukocytes – defense against potentially harmful microorganisms or molecules 3. Platelets - cell fragments involved in blood clotting Lymph contains - a liquid matrix (similar to plasma) - white blood cells Red blood cells Lymphocyte  Adipose Tissue Adipose tissue - specialized connective tissue - plays a role in energy homeostasis - composed of adipocytes (fat cells) - energy is stored in lipid droplets of adipocytes in the form of triglycerides - regulates energy homeostasis by secreting paracrine and endocrine substances Two types of adipose tissue: – White adipose tissue – Brown adipose tissue  Adipose Tissue Brown fat adipocytes - store lipids in the form of many droplets - have high metabolic activity White fat adipocytes - store lipids as a single large droplet - are metabolically less active - The number and type of adipocytes depends on the tissue and location, and vary among individuals in the population - When they accumulate in large numbers they are called adipose tissue  Adipose Tissue Adipose tissue-derived stem cells – capable for self-renewal and multipotential differentiation – adult stem cells  White Adipose Tissue - Predominant type in adults - Located subcutaneously - mammary glands - mesenteries - retroperitoneal spaces - visceral pericardium - orbits - bone marrow cavity - Functions in insulation, energy storage and hormone production - Adipocytes produce hormones like leptin, adiponectin and resistin  White Adipose Tissue Large, white and spherical, with flatened nucleus in the periphery of cell.  Brown Adipose Tissue Brown adipose tissue - present during fetal life and in infants but diminishes after the first decade of life - smaller cells, with nucleus in an eccentric position, but is not flat - cytoplasm is full of empty vacuoles (lipid is lost during tissue preparation) - Brown coloration due to high levels of iron-containing mitochondria - Brown adipocytes differentiate from mesenchymal stem cells - Metabolism of lipid in brown adipocytes generate heat in the process called thermogenesis (hibernating animals have large amounts) Brown Adipose Tissue  Summary 1. Types of Connective tissue cells 2. Type of protein fibers and principal functional components 3. Composition of ground substance 4. Classification of connective tissue 5. Classification of connective tissue proper into loose and dense 6. Supportive connective tissue 7. Adipose tissue as a special type of connective tissue Thank you!