An Overview of Connective Tissue

Connective Tissue • Connective Tissues consist of widely spaced cells suspended in an abundant extracellular matrix. – The volume of the extracellular matrix is greater than the volume of the cells. • Functions of Connective Tissues – connects organs to each other – divides body regions into compartments – provides support, leverage and protection (physical and immune) – covers and surrounds articular surfaces – stores nutrients – thermally insulates – absorbs shock – transports materials (water, nutrients, gases, waste, hormones) • Embryonic Connective Tissue – Mucoid Mesenchymal Tissue • Connective Tissue Proper A Classification Scheme for Connective Tissues – Loose (areolar) – Dense • Dense Regular • Dense Irregular – Reticular – Elastic • Specialized Connective Tissue – Adipose • Yellow and White • Brown – Hematopoietic • Supporting Connective Tissue – Cartilage • Hyaline Cartilage • Elastic Cartilage • Fibrocartilage – Bone Cells of Connective Tissues • All connective tissue cells are derived from mesoderm that developes in to mesenchymal cells as embryos develop. • Fibroblasts are the most abundant CT cell and they produce fibers and ground substance. • Adipocytes (fat cells) store triglycerides • Chondroblasts develop into chondrocytes as they produce cartilage • Osteoblasts develop into osteocytes as they produce bone • Hematopoietic Cells differentiate into blood cells – Macrophages and Mast Cells can leave the blood and enter the interstitial fluid between cells Cells of Connective Tissues Extracellular Matrix of Connective Tissue • The Extracellular Matrix is composed of: – Extracellular Tissue Fluid (mostly water, similar to blood plasma) – Protein Fibers – Ground Substance Protein Fibers of the ECM • Collagen Fibers – over a dozen distinct types of protein fibers – tough, flexible collagen fibers are abundant in tendons, ligaments, dermis of the skin, teeth, cartilage, bone • Reticular Fibers – thin, branched fibers that form a loose, interconnected network that hold cells, tissue fluid and ground substance – Located in distensible or spongy tissues (walls of blood vessels, dermis, lymph nodes, spleen, liver). Stain darkly with silver. • Elastic Fibers – also called yellow fibers because of their color in life – thin, straight fibers made of the protein elastin – stretch 150% resting length and recoil like a rubber band – give skin, lungs and arteries ability to stretch and recoil Ground Substance of ECM • Ground substance is a gelatinous or rubbery material mixed with CT fibers and found in between CT cells. • Molecules have a negative charge that attracts Na+ which holds water. the water and salt help regulate electrolyte balance in tissues and help resist tissue compression. • Ground substance consists of 3 classes of large molecules: – glycosaminoglycans (GAGs) • polymers of repeating disaccharides • important GAGs include chondroitin sulfate, glucosamine, hyaluronic acid, heparin • GAGs are especially abundant in cartilage, bone, tendon, joints, skin – proteoglycans • proteoglycans are GAGs linked to a protein core which form bottlebrush-shaped molecules (see structure on next slide) – adhesive glycoproteins • protein-carbohydrate complexes that stick cells to ECM • mark pathways for cell migration during development and healing • important glycoproteins include laminin and fibronectin GAGs, Proteoglycans and Proteoglycan Aggregates a proteoglycan Proteoglycan Aggregate The proteoglycan aggregate from cartilage, shown in the electron micrograph on the left, is formed from a hyaluronate backbone attached to proteoglycans. The middle diagram is of a small portion of the proteoglycan aggregate showing core proteins of proteoglycans attached to a long hyaluronate molecule. Chondroitin sulfate and keratan sulfate GAGs are linked to core proteins forming many different proteoglycans. GAGs have a repeating disaccharide structure. Embryonic Connective Tissue • The Mucoid Mesenchymal Tissue of Embryonic Connective Tissue is semifluid with thin reticular fibers and relatively abundant mesenchymal cells and blood vessels. Wharton’s jelly of the umbilical cord is an example. Wharton's Jelly Cyst in an Umbilical Cord Connective Tissue Proper • Loose Connective Tissue (Areolar Tissue) • Dense Connective Tissue – Dense Connective Tissues • Dense Regular Connective Tissue • Dense Irregular Connective Tissue • Reticular Connective Tissue • Elastic Connective Tissue Loose (Areolar) Connective Tissue • Scattered cells include thin fibroblasts, macrophages and mast cells. • Loose arrangement of thick, wavy collagen fibers (C) and thin dark-staining elastic fibers (EF). • Found under some types of epithelia including the mesentary of the digestive tract. • Contains nerves and blood vessels. EF C Dense Regular Connective Tissue • Densely, packed, parallel collagen fibers • Fibroblasts sandwiched between fibers • Forms the strong, resilient tissue of tendons and ligaments blood vessel Dense Irregular Connective Tissue • Densely woven collagen fibers with little open space and few cells • Withstands stresses applied in different directions • Forms the dermis of the skin and forms capsules around organs Reticular Tissue • Loose network of thin, branched reticular fibers. • Supports cells in vascular, filtering organs like the liver lymph nodes, spleen, thymus and bone marrow. • Fibers are stained very dark with silver. Elastic Tissue • Cross section of the aorta shows alternating layers of muscle, collagen fibers and thinner, darker staining elastic fibers. • Special stains containing solutions of metals like silver are needed to visualize elastic fibers. People with Ehlers-Danlos Syndrome usually have hyperelasticity of the skin as shown in these pictures. The unusually elastic skin can be stretched much further than normal skin because of defective collagen synthesis in connective tissues. This condition also causes skin to be easily bruised, heal poorly, and joints that are unusually flexible (hyperflexible, hypermobile). Specialized Connective Tissue • Adipose Tissues – Unilocular (Yellow or White Fat) – Multilocular (Brown Fat) • Hematopoietic Tissue Unilocular Adipose Tissue • White or yellow adipose cells are called unilocular adipocytes. • Unilocular adipocytes are large, empty-looking cells with the nucleus pressed against cell membrane (arrowhead). This shape is sometimes described as a “signet ring”. • Unilocular adipose is found beneath skin and surrounding organs. It is used for energy storage, insulation and for cushioning other tissues. Multilocular Adipose Tissue • Brown Adipose cells are called multilocular adipocytes. • Multilocular adipose is found in human babies and in hibernating animals. • Cells have many mitochondria and many small droplets of fat. Multilocular adipocytes produce heat by breaking down stored fat. Heat is transferred to the blood through abundant capillaries. Multilocular Adipose Tissue Distribution Hematopoietic Connective Tissue (Blood) leukocytes erythrocyte platelets • Formed Elements are the blood cells classified as erythrocytes, leukocytes and platelets. • Non-formed Elements are the components of the blood plasma Supporting Connective Tissue • Cartilage – Hyaline Cartilage – Elastic Cartilage – Fibrocartilage • Bone – Compact Bone – Spongy Bone Cartilage • Supportive connective tissue with a rubbery matrix • Cartilage is an avascular tissue (no blood vessels in the matrix) so cells rely on diffusion from a surrounding vascular membrane, the perichondrium, to deliver nutrients and remove wastes. (injured cartilage heals slowly if at all) • 3 types of cartilage distinguished by the fibers of the extracellular matrix: – hyaline – elastic – fibrocartilage Hyaline Cartilage • Fibrous, vascular capsule called the perichondrium covers a clear, non-fibrous matrix. • Chondrocytes produce the cartilage matrix. Each cell is in a pit called a lacuna. • Hyaline cartilage covers the ends of bones at all movable joints plus the sternal ends of ribs and is the supportive material in nose, larynx, trachea, bronchi. Elastic Cartilage • Fibrous, vascular perichondrium covers elastic cartilage • The cartilage matrix has a weblike mesh of elastic fibers among the lacunae • Provides flexible, resilient, support for the external ear and epiglottis Fibrocartilage • Chondrocytes in isolated clusters or rows in a matrix containing coarse, wavy collagen fibers. • Resists compression and absorbs shock • Found in the pubic symphysis of the pelvis and the intervertebral discs. Bone • Bone matrix stores the minerals calcium and phosphorus. • Compact bone provides physical support for leverage during muscle contraction. • Spongy bone, also called trabecular or cancellous, fills the ends of long bones and supports the bone marrow. • Compact bone always covers spongy bone. Compact Bone and Spongy Bone Spongy Bone Compact Bone Muscle Tissue • Cells that respond to stimuli by contracting • Function is to exert physical force on other tissues – move bones – moves blood through vessels – expel urine and feces • 3 types of muscle tissue: – skeletal – cardiac – smooth Skeletal Muscle • Long, cylindrical, unbranched cells with striations and multiple peripheral nuclei. • Function in movement, posture, breathing, speech, swallowing. Cardiac Muscle • Short, striated cells connected to each other with intercalated discs. • Usually one central nucleus per cell. • Sometimes cells are branched • Found in heart and functions to pump blood. Smooth Muscle • Short, fusiform cells; nonstriated with only one central nucleus. • Functions to control the diameter of openings in the gastrointestinal tract, respiratory tract, cardiovascular system and parts of the reproductive system. Neurons • Neurons may have long cell processes are usually surrounded by much smaller glial cells. • Neurons communicate with electrochemical signals at the tips of the axonal and dendritic processes. • Found in brain, spinal cord, nerves and ganglia.