Connective Tissue

1. Connective Tissue • Most abundant and widely distributed tissue type • Four mature classes • Connective tissue proper • Cartilage • Bone tissue • Blood • Mesenchyme is embryonic connective tissue • Specializes to give rise to all mature classes of connective tissue • Some mesenchymal cells remain as a source of new cells in mature connective tissue • Gel-like ground substance with fibers and star-shaped mesenchymal cells

3. Major Functions of Connective Tissue • Binding and support • Protection • Insulation • Transportation (blood)

4. Characteristics of Connective Tissue • Common origin (mesenchyme) • Varying degrees of vascularity • Cartilage is avascular & dense is poorly vascularized. The rest are have an ample supply of blood vessels. • Extracellular matrix: cells separated by non-living extracellular matrix (ground substance and fibers) • This matrix allows connective tissue to bear weight, withstand great tension & endure abuses (trauma and abrasion)

5. Structural Elements of Connective Tissue • Ground substance • Medium through which solutes diffuse between blood capillaries and cells • Components: • Interstitial fluid • Adhesion proteins (“glue”) • Proteoglycans • Protein core + large polysaccharides (chrondroitin sulfate and hyaluronic acid) • Trap water in varying amounts, affecting the viscosity of the ground substance

6. Structural Elements of Connective Tissue • Three types of fibers • Collagen (white fibers) • Strongest and most abundant type • Provides high tensile strength • Elastic • Networks of long, thin, elastin fibers that allow for stretch • Reticular • Short, fine, highly branched collagenous fibers

7. Structural Elements of Connective Tissue • Cells • Mitotically active and secretory cells = “blasts” • Mature cells = “cytes” • Fibroblasts in connective tissue proper • Chondroblasts and chondrocytes in cartilage • Osteoblasts and osteocytes in bone • Hematopoietic (blood) stem cells in bone marrow • Fat cells, white blood cells, mast cells, and macrophages

8. Cell types Extracellular matrix Ground substance Fibers • Collagen fiber • Elastic fiber • Reticular fiber Macrophage Fibroblast Lymphocyte Fat cell Capillary Mast cell Neutrophil Figure 4.7

9. Overview of Connective Tissues • For each of the following examples of connective tissue, note: • Description • Function • Location

10. Loose connective Areolar Adipose Reticular Dense connective Dense regular Dense irregular Elastic Connective Tissue Proper

11. (a) Connective tissue proper: loose connective tissue, areolar Description:Gel-like matrix with all three fiber types; cells: fibroblasts, macrophages, mast cells, and some white blood cells. Elastic fibers Function: Wraps and cushions organs; its macrophages phagocytize bacteria; plays important role in inflammation; holds and conveys tissue fluid. Collagen fibers Location: Widely distributed under epithelia of body, e.g., forms lamina propria of mucous membranes; packages organs; surrounds capillaries. Fibroblast nuclei Epithelium Photomicrograph: Areolar connective tissue, a soft packaging tissue of the body (300x). Lamina propria Figure 4.8a

12. (b) Connective tissue proper: loose connective tissue, adipose Description: Matrix as in areolar, but very sparse; closely packed adipocytes, or fat cells, have nucleus pushed to the side by large fat droplet. Function: Provides reserve food fuel; insulates against heat loss; supports and protects organs. Nucleus of fat cell Location: Under skin in the hypodermis; around kidneys and eyeballs; within abdomen; in breasts. Vacuole containing fat droplet Adipose tissue Photomicrograph: Adipose tissue from the subcutaneous layer under the skin (350x). Mammary glands Figure 4.8b

13. (c) Connective tissue proper: loose connective tissue, reticular Description: Network of reticular fibers in a typical loose ground substance; reticular cells lie on the network. Function: Fibers form a soft internal skeleton (stroma) that supports other cell types including white blood cells, mast cells, and macrophages. White blood cell (lymphocyte) Location: Lymphoid organs (lymph nodes, bone marrow, and spleen). Reticular fibers Spleen Photomicrograph: Dark-staining network of reticular connective tissue fibers forming the internal skeleton of the spleen (350x). Figure 4.8c

14. (d) Connective tissue proper: dense connective tissue, dense regular Description: Primarily parallel collagen fibers; a few elastic fibers; major cell type is the fibroblast. Collagen fibers Function: Attaches muscles to bones or to muscles; attaches bones to bones; withstands great tensile stress when pulling force is applied in one direction. Location: Tendons, most ligaments, aponeuroses. Nuclei of fibroblasts Shoulder joint Ligament Photomicrograph: Dense regular connective tissue from a tendon (500x). Tendon Figure 4.8d

15. (e) Connective tissue proper: dense connective tissue, dense irregular Description: Primarily irregularly arranged collagen fibers; some elastic fibers; major cell type is the fibroblast. Nuclei of fibroblasts Function: Able to withstand tension exerted in many directions; provides structural strength. Location: Fibrous capsules of organs and of joints; dermis of the skin; submucosa of digestive tract. Collagen fibers Fibrous joint capsule Photomicrograph: Dense irregular connective tissue from the dermis of the skin (400x). Figure 4.8e

16. (f) Connective tissue proper: dense connective tissue, elastic Description: Dense regular connective tissue containing a high proportion of elastic fibers. Function: Allows recoil of tissue following stretching; maintains pulsatile flow of blood through arteries; aids passive recoil of lungs following inspiration. Elastic fibers Location: Walls of large arteries; within certain ligaments associated with the vertebral column; within the walls of the bronchial tubes. Aorta Photomicrograph: Elastic connective tissue in the wall of the aorta (250x). Heart Figure 4.8f

17. Connective Tissue: Cartilage • Three types of cartilage: • Hyaline cartilage • Elastic cartilage • Fibrocartilage

18. (g) Cartilage: hyaline Description: Amorphous but firm matrix; collagen fibers form an imperceptible network; chondroblasts produce the matrix and when mature (chondrocytes) lie in lacunae. Function: Supports and reinforces; has resilient cushioning properties; resists compressive stress. Location: Forms most of the embryonic skeleton; covers the ends of long bones in joint cavities; forms costal cartilages of the ribs; cartilages of the nose, trachea, and larynx. Chondrocyte in lacuna Matrix Costal cartilages Photomicrograph: Hyaline cartilage from the trachea (750x). Figure 4.8g

19. (h) Cartilage: elastic Description: Similar to hyaline cartilage, but more elastic fibers in matrix. Function: Maintains the shape of a structure while allowing great flexibility. Chondrocyte in lacuna Location: Supports the external ear (pinna); epiglottis. Matrix Photomicrograph: Elastic cartilage from the human ear pinna; forms the flexible skeleton of the ear (800x). Figure 4.8h

20. (i) Cartilage: fibrocartilage Description: Matrix similar to but less firm than that in hyaline cartilage; thick collagen fibers predominate. Function: Tensile strength with the ability to absorb compressive shock. Location: Intervertebral discs; pubic symphysis; discs of knee joint. Chondrocytes in lacunae Intervertebral discs Collagen fiber Photomicrograph: Fibrocartilage of an intervertebral disc (125x). Special staining produced the blue color seen. Figure 4.8i

21. Epithelial Membranes • Cutaneous membrane (skin) (More detail with the Integumentary System, Chapter 5) • Mucous membranes • Mucosae • Line body cavities open to the exterior (e.g., digestive and respiratory tracts) • Serous membranes • Serosae • Line internal body cavities (pleural membranes, visceral membranes, peritoneum etc.)

22. Cutaneous membrane (skin) (a) Cutaneous membrane (the skin)covers the body surface. Figure 4.11a

23. Mucosa of nasal cavity Mucosa of mouth Esophagus lining Mucosa of lung bronchi (b) Mucous membranes line body cavitiesopen to the exterior. Figure 4.11b

24. Parietal peritoneum Parietal pleura Visceral pleura Visceral peritoneum Parietal pericardium Visceral pericardium (c) Serous membranes line body cavitiesclosed to the exterior. Figure 4.11c

25. Tissue Injury • Tissue injury = penetration of one of the bodies primary defense barriers (skin & mucosae) • Injury triggers two types of responses: • Inflammation: rapid, but not specific • Immune: slower, but specific • Tissue repair involves two major processes: • Regeneration: replace damaged tissue with the same type of tissue • Fibrosis: production of fibrous connective tissue called scar tissue • Which process occurs depends on the location and severity of the tissue damage

26. Steps in Tissue Repair 1. Inflammation • Trauma triggers the release of inflammatory chemicals from injured cells as well as macrophages & mast cells • These chemicals trigger the dilation of blood vessels • Dilation increases vessel permeability, allowing WBCs & plasma w/ clotting proteins and antibodies to seep into injured area • Clotting occurs, stopping the loss of blood, holding edges of wound together and isolating it from bacteria, toxins etc.

27. Scab Epidermis Blood clot inincised wound Vein Migrating whiteblood cell Inflammatorychemicals Artery 1 Inflammation sets the stage: • Severed blood vessels bleed and inflammatory chemicals arereleased. • Local blood vessels become more permeable, allowing whiteblood cells, fluid, clotting proteins and other plasma proteinsto seep into the injured area. • Clotting occurs; surface dries and forms a scab. Figure 4.12, step 1

28. Steps in Tissue Repair 2. Organization and restored blood supply (beginning of actual repair) • The blood clot is replaced with granulation tissue, which lays down a new capillary bed • Fibroblasts within the granulation tissue produce growth factors, collagen fibers & contractile proteins to pull edges of wound together • Epithelium begins to regenerate • Debris is phagocytized by macrophages and underlying fibrous patch becomes scar tissue resistant to infection because it produces bacteria inhibiting fibers

29. Regeneratingepithelium Area ofgranulationtissueingrowth Fibroblast Macrophage 2 Organization restores the blood supply: • The clot is replaced by granulation tissue, which restoresthe vascular supply. • Fibroblasts produce collagen fibers that bridge the gap. • Macrophages phagocytize cell debris. • Surface epithelial cells multiply and migrate over thegranulation tissue. Figure 4.12, step 2

30. Steps in Tissue Repair 3. Regeneration and fibrosis • The scab detaches as epithelial tissue regenerates • Fibrous tissue matures; epithelium thickens and begins to resemble adjacent tissue • Results in a fully regenerated epithelium with underlying scar tissue NOTE: In “pure infections” where there is no wound, puncture or scrape, repair is carried out by regeneration alone and there is generally not any clot formation or scarring.

31. Regeneratedepithelium Fibrosedarea 3 Regeneration and fibrosis effect permanent repair: • The fibrosed area matures and contracts; the epitheliumthickens. • A fully regenerated epithelium with an underlying area ofscar tissue results. Figure 4.12, step 3

32. Developmental Aspects of Tissues • There are three primary germ layers: ectoderm, mesoderm, and endoderm • Formed early in embryonic development (by the end of the second month) • Specialize to form the four primary tissues • Nerve tissue arises from ectoderm (mitosis basically stops after birth) • Muscle and connective tissues arise from mesoderm (continue to divide after both) • Epithelial tissues arise from all three germ layers (continue to divide after both) Muscle and connective tissue (mostly from mesoderm) 16-day-old embryo (dorsal surface view) Ectoderm Nervous tissue (from ectoderm) Mesoderm Endoderm Epithelium