1. Chapter 5 – Tissues
Connective Tissues

2. General Characteristics of Connective Tissue
Most abundant type of tissue by weight.
Bind structures, provide support and protection, serve as frameworks, fill spaces, store fat, produce blood cells, protect against infections, and help repair tissue damage.
Can be rigid or flexible.
Usually have a good blood supply (for nutrients).

3. General Structure of Connective Tissue
Cells are farther apart than cells in epithelial tissue
Have an abundance of extracellular matrix
Extracellular matrix consists of fibers and a ground substance that varies from fluid to solid
The ground substance binds, supports, and provides a medium through which substances can be transferred between blood and cells in the tissue

4. Major Cell Types in Connective Tissue
Cells in connective tissue are divided into fixed cells and wandering cells.
Fixed cells are those that reside in the connective tissue for an extended period of time.
Wandering cells are those that move through and appear in connective tissue temporarily…usually in response to an injury or infection.

5. Major Cell Types in Connective Tissue
Fixed cells include:
Fibroblasts
Most common kind of fixed cell in connective tissues
Large and star-shaped
Produce fibers by secreting proteins into the extracellular matrix
Mast cells
Usually located near blood vessels in connective tissues
Widely distributed
Release heparin (a compound that prevents blood clotting) and histamine (a substance that promotes reactions associated with inflammation and allergies)

6. Fibroblast

7. Mast Cell

8. Major Cell Types in Connective Tissue
Wandering cells include:
Macrophages (or histiocytes)
Originate as white blood cells
Usually attached to fibers but can also detach and move about
Specialized for phagocytosis and so are important in immunity and defense against infection

9. Macrophage

10. Connective Tissue Fibers
3 types of tissues are produced by fibroblasts:
Collagenous fibers
Elastic fibers
Reticular fibers (least abundant)

11. Connective Tissue Fibers
Collagenous fibers
Composed of thick threads of the protein collagen (which is the major structural protein of the body)
Grouped in long, parallel bundles
Flexible but only slightly elastic
Have really good tensile strength (resistance to a pulling force)
Important component of body parts that hold structures together such as ligaments and tendons
Tissue with lots of collagenous fibers is termed dense connective tissue and appears white
Tissue with very few collagenous fibers is termed loose connective tissue

12. Connective Tissue Fibers
Elastic Fibers
Composed of the spring-like protein elastin
Fibers branch and form complex networks
Fibers are weaker than collagenous fibers but are very elastic (easily retain their shape after being stretched and deformed)
Normally found in body parts that are stretched such as vocal cords and the air passages of the respiratory system
Sometimes called yellow fibers as this is the color of tissues that have lots of elastic fibers

13. Collagenous and Elastic Fibers

14. Connective Tissue Fibers
Reticular Fibers
Composed of very thin collagenous fibers
Highly branched
Form delicate supporting networks in tissues such as those of the spleen

16. Categories of Connective Tissues
Connective tissue is divided into two categories:
Connective tissue proper
Includes loose connective tissue, adipose tissue, reticular connective tissue, dense connective tissue, and elastic connective tissue
Specialized connective tissue
Include cartilage, bone, and blood

18. Connective Tissue Proper
Loose connective tissue (or areolar tissue)
Forms delicate thin membranes throughout the body
Cells are mainly fibroblasts and are spread far apart with a gel-like ground substance between them
The ground substance contains many collagenous and elastic fibers that are secreted by the fibroblasts
Binds the skin to underlying organs, fills spaces between muscles, and lies beneath most layers of epithelium (remember, connective tissue provides the nutrient source for epithelial tissue)

19. Loose Connective Tissue

20. Connective Tissue Proper
Adipose Tissue
Or…fat
Has cells called adipocytes that store fat in droplets within their cytoplasm
Every person is born with a certain number of adipocytes
At first these cells resemble fibroblasts
As the cells get bigger (through accumulation of fat) their nuclei get pushed to one side
When groups of these cell become so large that they crowd out other types of cells, adipose tissue is formed
If a person goes through a period if fasting, the cells may loose fat and shrink, resembling fibroblasts again
Lies beneath the skin (for insulation and energy storage), in spaces between muscles, around the kidneys and certain joints (for cushioning), behind the eyeballs, in certain abdominal membranes, and on the surface of the heart

21. Adipose Tissue

22. Connective Tissue Proper
Reticular Connective Tissue
Composed of thin, collagenous fibers in a 3-D network (called reticular fibers)
Help provide the framework for specific internal organs, such as the spleen, liver, and lymphatic organs

23. Reticular Connective Tissue

24. Connective Tissue Proper
Dense Connective Tissue
Composed of many closely packed, thick collagenous fibers, a fine network of elastic fibers and a few cells (which are mostly fibroblasts)
May be classified as regular or irregular depending on how the fibers are organized
Regular dense connective tissue has collagenous fibers that are very strong, which allows the tissue to withstand pulling forces
Abundant in tendons and ligaments
Blood supply is poor which slows the time for tissue repair (such as in a sprained joint)
Irregular dense connective tissue has fibers that are thicker, interwoven, and more randomly organized, allowing the tissue to sustain tension exerted from many different directions
Found in the dermis (inner layer of the skin)

25. Dense Connective Tissue

26. Connective Tissue Proper
Elastic Connective Tissue
Composed mainly of yellow, elastic fibers in parallel strands or in branching networks with collagenous fibers and fibroblasts in between
Found in the attachments between bones of the spinal column (ligamenta flava) and in layers within the walls of certain hollow internal organs (including the larger arteries), some portions of the heart, and the larger airways

27. Elastic Connective Tissue

28. Specialized Connective Tissue
Cartilage
Provides support, frameworks and attachments, protects underlying tissues, and forms structural models for many developing bones
Has abundant extracellular matrix composed largely of collagenous fibers embedded in a gel-like ground substance that is rich in a protein-polysaccharide complex (chondromucoprotein) and contains a large volume of water
Cartilage cells, called chondrocytes, occupy small chambers called lacunae that lie completely within this matrix
Because cartilage tissue lacks a direct blood supply, nutrients must be obtained through diffusion from the perichondrium, which is a covering of connective tissues that surround cartilagenous structure
This lack of direct nutrients causes low levels of cell division and longer healing times

29. Specialized Connective Tissue
Cartilage
Cartilage can be divided into three types based on differences in the extracellular matrix
Hyaline cartilage
Has very fine collagenous fibers in the extracellular matrix
Most common type
Found on the ends of bones in many joints, in the soft part of the nose, and in the supporting rings of the respiratory passages
Important in the development and growth of most bones
Parts of an embryo’s skeleton actually begin as hyaline cartilage “models” that bone slowly replaces
Elastic cartilage
Has a dense network of elastic fibers in its extracellular matrix (making it flexible)
Provides the framework for the external ears and parts of the larynx
Fibrocartilage
Has many large collagenous fibers in its extracellular matrix (making it very tough)
Provides a “shock absorber” for structures that are subjected to pressure, such as vertebrae, knees, and the pelvic girdle.

30. Hyaline Cartilage

31. Elastic Cartilage

32. Fibrocartilage

33. Specialized Connective Tissue
Bone (osseous tissue)
Most rigid type of connective tissue
Due to mineral salts (such as calcium carbonate and calcium phosphate) and abundant flexible collagenous fibers being located between cells
Supports body structures internally, protects vital structures in the thoracic and cranial cavities, serves as an attachment for muscles, forms blood cells in red marrow, and stores and releases inorganic chemicals such as calcium and phosphorous

34. Specialized Connective Tissue
Bone (osseous tissue)
Structure
Bone matrix forms thin layers called lamellae that are located in concentric patterns around capillaries that are located within tiny longitudinal tubes called central (Haversian) canals.
Each central canal contains a blood vessel, so each bone cell has a close supply of nutrients
In addition, each bone cell has many cytoplasmic processes that extend outward, pass through minute tubes in the extracellular matrix called canaliculi, and are attached by gap junctions to the membranes of nearby cells, leading to rapid movement of materials between blood vessels and bone cells and rapid healing.
Bone matrix is deposited by cells called osteoblasts (which mature into osteocytes) that are located in lacunae
Osteoblasts are evenly spaced within the lamellae and also form concentric circles
Together, the osteocytes, layers of extracellular matrix and the central canals around which they are clustered form a cylindrical shaped unit called an osteon, or Haversian system
Many of these units become cemented together to form the substance of bone

35. Bone Tissue

36. Specialized Connective Tissue
Blood
Composed of cells suspended in a fluid extracellular matrix called plasma
Most blood cells form in special tissues called hematopoietic tissues in the red marrow that is located in the hollow part of certain bones
Types of blood cells
Red blood cells
Transport gases
Function completely within the blood vessels
White blood cells
Fight infection
Typically migrate from the blood through capillary walls to connective tissues where they carry out their major activities and usually remain until they die
Platelets
Involved in blood clotting

37. Blood Tissue