Module 4.3: Connective Tissue

Learning Outcomes

  • Compare and Contrast the roles of individual cell and fiber types within connective tissues

  • Identify the different types of connective tissue, and describe where in the body they are found

  • Describe the functions of each type of connective tissue, and correlate function with structure in each tissue type

Connective Tissue functions

Connecting and Binding:

  • Connects tissues to structures, like anchoring organs in place

Support:

  • Bone and cartilage support the weight of the body

Protection:

  • Bones provide protection for organs, fat and cartilage provide shock absorption, and many parts of the immune system are found within the connective tissues

Transport:

  • Blood is a fluid connective tissue that transports nutrients

  • Made up of loosely packed cells, surrounded by protein fibers, all within ground substance

  • Although diverse in function, all of their ECM plays an extensive role

  • Two classifications: Connective tissue proper, and specialized connective tissue

  • They differ in the types of cells and the components of the ECM

Connective Tissue Proper

  • Resident cells inhabit a tissue permanently, while migrant cells go where ever their function is needed

Cells of Connective Tissues

Fibroblasts (-blast = immature)

  • Most common resident cell in connective tissue

  • Is a mature cell with properties of an immature cell

  • produce ground substance, protein fibers, and other elements of ECM

  • Are close to collagen fibers, as they produce collagen proteins

Adipocytes

  • The cytoplasm is most commonly dominated by a single large inclusion containing lipids

  • The nuclei and other organelles are pushed to the perimeter and hard to see

Mast Cells

  • Largest resident cell in connective tissue

  • Cells of the immune system that have cytosolic inclusions (or granules), containing inflammatory mediators (like histamine)

  • When stimulated, they release their granules and cause inflammation, signaling to activate the immune system and get more immune cells

Phagocytes

  • ingest foreign substances, microorganisms, and dead/damaged cells, by phagocytosis

  • 2 types

  • macrophages: either resident or migrant cells in connective tissue

  • neutrophils: migrant immune cells

Other Immune Cells

  • Other cells migrate in and out of connective tissue based on their needs

Types of Connective Tissue Proper

  • Four basic types:

Loose Connective Tissue

  • Also known are areolar connective tissue

  • Mace mostly of ground substance

  • all protein fibers, fibroblasts, and adipocytes are suspended in it, thus allowing the ECM to be loose

  • Found to the epithelium of the skin, in the membranes lining body cavities, and layers the walls of hollow organs

  • Supports blood vessels allowing nutrients and oxygen to diffuse up

  • Houses many immune cells to protect, in case the epithelium is breached

Dense Connective Tissue

  • Composed primarily of protein fibers, hence why it is also called fibrous connective tissue, and ECM is coarse and tough

  • Grouped in 3 classes

  • Dense Irregular Connective tissue

    • Predominant fiber is collagen

    • Their disorganization makes them strong resistant to tension in all 3 planes

    • Found in organs subjected to high tension: Dermis, around organs and joints

  • Dense Regular Collagenous Connective Tissue

    • Thick collagen fiber bundles, arranged in one plane

    • They are very strong in that single plane but weak in others

    • found in tendons (unites bone to muscle), and ligaments (unites two bones)

  • Dense Regular Elastic Connective tissue

    • Often shorted to elastic tissue

    • Run parallel, much like the regular connective tissue

    • and orientated kind of wavy-ish

    • Elastic is thinner than collagen

    • Found in certain organs to stretch and lining of large blood vessels, like the aorta and certain ligaments like the spine

Reticular Tissue

  • Named because of the presence of many reticular fibers produced by surrounding fibroblasts

  • interweave to form “meshes” that support blood vessels and lymphatic vessels

  • Found in lymph nodes and spleen to trap foreign and old cells, and house white blood cells

  • forms a part of the basement membrane that supports the epithelia and structure of the liver and bone marrow

Adipose Tissue

  • Consists of adipocytes, fibroblasts an ECM

  • Though, initially, the collagen fibers may be predominant, the adipocytes can increase in size and outcompete the space used

  • They insulate, shock absorption, protection, and energy reserve

  • White adipose tissue consists of adipocytes with a single large lipid inclusion in their cytosol

  • They are found in the hypodermis (deep to the skin), on top of the dense superficial facia, this fat is also called subcutaneous fat

  • Found mainly in the abdomen, breast, hips, buttocks, and thighs, also surrounding the heart and abdominal organs (visceral fat)

  • Brown adipose tissue have multiple adipocytes and appear brown due to extensive mitochondria and blood supply. Oxidizes fatty acids 20 times faster

Specialized Connective Tissue

  • Three types: Cartilage, Bone or Osseous, and Blood

Cartilage

  • Found between joints, in ears, nose, and certain respiratory pathways

  • Its solid gel-like ECM, containing many glycosaminoglycans, proteoglycans, collagen fibers, and elastic fibers, allow to be tough and flexible

  • Contains two major cell types:

  • Chondroblasts

    • actively divide by mitosis and make most of the ECM of cartilage

    • As they are surrounded by their own ECM they eventually become inactive and mature chondrocytes

  • Chondrocytes

    • Eventually inhabit small cavities of the ECM called lacunae (lacuna = cavity)

  • Is essentially avascular, as almost no blood vessels are in it, thus oxygen and nutrients must diffuse through the outer sheath of dense irregular connective tissue called the perichondrium

  • Thus, this slow diffusion process limits thickness to only a few millimeters

  • Divided into 3 classes, based on composition of the ECM

  • Hyaline Cartilage (Hyaline = glass-like):

    • Most abundant

    • ECM has large amount of ground substance with a fine type of collagen fibers, thus giving them a bluish-gray glassy appearance

    • covers the end of bones where they form joints (articulations, so hyaline is also called articulate cartilage)

    • smooth, allowing for little friction between bones and distributes mechanical tension more equally

    • Is used where strong yet flexible cartilage is needed, like the connection of the sternum to the ribs, framing of the respiratory tract, and nose.

  • Fibrocartilage:

    • The ECM is dominated by collagen fibers, little room for ground substance

    • The collagen fibers are there because, in addition to the chondrocytes and chondroblasts, there are fibroblasts

    • have little elastic fibers

    • This composition gives great tensile strength, and some flexibility

    • Found in fibrous joints and intervertebral discs; also articular discs (allows two bones to fit better)

    • differs from the other two types of cartilage because, it lacks a perichondrium (outer sheath of dense irregular connective tissue that surrounds cartilage)

  • Elastic cartilage:

    • Limited to the external ear, and parts of the framework of the larynx

    • filled with elastic fibers, allowing it to vibrate to assist in the detection of sound and production of sound

Bone

  • 35% of its mass consists of organic substances (collagen fibers, and a type of ground substance called osteoid)

  • 65% is inorganic (calcium phosphate crystals)

  • Mature bone tissue contains 3 cell types:

  • Osteoblasts:

    • carry out bone deposition (synthesize the organic parts of the ECM, which is secreted by exocytosis, and produces chemicals for calcium salts to deposit within the ECM)

    • found on outer surface of the bone

    • near a dense irregular connective tissue covering (periosteum)

  • Osteocytes:

    • once osteoblasts are surrounded in the ECM they secrete, they reside in lacunae and become osteocytes (mature bone cells)

    • produce substances for bone maintenance

  • Osteoclasts (-clasia = broken):

    • large and multinucleated

    • carry out bone resorption

    • secrete hydrogen ions and enzymes to break down the parts of the ECM

Blood

  • ECM of blood is called plasma

  • consists largely of water, dissolved solutes, and proteins

  • does not have protein fibers, but smaller proteins to transport and aid in clotting

  • Contains cellular fragments called platelets to aid in clotting

  • two main types of blood cells:

Erythrocytes (erythro- = red)

  • Bind and transport oxygen

Leukocytes (leuko- = white)

  • Function in immunity