Chapter 4 · Tissues · Topic 25

Connective tissue

A&P IanatomyRead the notes

1Why this matters

Marcus, 34, twisted his knee in a basketball game and tore a C-shaped cartilage pad inside it, a meniscus. The same week he cut his forearm on a car door. The cut closed in ten days. His surgeon warns that the inner part of the meniscus tear will probably never heal on its own. Both injuries hit connective tissue. The difference comes down to what each tissue is made of and whether blood vessels run through it.

2What this builds on

3Quick check before you start

1. Which is the fluid part of blood?

  1. Plasma
  2. Platelets
  3. Red blood cells
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Plasma is the liquid part of blood, mostly water with dissolved salts, nutrients and proteins. Red cells, white cells and platelets are suspended in it.

  • Correct: Plasma:
  • Platelets:
  • Red blood cells:

2. A cell that can divide and then become several different kinds of specialized cell is called what?

  1. A red blood cell
  2. A stem cell
  3. A fully differentiated cell
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A stem cell can divide to make more of itself and can differentiate into specialized cells. Fully differentiated cells, such as red blood cells, have already specialized.

  • A red blood cell:
  • Correct: A stem cell:
  • A fully differentiated cell:

3. Which of the four primary tissue types is made of tightly packed cells with almost no material between them?

  1. The connective type
  2. Epithelial tissue
  3. Nervous tissue
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Epithelium is sheets of cells packed edge to edge, held by cell junctions. The connective type is the opposite: few cells in a lot of material between them.

  • The connective type:
  • Correct: Epithelial tissue:
  • Nervous tissue:

4How it works, step by step

  1. Cartilage has no blood vessels.Oxygen and nutrients reach its chondrocytes only by diffusing through the matrix from vessels outside it.
  2. Diffusion through a thick, dense matrix is slow.Chondrocytes are few, spread out and slow to divide or build new matrix.
  3. After a tear, only a few slow cells with a poor supply are near the damage.New matrix is laid down very slowly, and the inner part of a torn meniscus, farthest from any vessel, often never fills in.
  4. Bone, by contrast, is full of blood vessels that deliver cells and nutrients to a break.A broken bone usually heals in weeks, while torn cartilage may need surgery.

5Core concepts

Structure and function

6A common mistake

The wrong idea: Blood can't be a connective tissue, because it is a liquid with no fibers.

What actually happens: Connective tissue is defined by cells scattered in a large amount of extracellular matrix. Blood fits. Plasma is its liquid matrix, and its fiber-forming protein is there all along in dissolved form; it turns into threads when a blood clot forms.

7Check yourself

Anything you miss goes into your review queue.

1. What feature defines connective tissue?

  1. Sheets of cells packed edge to edge
  2. Few cells scattered in a large amount of extracellular matrix
  3. Cells that shorten and pull when they are stimulated
  4. A free surface facing a lumen or the outside
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Connective tissue is mostly matrix: ground substance and fibers made by relatively few, scattered cells. The matrix gives each connective tissue its properties.

  • Sheets of cells packed edge to edge: Cells packed edge to edge describe epithelium, the opposite arrangement.
  • Correct: Few cells scattered in a large amount of extracellular matrix: Correct. Scattered cells in abundant extracellular matrix define connective tissue.
  • Cells that shorten and pull when they are stimulated: Shortening to pull is the job of muscle, the tissue type covered in the next topic.
  • A free surface facing a lumen or the outside: A free apical surface facing a lumen belongs to epithelium. Connective tissue sits inside the body with no free surface.

2. Which cell makes the fibers and ground substance of connective tissue proper?

  1. Fibroblast
  2. Macrophage
  3. Adipocyte
  4. Chondrocyte
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Fibroblasts (fibro- = fiber, -blast = builder) make collagen, elastic and reticular fibers and the ground substance. A less active one that maintains the matrix is called a fibrocyte.

  • Correct: Fibroblast: Correct. Fibroblasts build the matrix of connective tissue proper.
  • Macrophage: Macrophages engulf debris and bacteria. They clean the tissue; they don't build its matrix.
  • Adipocyte: Adipocytes store triglyceride in fat droplets.
  • Chondrocyte: Chondrocytes make the matrix of cartilage, a supportive connective tissue, not connective tissue proper.

3. The dermis of your skin is stretched in many directions as you move. Which tissue fits that job?

  1. Dense regular connective tissue
  2. Reticular tissue
  3. Hyaline cartilage
  4. Dense irregular tissue
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Dense irregular tissue has thick collagen bundles running in many directions, so it resists pulls from any direction. That is the dermis's tissue.

  • Dense regular connective tissue: Dense regular tissue resists pulling in one direction only. It suits tendons and ligaments.
  • Reticular tissue: Reticular tissue is a delicate net that supports soft organs such as the spleen. It is not strong.
  • Hyaline cartilage: Hyaline cartilage resists squeezing and is avascular. The dermis is a flexible, well-supplied layer.
  • Correct: Dense irregular tissue: Correct. Collagen bundles running every way resist stretch from any direction.

4. An athlete tears the inner part of a knee meniscus and, in the same month, breaks a bone in her wrist. The bone heals in eight weeks; the meniscus tear does not heal. What best explains the difference?

  1. The meniscus is made of elastic cartilage, which tears more easily
  2. Bone has no cells, so it simply sets like cement
  3. The inner meniscus lacks blood vessels; bone has many
  4. Fibrocartilage has a thick perichondrium that blocks healing
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Cartilage is avascular, so repair depends on slow diffusion and a few slow-dividing chondrocytes. Bone has a rich blood supply that brings cells and nutrients to a break, so it heals in weeks.

  • The meniscus is made of elastic cartilage, which tears more easily: Menisci are fibrocartilage, not elastic cartilage. The healing problem is the missing blood supply.
  • Bone has no cells, so it simply sets like cement: Bone is living tissue with cells throughout its matrix; its blood supply is what lets it heal.
  • Correct: The inner meniscus lacks blood vessels; bone has many: Correct. No vessels in the inner meniscus means little repair; bone's rich supply supports healing.
  • Fibrocartilage has a thick perichondrium that blocks healing: Fibrocartilage has no perichondrium at all. A perichondrium would actually help, since it carries vessels and cells.

5. An intervertebral disc is squeezed by your body weight and pulled as you bend and twist. Why is it made of fibrocartilage rather than hyaline cartilage?

  1. Thick collagen bundles resist pulling too
  2. Fibrocartilage has blood vessels that speed repair
  3. Fibrocartilage is the most flexible type of cartilage
  4. Hyaline cartilage disappears after birth
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All cartilage resists squeezing because its ground substance holds water. Fibrocartilage adds thick bundles of collagen, which resist pulling. A disc faces both, so it is fibrocartilage.

  • Correct: Thick collagen bundles resist pulling too: Correct. Collagen bundles add strength in tension to cartilage's resistance to compression.
  • Fibrocartilage has blood vessels that speed repair: Cartilage matrix has almost no blood vessels. Only the outer rim of pads such as the menisci gets a small supply.
  • Fibrocartilage is the most flexible type of cartilage: Elastic cartilage is the most flexible type. Fibrocartilage is the toughest.
  • Hyaline cartilage disappears after birth: Hyaline cartilage is the most common cartilage in adults too, for example on bone ends and in the trachea.

6. Blood is classed as a connective tissue. Which part of blood plays the role of the extracellular matrix?

  1. Red blood cells
  2. Platelets
  3. Plasma
  4. White blood cells
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In any connective tissue, cells sit in an extracellular matrix. In blood, that matrix is plasma, a liquid of water, salts, nutrients and dissolved proteins.

  • Red blood cells: Red cells are the most numerous cells of blood. They sit in the matrix; they are not the matrix.
  • Platelets: Platelets are cell fragments carried in the matrix.
  • Correct: Plasma: Correct. Plasma is blood's liquid matrix.
  • White blood cells: White cells are cells carried by blood, not its matrix.

7. This description of how cartilage gets its supplies contains one wrong step. Find it.

  1. Blood vessels run through the perichondrium around most cartilage
  2. Oxygen and nutrients leave those vessels
  3. Capillaries inside the matrix deliver them to each chondrocyte
  4. Chondrocytes in the center are the farthest from the supply
  5. Repair of damaged cartilage is slow
Show the answer

Cartilage is avascular. There are no capillaries in its matrix; oxygen and nutrients must diffuse through the matrix to reach the chondrocytes.

  • Blood vessels run through the perichondrium around most cartilage: This step is right: the perichondrium carries the vessels that supply most cartilage.
  • Oxygen and nutrients leave those vessels: This step is right: supplies leave the vessels outside the cartilage.
  • Correct: Capillaries inside the matrix deliver them to each chondrocyte: This is the error. Cartilage has no capillaries; supplies diffuse through the matrix.
  • Chondrocytes in the center are the farthest from the supply: This step is right: diffusion makes the central cells the least supplied.
  • Repair of damaged cartilage is slow: This step is right: a poor supply and few slow cells make repair slow.

8Summary

Connective tissue is cells scattered in a large extracellular matrix of ground substance and fibers: collagen fibers resist pulling, elastic fibers stretch and recoil, and reticular fibers form nets. Fibroblasts build the matrix, and macrophages engulf debris. Loose connective tissue (areolar, adipose, reticular) cushions, fills, stores fat and supports soft organs. Dense regular tissue forms tendons, ligaments and aponeuroses; dense irregular tissue forms the dermis and organ capsules. Cartilage (hyaline, fibrocartilage, elastic) is avascular and heals poorly; bone is mineralized and well supplied. Blood is a fluid connective tissue with plasma as its matrix. Most of these arise from mesenchyme, which comes mainly from the embryo's middle layer.

9What comes next