H.A. 101: Connective Tissue

Classification Of Connective Tissue

Connective tissue is one of the major tissue groups and is the most abundant tissue in the entire body. Within the category of connective tissue, there are three main components. Connective tissue has specialized cells, extracellular protein fibers, and a matrix.

The matrix of Connective tissue is the term used to describe the extracellular component of all connective tissue. This matrix can be made of fibrous proteins and a ground substance. The ground substance is the fluid portion of the matrix. In bone the matrix is calcified. This is one of the reasons it has a solid characteristic.

So, what is the function of Connective tissue? Connective tissue has many functions ranging from defense against pathogens and microorganisms to transporting fluid and materials. Other functions of Connective tissue are creating the structural framework of the body, protecting, supporting, and connecting tissues, and storing energy. Based on the location and structure of the Connective tissue it will have a different function.

To classify Connective tissue to understand its functions and characteristics we can categorize them into three groups. These three groups are:

  1. Connective Tissue Proper
  2. Fluid Connective Tissue
  3. Supporting Connective Tissue

Connective tissue proper is defined as tissues that have a matrix of fibers. These include loose and dense fibers. Loose connective tissue proper creates a loose framework and leads to tissue-like Areolar tissue, Adipose tissue, and Reticular tissue. Dense connective tissue proper is a connective tissue that has tightly packed fibers. These tissues include dense regular, dense irregular, and elastic tissues.

Fluid connective tissue is a connective tissue that has a fluid matrix. Many people think of tissue being solid, but this is highly untrue. This category of Connective tissue includes Blood and Lymph.

Supporting Connective tissue is similar to connective tissue proper but, it has a matrix of fibers and in some tissues, it has insoluble calcium salts. Supporting connective tissue that does not have insoluble calcium salts can be classified as Cartilage while those that do can be called Bone.

Connective Tissue Proper Cells and Fibers

Connective tissue proper can be divided into two classes of cells.

  1. Fixed Cells
  2. Wandering Cells

Fixed cells are cells that remain within connective tissue proper. These cells include:

  • Fibroblasts- contribute to the production of connective tissue
  • Fibrocytes- Maintain the fibers of connective tissue and matrix
  • Fixed macrophages- destroy pathogens and damaged cells using phagocytosis
  • Adipocytes- Stores fats within connective tissue
  • Mesenchymal cells- Stem cells within connective tissue that differentiates into other cell types
  • Melanocytes- Found in the skin and eyes and produces as well as contains pigment called melanin.

Wandering cells are cells that do not remain within connective tissue proper. This means that they are not fixed to a specific tissue and can travel between tissues. These include:

  • Free macrophages- Mobile defense cells, derived from monocytes, that use phagocytosis to engulf damaged cells and pathogens.
  • Mast cells- Stimulate inflammation
  • Lymphocytes- White blood cells that are a vital part in the immune response
  • Neutrophils- Another types of white blood cell that mobilize during infection

Connective tissue proper has three different fibers that are associated with it. These are:

  1. Collagen fibers
  2. Reticular fibers
  3. Elastic fibers

Collagen fibers can be defined as flexible yet strong fibers. Although it is highly flexible it also has great tensile strength. Tensile strength is the ability to resist breaking under tension. Its flexibility does not mean it’s stretchable, however. For this reason, Collagen fibers can be found in tissues like tendons and ligaments. They function to hold tissue together even during normal body movements.

Reticular fibers are a network of randomly branched collagenous fibrils. The goal of random branching is to increase surface area. This helps achieve its function of supporting organs and many other tissues.

Elastic fibers contain a specific type of protein called elastin. Elastin is an extracellular protein that specializes in creating recoil and extensibility in many tissues. Things such as the heart, large arteries, vessels, and the lungs have this protein due to their need to expand and recoil. Elastic fiber’s primary needs are elasticity and resilience.

Connective Tissue Proper Loose Connective Tissues

Within loose Connective tissues there are two categories

  1. Loose fibers (Areolar Tissue, Adipose Tissue, Reticular Tissue)
  2. Dense fibers (Dense Regular, Dense Irregular, Elastic)

Loose Connective Tissue Proper is defined as having a more complex extracellular matrix that consists of mainly ground substance and two main types of protein fibers (Elastic and Reticular).

Areolar Tissue is located deep within the dermis of the skin. It can be found around vessels, nerves, and between muscles. The function of this tissue is to connect the skin to the muscle and provide some support and independent movement. It also helps cushion organs and acts as a defense mechanism for pathogens. Its matrix consists mainly of fibers (elastic and collagen)

Adipose tissue is located in your buttocks, hypodermis, and around many of your organs. The goal of this very unique type of tissue is to provide insulation and cushion for the body. This tissue can be categorized into two different groups based on its color. Brown adipose tissue is usually activated when you become cold. It burns the fat and creates heat for your body to stay warm. White adipose tissue is the predominant type of fat found in the body and it functions to insulate and protect the body.

Reticular tissue can be found in the kidneys, appendix, bone marrow, liver, spleen, tonsils, and many more. The matrix of this tissue consists of reticular fibers and its function is to support organs and the framework of the body.

Dense connective tissue can be defined as fibers that are densely packed. This usually means that there is less ground substance than fibers or that the fibers are very close in proximity. There are two types of dense connective tissue. The first is Dense Regular and the second is Dense Irregular.

Dense Regular connective tissue can be found in the ligaments, tendons, aponeuroses, and elastic tissue. The function of this tissue is to provide attachment to muscles and other tissues. The tendon functions to connect muscle to bone. The aponeuroses function to connect muscle to muscle or connect an entire muscle by covering it. Ligaments connect bone to bone and finally, elastic tissue stabilizes the vertebrae of your spine.

Dense Irregular connective tissue is found near nerves and muscle sheaths. Here they provide the strength these areas need and form a fibrous capsule around the organs. This capsule helps prohibit overexpansion of the organs like in the bladder.

Fluid Connective Tissues

Fluid connective tissue can be categorized as Blood and Lymph. This tissue has a watery/syrupy matrix that suspends its cells within.

Blood can be found within the circulatory system within blood vessels. Blood contains a mixture of many different cells that are vital to keeping the body alive. Erythrocytes (Red Blood Cells) are responsible for transporting oxygen as well as carbon dioxide around the body and through blood vessels. Leukocytes (White Blood Cells) are a category of cells that help fight infection or disease. They consist of Monocytes, Lymphocytes, Eosinophils, Neutrophils, and Basophils. Thrombocytes or Platelets are membrane-enclosed packets of cytoplasm. They function in the process of blood clotting and coagulation to fix broken blood vessels. Finally, the liquid matrix all of these cells float in is called Plasma. It makes up 55% of blood and contains water, salts, proteins, antibodies, and more.

Lymph is located within the Lymphatic system where it travels through vessels into lymph nodes, the spleen, and more. The function of lymph is to house cells that can detect and fight infection and disease. It is a critical part of the immune response. Within lymph, there are cells called Lymphocytes. These cells develop into T cells and B cells.

Supporting Connective Tissue

Supporting connective tissues functions to support the body by creating a strong framework of tissue. Within this tissue, there is cartilage and bone.

Cartilage is a white flexible supportive connective tissue found in the trachea, ear, respiratory tract, and articulating surfaces of joints. The cells of this tissue are called chondrocytes. The matrix of this tissue is considered a gel and is made of chondroitin sulfate. It also has a fibrous perichondrium that consists of a cellular layer and a fibrous layer. This tissue is avascular and the cells reside in an area called the lacunae.

Cartilage is more common in the developing human because it is involved in appositional and interstitial growth. Appositional growth helps increase the cellular dimensions of the body while interstitial growth creates additional matrix material.

There are many different types of cartilage tissue as well. Since it is so important in the structure of the body, this tissue has adapted to meet the needs of different locations.

Hyaline cartilage is specialized for flexible support and reducing the friction movement may cause in the body. It can be found in areas such as the ribs, sternum, connections within the joints of the elbow and knee, and trachea. This cartilage provides a flexible and stiff support on bony surfaces.

Elastic cartilage is usually found in the auricle of the ear, epiglottis, and cuneiform of the larynx. It functions for flexible support while withstanding distortion and recoil.

Fibrous cartilage can be found in the pads of the knee joints, between the spinal vertebrae, and the pubic symphysis. Here, it functions to resist compression and absorb the shock that the body might experience. Its goal is to prevent bone-to-bone contact that might result in damage.

Bone is a solid matrix made of calcium phosphate. This tissue makes up the skeletal system and its function is to support the body and provide strength. It has a sold matrix called the lamellae. Everybody is covered with a layer of protection called the periosteum. The cells of the bone are called osteocytes. They reside in a circular region within the bone called the Osteon. This area also contains the central canal, lacunae, canaliculi, and lamellae.

There are two types of bone, compact bone, and Spongy bone. Compact bones have trapped blood vessels within the matrix while Spongy bones do not.