Connective tissues

Term connective tissue was introduced in 1830 by Johannes Peter Muller. The tissue was already recognized as a distinct class in the 18th century

Connective tissues are the most abundant and widely distributed tissues of complex animals. These are mesodermal in origin. They perform the special function of linking and supporting, protecting other tissues/organs of the body.

Features of Connective tissues:

The connective tissues have variations from soft connective tissues to specialized types like cartilage, bone, adipose, and blood.

  • In All connective tissues, the cells secrete fibers of a structural protein called collagen or elastin except blood. The fiber function of providing strength, elasticity, and flexibility to the tissue.
  • The modified polysaccharide secreted by these cells, that accumulates between cells and fibers and acts as a matrix.
Fibers Location and Purpose Component
Collagen fiber Located in the tendon, ligament, skin, cornea, cartilage, bone, blood vessels, gut, and intervertebral disc.

Purpose: Bind bones and other tissues to each other

Alpha polypeptide chains
Elastic fiber Located in the extracellular matrix

Purpose: Allow organs like arteries and lungs to recoil

Elastic microfibril and elastin
Reticular Fibre Located in the liver, bone marrow, and lymphatic organs

Purpose: Form a scaffolding for other cells

Type III collagen
Classification of connective tissue:
  1. Proper connective tissue: These are divided into Loose connective tissue and dense connective tissue.
    1. Loose connective tissue
      1. Areolar tissue
      2. Adipose tissue
    2. Dense connective tissue
      1. Dense regular connective tissue
      2. Dense irregular connective tissue
  2. supportive connective tissue: These tissues are divided into-
      1. Cartilage
      2. Bone
  3. Vascular connective tissue: these are divided into-
      1. Blood
      2. Lymph
connective tissues: Areolar connective tissue, adipose connective tissue, fibrous connective tissue, blood

connective tissues

Proper Connective tissue

The proper connective tissues consist of a viscous gel-like matrix composed of proteoglycans. It is divided into two types:

  1. Loose Connective Tissue: in this type, the cells and fibers are loosely arranged in a semifluid matrix. These are of the following types:
    • Areolar tissue: It occurs beneath the epithelial of many hollow visceral organs, skin, and blood vessels (arteries and veins). These tissues contain different types of cells like fibroblast, macrophages, and mast cells. It functions as a support framework for epithelium.
    • Adipose Tissue: Mainly located beneath the skin. They specialized to store fat, (Nutrients that are not used immediately are converted into fats and stored in adipose tissue.) and reduce heat loss through the skin.
  2. Dense Connective tissue: In this tissue, the Fibres and fibroblasts are compactly packed, with very little matrix. It is classified as
    • Dense regular connective tissue: In the dense regular connective tissues, the collagen fibers are present in rows between many parallel bundles of fibers. It is also of two types – Tendons – attached skeletal muscles to bones, and ligaments – attached one bone to another are examples of this tissue.
    • Dense irregular connective tissue: Dense irregular connective tissue has fibroblasts and many fibers (mostly collagen) that are oriented differently This tissue is present in the skin. Cartilage, bones, and blood are various types of specialized connective tissues
Supportive Connective Tissue

The Supportive connective tissues functions for support, they are of two types:

  1. Cartilage: The cells of cartilage are enclosed in small cavities within the matrix secreted by them.
    • Its intracellular material is solid and pliable, resists compression. Chondrin protein secreted by the cell which is flexible named chondrocytes.
    • It mostly contains collagen fiber.
    • Cartilages present in the tip of the nose, outer ear joints, between adjacent bones of the vertebral column, limbs, and hands in adults.
  2. Bone: It is solid, rigid connective tissue. Bones have a hard and non-pliable ground substance rich in calcium salts and collagen fibers which give bone its strength.
    • The bone is the main tissue that provides a structural frame to the body.
    • It supports and protects softer tissues and organs. The bone cells (osteocytes) are present in the spaces called lacunae.
    • Limb bones, such as the long bones of the legs, serve weight-bearing functions. They also interact with skeletal muscles attached to them to bring about movements.
    • The bone marrow in some bones is the site of the production of blood cells. The Bone tissue consists of four parts:
      • Periosteum: It covers the bone completely; it is the dense white fibrous sheath.
      • Matrix: It occurs in lamellae in the form of layers. it is of three types: Haversian lamellae, concentric lamellae, and interstitial lamellae. False irregular space lacunae occur in the matrix; these lacunae are occupied by flat bone cells or osteocytes.
      • Endosteum: It is located outer to the bone marrow cavity, consists of osteoblast. the osteoblast is bone-forming cells, that secret ossein protein in the matrix.
      • Bone marrow: The soft fatty tissue, classified into two types:
        1. Red bone marrow: red due to the presence of RBCs
        2. Yellow bone marrow: Yellow due to the presence of adipocytes.
Vascular Connective Tissue (Fluid connective tissue)

Vascular connective tissues are specialized, which circulate through the cardiovascular system. It is differentiated as blood and lymph.

  1. Blood: Blood is a fluid connective tissue containing plasma, red blood cells (RBC), white blood cells (WBC), and platelets.
    • It helps in the transport of various substances. (will read about blood in a separate post)
  2. Lymph: It is a colorless fluid connective tissue consisting of plasma and lymphocytes. It does not consist of RBCs.
    • Lymph comprising interstitial villi called lacteals which mainly function for the absorption of digested food.

    Note***: We will read blood and lymph in a separate post.

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