Connective Tissue Notes

Connective Tissue

Connective tissue is the most abundant of the four types of body tissue, serving junction and support functions. It is highly polymorphic but shares common features:

  • Cells

  • Fibers

  • Fundamental substance

Connective Tissue Cells

  • Fixed cells: Low or no mobility

    • Fibroblasts

    • Adipocytes

  • Mobile cells (free, of blood origin):

    • Macrophages

    • Lymphocytes

Fibroblasts

Main cells of connective tissue.

  • Young, very active, spindle-shaped or stellate cells, 20 to 30 μm long.

  • Cytoplasm rich in organelles: RER, free ribosomes, and a well-developed Golgi apparatus.

  • Reflects intense synthesis of protein precursors, fibers, and ground substance.

Adipocytes

A large cell (120 μm in diameter) specialized in storing lipids, mostly grouped in adipose tissue.

  • White fat adipocytes: A large lipid vacuole occupying almost the entire cytoplasm; the only type present in adult humans.

  • Brown fat adipocytes: Very small size and multiple lipid vacuoles.

Macrophages

Polymorphic cells originating in the bone marrow that pass through the blood as monocytes.

  • Function: Defending the body in the connective tissues.

  • Large cells with cytoplasmic extensions (pseudopodia).

  • Cytoplasm rich in synthetic organelles.

  • Actions:

    • Phagocytosis

    • Secretion of toxic substances

    • Triggering immune reactions

Fibers

Collagen Fibers

The thickest of the connective fibers.

  • Stretchy but not elastic.

  • Give connective tissue its resistance to mechanical forces and its strength.

  • Collagen is an insoluble fibrous protein synthesized by:

    • Dermal fibroblasts

    • Cartilage chondrocytes

    • Endothelial cells and smooth muscle cells of vascular walls

    • Corneal keratocytes

    • Bone osteoblasts

Elastic Fibers

Characterized by their elasticity.

  • Strong and extensible, returning to their initial length after stretching.

  • Elastin is the main constituent.

  • Abundant in the connective tissue of the skin and the walls of arteries.

Reticulin Fibers

Recently formed collagen fibrils with a carbohydrate-lipid layer that prevents their transformation into collagen fibers.

  • Located in the embryonic connective tissue and certain organs (liver, endocrine glands).

Fundamental Substance

A homogeneous substance occupying the spaces between the fibers and the cells.

  • A very water-rich environment containing small dissolved molecules (mineral salts, sugar, polypeptides, etc.) and large protein macromolecules.

Function of Connective Tissues

Depending on its location and subtype, connective tissue has 3 main functions:

  1. Support and cohesion: Linked to its richness in fibers.

  2. Nutritional and exchange functions: Linked to its richness in extracellular matrix and vessels.

  3. Defense function: Linked to the presence of passing immune cells.

Classification of Connective Tissues

Classified according to morphological criteria, based on the relative proportions of their constituents.

Embryonic Connective Tissue

  • Mesenchyme

    • Present in the embryo and gives rise to all other connective tissues.

    • Cells are irregular, the ground substance is semi-liquid, and the fibers are reticulated.

  • Mucous Connective Tissue (Mucoid)

    • Found in the umbilical cord.

    • Very poor in cells and very rich in ground matter.

    • The cells are stellate.

    • The ground substance is hydrated and gelatinous.

    • The fibers are collagen fibers, very fine and dispersed.

Adult Connective Tissue

  • Loose Connective Tissue

    • Widespread in the body, plays a role of support and defense.

    • Random Tissues

      • Located under the epithelium, surrounds the glands and the vessels.

      • Plays a mechanical role of support, nutrition by the vessels that it carries, and defense of the organism by the cells that it contains.

    • Adipose Tissues

      • A loose connective tissue with a predominantly cellular structure; contains adipocytes adapted for storing fat.

      • White Adipose Tissue

        • Distributed throughout the body, especially in the deep layers of the skin.

        • Acts as an energy store, thermal insulator, and cushion against mechanical shock.

      • Brown Adipose Tissue

        • Found in newborn mammals, where it plays a role in regulating body temperature.

        • Only small amounts are found in adults.

    • Reticular Tissues

      • Very rich in type III collagen fibers called reticulin.

      • Forms the stroma of the spleen, bone marrow, and liver.

  • Dense Connective Tissue

    • Tissues in which fibers predominate; cells are few in number, almost exclusively fibrocytes.

    • Dense (Fibrous) Oriented Connective Tissue

      • Dominated by collagen fibers.

      • The parallel collagen fibers follow unidirectional paths.

      • Examples: ligaments (bone-bone connection) and tendons (muscle grip for the skeleton).

    • Dense (Fibrous) Unoriented Connective Tissue

      • Made up of numerous, dense collagen fibers without any specific orientation.

      • Examples: Envelopes of organs subjected to multidirectional forces.

  • Blood

    • A specialized connective tissue of mesenchymal origin.

    • Comprises a fundamental substance (plasma) within which cells bathe.

    • Circulates in a closed network (the circulatory system) thanks to cardiac contractions.

    • Role of Blood Tissue:

      • Transport of respiratory gases (by red blood cells), hormones, nutrients.

      • Body defense (by leukocytes).

      • Healing and hemostasis (by platelets).

      • Waste evacuation to elimination organs.

    • Blood Composition: Cells and plasma.

      • Plasma

        • Yellowish, viscous liquid made up of water, mineral salts, and organic molecules (carbohydrates, lipids, and proteins like albumin, antibodies, fibrinogen, etc.).

      • Cellular Elements

        • Formation of blood cells is called hematopoiesis and takes place in the bone marrow.

        • Red Blood Cells (Erythrocytes)

          • Concave discs without a nucleus or organelles.

          • Born in the bone marrow, die in the spleen, with a lifespan of 120 days.

          • Contain hemoglobin, which carries oxygen to the organs.

        • White Blood Cells (Leukocytes)

          • Cells with a nucleus, numbering 4,000 to 10,000/mm310,000/mm^3 of whole blood.

          • Two main groups: granular leukocytes and hyaline leukocytes.

          • Granular Leukocytes (Granulocytes or Polymorphonuclear Leukocytes)

            • Multi-lobed nucleus.

            • Specific cytoplasmic granules.

            • Neutrophils

              • Most numerous (45 to 70% of all leukocytes).

              • Diameter of around 10 to 12 μm.

              • Nucleus has 2 to 5 lobes (most often three).

              • Cytoplasm contains small granules rich in lysosomal enzymes.

              • Short lifespan (one to four days).

              • Involved in the body's defense processes (against bacterial agents).

            • Eosinophilic Polymorphonuclear Leukocytes

              • Representing 1 to 3% of all leukocytes.

              • Diameter of 12-15 μm.

              • Presence of specific eosinophilic granules.

              • Nucleus is usually less lobed (2 lobes).

              • Short lifespan in the blood.

              • Could phagocytose antigen-antibody complexes.

            • Basophils

              • Representing 0 to 0.5% of all leukocytes (10 to 50/mm350/mm^3).

              • Diameter of around 15-18 μm.

              • Irregular, clover-shaped nucleus often masked by specific basophilic granules.

              • Granules are rich in histamine and heparin.

              • Poorly understood functions (perhaps a role in delayed hypersensitivity reactions).

          • Hyaline Leukocytes

            • Regular, non-polylobed nucleus, and do not possess specific granules.

            • Two main categories: lymphocytes and monocytes.

            • Lymphocytes

              • Represent 20 to 40% of all leukocytes (i.e., 1500 to 4000/mm34000/mm^3).