Connective Tissue: Types and Functions

• Connective tissue is formed of cells, fibers, and extracellular matrix.
• Extracellular matrix is where cells and fibers are embedded.
• Connective tissue develops from mesoderm, which gives rise to mesenchymal tissue.
• Connective tissue is formed of undifferentiated mesenchymal cells (UMCS) and matrix.
• Connective tissue is rich in blood vessels, nerves, and lymphatics.

Types of Connective Tissue (According to Matrix)

• Soft connective tissue is connective tissue proper.
• Rubbery connective tissue is cartilage.
• Hard connective tissue is bone.
• Fluid connective tissue is blood.

Connective Tissue Fibers

• There are elastic, collagenous, and reticular fibers.

Types of Connective Tissue Proper

• Loose (areolar) connective tissue is one type of connective tissue proper.

Sites for Loose (Areolar) Connective Tissue

• Under epithelium, subserous, dermis of skin, and submucosa.
• Around blood vessels.
• Fills spaces between other tissues.

Functions of Loose (Areolar) Connective Tissue

• Supports epithelium, blood vessels and nerves.

Adipose Connective Tissue

• Consists of a large number of fat cells forming lobules.
• Lobules separated by connective tissue of collagen and elastic fibers.

Types of Adipose Connective Tissue

• White adipose CT
• Brown adipose CT.

White Adipose Tissue:

• Has cells that are large, each with a single droplet of lipid.
• Cells have few mitochondria and a flat, peripheral nucleus.
• Functions in synthesizing and storing fat, supporting kidneys and blood vessels, and insulation.
• Located under the skin, around kidneys and blood vessels, and in the mesentery.

Brown Adipose Tissue:

• Has cells that are small, with many droplets of lipids.
• Cells have many mitochondria and a spherical central nucleus.
• Color is brown due to rich blood vessels.
• Cells are rich in mitochondria with cytochrome pigment.
• Generates heat.
• Located in foetuses and newborns in the interscapular region, axilla, and mediastinum.
• In adults, only found around the thoracic aorta.

Reticular Connective Tissue

• Found in the stroma of lymphatic organs, bone marrow, and glands like the liver.

• Supports cells in its sites.

• Forms a network of interlacing reticular fibers and reticular cells.

• Functions as a filter in the spleen and lymph nodes.

Mucoid Connective Tissue

• Formed of mesenchymal cells and fibroblasts.
• Has a jelly-like ground substance rich in mucus and hyaluronic acid.
• Found in the umbilical cord, pulp of growing teeth, and vitreous humour of the eye.

White Fibrous Connective Tissue

• Formed of fibroblasts and collagen fibers with minimal ground substance.
• Has regular and irregular types.

Regular White Fibrous Connective Tissue

• Site is tendons and cornea.
• Resists stretch in one direction.

Irregular White Fibrous Connective Tissue

• Site is periosteum, perichondrium, and sclera.
• Resists stretch in different directions.

Yellow Elastic Connective Tissue

• Found in the aorta, large vessels, bronchi, and bronchioles.
• Also found in ligamentum nuchae and ligamentum flavum.

Connective Tissue Proper Cells

• Has fixed (stable, long lived) cells like fibroblasts, fat cells, and mesenchymal cells.
• Also has free (transient, short lived) cells like plasma cells, mast cells, and leukocytes.

Undifferentiated Mesenchymal Cells (UMCs)

• Shape is small and branched.
• Has pale basophilic cytoplasm and a central large oval pale nucleus.
• Has many ribosomes but few other organelles.
• Can differentiate into other types of CT cells, blood cells, smooth muscle fibres, and endothelial cells.

Pericytes

• Are branched with long processes along the longitudinal axis of blood capillaries.
• Has pale basophilic cytoplasm and a central large oval pale nucleus.
• Has few organelles.
• Forms gap junctions with endothelial cells.
• In case of injury, can differentiate into smooth muscles, endothelial cells and fibroblasts.
• Can cause vaso-constriction by contraction.

Active Fibroblasts

• Are stellate with many long processes.
• Active fibroblasts have deeply basophilic cytoplasm.
• Active fibroblasts have a large oval pale nucleus with prominent nucleolus.
• Have well-developed rough endoplasmic reticulum and Golgi apparatus.
• Nucleus with with fine chromatin

Synthesis and Functions of Active Fibroblasts

• Synthesizes C.T. fibers and matrix.
• Produce growth factors that influence cell growth, differentiation, and wound healing.

Inactive Fibroblasts (Fibrocytes)

• A spindle shape with few processes.
• Has pale basophilic cytoplasm and a small dark nucleus.
• Has fewer organelles.
• Nucleus with condensed chromatin
• It can become active in case of wound healing.

Reticular Cells

• Small, stellate cells with many long thin processes.
• Has pale basophilic cytoplasm.
• Synthesizes reticular fibers.
• Can change to phagocytic cells.
• Acts as an antigen presenting cell to activate lymphocytes.

Unilocular Adipocytes

• Are Large oval cells of 50 – 150 μm.
• Eccentric and flattened nucleus
• Store fat in the form of one large droplet.
• Lipid droplets are removed during staining leaving an appearance of a signet ring appearance.
• Can be stained with Sudan III (orange).
• Have more free ribosomes and a few filamentous mitochondria.
• Synthesizes and stores fat for support, heat insulation, and energy release.

Multilocular Adipocytes

• Structure includes smaller with a spherical and central nucleus
• LM includes fat present as small droplets, and there is no signet ring appearance.
• Nucleus pigmented by cytochrome pigments in mitochondria.
• Its function is breakdown of fat to release heat.

Pigment Cells

• Small and branched with granular cytoplasm and a central rounded nucleus.
• In the cytoplasm, you will find Melanin granules called melanosomes
• Carry melanin for skin and eye color and protect from ultraviolet rays.

Mast cells

• Large and oval with a basophilic cytoplasm filled with basophilic granules.
• They have a specific staining where the granules can be stained metachromatically
• Small central rounded nucleus
• Secrete Heparin, an anticoagulant.
• They secrete Histamine for vasodilation.
• They secrete Esinophilic chemotactic factor which attracts eosinophil to site of allergy.

Plasma cells

• Are large and oval with basophilic cytoplasm and a central spherical eccentric nucleus
• Plasma cells are synthesisers and secretors of antibodies.

Macrophages

• Are large and branched with pseudopodia
• Macrophages have pale basophilic cytoplasm and a single dark eccentric kidney-shaped nucleus.
• Phagocytose microorganisms
• Can fuse to form multinucleated giant cell
• Acts as antigen presenting cell to activate B- lymphocytes to form antibodies
• Produce enzymes and cytokines that help in defence & repair.

Extravasated Lecocytes

• Are leucocytes that leave the blood through capillaries to perform their immune function.