Epithelium

Formed from the three embryonic germ layers

• Ectoderm
• Endoderm
• Mesoderm (least used layer):
• Mesenchymal cells, derived from mesoderm, are stellate in shape
• Undergo changes to form connective tissue

Connective Tissue:

Differentiated by

• Presence or absence of fibers (collaged, elastin, and/or reticular)
• Types of cells present (fibrocytes, WBCs, RBCs, platelets, chrondrocytes, osteocytes)
• Nature of the interstitual matrix (fluid-like, pus-like, gel-like, hard-like)

Collagen Fiber

Tensile Strength

• Appearance:
• White, ribbon-like fibers; runs in all directions
• Function/Characteristics
• Most common type of connective fibers, occuring in all connective tissue
• Possesses tensile strength
• Can be dissolved by weak acids
• If boiled, yields a gelatin substance
• Structure:
• Made up of smaller units called fibrils, made up of microfibrils, which are composed of macromolecules--tropocollagens

Elastic Fiber

Mechanical Support

• Appearance:
• coiled, spring-like; usually thinner than collagen fibers
• Chemical Composition:
• macopolysaccharide and protein referred to as elastin
• Structural Composition:
• amorphous homogenouos gel (elastin) and fibrolar (microfibrils, ~110A in dia.)

Reticular Fiber

Mechanical Support

• Appearance:
• Branch-like, finer than collagen
• Function/Characteristics:
• Strongest fiber, forms the interstitial matric and holds cells together
• Less affinity for acid dyes--silver nitrate stain used instead to locate reticular fiber (agyrophilic, silver-loving)
• If boiled, yields reticulum

• Factors that influence combination of cells found in tissue samples:
• Tissue Age
• Tissue Growth
• Tissue Repair
• Inflammatory Response to Injury

Fixed Cells

Synthesis of Fiber and Matrix

Fibrocytes/Fibroblasts

Repair after Injury

• Appearance:
• Look like nerve cells, shape depends on physical state
• Resting-small nucleus
• Active-entire cell enlarged
• Function:
• Produce fibers and ground substance (matrix)
• Characteristics:
• Common to all types of connective tissue
• Population of fibroblasts not constant
• In injury, new fibroblasts are recruited from an undifferentiated mesenchyme cell
• Cells migrate to site of injury and divide in preparation for wound repair
• Secrete a glycoprotein and a muscopolysaccharide--basis of ground substance
• Mesenchymal cells also will produce collagen--quantity will increase to such a degree that scar tissue is formed
• Demonstrates the ability of CT to repair

Mesenchymal Cells

Differentiation

• Appearance:
• Smaller than fibroblasts
• Function:
• Believed to be primitive cell
• Stimulation with toxins produces various types of cells (differentiation)
• Believed to be precursors of adipose tissue

Adipose Tissue

Storage of Triglycerides

• Appearance:
• Can occur singularly or in groups, bound by reticular fibers
• Function:
• Storage of nutrients in the form of triglycerides
• Adipose tissue--when adipose cells predominate over fiber and ground substance

Histiocytes (Macrophages)

Phagocytosis

• Function:
• Cells that specialize in phagocytosis, may also be wandering cells
• Characteristics:
• Majority of cells are stationary
• During inflammatoin, macrophages are known to detach their protoplasmic processes, detach themselves from fiber and become mobile

• Characteristics:
• Move in and out of connective tissue via matrix
• Function:
• To remove debris after tissue injury
• First line of defense against invasion of micro-organisms

• Include some cells that migrate from the blood:
• Neutrophils
• Basophils
• Eosinophils
• Monocytes
• Lymphocytes

Plasma Cells

Produce Antibodies

• Function:
• Resistance to disease, principle producers of antibodies
• Characteristics:
• These cells do not immigrate from the blood, but differentiate from lymphocytes

Mast Cells

Chemotactic

• Appearance:
• Large, round cells with pale nuclei, cannot see nuclei, size and shape vary
• Produce:
• Heparin--antocoagulant
• Histamine--vasodilator
• Serotonin--vasoconstrictor

Blood

Exchange of Metabolites

• Interstitial Matrix:
• Fluid-like
• Cells:
• Formed elements
• Erythrocytes
• Leukocytes
• Platelets
• Blood is considered a specialized form of connective tissue:
• Provides communication between the deep tissues and lungs
• Means of hormonal distribution

Erythrocytes (RBC)

Transport of Gasses

• Function/Characteristics:
• Lack a nucleus, mitochondria, and other organelles (not considered a true cell)
• Biconcave, disc-shaped cells
• Contains a respiratory protein, hemoglobin (33% mass)
• Carries oxygen, involved in transport of gasses
• Erythropoeisis
• Takes longer to form, requires higher concentration
• Loss of nuclear region to increase surface area (biconcave), enhances cell's ability to transport gasses

• Function/Characteristics
• Contain a nucleus and other organelles (true cells)
• Exhibit a limited degree of amoeboid movement
• Five kinds of leukocytes in blood, categorized according to:
• Presence or absence of specific cytoplasmic granules (granulated or non-granulated)
• According to shapes of their nuclear region (mono or polymorphonuclear leukocytes)
• Stain affinity (pertains to granular leukocytes only)

Granular Leukocytes 

(Grandulocytes)

Neutrophils

Bacteria

55-65%

• Appearance:
• 3-5 lobes in nuclear region, connected by a very thin strand of chromatin material
• Function/Characteristics:
• Phagocytic to bacteria and particular material
• 1^st line of defense of the body against bacterial (microbial) invasion
• Cytoplasmic Region Contains:
• Primary/Azurophilic granules
• Large, electon-dense granules
• Contain perodixases and hydrolases
• Specific granules
• Less electron-dense
• Contain alkaline phosphatase and anti-bacterial substance--phagocytin
• Heterophilic granules
• Smaller than specific granules
• Contain higher concentration of acid hydrolases

Eosinophils

Allergies or Parasites

1-3%

• Appearance:
• Bilobal with large granules (pomegranite-like)
• Function/Characteristics:
• Phagocytize antibody-antigen complexes
• Increase during allergic or parasitic infection
• Increase due to chemotactic factors produced by basophils and lymphocytes

Basophils

Chemotactic

.5%

• Appearance:
• Covered in metachromatic granules (blackberry-like)
• Function/Characteristics:
• Not clear, but believed to have a chemotactic function--responds to chemical stimulus
• Contains granules that are composed of:
• Histamine: vasodilator
• Heparin: anti-coagulant
• Seratonin: vasoconstrictor

Non-Granular Leukocytes

Non-graular

Monocular

Monocytes

3-8%

• Appearance:
• Largest of WBCs (2.5x RBC) with a nuclear region shaped like a kidney bean
• Function/Characteristics:
• Migrate in blood and act as scavengers; play defensive role in phagocytosisand intracellular digestion of invading micro-organisms (2^nd line of defense against microbial invasions)
• Essential for processing of antigen to development of antibodies
• Can become macrophages

Lymphocytes

20-35%

• Appearance:
• Smallest of WBC, nuclear region is slightly larger than the entire RBC
• 2 Distinct categories of lymphocytes based on:
• Background of lymphocytes (origin vs. rest stops)
• Life span (days to weeks vs. months to years)
• Function potential (susceptibility to certain drugs)

• Develop in the bone marrow --> Thymus --> Maturation OR Proliferation
• May re-enter blood stream and return to bone marrow OR go to peripheral lymphoid organs (lymph node, thymus, tonsils, spleen, bone marrow) where they live from months to years

B-Lymphocytes

Synthesis of Antibodies

• Do not pass through the thymus but go directly to lymphoid tissue via the blood stream
• Survive only a few days to weeks
• When antigens are present in the body, B lymphocytes differentiate into plasma cells, which synthesize the antibodies

• Amounts to 10-15% of circulating lymphocytes. They attack:
• Virus-infected cells
• Transplanted cells
• Cancer cells without previous stimulation

Note: T-lymphocytes and macrophages play an important role by bringing antigens into contact wiht B-lymphocytes.

Monocytes could be a precursor to macrophages 

Platelets/Thrombocytes

Blood-clotting

• Structure:
• Thrombocytes have nuclei, platelets do not
• Function:
• Blood clotting mechanism (coagulation)
• Platelets/Injured tissue produce Thromboplastin (enzyme), which converts:
• Prothrombin --> Vit. K, Ca++ -->Thrombin (enzyme), which converts:
• Blood protein Fibrinogen --> Fibrin (clot) --with all 3 fiber types
• Megakaryocytes
• Found only in bone marrow, gives rise to platelets 
• Cytoplasmic regin undergoes fragmentation at maturity leaving behind nuclear region
• Nuclear region is eliminated by reticuloendothelial system (cleanup system) consisting of phagocytes (microphages and macrophages) throughout the body
• Thrombopoiesis
• Polyploidy state--cell swelling sue to undergoing karyokinesis without cytokinesis

• Lymphoid Elements
• Mononuclear leukocytes-monocytes ad lymphocytes (do not have lobes in nuclear region)
• Lymphopoiesis-Agranular/mononuclear leukocyte formation in the lymphoid organs or bone marrow
• Myeloid Elements
• Platelets, erythrocytes, and granular leukocytes
• Myelopoesis-myeloid element formation in the bone marrow