Histo.9

• Blood

  A specialized connective tissue consisting of cells and plasma

  
• Blood cells

  • Erythrocytes (red blood cells)
  • Leukocytes (white blood cells)
  • Platelets

  
• Erythrocytes (red blood cells)

  • Terminally differentiated structures lacking nuclei and completely filled with the O2-carrying protein hemoglobin
  • Flexible biconcave discs, approximately 7.5 µm in diameter, 2.6-µm thick at the rim, but only 0.75-µm thick in the center
  • Normally quite flexible, which permits them to bend and adapt to the small diameters and irregular turns of capillaries

  
• Erythrocyte plasmalemma

  Consists of about 40% lipid, 10% carbohydrate, and 50% protein
• Erythrocyte membrane proteins

  • Band 3 protein
  • Glycophorin A
• Erythrocyte submembranous meshwork

  • Stabilizes the membrane
  • Maintains the cell shape
  • Provides the cell elasticity required for passage through capillaries
• Leukocyte groups

  • Granulocytes
  • Agranulocytes
• Granulocytes

  • Possess two major types of abundant cytoplasmic granules: lysosomes and specific granules
  • Have polymorphic nuclei with two or more distinct lobes
  • Have poorly developed Golgi complexes and rough ER, and few mitochondria, depending largely on glycolysis for their energy needs
• Agranulocytes

  • Lack specific granules, but do contain some azurophilic granules (lysosomes)
  • Have spherical or indented but not lobulated nuclei
• Neutrophils

  • 12-15 µm in diameter, with nuclei having two to five lobes linked by thin nuclear extensions
  • Have two main types of cytoplasmic granules: azurophilic primary granules/lysosomes and specific secondary granules
• Eosinophils

  • About the same size as a neutrophil or slightly larger, but with a characteristic bilobed nucleus
  • Have abundant large, acidophilic specific granules typically staining pink or red
• Basophils

  • 12-15 µm in diameter but make up less than 1% of circulating leukocytes
  • Have a nucleus divided into two irregular lobes, with large specific granules overlying the nucleus usually obscuring its shape
  • Have fewer, larger, and more irregularly shaped specific granules than other granulocytes
• Lymphocytes

  • The most numerous type of agranulocyte in normal blood smears
  • Typically the smallest leukocytes, constituting approximately one-third of these cells
  • Small lymphocytes have spherical nuclei with highly condensed chromatin and only a thin surrounding rim of scant cytoplasm
  • Larger lymphocytes have larger, slightly indented nuclei and more cytoplasm that is slightly basophilic, with a few azurophilic granules, mitochondria, free polysomes, and other organelles

  
• Monocytes

  • Larger agranulocytes (12-15 µm in diameter) with distinctly indented or C-shaped nuclei
  • Have basophilic cytoplasm containing many small lysosomal azurophilic granules
  • Have mitochondria, small areas of rough ER, and a Golgi apparatus involved in the formation of lysosomes
• Platelets

  • Very small non-nucleated, membrane-bound cell fragments only 2-4 µm in diameter
  • Play a role in controlling blood loss (hemorrhage) and in wound healing

  
• Platelet structure

  • Peripheral-transparent zone (hyalomere)
  • Central-dense granular zone (granulomere)
  • Sparse glycocalyx surrounding the platelet plasmalemma involved in adhesion and activation during blood coagulation
• Hematopoiesis

  1. Embryonic sites: yolk sac, liver, spleen, and lymph nodes

  2. Adult site: red bone marrow (skull, sternum, ribs, vertebrae, pelvis)

  3. Common pluripotential stem cell forms pluripotential myeloid and lymphoid stem cells

  4. Myeloid stem cells give rise to erythrocytes, eosinophils, neutrophils, basophils, monocytes, and megakaryocytes

  5. Lymphoid stem cells give rise to B lymphocytes and T lymphocytes
• Lymphatic vessels

  • Carry lymph, the fluid that re-enters the circulation from the tissues
  • Lymphatic capillaries join together to form larger lymphatic vessels, with the thoracic duct being the largest lymphatic vessel in the body
• Lymph

  • A transudate from blood, containing the same proteins as in plasma but in smaller amounts and in somewhat different proportions
  • Suspended in lymph are cells that are chiefly lymphocytes, most of which are added as lymph passes through lymph nodes
• Lymphoid tissue classification

  • Diffuse lymphoid tissue (diffusely arranged lymphocytes and plasma cells)
  • Dense lymphoid tissue (aggregations of lymphocytes arranged in nodules)
  • Discrete lymphoid organs (thymus, lymph nodes, spleen, tonsils)
  • Mucosa-associated lymphoid tissue (MALT) (small numbers of lymphocytes in relation to the mucosa of the respiratory, alimentary and urogenital tracts)