COMPARATIVE ASPECTS - BLOOD

—      Nonmammalian vertebrates - RBCs are nucleated, elliptical in shape and larger (largest = Congo eel Amphiuma, 60-80 micrometers)

—      Other Mammals - RBCs vary in size (smallest is Musk Deer 2.5 micrometers) and in shape (discoid to elliptical)

—      Nonmammalian vertebrates have Thrombocytes instead of platelets; these are spindle-shaped cells involved in clotting

—      Nonmammalian verts. have Hemoblast/Hemocytoblast circulating in bloodstream = blood cell-producing (thrombocytes and RBCs) cell; high numbers exist in Hagfish (primitive vertebrate), lower numbers in frogs (more advanced) very low numbers in amniotes (advanced). Reduction in number of circulating hemoblastsdue to appearance of hemopoietic organs in more adv. verts.

—      Nonmammalian verts. have Heterophils = neutrophil w/ increased eosinophilia (rod-shaped eosinophilic granules)

SUMMARY

HEMOPOIESIS AND HEMOPOIETIC TISSUES - blood cells have relatively short life spans and are not capable of cell division so new production is constantly required

Hemopoiesis = formation of blood cells, Erythropoiesis = RBC production
Granulopoiesis = granular WBC production
Hemopoietic Tissues = specialized CT derived from mesenchyme, responsible for production of new blood cells; 2 Types:

1)       Myeloid Tissue (Red Bone Marrow) = responsible for production of most blood cell types

2)       Lymphatic Tissue (Thymus, etc.) = responsible for formation of T-lymphocytes, proliferation of B-lymphocytes, immune defenses (lymph nodes & nodules, spleen – generate lymphocytes upon antigen stimulus

—      Embryonically, blood cells are derived from yolk sac, mesenchyme, blood vessels, liver, spleen and bone marrow

Theories of Hemopoiesis:

1.       Monophyletic Theory (accepted by majority) = all blood cells arise from a single common stem cell (hemocytoblast)

2.       Diphyletic Theory = lymphocytes and monocytes derived from one stem cell (lymphoblast), granular leukocytes and RBCs from another stem cell (myeloblast)

3.       Polyphyletic Theory = a separate stem cell exists for each blood cell type

—      Difficult in determining which is correct lies in the fact that the earliest stages of blood cell formation involve progenitor cells that cannot be readily identified under the microscope.

Generalized Scheme of Blood Cell Formation (Monophyletic Theory)
- Differentiating blood cells can be grouped into 3 general categories:

1) Pluripotential Stem Cells
2) Restricted Progenitor Cells
3) Functional Blood Cells

Hemocytoblast = Pluripotential Stem Cell - has capacity to give rise to several different lines of specialized cells

- Nucleus is undifferentiated (cells look like large lymphocyte) with dense accumulations of chromatin material; basophilic cytoplasm due to presence of ribosomes; capable of extensive proliferation by mitotic division, so can serve aslifelong source of potential blood cells

Restricted Progenitor Cells = prospective fate of various progenitor cells becomes increasingly restricted: first to myeloid or lymphoid (lymphocytes & monocytes) and finally to progenitor cells specific for a certain blood cell

—      These cells are still relatively undifferentiated and cannot be identified under the microscope

—      Proliferation and differentiation of restricted progenitor cells is under control of regulating factors specific to each cell lineage

Eventually, progenitor cells become recognizable under microscope; at this stage they are termed blood cell precursors, which then mature into functional blood cells

ERYTHROPOIESIS = RBC Formation
- Principal processes involved in differentiation of RBC:

1) Reduction in size
2) Condensation and eventual loss of nuclear material and organelles
3) Acquisition of Hb via protein synthesis
4) Loss of proliferation capabilities

- Stages in Erythropoiesis:

1)       Proerythroblast = large cells (20-30 micrometers), large spherical nucleus, prominent nucleoli, basophilic cytoplasm

2)       Basophilic Erythroblast = still large (15-20 micrometers), nucleus round and proportionately smaller, basophilic cytoplasm, chromatin more condensed

3)       Polychromatophilic Erythroblast = 12-15 micrometers in diameter, cytoplasm with diffuse basophilic (blue = ribosomes) and acidophilic (pink = Hb) staining,nucleus spherical and proportionately smaller with chromatin more condensed
(Stages 1-3 are capable of proliferation)

4)       Normoblast = 8-10 micrometers, nucleus small and dark-staining (condensed heterochromatin), cytoplasm pinkish-blue to pink

5)       Reticulocyte (polychromatophilic erythrocyte) = 8-10 micrometers, no nucleus (extrusion has occurred), more acidophilic cytoplasm but still some basophilia due to ribosomes

6)       Mature Erythrocyte = cytoplasm stains completely acidophilic

—      Differentiation and maturation takes about 3 days for RBCs

—      Erythropoiesis stimulated by release of erythropoietin from kidney in response to tissue hypoxia resulting from removal of old RBCs. Erythropoietin stimulates Pluripotential stem cells to differentiate into RBCs.

GRANULOPOIESIS = Granular WBC formation
- Principal processes:

1) modest reduction in size
2) increasing darkening and lobation of nucleus
3) accumulation of specific granules

- Differentiation and maturation of granulocytes requires 14 days or more
- Stages in Granulopoiesis:

1)       Myeloblast = relatively large 15-21 micrometers; nucleus pale reddish-blue, occupies about 2/3 of cell > 2 nucleoli present; cytoplasm lacks granules - stains medium to light blue

2)       Promyelocyte = usually larger than myeloblast (18-30 micrometers); nucleus oval, chromatin condensed; bluish cytoplasm with some azurophilic granules (primary granules)

3)       Myelocyte = 12-15 micrometers; nucleus oval or flattened on one side, chromatin in dark reddish-purple clumps; cytoplasm filled with specific granules

(Stages 1-3 are capable of proliferation and granule production)

4)       Metamyelocyte =12-15 micrometers; nucleus kidney-shaped, chromatin coarse and densely clumped; specific granules fill cytoplasm

5)       Band Forms = slightly smaller; nucleus U-shaped; coarse clumps of chromatin, specific granules fill cytoplasm

6)       Mature Granulocyte = functioning adult cell

a.       Various growth factors control development of granulocytes and other blood cell types

b.        Negative feedback mechanism regulates release from marrow, involves releasing factors, including blood proteins and hormones (glucocorticoids and androgens)

MEGAKARYOCYTES AND PLATELET FORMATION
- Megakaryocytes = giant cells (30-100 micrometers) of red bone marrow
- Stages in Development:
Hemocytoblast → Megakaryoblast →  Megakaryocyte

1)        Megakaryoblast = nucleus large and often indented, peripheral heterochromatin dense, cytoplasm basophilic
- Transition to Megakaryocyte by multiple mitotic division without cytoplasmic division

2)       Megakaryocyte = complexly lobed nucleus, cytoplasm with patchy basophilia and numerous azurophilic granules
- Platelet formation involves extension of cytoplasmic processes from megakaryocyte which become pinched off as platelets. Part of cytoplasm subdivided by tiny tubular channels - platelets form by fragmentation along these channels

DEVELOPMENT OF LYMPHOID ELEMENTS

 

—      Occurs in lymphoid tissue (thymus, lymph nodes and nodules, spleen) and to some degree in myeloid tissues as well

—      Morphological evidence of differentiation is not marked

—      Lymphocytes derived from lymphoblasts:
Hemocytoblast  →  Lymphoblast  →  Prolymphocytes  →  Lymphocytes

—      Maturation involves:

1)       Condensation of chromatin

2)       Reduction in size

—      In adult mammals, most lymphocytes arise by proliferation from previously existing lymphocytes within lymphoid tissue - generally in response to invasion by foreign antigen (B-cells from hemocytoblasts)

 

—      Monocytes develop from stem cell in bone marrow
Monoblast →  Promonocyte →  Monocyte

—      Maturation involves:

1) Reduction in size
2) Appearance of a very few azurophilic granules

COMPARATIVE ASPECTS OF HEMOPOIETIC TISSUES

—      Hemopoietic organs in nonmammal vertebrates include:

1)       Spleen

2)       Bursa of Fabricius (Birds) - diverticulum extending dorsally from cloaca; produces
B-lymphocytes

3)       Typhlosole (lamprey) = infolding of mesentery into ventral gut

4)       Gonads - granulopoietic in elasmobranchs and lungfishes

5)       Liver - important granulopoietic tissue in teleosts, amphibians, reptiles

6)       Kidney - hemopoietic in some teleost fish

—      Bone Marrow first appears in Anurans (frogs and toads) - only involved in WBC production; becomes involved in granulopoiesis + erythropoiesis in marrow of amniotes

—      Embryonically, bone marrow enters "cartilage model" with bone-forming bud as primitive mesenchymal cells

RED BONE MARROW

—      Composed of hemopoietic cords (loose aggregations of developing blood cells), stroma of reticular CT, and sinusoidal capillaries (whereby blood cells gain access to circulation)

—      Hemopoietic cells often arranged into colonies of cells producing myeloid or lymphoid lines, granulocytes or monocytes, or individual blood cells types

To Lecture 7