Hematopoiesis is the process by which immature precursor cells develop into mature blood cells.  The currently accepted theory on how this process works is called the monophyletic theory which simply means that a single type of stem cell gives rise to all the mature blood cells in the body.  This stem cell is called the pluripotential (pluripotent) stem cell.

SITES OF HEMATOPOIESIS

Age of animal Site of hematopoiesis
Embryo yolk sac then liver
3rd to 7th month spleen
4th and 5th months marrow cavity - esp. granulocytes and platelets
7th month marrow cavity - erythrocytes
Birth mostly bone marrow; spleen and liver when needed
Birth to maturity number of active sites in bone marrow decreases but retain ability for hematopoiesis
Adult bone marrow of skull, ribs, sternum, vertebral column, pelvis, proximal ends of femurs

STRUCTURE & FUNCTION OF BONE MARROW

Bone marrow has a vascular compartment and an extravascular compartment.  The vascular compartment is supplied by a nutrient artery which branches into central longitudinal arteries which send out radial branches that eventually open into sinuses.  These sinuses converge into a central vein that carries the blood out of the bone marrow into the general circulation.  Hematopoiesis takes place in the extravascular compartment.  The extravascular compartment consists of a stroma of reticular connective tissue and a parenchyma of developing blood cells, plasma cell, macrophages and fat cells.  The high activity of the bone marrow is demonstrated by its daily output of mature blood cells: 2.5 billion erythrocytes, 2.5 billion platelets, 50-100 billion granulocytes. The numbers of lymphocytes and monocytes is also very large.

Bone marrow is the site for other important activities in addition to hematopoiesis.  These include the removal of aged and defective erythrocytes and the differentiation of B lymphocytes.  It is also the site of numerous plasma cells.

 

THE PROCESS OF HEMATOPOIESIS

The monophyletic theory of hematopoiesis states that pluripotent stem cells multiply to produce more pluripotent stem cells, thus ensuring the steady and lasting supply of stem cells.  Some of the pluripotent stem cells differentiate into precursor cells that are at least partially committed to become one type of mature blood cell.   

Pluripotent stem cells multiply slowly into one of five possible unipotential stem cells which then multiply rapidly into the precursor of the specific mature blood cell for which they are destined.  

 

Although the pluripotent stem cells and the unipotential stem cells cannot be distinguished from one another histologically, the precursor cells can be distinguished with a trained and practiced eye.  

 

General Features:  Understanding the general process of hematopoiesis will be extremely helpful in distinguishing and identifying the different cells in a bone marrow smear or in an intact bone marrow preparation.  Basically an immature, precursor cell goes from a cell that is making lots of protein to a cell that is making much less protein.  

Since structure is (always) related to function, the structure of the precursor cell changes as it goes from making more protein to making less protein.  Thus, a cell that is making a lot of protein will have a nucleus containing dispursed or active chromatin, i.e., that is being transcribed actively.  When this cell is making less protein, the chromatin is condensed or clumped because it is not being transcribed.  Likewise, a cell that is making a lot of protein will have many and large nucleoli, the site of ribosomal RNA synthesis and assembly; as protein secretion decreases there are smaller and fewer nucleoli. Cells with high protein synthetic activity have more ribosomes in their cytoplasm and consequently the cytoplasm stains more basophilic (hematoxylin staining of the RNA in ribosomes). Cells with lower protein synthetic activity have fewer ribosomes, thus less basophilic staining with hematoxylin leaving the cytoplasm appearing more acidophilic due to the eosin staining of cytoplasmic proteins.  Cells with high protein synthetic activity the Golgi apparatus is highly developed, occupies much of the cytoplasm thus pushing the nucleus off to one side (acentric nucleus).  Cells with low protein synthetic activity have a smaller Golgi and the nucleus tends to be more centrally located.  

The chart below summarized these features.

 

ERYTHROPOIESIS

As the cells are maturing In the erythrocytic series, the cells are usually getting smaller, the nucleus is becoming smaller and more condensed and is eventually lost, and the cytoplasm is becoming more pink rather than blue.  

The cells in the developing erythrocyte series are as follows:  

  • Unipotent stem cell: not shown, cannot be distinguished from other unipotent stem cells histologically

 

hemoRBC1F.jpg (13974 bytes)
 

  • Proerythroblast: nucleus still rather large, taking up most of the cell; nucleus not condensed; cytoplasm still very blue or basophilic

  • Basophilic erythroblast: not shown; very difficult to distinguish from the proerythroblast

 

  • Polychromatophilic erythroblast: nucleus is more condensed than that of the proerythroblast; cytoplasm less blue, more grayish

  • Orthochromatophilic erythroblast: nucleus more condensed, smaller than that of previous cells and looks pyknotic by comparison; cytoplasm beginning to take on a more pinkish cast

hemoRBC4F.jpg (13707 bytes)

 

  

Micrographs from bone marrow smear from adult dog. 
(Lab slide 38A)

hemoRBC1F.jpg (13974 bytes)

 

hemoRBC2F.jpg (12408 bytes)

 

hemoRBC3F.jpg (12569 bytes)

  • Reticulocyte: no nucleus; cytoplasm still stains somewhat bluish due to presence of remnants of polyribosomes

  • Erythrocyte: mature erythrocyte has no nucleus (in mammals); cytoplasm stains very pink due to lack of ribosomes and presence of high amounts of protein, i.e., hemoglobin

 

GRANULOCYTE DEVELOPMENT

 
  • Unipotent stem cell: not shown, cannot be distinguished from other unipotent stem cells histologically

hemoNeu1F.jpg (10992 bytes)

  • Myeloblast: large cell with blue-staining cytoplasm; large nucleus; often as in this example, a clear area near the nucleus can be seen - this is where the rather large Golgi is located
     

 

 

 

  • Promyelocyte: example not shown; still a rather large cell with azurophilic (not specifically stained) granules




  • Myelocyte: example not shown; overall cell still rather large; nucleus still round without indentation; granules staining appropriately for the series,   i.e., pink for eosinophilic, blue for basophilic, neutral for neutrophilic 

 

 

Micrographs from bone marrow smear from adult dog.
 (Lab slide 38A)




hemoEosino1F.jpg (12662 bytes)

  • Metamyelocyte: cell is about the size of a mature granulocyte; nucleus with slight indentation; granules present that stain appropriately for the series, i.e., pink for eosinophilic, blue for basophilic, neutral for neutrophilic

hemoNeu2F.jpg (12618 bytes)
  

  • Band cell: cell is about the size of a mature granulocyte; nucleus with definite indentation - looks like a horseshoe; prominent granules that stain appropriately for the series 
  • Mature (segmented) granulocyte: cell is mature and looks like normal, mature granulocytes in the blood with lobed nucleus and prominent granules that stain appropriately for the series

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MONOCYTES

Not responsible for knowing the sequence of development of monocytes.

LYMPHOCYTES

Since mature lymphocytes look essentially like their precursor cells and pluripotent stem cells, intermediate forms cannot be identified histologically.

PLATELETS

Platelets, also called thrombocytes, play an important role in hemostasis by.  
  • plugging holes in blood vessels to prevent bleeding 
  • promoting formation of clots to further prevent bleeding 
  • helping to repair damaged blood vessels 

 

Platelet granules contain the secretory material that platelets produce to help repair damaged blood vessels, growth factor and many other proteins. Some of these are:

  • platelet factor 4 - regulates vascular permeability, calcium mobilization from bone, chemotaxis of monocytes and neutrophils
  • beta thromboglobulin - function unknown; used to monitor activation of platelets in some diseases
  • coagulation factors - fibrinogen, factor V, factor VIII
  • fibronectin, thrombospondin, platelet-derived growth factor - all may be involved in repair of damaged blood vessels
  • serotonin (taken up from plasma and stored in granules)
  • lysosomal enzymes such as hydrolases  

 

 

Platelets are formed in the bone marrow from megakaryocytes (30-100 µm diameter), very large cells with a polyploid, multilobed nucleus.  Platelets are released from fragmenting megakaryocytes in at least two ways:

  • extension of pseudopodia through the wall of the sinuses; pseudopodia contain "strings" of platelets that are pinched off and released into the circulation
  • passage of mature megakaryocyte into circulation and fragmenation in the pulmonary vascular bed

 

 

Micrograph of smear of monkey blood; Wright's stain (Lab slide L). 


Platelets appear as round, oval or biconcave disks and have a diameter of about 1.5 to 3.5 µm.  They are somewhat difficult to see in blood smears because of their small size and because they are often clumped together. Despite their small size, they contain all of the normal organelles and are rich in granules that are difficult to resolve with the light microscope but can be easily seen with the electron microscope. 

mega1F.jpg (13269 bytes)


Micrograph of fetal bone of a cat (Lab slide 3). 
Arrows indicates two of the lobes of  nucleus of the megakaryocyte.

Copyright 2002 Charlotte L. Ownby
Histology Part 2 Index