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.
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.
|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
The cells in the developing erythrocyte series are as follows:
Micrographs from bone marrow smear from adult dog.
Micrographs from bone marrow smear from adult dog.
Not responsible for knowing the sequence of development of monocytes.
Since mature lymphocytes look essentially like their precursor cells and pluripotent stem cells, intermediate forms cannot be identified histologically.
also called thrombocytes, play an important role in hemostasis
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:
Platelets are formed in the bone marrow from megakaryocytes (30-100 Ám diameter), very large cells with a polyploid, multilobed nucleus. Platelets are released from fragmenting megakaryocytes in at least two ways:
Micrograph of smear of monkey blood; Wright's stain (Lab slide L).
Copyright 2002 Charlotte L. Ownby
Histology Part 2 Index