Pathology > Basic Hematology > Normal Hematopoiesis > Introduction

Introduction to Hematopoiesis

 

The blood consists of several types of leukocytes, erythrocytes and platelet suspended in plasma. The cellular elements of the blood are produced in extravascular tissues such as the bone marrow and lymphoid tissues. After delivery into the circulating blood ,the cellular elements may function within the vascular system or after passing into tissues. Blood elements have fixed lifespans and when they die are removed from the circulation and replaced by new cellular elements. The concentration of the various cellular elements of the blood are maintained within strict limits.

Plasma constitutes greater than one-half of the blood volume and is largely made up of water. In addition to dissolved ions,plasma contains carrier proteins (such as albumin, transferrin, haptoglobin, transcobalmin, and lipoproteins), coagulation proteins, and immunoproteins (immunoglobulin and complement) as well as growth factors, interleukins and numerous other compounds.

Hematopathology

Which of the following statements regarding plasma carrier proteins is FALSE?

A. Transferrin transports iron to red blood cells
B. Albumin is the single most abundant plasma protein
C. The accumulation of copper results in the disease state known as Wilson's disease
D. Alpha-1-antiprotease and alpha-2-macroglobulin neutralize proteolytic enzymes released from neutrophils
E. Haptoglobin binds and reduces methemoglobin

Blood* is separated into its major components by centrifugation. The red blood cells are at the bottom. On top of these is the buffy coat (a thin layer of white blood cells and platelets) and the remaining plasma.

Separation of blood by centrifugation is the basis for the HEMATOCRIT - the ratio of the volume of the erythrocytes to that of the whole blood, expressed as a decimal fraction.

*Anticoagulated blood (the anticoagulant is usually EDTA - a Ca++ chelator), prevents formation of a fibrin clot in the tube. The protein, water and salts forced out from between the fibrin strands by centrifugation is known as serum.

The Hematocrit (Hct) is sometimes referred to as the Packed Cell Volume or volume of packed red cells. A spun or centrifuged Hct contains a small (2%) amount of trapped plasma between red cells.

Estimate the Hct for the microhematocrit shown on the lower right.

Normal Hct Values:

adult man

.42 - .54 *

adult woman

.37 - .44 *

term newborn

.53 - .68

infant (3m)

.30 - .38

child (10y)

.37 - .44

* known approximate range

The Hct varies with age and sex.

Hematopathology

Right, the Hct you estimated from the previous card was about 0.45 !

Why is the normal adult Hct generally in the range of .40 - .45 ?

A. maximum erythroid cell production
B. optimal O2 carrying capacity and serum viscosity
C. hemoglobin threshold exceeded past 0.45
D. all of the above
E. none of the above

What is the origin of the cellular elements of the blood ? All blood cells appear to originate from a common progenitor cell or pluripotent. This cell matures into cells that are committed to one or another of the major cell lines (committed stem cell). Purported stem cells can be recogized only by immunologic means as morphologically they resemble immature lymphocytes. Stem cells are self-replicating in that only one of the daughter cells differentiates. The other remains a stem cell. Thus the number of stem cells remains constant. Thus the number of stem cells remains constant.

Most of the early hematopoietic development and maturation takes place in the bone marrow. The marrow may be considered a large organ weighing from 1600-3700 gr. Hematopoiesis is found in only certain bones of which the major sites are the vertebra, sternum, pelvis, ribs, skull, scapula,and proximal long bones.

The bone marrow is made up of hematopoietic cells and stromal cells including fat cells and fibrocytes, with extracellular connective tissue, and sinusoidal and vascular spaces. The normal marrow contains 30-70% hematopoietic cells,while the rest is largely fat. In hematopoietically active marrows the amount of fat decreases whereas the inverse occurs in inactive marrows.

Approximately 70% of the marrow cells are myeloid and 25% are erythroid. All stages of maturation are found in the marrow but egress from the marrow is normally limited by stage of development.

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