Introduction
 |
Before study more about Sickle Cell Anemia. We should needs to know what is the morphology of normal red blood cells & sickle blood cells.
Normal Red Blood Cell (RBC) Morphology
In order to properly understand and identify the abnormal red blood cell morphologies associated with RBC disorders, the laboratorian must first become competent in normal RBC characteristics.
Normally, RBCs will display a defined morphology in the peripheral blood. Mature RBCs, under normal circumstances, are round, biconcave disc-shaped, anuclear cells measuring approximately 7-8 microns in diameter with an internal volume of 80-100 fL. The term used to express RBCs of normal size is normocytic. Mature red blood cells, under normal circumstances, will also have an appropriate hemoglobin content (a normal MCH and MCHC), giving them a red-orange appearance on Wright-stained smears. These cells will display a central pallor (lighter area inside of the cell) no larger than 3 microns in diameter. This normal morphology is indicated by the term normochromic. It is paramount for RBCs to contain an adequate amount of hemoglobin for the purpose of transporting oxygen to the tissues and carbon dioxide back to the lungs. An example of a normocytic, normochromic peripheral blood picture is shown on the right.
In addition, the RBC membrane plays a key role in allowing the deformability of the cell to take place in order to travel through smaller vessels. Normally functioning RBCs survive for approximately 120 days in the peripheral blood circulation before being removed by the liver or spleen. Under normal circumstances, the body produces enough RBCs each day to account for the senescent (old) cells that are removed.
Certain disease states can alter the normal RBC characteristics described above. This course will illustrate and correlate various RBC morphologic changes with the specific disease states or conditions, which they are associated.
The hemoglobin molecule is a tetramer consisting of 4 polypeptide chains, known as globins, which are usually:
- 2 alpha chains that are each 141 amino acids long
- 2 beta chains that are each 146 amino acids long
Hemoglobin has a quaternary structure characteristic of many multi-subunit globular proteins. Most of the amino acids in hemoglobin form alpha helices, connected by short non-helical segments. Hydrogen bonds stabilize the helical sections inside this protein, causing attractions within the molecule, folding each polypeptide chain into a specific shape.Hemoglobin's quaternary structure comes from its four subunits in roughly a tetrahedral arrangement.
Genetics Of Hemoglobin
Hemoglobin consists mostly of protein subunits (the "globin" chains), and these proteins, in turn, are folded chains of a large number of different amino acids called polypeptides. The amino acid sequence of any polypeptide created by a cell is in turn determined by the stretches of DNA called genes. In all proteins, it is the amino acid sequence which determines the protein's chemical properties and function.
There is more than one hemoglobin gene. The amino acid sequences of the globin proteins in hemoglobins usually differ between species. These differences grow with evolutionary distance between species. For example, the most common hemoglobin sequences in humans and chimpanzees are nearly identical, differing by only one amino acid in both the alpha and the beta globin protein chains. These differences grow larger between less closely related species.
Haemoglobin is a protein consisting of four polypeptide chains, two α-chains, and two β-chains folded round each other. The two α-chains of haemoglobin are identical, as are the two β-chains. In haemoglobin, an oxygen transport protein, the binding of oxygen is cooperative (as each oxygen is bound, it becomes easier for the next one to bind) and is modulated by ligands such as H+, CO2 and BPG (2,3-bisphosphoglycerate, sometimes referred to as DPG).
The Haem group is a porphyrin derivative containing an iron atom, which undergoes transition from Fe2+ to Fe3+ on binding O2, which results in allosteric quaternary structural changes in the globin chains. It is this ferrous iron atom (located centrally within each haem group) which gives Hb its red appearance. A Haem group is attached to each of the four chains.
Hb is a red-coloured protein found in erythrocytes. It is composed of a prosthetic group called haem, and a globin protein. The globin consists of four polypeptide subunits, in two pairs. Different types of Hb (all found in adult blood) contain different types of polypeptides:
Type
|
Peptide chains
|
% in adult blood
|
% in fetal blood
|
HbA
|
α2, β2
|
97
|
10-50
|
HbA2
|
α2, delta2
|
2.5
|
trace
|
HbF
|
α2, gamma2
|
0.5
|
50-90
|
During early human development, HbF is normally replaced by HbA within 6 months of birth unless the production of polypeptide chains is abnormal, e.g. in thalassaemia and other haemoglobinopathies including Sickle-cell anaemia (HbS). HbF, which consists of gamma chains, have a higher affinity for oxygen due to a less positive charge in the gamma chains. This makes BPG bind less readily and as a result stabilising the oxy form of HB.
Hb tetramer molecule can bind four O2 molecules. The binding of one O2 molecule increases the affinity of further binding, resulting in an oxygen-haemoglobin dissociation curve, which is sigmoidal. In deoxyhaemoglobin, a water molecule is attached to each of the four iron atoms.
On oxygenation (this effect is sometimes known as co-operative) Hb undergoes conformational changes as some of the weak non-covalent bonds, holding the deoxy (Tense form) structure together, are broken. The oxygenated form of Hb is also referred to as the Relaxed form. The co-operative effect in oxygenation is due to the binding of one oxygen molecule to one iron atom in one of the 4 chains of Hb facilitates the binding of another oxygen molecule to a second iron atom in the same molecule and so on.
