Megaloblastic Anemia

Question 1

Megaloblastic anemia is due to a lack of either ____ or ____ and results in
macrocytosis

Answer 1

Megaloblastic anemia is due to a lack of either Vitamin B12 or folic acid and results in
macrocytosis

Question 2

How does Vitamin B12 or cobalamin correlate to the structure of heme?

Answer 2

Vitamin B12 or cobalamin has a structure somewhat similar to that of heme, but here we have cobalt in the center instead of iron, that’s why we refer to it as cobalamin.

Question 3

How is dietary cobalamin acquired?

Answer 3

Vitamin B12 is the only vitamin exclusively synthesized by microorganisms.
Generally, bacteria and yeast produce it regularly and deliver it to the animal and from the animal; we get it by ingesting food of animal origin (food chain).
The main sources of Vitamin B12 in the human diet are meat, liver, fish, dairy products and eggs (where it is found in large amounts). Note that Vitamin B12 is resistant to heat —-> we can get it even in cooked animal products.

Question 4

Why do pure vegetarians suffer from Vitamin B12 deficiency?

Answer 4

Since Vitamin B12 is not present in the plant kingdom, pure vegetarians suffer from Vitamin B12 deficiency.

Question 5

How much Vitamin B12 needs to be taken daily?

Answer 5

We daily lose 1-3ųg of Vitamin B12 in the intestines and in the urine, therefore, we need to take that much of Vitamin B12 daily.

Question 6

How much Vitamin B12 is stored in the body? How long can a person rely on the stored vitamin B12 before developing deficiency?

Answer 6

Vitamin B12 is stored primarily in the liver, and we have around 3-4mg of it in the stores, therefore, in contrast to folate deficiency, it takes around 3 years to develop Vitamin B12 deficiency after absorption of dietary Vitamin B12 stops.

Question 7

How is Vitamin B12 absorbed?

Answer 7

For Vitamin B12 to be absorbed, it has to be bound to a factor released by the normal
stomach (parietal cells) referred to as intrinsic factor (IF) and here Vitamin B12 is the
extrinsic factor. The IF binds to the extrinsic factor (Vitamin B12) and both go to the
terminal ileum which is the last portion of the small intestine where Vitamin B12
absorption occurs. Once in the ileum, this Vitamin B12 – IF complex then adheres to specific receptors sites found on the epithelial cells of the ileum (these receptors are for the IF).

Following this, Vitamin B12 enters the epithelial cells and is absorbed, whereas the IF
leaves. Once absorbed, Vitamin B12 is transported in the plasma bound to a group of proteins, called transcobalamin I (TC I) and transcobalamin II (TC II). 90% of newly absorbed Vitamin B12 is bound to TC II, which serves as the chief transport protein, rapidly delivering the vitamin to the liver, hemopoietic cells and other dividing cells. Some Vitamin B12 attaches to TC I which accounts for the endogenous serum Vitamin B12 level, but it does not deliver Vitamin B12 to cells in contrast to the delivery protein TC II.

Question 8

What happens if someone has a disease on the stomach or terminal ileum?

Answer 8

The stomach (produces IF) and the terminal ileum are crucial for the absorption of Vitamin B12. Therefore, he will have a problem in Vitamin B12 absorption and he can develop megaloblastic anemia as a consequence.

Question 9

What results from the lack of Intrinsic Factor?

Answer 9

Pernicious Anemia

Question 10

What is pernicious anemia and what is it typically caused by?

Answer 10

Pernicious anemia is an autoimmune condition that prevents the body from absorbing vitamin B12 and it is caused by:

• Stomach atrophy (shrinkage)
• antibody production against parietal cells
• antibody production against IF
• Surgical removal of parts of the stomach

Question 11

Which is the chief transport protein of Vitamin B12?

Answer 11

90% of newly absorbed Vitamin B12 is bound to TC II, which serves as the chief transport protein, rapidly delivering the vitamin to the liver, hemopoietic cells and other dividing cells.
Some Vitamin B12 attaches to TC I which accounts for the endogenous serum Vitamin B12 level, but it does not deliver Vitamin B12 to cells in contrast to the delivery protein TC II.

Question 12

How is Folic acid acquired and metabolized?

Answer 12

Folic acid does not exist in the human body. Humans are unable to synthesize folic acid and depend on absorption of dietary folate. Folic acid is present in a wide variety of foods, such as eggs, milk, leafy vegetables, yeast, liver and fruits, but folic acid is very sensitive to heat —> eat fresh green vegetables + raw liver and meat.

Folic acid passes from the stomach —> small intestine. It needs nothing in the stomach to accompany it and absorption occurs in the jejunum. Folate is then rapidly removed from plasma to cells and tissues for utilization.

Question 13

How much intake of folate is needed daily? How much is stored and for how long?

Answer 13

Daily intake: 100-300ug (1/3 of a mg)
The store of folate in the body is enough only for 3 months and storage occurs in the liver (10-12 mg)

Question 14

What is the relation of vitamin B12 and Folate in the uridylate thymidilate pathway? What happens in case of deficiency in either vitamin B12 or folate?

Answer 14

Uracil is absorbed directly and exogenously
then it is used in the RNA to transcribe its complement being thymine in a process called uridylate thymidilate pathway which requires both vitamin B12 and Folate.
–> Any deficiency in either vitamin B12 or folate causes a reduced thymine and ultimately DNA production

Question 15

What primarily causes the erythroblasts to turn into megablasts?

Answer 15

The lack of effect of Vitamin B12/folate deficiency on hemoglobin/protein synthesis.

This is because only DNA is affected, while the RNA remains intact. Therefore, when DNA synthesis is prolonged while RNA is active, the cytoplasm goes on producing proteins, and it will be enlarged leading to the formation of macrocytic cells
(Megalos). This is called nuclear/cytoplasmic dissociation or asynchrony.

Question 16

What would happen if DNA synthesis is incomplete rather than prolonged?

Answer 16

If the DNA synthesis is incomplete, then the genetic material of the cell will be
disturbed and the cell will no more survive —> the cells will therefore be eliminated
progressively from the BM before they mature. This is referred to as ineffective
erythropoiesis which may reach 80-90% in very severe cases.

Question 17

What is seen in the peripheral blood in megaloblastic anemia? (Indices, counts, morphology)

Answer 17

PERIPHERAL BLOOD
- MCV high; usually >100 fl and can reach 130 fl —› macrocytic rbcs
- MCH high indicating increased Hb proportion in RBCs
- MCHC normal
- Pancytopenia (Decreased erythrocytes, leukocytes, and platelets production due to impaired DNA synthesis and inability to replicate)
- Low Hb and Hct (anemia)
- Anisocytosis and Poikilocytosis
- Macrocytic normochromic cells
- Tear drop cells and fragments of RBCs (schistocytes) due to innefective erythropoiesis and subsequently intravascular hemolysis
- Nucleated RBCs with basophillic stippling seen in moderate or severely anemics (also due to innefective erythropoeisis)
- Howel Jolly bodies (due to innefective DNA synthesis resulting in nuclear reminants)
- Cabot rings (rarely)
- Hypersegmented neutrophils/shift to the right (due to innefective leukopoiesis)
- Reticulocytes: Normal to low (later stages of the erythroid series are depleted)

Question 18

What are the B.M. Changes in megaloblastic anemia?

Answer 18

However, the B.M. shows erythroid hyperplasia in the early erythroid precursors ONLY. Late stages of the erythroid series (like reticulocytes) are either normal
decreased in amount, due to the lack of DNA.

The erythroid hyperplasia causes a significant decrease in the M/E ratio, which is the ratio of myeloid or granulytic cells to erythroids. Normally, the bone marrow
produces a higher proportion of granulocytes compared to erythrocytes resulting in a higher M/E ratio. However, erythroid hyperplasia resulting from compensation to the hemolysis of megaloblasts, results in a decreased M/E ratio of 1/1 or even 1/3.

Megaloblastoid changes:
The most characteristic feature of megaloblastic anemia is the abnormally large erythroblasts -→> Megaloblasts
Usually, in the early stages of erythropoiesis which involves cellular division, the DNA is replicated and condensed inside the cytoplasm.
However, due to impaired DNA synthesis, the DNA remains uncondensed in early precursors (proerythroblast, early and intermediate erythroblast) and manifests as networks of thin chromatin called nuclear sieving.
On the other hand, the later stages of erythropoiesis do not involve division but only maturation. This allows the DNA to remain condensed but only around the circumference of the nucleus, which manifests as a nucleus clock face.

Myeloid changes:
The myeloid series also manifests in a similar way to the megaloblasts. In the early myeloid series, the myeloblasts are uncondensed and display nuclear sieving.
The metamyelocytes and bands are enlarged just like the megaloblasts. This is because they are unable to mature (get smaller) - › increase tremendously in size

Megakaryocyte changes:
Megakaryocytes (platelet precursor) are shown even larger in size and have segmented nuclei in megaloblastic anemia

Question 19

What is the key diagnostic factor for megaloblastic anemia?

Answer 19

The enlarged metamyelocytes and bands are the key characteristic diagnostic factor for megaloblastic anemia. This is because while macrocytosis and megaloblasts are usually present in large amounts, they can be masked by Fe deficiency anemia (microcytosis) if it is also present. Meanwhile, myeloids are not affected.
Therefore, a presentation of microcytic hypochromic RBCs + giant metamyelocytes + giant biands in the B.M. is generally a sufficient diagnosis for Fe deficiency anemia combined with Vitamin B12 or folate deficiency (usually due to malabsorption)

Question 20

What is the treatment for megaloblastic anemia?

Answer 20

Vitamin B12/ folic acid is administered. Few days later ⬆️⬆️ reticulocyte count