Etiology: why does deficiency occur?
Vitamin B12 deficiency is relatively rare. The main reason lies in impaired intestinal absorption. Several key factors are distinguished:
- Autoimmune mechanisms. This is the most frequent cause (over 75% of cases). The body begins to produce antibodies (Abs) against intrinsic factor (Castle's factor), the vitamin B12 / intrinsic factor complex, or the proton pump proteins of gastric parietal cells. The consequence of the latter is the development of hypoacid gastritis.
- Gastrointestinal diseases accompanied by malabsorption syndrome.
- Fish tapeworm infestation, which actively and competitively consumes the vitamin.
- Severe dysbiosis.
Folate deficiency is much more widespread. It is triggered by:
- Alimentary factor (low vitamin content in food).
- Insufficient transplacental transfer to the fetus (when maternal folate levels are low).
- Genetic defects disrupting folate transport and metabolism.
- Increased body demand for the vitamin (e.g., during severe infections).
Pathogenesis of megaloblastic hematopoiesis
A key link in the development of the disease is the disorder of thymidine nucleotide synthesis. Because of this, DNA replication is critically impaired, leading to a sharp decrease in the division rate of bone marrow cells. The outcome of this process is pancytopenia—a drop in the levels of all blood form elements.
The formation of the pathological cell—the megaloblast—occurs through a special mechanism. It is important to understand that protein synthesis in precursor cells is completely unaffected. So-called nuclear-cytoplasmic dyssynchrony occurs. Due to the DNA replication glitch, nuclear maturation is greatly slowed down. Meanwhile, the cytoplasm continues to mature normally, and erythroblasts continuously accumulate hemoglobin. Because the cell cannot divide on time, it grows pathologically large.
Bone marrow and biochemical changes
Pathological precursor cells—megaloblasts—appear in the bone marrow, along with a large number of megalocytes. A picture of ineffective erythropoiesis develops:
- Degeneration of megaloblast nuclei is observed.
- Normal maturation of myeloid cells is impaired.
Since megaloblasts and the erythrocytes formed from them are defective, they undergo destruction (hemolysis). In biochemical blood analysis, this is reflected by the development of bilirubinemia.
Peripheral blood: quantitative and morphological shifts
A general blood count reveals marked erythropenia, which often progresses to pancytopenia (decrease in all blood elements). Megaloblasts and megalocytes are detected in the blood smear.
Erythrocyte morphology undergoes major changes:
- Anisocytosis: macrocytes and giant megalocytes predominate in the blood.
- Poikilocytosis: cells lose their normal shape.
- Hyperchromia: erythrocytes are oversaturated with hemoglobin, and a high color index is noted.
- Polychromatophilia: erythrocytes acquire the ability to be stained simultaneously by both acidic and basic dyes.
- Basophilic stippling of erythrocytes.
- Presence of nuclear remnants inside erythrocytes: Howell-Jolly bodies and Cabot rings are detected.
Briefly: answers to topical questions
What neurological disorders are specific to vitamin B12 deficiency anemia?
A specific neurological disorder in vitamin B12 deficiency anemia is funicular myelosis. The disease is characterized by the development of pathological neuropsychiatric symptoms.
The pathogenesis of nervous system damage includes the following processes:
- Accumulation of methylmalonic acid (a metabolite that is toxic to nerve cells).
- Decreased myelin content in nerve fibers.
How does the reticulocyte count in peripheral blood change in megaloblastic anemias?
Absolute reticulocytopenia is observed in the general clinical peripheral blood analysis in vitamin B12 deficiency anemia. Despite the overall decrease in their count, an increase in the immature reticulocyte fraction is often noted.
In which part of the intestine does the absorption of the vitamin B12-intrinsic factor complex take place?
Absorption of the stable vitamin B12 and intrinsic factor (Castle's factor) complex occurs in the small intestine, predominantly in the ileum.
It is in this section that cubilin—a specific receptor protein for intrinsic factor—is localized. During absorption through the intestinal wall, the formed complex breaks down, after which vitamin B12 enters the bloodstream.
To easily remember the mechanism of megaloblast formation, imagine a "greedy cell that forgot how to divide." It continues to actively accumulate hemoglobin (protein synthesis is normal), but due to a DNA breakdown, it cannot divide in two. As a result, the cell simply balloons to giant proportions.
Class and exam questions
Why does vitamin B12 deficiency often lead to hypoacid gastritis?
In most cases, the deficiency is autoimmune in nature. The body produces antibodies against the proton pump proteins of gastric parietal cells, leading to hypoacid gastritis.
What is the essence of nuclear-cytoplasmic dyssynchrony?
This is a pathological condition in which, due to impaired DNA replication, the cell nucleus matures extremely slowly, while the cytoplasm continues to develop normally and accumulate hemoglobin.
What is the cause of bilirubinemia in megaloblastic anemias?
The increase in blood bilirubin levels is caused by active hemolysis (destruction) of defective megaloblasts in the bone marrow and pathologically altered erythrocytes in the bloodstream.
What specific inclusions can be found in erythrocytes in this pathology?
Erythrocytes with nuclear material remnants—Howell-Jolly bodies and Cabot rings—as well as cells with basophilic stippling, are found in the peripheral blood smear.
What else to study on this topic
- Mechanisms of malabsorption syndrome development in gastrointestinal diseases.
- The role of Castle's intrinsic factor in vitamin B12 absorption.
- Genetic defects affecting folate transport and metabolism.
- Features of megaloblast nuclear degeneration against the background of ineffective erythropoiesis.
- Pathogenesis of impaired myeloid cell maturation and pancytopenia development.
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