General characteristics and morphology of hematins
The formation of hematins is based on the chemical process of oxyhemoglobin hydrolysis. As a result of this transformation, pigments possessing a number of unique physicochemical and morphological properties are formed.
Upon microscopic examination, hematins are visualized as characteristic structures. They can take the form of rhomboid crystals or appear as clusters of small granules. The color palette of these formations varies from rich dark brown to deep black.
The most important diagnostic criterion for hematins is their behavior in polarized light. These pigments exhibit the property of anisotropy—they produce distinct birefringence. Furthermore, a fundamental property of hematins is that the iron in their structure is strictly in a bound state.
Classification of hematins
In pathomorphology, when studying hemoglobinogenic pigments, it is customary to distinguish three main forms of hematins:
- Hemomelanin (also known as malarial pigment or hemozoin).
- Hydrochloric acid hematin (often referred to in literature as hemin).
- Formalin pigment.
Etiology and pathogenesis of malarial pigment
Hemomelanin has an infectious etiology. Its appearance is inextricably linked with the life cycle of the malarial parasite belonging to the genus Plasmodium.
The pathogenesis of the formation of this pigment represents a strict sequence of events:
- At the first stage, the merozoites of the parasite actively penetrate inside erythrocytes.
- Once inside, the parasite hydrolyzes hemoglobin.
- During this process, a specific amorphous brown pigment is formed from the prosthetic group of the hemoglobin molecule.
- As the pigment accumulates, the affected erythrocytes are destroyed, resulting in the release of the pigment into the bloodstream.
- At the final stage, the circulating pigment undergoes phagocytosis by macrophages.
Localization and macroscopic picture
Following phagocytosis by macrophages, systemic distribution of hemomelanin occurs in the organs of the reticuloendothelial system (RES). The main targets for deposition are the spleen, liver, bone marrow, and lymph nodes.
Of particular note is the localization of the pigment in the central nervous system. During the development of cerebral malaria, hemomelanin is found directly within the glial cells of the brain.
The macroscopic picture of the affected organs is quite specific. Due to the massive accumulation of pigment, the tissues lose their natural color and acquire a pronounced slate-gray hue. It should also be noted that alongside the malarial pigment, concomitant deposition of other pigments is observed in the tissues.
Briefly: answers to topical questions
In what pathological processes in the body is hydrochloric acid hematin formed?
Hydrochloric acid hematin (hemin) is formed in the stomach during gastrointestinal bleeding as a result of the interaction of hemoglobin with enzymes and hydrochloric acid. The main pathological processes leading to its formation include:
- Esophageal variceal bleeding—acting as a marker of portal hypertension.
- Mallory-Weiss syndrome—tears of the mucosa at the gastroesophageal junction.
- Tumors of the stomach and duodenum—in the stage of decay or with vessel arrosion.
- Gastric erosions and ulcers—the pigment stains their base brown.
At a moderate bleeding rate, the pigment gives coffee-ground emesis its characteristic appearance.
Under what fixation conditions is formalin pigment formed?
Formalin pigment is formed as an artifact during tissue fixation in acidic formalin. This pigment is a derivative of hematin and macroscopically appears as dark brown needles or brownish granules.
To prevent its formation, fixation conditions must be strictly observed: this pigment does not form if the pH of the formalin solution used exceeds 6.0.
What concomitant pigments are deposited in tissues along with the malarial pigment?
Along with the malarial pigment (hemomelanin), hemosiderin is deposited in the tissues of the reticuloendothelial system organs. Upon erythrocyte destruction, hemoglobin fragments are engulfed by macrophages, where hemosiderin formation takes place.
The combined accumulation of these pigments leads to the development of hemomelanosis and hemosiderosis, imparting a dark gray or nearly black color to the affected organs (liver, spleen, bone marrow). Microscopically, the liver reveals deposition of hemomelanin and hemosiderin in hyperplastic stellate reticuloendothelial cells (Kupffer cells).
For quick memorization of the classification of hematins, use the acronym HSF: Hemomelanin, Saline (hydrochloric acid) hematin, Formalin pigment.
Class and exam questions
What is the nature of hematins and how are they formed?
Hematins are iron-containing pigments. Their formation occurs as a result of the hydrolysis of oxyhemoglobin.
How can hematins be identified in polarized light?
The main optical sign is their anisotropy. Dark brown or black crystals of hematins exhibit double refraction.
What is the mechanism of formation of malarial pigment?
Merozoites of Plasmodium penetrate erythrocytes and cause hemoglobin hydrolysis. An amorphous brown pigment is formed from its prosthetic group, which, after erythrocyte destruction, enters the bloodstream and is phagocytosed by macrophages.
Which organs change color in malaria and why?
RES organs are affected (spleen, liver, bone marrow, lymph nodes). Due to the accumulation of hemomelanin, they acquire a specific slate-gray color.
What else to study on this topic
- Hydrochloric acid hematin (hemin): structural features and properties.
- Formalin pigment: conditions of formation in tissues.
- Study of other pigments whose deposition is observed concomitantly with malarial pigment.
- Pathomorphological changes in glial cells of the brain during cerebral malaria.
- The role of macrophages in the phagocytosis of hemoglobinogenic pigments.
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