Erythrophagocytosis of the red pulp is the removal of red blood cells from the blood by macrophages within the tissue. The red pulp is primarily filled with venous sinuses containing red blood cells and occasionally white blood cells. These sinuses are lined with elongated, rod-shaped endothelial cells. The cells have slits on their sides, which allow red blood cells to enter and exit the sinuses. They drain into the splenic vein. The red pulp also contains parenchyma, lymphocytes, macrophages, and plasma cells.
Blood entering the spleen is distributed through trabecular arteries, which branch throughout the parenchyma. The central arteriole is surrounded by lymphoid cells and may drain directly into the venous sinuses or the red pulp cord. This tissue is home to macrophages and other immune cells that phagocytose blood cells and other cellular debris. This process is initiated when blood cells are in contact with resident spleen cells and specialized macrophages.
The EPC rate of ht mice is much higher than that of control mice. The EPC rate is higher in the red pulp than in the peripheral tissues. Moreover, the level of erythrophagocytosis in the peripheral tissues of the mice is higher.
Hemosiderin deposits in macrophages
Hemosiderin deposits are a characteristic pigment of red pulp macrophages containing iron. Hemosiderin is derived from the hemoglobin of phagocytized erythrocytes, which are also found in the spleen. Hemosiderin is more prominent in females than males. Other pigments found in macrophages include lipofuscin and ceroid.
Hemosiderin is a product of heme degradation. Macrophages engulf the heme and remove it from tissues. This process is prolonged in severe trauma, where erythrocytes are broken down for a long time. In such cases, hemosiderin-laden macrophages are present in the blood, leading to chronic bleeding.
The central artery is shown in cross-section on many atlases and texts. It is surrounded by the PALS, a sheath that runs the length of the main street. In contrast, the white pulp, or marginal zone, is less well-preserved, so detecting it in well-preserved sections isn’t easy. The marginal site contains reticular cells, antigen-presenting cells, and lymphocytes that may play a role in antigen-trapping.
In black mice, the spleen contains melanocytes with melanin. These cells are usually located in the red pulp and trabeculae.
The histology of Hodgkin’s disease and red-pulp cord is quite similar. The lines contain a mixture of white and red pulp with red pulp involving sinusoids. In the threads, residual lymphoid follicles are present. The cords may also exhibit dilated sinuses.
Patients with LPL have a more extensive disease characterized by greater IgM proteinemia and hyperviscosity. In addition, the spleen is secondarily involved with mild enlargement. LPL is associated with an MYD88 L265P somatic point mutation found in nearly 90% of patients. This somatic mutation has potential utility in the workup of splenic B-cell lymphomas.
While there is no consensus on phenotypic criteria for SMZL, it is considered the most frequent lymphoma of the spleen. The morphological characteristics of SMZL include an enlargement of the white pulp and a biphasic or monophasic marginal zone pattern. The presence of red pulp cells is variable and may show a nodular appearance.
In addition, there are specific other histological findings that are not related to Hodgkin’s disease. Some lymphomas, such as lymphoplasmacytic leukemia, exhibit predominantly red pulp histology. While the World Health Organization (WHO) classifies red pulp cord lymphoma as an unclassifiable disorder, prior history of lymphoma and specific sites of involvement may help narrow the differential diagnosis. A peripheral blood smear and blood flow cytometry can help determine the exact diagnosis.
SCID mice are homozygous for the Prkdcscid mutation in DNA-PKcs, a gene required to mature B and T-cells. This mutation results in decreased numbers of mature B and T-cells, essential for developing a functional immune system. SCID mice also have reduced spleen sizes compared to wild-type mice. Their white pulp pulps contain few lymphocytes and are dominated by macrophages.
The SCID-hu mice display a limited xenogeneic immune response to HIV infection. These mice develop fewer human CCR5-carrying cells. In addition, human PBL-SCID mice have a limited repertoire of fetal tissues transplantable to human donors. These mice exhibit restricted HIV pathology in SCID-hu mice, whereas hu-PBL-SCID mice develop only small amounts of human CCR5-carrying cells in their fetal tissues.
CXCR4 is required for the normal distribution of plasma cells near blood vessels. CXCL12 is not restricted to a splenic red pulp, as it is expressed in lymph node medullary cords and splenic red pulp. However, it does regulate the location of plasma cells and the function of the bone marrow.
The CXCR4-/ chimeras have reduced levels of IgG-secreting cells and have reduced numbers of IgD-secreting cells. The red pulp also harbors a large population of macrophages, which phagocytose age-related red blood cells. This process is critical for the turnover of red blood cells and the recycling of iron.
Red pulp cord macrophages are a highly diverse population mainly present in the spleen. These immune cells have multiple functions, including phagocytosis of injured or senescent erythrocytes and other blood-borne particles. They are generally maintained throughout a person’s life and are independent of circulating monocytes and bone marrow-derived hematopoietic cells.
The red pulp comprises two central tissues: reticular tissue and sinusoids. These two tissues are linked together by a network of stromal cells. The reticular tissue contains numerous macrophages, lymphocytes, and polymorphs. It also includes a large number of platelets. The reticular tissue also has groups of plasma cells located along the trabeculae and marginal zone.
The function of tissue-resident macrophages needs to be better understood. Although tissue-resident macrophages display similar gene expression changes in response to injury, monocyte-derived macrophages show disproportionately more significant changes in physiologic measures. They play a central role in the integrity of the submucosa.
Currently, research is underway to understand the functions of macrophages better. Single-cell transcriptomics, spatial transcriptomics, and advanced lineage tracing studies characterize tissue-resident macrophage populations. These studies have the potential to inform treatment decisions by identifying specific phenotypes.
In addition to their inflammatory role, macrophages also play an essential role in tissue repair and regeneration. Their differentiation is governed by factors present in specific tissue microenvironments. These factors are often unique to cells or molecules expressed during injury. These factors convert macrophages from an inflammatory to a resolution phenotype. The transformation occurs continuously during an injury.