Red marrow (Myeloid tissue) is composed of stroma of reticular cells and fibres with free cells located in the meshwork. The stroma also shows varying number of fat cells. Majority of free cells in bone marrow are the developmental stages of the granulocytes and a few lymphocytes, monocytes and plasma cells. Sinusoids are numerous and form anastomosing networks.
Haemocytoblasts
- They are large cells up to 15 microns in size. The nucleus is large and rounded and the cytoplasm is deeply basophilic. They constitute normally 0.3 to 0.5 percent of the cells of marrow.
ErythroblastsThese are the precursors of erythrocytes and undergo a series of divisions and transformation before the erythrocytes is formed. The important stages in erythrocyte formation are;
- Proerythroblasts or Basophilic erythroblast
- These are large round cells with spherical nucleus containing angular particles or chromatin and the cytoplasm is homogenous and is intensely basophilic.
- Polychromatophilic erythroblasts
- These arise by mitotic division of proerythroblasts. There is an increase in the haemoglobin in successive stages so that erythroblasts at successive stages show less of basophilia and more of acidophilia. Hence these cells are called poly-chromatophilic erythroblasts. Chromatin becomes more regularly arranged. The cells are smaller than proerythroblasts and larger than normoblasts.
- Normoblasts
- These are derived from polychromatophilic erythroblasts after a number of divisions. The cytoplasm has more haemoglobin and stains like erythrocytes. The cells are slightly larger than erythrocytes. Their nucleus is smaller and dense in chromatin and stain intensely. The normoblasts extrude their nuclei to become erythrocytes. The youngest erythrocyte shows a delicate reticulum and so it is called reticulocyte. The reticulum disappears before reticulocytes leave the marrow.
- Erythrocyte
Myelocytes
- These differentiate from myeloblasts. These constitute 12 percent of the marrow cells. These cells show an increase in the granules and increase in the density of chromatin of the nuleus. The earliest myelocytes are known as Promyelocytes. They are large cells with rounded or oval nucleus and basophilic cytoplasm. The cytoplasm contains a few granules of one of the specific types (neutrophilic or acidophilic) but because of the few granules promyelocytes are not classified into three groups. The promyelocytes proliferate and differentiate into myelocytes proper. The cells become smaller in size show an increase in the number of granules, decrease in the basophilia of cytoplasm and increase in the density of the chromatin of the nucleus.
- Neutrophilic, esoinophilic and basophilic myelocytes are distinguished. The myelocytes after a number of generations, differentiate into metamyelocytes. They have an indented or kidney shaped nucleus and the cytoplasm showing numerous specific granules of each type. The metamyelocytes differentiate into mature leucocytes, which enter the blood stream.
Megakaryocyte
- These are also called as giant cells and are large (30-100 microns) rounded cells but the cytoplasm may show irregular processes. The nucleus consists of many lobes connected by short stalks. Processes of cytoplasm of their cell extend through the walls of sinusoids and by constriction and segmentation.
