Bone marrow consists of various haemopoietic cells and their precursors, reticular cells and reticular fibers, endothelial lined sinusoids and adipocytes. Yellow marrow is limited to three types of cells; reticular cells, connected with endosteum and blood capillaries, endothelial cells of capillary walls and sinusoids and adipocytes
Adipocytes occupy space as haemopoiesis recedes and give up space as the demand for expansion of red marrow occurs in response to continuous blood loss or haemolytic anemia. Transition from yellow to red marrow takes place in response to the hormone erythropoietin
There exist subsets of the pluripotential stem cells and committed progenitor cells of various cell lineages
Myeloid and lymphoid cells originate separately but from a common progenitor cell
Erythrocytes, all leukocytes, macrophages, mast cells and megakaryocytes originate from pluripotential stem cell, pluripotential hemopoietic stem cell, capable of differentiation into myeloid stem cell and lymphoid stem cell.
2. Colony forming unit-erythroid (CFU-E). BFU-E is progenitor of CFU-E
BFU-E produces large erythroid colonies in large amounts of erythropoietin or humoral factor called burst promoting activity (BPA). CFU-E forms small colonies in 48 hours in a small amount of erythropoietin
committed progenitor cells of the erythrocyte, granulocyte-macrophage, eosinophil, and megakaryocyte are regulated by specific stimulations, which are glycoproteins containing neuraminic acid. Erythroid progenitor cells are stimulated by erythropoietin. Differentiation of pluripotential stem cell into BFU-E is enhanced by BPA and T-cell product’s IL-I, IL-3
Soluble substances of macrophages, probably IL-12, stimulate erythrocyte production by promoting growth in early (BFU-E) and late (CFU-E) committed erythroid progenitors and through cell to cell contacts
A lymphocyte-like primitive undifferentiated stem cell exists in bone marrow, pluripotential stem cell ( CFU-S ), gives rise to unipotential cells that commit to erythrocytic, granulocytic, monocytic or megakaryocytic series
Erythropoiesis begins when pluripotential stem cells differentiate into two erythroid progenitor cells, burst forming unit-erythroid ( BFU-E ) and its progeny, colony forming unit –erythroid ( CFU-E ), which under erythropoietin (Ep) gives rise to erythroid precursors, rubriblasts
Rubriblasts undergo 4-5 mitotic divisions to produce mature erythrocytes
These divisions occur at each stage of rubriblast, prorubricyte, basophilic rubricyte and polychromatic rubricyte
Metarubricytes do not divide
Sequential maturation involves loss of nucleoli and mitotic capacity, progressive decrease in size and cytoplasmic basophilia and increase in nuclear chromatin condensation, chromasia leading to normochromasia with hemoglobin synthesis
Fig: An erythrobastic island having corona of erythroid cells, encircling a reticulum cell (nurse cell) where hemosiderin occurs in its cytoplasm. Note nucleoli of early erythroid series and nucleus of reticulum cell (NRC).
Cytoplasm of reticulum cell contains hemosiderin granules and interdigitates between erythroid cells, thus nourishing them (nurse cell) providing ferritin for use in heme synthesis and may be a stimulant for further rubriblasts
Maturing erythroid cell move away from reticulum cell to become reticulocytes. Reticulum cells also are phagocytic, engulfing free nuclei of metarubricytes and aged red cells
Largest haematopoietic cells (4.7x10 -13 l or 4,700 fl) volume, 10-65 μm diameter
Megakaryocytes classified into 3 types, stages or groups based cytoplasm and nucleus; megakaryoblast (Stage I or Group I) cell, promegakaryocyte (Stage II or Group II) cell and megakaryocyte (Stage III) cell. Megakaryocytes are categorized as granular (Stage III) and mature or productive (Stage IV).
Fig. 2: Human bone marrow: Red bone marrow shows great cellularity, developing erythrocytic and myelocytic cells and large round clear fat cells. A megakaryocyte (a), larger venous sinuses (d), bone (b), periosteum (c). HE x26
Fig. 3: Rabbit bone marrow: Red bone marrow showing great cellularity, developing erythrocytic and myelocytic cells, sinusoids (b), large clear fat cells. Megakaryocytes are largest (except for fat cells) in bone marrow with abundant cytoplasm and multilobulated nuclei (a). HE x 52
Fig. 4: Rabbit bone marrow: R abbit injected intraperitoneal with lithium carmine (red). Reticular cells around sinuses are phagocytic, have actively ingested red dye. Phagocytized dye in their cytoplasm follows elongated cytoplasmic processes which form sinusoidal lining (arrows). HE x 100