Histology Module (201) Hemopoiesis PDF

HISTOLOGY Module (201) Hemopoiesis  (Hemopoiesis) It is the development of blood cells occurring in: ** Myeloid tissue (Bone marrow) ** Lymphoid tissue (Thymus gland) Myeloid tissue (Bone marrow) Types: 1- Red bone marrow (active) 2- Yellow bone marrow (inactive) ...

HISTOLOGY Module (201) Hemopoiesis  (Hemopoiesis) It is the development of blood cells occurring in: ** Myeloid tissue (Bone marrow) ** Lymphoid tissue (Thymus gland) Myeloid tissue (Bone marrow) Types: 1- Red bone marrow (active) 2- Yellow bone marrow (inactive)  Active (red) bone marrow Sites: In foetus: in most of the bone. In adults: for example flat bones & vertebrae. Structure: A- Stroma: reticular fibers containing: C.T cells: Reticular cells, fibroblasts and fat cells (the largest cells in the bone marrow) Pericytes, bone cells, endothelial cells and Matrix: formed of collagen, glycoproteins (hemonectin) B- Blood sinusoids: Irregular wide blood channels. Lined with endothelial cells with no basement membrane (replaced by reticular fibers) Surrounded by macrophages which extend their pseudopodia into the lumen of the sinusoids to phagocytose foreign bodies. C- Free cells: These are blood cells in various degree of development. Function of red bone marrow: Formation of blood cells. Destruction of old RBCs by the macrophages. Storage of fat in the fat cells.  Inactive (yellow) bone marrow Sites: shaft of long bone Structure: Stroma Large number of fat cells No free cells Function: Storage of fat. On need (haemorrhage, haemolysis), it is changed into active red marrow.  Erythropoiesis (Development of RBCs) Steps: 1- Undifferentiated Mesenchymal Cells (U.M.C): Shape: branches, stellate shape. Nucleus: oval, pale Cytoplasm: pale basophilic 2- Colony Forming Unit (C.F.U): Origin: U.M.C Shape: rounded Size: 8-10 µm Nucleus: rounded, pale Cytoplasm: as UMC 3- Proerythroblasts: Size: 13-15 µm. Nucleus: rounded, pale and has two nucleoli. Cytoplasm: basophilic. 4- Basophilic erythroblasts: Size: 11-13 µm Nucleus: smaller and darkly stained. Cytoplasm: more basophilic due to increased ribosomes. 5- Polychromatophilic erythroblasts: Size: 9-11 µm.. Cytoplasm shows: Acidophilia due to HB formation Basophilia due to some ribosomes. 6- Normoblasts: Size: 8-10 µm Nucleus: pyknotic then lost. Cytoplasm: acidophilia sue to more HB. 7- Reticulocytes: Size: slightly larger than RBCs. Nucleus: lost Cytoplasm: pale acidophilic, contains remnants of polyribosomes in the form of reticulum in the center of the cell.  May appear in the blood (not more than 2%) Increased number of reticulocytes in the blood occurs in haemolytic anemia, haemorrhage. 8- Mature erythrocyte: They enter the blood sinusoids  Development of granular leucocytes (Neutrophils, Eosinophils and Basophils) Site: in the red bone marrow Steps: 1- U.M.C 2- C.F.U 3- Myeloblasts: Size: slightly larger than CFU. Nucleus: large, rounded and pale. Cytoplasm: basophilic, non granular 4- Promyelocytes: Size: the largest cell in the granular leucocytes series. Nucleus: large, pale and prominent nucleolus. Cytoplasm: basophilic and contain fine non specific (azurophilic) granules. 5- Myelocytes: Size: become smaller Nucleus: become indented Cytoplasm: appearance of specific granules which differentiate the myelocytes. 6- Metamyelocytes: Size: smaller Nucleus; smaller, dark and kidney shaped. Cytoplasm: contains more specific granules. 7- Mature granulocytes  Development of platelets Site: in the red bone marrow Steps: 1- U.M.C 2- C.F.U 3- Megakaryoblasts: Size: large Nucleus: indented with clear nucleoli. Cytoplasm: basophilic. 4- Promegakaryocyte: Size: larger Nucleus: large, lobulated 5- Megakaryocyte: Size: very large (40 µm) Nucleus: large, lobulated and dark stained. 6- Platelets formation: ***by one of the following mechanisms: A- Numerous invaginations of the cell membrane extend through the cytoplasm dividing the cytoplasm into areas that will be shed as platelets. B- Megakaryocytes can extend pseudopodia into the blood sinusoids then, the platelets are separated from the tips of these pseudopodia.