HISTOLOGY 1.8.: HEMATOPOIESISHISTOLOGY 1.8.: HEMATOPOIESIS

I.I. Prenatal:Prenatal:

• Blood cell generation begins in the wall of the yolk sac during Blood cell generation begins in the wall of the yolk sac during intrauterine life: primitive erythroblasts derive from mesenchyme.intrauterine life: primitive erythroblasts derive from mesenchyme.

- Later the hematopoietic cells migrate to the liver.Later the hematopoietic cells migrate to the liver.

- Then the bone marrow, spleen, lymph nodes and thymus of theThen the bone marrow, spleen, lymph nodes and thymus of theembryo are seeded with hematopoietic stem cells from the liverembryo are seeded with hematopoietic stem cells from the liverand become engaged with hematopoiesis.and become engaged with hematopoiesis.

II. Postnatal hematopoiesisII. Postnatal hematopoiesis

The major site of hematopoiesis is the bone marrow, it also persistsThe major site of hematopoiesis is the bone marrow, it also persistsin the liver and spleen for a few weeks after birth (potential is retained).in the liver and spleen for a few weeks after birth (potential is retained).

Early in life all the marrow is active, with age the demand for Early in life all the marrow is active, with age the demand for blood cells decreases: red marrow is replaced by resting yellow blood cells decreases: red marrow is replaced by resting yellow marrow.marrow.

Red bone marrow is present in the adult animal:Red bone marrow is present in the adult animal:sternumsternumvertebraevertebraeribsribsskullskullpelvispelvisepiphyses of long bonesepiphyses of long bones

Structure of the bone marrowStructure of the bone marrowThe hematopoietic compartment consists of irregular The hematopoietic compartment consists of irregular anastomosing cords that lie between vascular sinuses.anastomosing cords that lie between vascular sinuses.The marrow lacks lymphatic vesselsThe marrow lacks lymphatic vesselsInnervation: vasomotor nerves around the blood vesselsInnervation: vasomotor nerves around the blood vessels

Bone marrow in sternum

Hematopoietic compartment

Bone marrow smear

Blood cells are produced in the hematopoietic compartment and Blood cells are produced in the hematopoietic compartment and reach the bloodstream by crossing the wall of vascular sinuses.reach the bloodstream by crossing the wall of vascular sinuses.

Adventitial reticular cells:Adventitial reticular cells: meshwork that support hematopoietic cells and provide special meshwork that support hematopoietic cells and provide special microenvironment that influence the development of the various microenvironment that influence the development of the various stem cells. stem cells. They may fill with fat and transform into adipose cells when They may fill with fat and transform into adipose cells when hematopoiesis decreases.hematopoiesis decreases.

Marrow hematopoiesis:Marrow hematopoiesis: is most active in areas close to the bone. is most active in areas close to the bone.

Erythropoietic cells and megakaryocytes are close to vascular Erythropoietic cells and megakaryocytes are close to vascular sinuses.sinuses.Granulopoietic cells are deep within the cords, away from the vascular Granulopoietic cells are deep within the cords, away from the vascular sinuses.sinuses.

Hematopoietic stem cells:Hematopoietic stem cells:

Undifferentiated cells are generally larger than mature cells, Undifferentiated cells are generally larger than mature cells, they have large euchromatic nuclei (large nuclear-to-cytoplasmic ratio).they have large euchromatic nuclei (large nuclear-to-cytoplasmic ratio).The marrow contains several types of self-replicating stem cells. The marrow contains several types of self-replicating stem cells. Their morphology resembles that of lymphocytes, but their proliferating Their morphology resembles that of lymphocytes, but their proliferating capacity differs: capacity differs: pluripotentpluripotent

multipotentmultipotent stem cellsstem cellsunipotentunipotent

Embryonic yolk sac, fetal liver,spleen, bone marrow: Embryonic yolk sac, fetal liver,spleen, bone marrow: primitive stem cells (pluripotent)primitive stem cells (pluripotent)

Lymphoid stem cellsLymphoid stem cells myeloid stem cells (multipotent)myeloid stem cells (multipotent)

LymphocytesLymphocytes erythrocyticerythrocytic granulocyticgranulocytic monocyticmonocytic megakaryocyticmegakaryocytic(They are all unipotent cell types)(They are all unipotent cell types)

Hematopoietic and stromal stem cell differentiationHematopoietic and stromal stem cell differentiation

(Rubriblast)

(Prorubricyte)

(Rubricyte)

(Metarubricyte)

CFU-E (Colony-forming unit-erythrocyte)CFU-E (Colony-forming unit-erythrocyte)

16-22 m

14-18 m

12-15 m

10-12 m

Erythron:Erythron: mass of circulating erythrocytes + marrow erythropoietic tissue mass of circulating erythrocytes + marrow erythropoietic tissue

Efficacy of erythropoiesis in a dog: 1 million/secondEfficacy of erythropoiesis in a dog: 1 million/second

Erythroblastic islands:Erythroblastic islands: group of erythrocytic cells within the bone marrow group of erythrocytic cells within the bone marrow organized around a macrophage (more mature ones at the periphery).organized around a macrophage (more mature ones at the periphery).

1. Rubriblast (arrow)1. Rubriblast (arrow)largest, with deep blue cytoplasmlargest, with deep blue cytoplasmand round euchromatic nucleusand round euchromatic nucleus

2. Prorubricyte (arrowhead)2. Prorubricyte (arrowhead)Similar, but smaller, no nucleoliSimilar, but smaller, no nucleoli

4. Metarubricyte: the smallest nucleated erythrocyte withpicnotic nucleus

3. Basophilic rubricyte:3. Basophilic rubricyte: nucleus with nucleus with clumped chromatinclumped chromatin

4. 3.

Erythrocyte kinetics:Erythrocyte kinetics:Development of rubriblasts to mature erythrocytes: 5-7 daysDevelopment of rubriblasts to mature erythrocytes: 5-7 daysReticulocytes: normally 1-2 days maturation within the bone marrowReticulocytes: normally 1-2 days maturation within the bone marrowRegulation of erythrocyte formation: cellular and humoral factorsRegulation of erythrocyte formation: cellular and humoral factors

erythropoietin: key-molecule produced in the kidneyerythropoietin: key-molecule produced in the kidney

ReticulocyteReticulocyte

Mature mammalian erythrocytesMature mammalian erythrocytes

GranulopoiesisGranulopoiesisOccurs in clusters away from the vascular sinuses in the midportion Occurs in clusters away from the vascular sinuses in the midportion

of the hematopoietic compartment of the marrowof the hematopoietic compartment of the marrow

CFU-GM: colony-forming units-granulocyte-monocyte (bipotent)CFU-GM: colony-forming units-granulocyte-monocyte (bipotent)

Neutrophil and /or monocyte progenitor cellsNeutrophil and /or monocyte progenitor cells

15-18 m

18-22 m

10-15 m

Myeloblast:Myeloblast: ovoid or spherical cell, spherical euchromatic nucleus,ovoid or spherical cell, spherical euchromatic nucleus,light blue cytoplasm.light blue cytoplasm.

Promyelocyte:Promyelocyte: larger, similar nucleus,more cytoplasm, with azurophiliclarger, similar nucleus,more cytoplasm, with azurophilicgranules.granules.

Myelocyte:Myelocyte: spherical to slightly indented nucleus, some chromatin spherical to slightly indented nucleus, some chromatin condensation, specific and azurophilic granules. condensation, specific and azurophilic granules. Specific granules indicate the type of granulocyte to beSpecific granules indicate the type of granulocyte to bedeveloped.developed.

Metamyelocyte:Metamyelocyte: indented, kidney-shaped heterochromatic nucleus, indented, kidney-shaped heterochromatic nucleus, specific granules have their characteristic colours.specific granules have their characteristic colours.

Band-form:Band-form: further nuclear indentation, C,-S,-V-shaped nuclei.further nuclear indentation, C,-S,-V-shaped nuclei.

Mature forms:Mature forms: marked segmented nuclei and specific granulesmarked segmented nuclei and specific granules

Granulocyte kinetics:Granulocyte kinetics:

Compartments of granulocytic cells:Compartments of granulocytic cells:proliferative proliferative (mitotic) with myeloblasts,(mitotic) with myeloblasts,promyelocytes, myelocytespromyelocytes, myelocytesmaturative maturative (postmitotic) with metamyelocytes,(postmitotic) with metamyelocytes,band neutrophilsband neutrophilsreserve reserve (storage) with mature neutrophils(storage) with mature neutrophils

Production time for granulocytes: 5-7 daysProduction time for granulocytes: 5-7 days

Compartments of blood neutrophils: circulating (6-14 hours)Compartments of blood neutrophils: circulating (6-14 hours) marginatingmarginating

Total neutrophil pool in the blood is replaced at least twice a day.Total neutrophil pool in the blood is replaced at least twice a day.

The production and kinetics of eosinophils, basophils and monocytesThe production and kinetics of eosinophils, basophils and monocytesare similar to those of neutrophils.are similar to those of neutrophils.

Formation of monocytes: monocytopoesisFormation of monocytes: monocytopoesis

CFU-M: monocyte progenitor (common ancestor with neutrophils)CFU-M: monocyte progenitor (common ancestor with neutrophils)

MonoblastMonoblast PromonocytePromonocyte MonocyteMonocyte

Azurophilic granulesAzurophilic granules Final shape and size of nucleusFinal shape and size of nucleus

Monocytes reside in the blood stream for 24 hours, then enter Monocytes reside in the blood stream for 24 hours, then enter the connective tissue as CT macrophages.the connective tissue as CT macrophages.

Their development is still not fully understood.Their development is still not fully understood.

Thrombopoiesis:Thrombopoiesis:

CFU-MK:CFU-MK: colony-forming unit-megakaryocyte: large spherical colony-forming unit-megakaryocyte: large spherical euchromatic nucleuseuchromatic nucleus

Megakaryoblast:Megakaryoblast: undergoes endomitosis, thus, only the nucleus divides undergoes endomitosis, thus, only the nucleus dividesPromegakaryocyte:Promegakaryocyte: multilobed nucleus, lot of cytoplasm multilobed nucleus, lot of cytoplasmMegakaryocyte:Megakaryocyte: largest hematopoietic cell in the marrow (40-100 largest hematopoietic cell in the marrow (40-100 m)m)

Platelet kinetics:Platelet kinetics:

Platelets are produced by fragmentation of the megakaryocyticPlatelets are produced by fragmentation of the megakaryocyticcytoplasm along demarcation membranes.cytoplasm along demarcation membranes.

Place of megakaryocytes in the bone marrow:Place of megakaryocytes in the bone marrow: against the sinus wall. against the sinus wall.They shed their platelets directly into the bloodstream, or extend theirThey shed their platelets directly into the bloodstream, or extend theircytoplasmic projections through the endothelium into the sinus lumen.cytoplasmic projections through the endothelium into the sinus lumen.

Life span of platelets:Life span of platelets: 9-12 days in domestic species. 9-12 days in domestic species.

Place of storage:Place of storage: spleen spleen

Regulation of platelet formation:Regulation of platelet formation: local regulatory factors and local regulatory factors and thrombopoietin produced in the kidney.thrombopoietin produced in the kidney.

LymphopoiesisLymphopoiesis

Lymphoid stem cells: pre-B-lymphocyte B-lymphoblast B-lymphocytepre-T-lymphocyte T-lymphoblast T-lymphocyte

Stem cells derive from the bone marrow.Maturation process for B-lymphocytes: bone marrow, GALT

(bursa-equivalent organs)Maturation process for T-lymphocytes: thymus

Thank you for your attention!Thank you for your attention!