Hematopoiesis in the Embryo

Hematopoiesis in the Embryo

I. Hematopoiesis in the Yolk Sac

– The first hematopoiesis in humans occurs in the yolk sac wall, beginning in the third week of gestation.

– The first blood cells form from Wolff-Pander islands, which are formed when mesenchymal cells lose their branches and congregate into clumps.

– The peripheral cells of the blood islands differentiate into flat cells, while the remaining part differentiates into primitive blood cells.

– Primitive blood cells are large, basophilic, round-shaped with a nucleus.

– Primitive blood cells divide and differentiate into primitive erythroblasts.

– Primitive erythroblasts later differentiate into megaloblasts and secondary erythrocytes.

– Megaloblasts become polychromatophilic erythroblasts when hemoglobin appears in the cytoplasm.

– The developmental sequence of megaloblasts is: megaloblast ? polychromatophilic erythroblast ? acidophilic erythroblast.

– The presence of both nucleated and non-nucleated erythrocytes is due to some types of erythroblasts undergoing nuclear degeneration and releasing them outside.

– Hematopoiesis in the yolk sac wall produces primary erythrocytes and secondary erythrocytes in the blood vessels, and neutrophils and eosinophils outside the blood vessels.

II. Hematopoiesis in the Liver

– The liver begins to form in the 3rd-4th week of gestation and becomes the center of hematopoiesis from the 5th week.

– Hematopoietic stem cells migrate from the yolk sac to the liver and proliferate rapidly, forming primary erythrocytes.

– In addition to hematopoiesis, the liver also produces eosinophils, megakaryocytes, and neutrophils.

– Hematopoiesis in the liver occurs outside the blood vessels, along the capillaries in the hepatic lobules.

III. Hematopoiesis in the Spleen

– The spleen begins to form at the end of the 4th week of gestation.

– Hematopoietic stem cells from the yolk sac or liver migrate to the spleen, making it a pluripotent hematopoietic organ.

– Erythrocyte and leukocyte production in the spleen reaches its peak in the 5th month of gestation.

– After the 5th month, the spleen mainly produces lymphocytes.

IV. Hematopoiesis in the Bone Marrow

– Bone marrow begins to form in the 2nd month of gestation.

– By the 12th month, the bone marrow exhibits erythroblasts and myeloblasts.

– Bone marrow is the source of blood stem cells for the thymus and peripheral cells.

V. Erythropoiesis

– The process of erythropoiesis goes through stages: proerythroblasts, basophilic erythroblasts, polychromatophilic erythroblasts, acidophilic erythroblasts, reticulocytes, and erythrocytes.

– In adults, erythropoiesis is carried out by the division of basophilic erythroblasts.

– Reticulocytes and erythrocytes are blood cells that enter the bloodstream.

– During erythrocyte development, the cell size decreases, and the cytoplasmic properties change from basophilic to acidophilic.

VI. Granulopoiesis

– The process of granulopoiesis goes through stages: myeloblast, promyelocyte, myelocyte, metamyelocyte, and granulocyte.

– The myelocyte stage is when the cell can divide into the neutrophil, eosinophil, and basophil lines.

– Promyelocyte is the stage when the first azurophilic granules appear, secreted in the Golgi apparatus.

– Myelocyte is the stage when more azurophilic granules are secreted, and the Golgi apparatus begins to form specialized granules.

– Metamyelocyte is the stage when many specialized granules appear, azurophilic granules are scattered, and the Golgi apparatus degenerates.

Note:

– This article is written based on the information provided, and there may be some incomplete or inaccurate information.

– To understand hematopoiesis better, you should consult further specialized medical materials.