Within these islands 2 populations of cells exist: peripheral and core. These blood islands extend and fuse together making a primordial vascular network. Stem cells that form blood cells (Hematopoietic Stem Cells, HSCs) change their location during development moving from tissue to tissue until their adult bone marrow location is formed and populated.Īngioblasts initially form small cell clusters (blood islands) within the embryonic and extraembryonic mesoderm. The fetal white blood cells (neutrophils, monocytes, and macrophages) develop, though mononuclear phagocytes do not mature until after birth. Granulocyte-colony stimulating factor (G-CSF) may initiate neutrophil production, with neutrophils first appearing in the clavicle marrow at 10 - 11 weeks. The clavicle is also one of the first fetal bone to contain marrow. " If the onset can be recognized in a given specimen, that specimen is straightway classed as a fetus." In 1949 the embryologist George Streeter used the replacement of cartilage within the humerus by bone marrow as an arbitrary definition of the embryo to fetus transition. At the time when blood first forms there are no bones, and it is only with bone development that we see bone marrow formation and relocation of blood stem cells. In the adult, these stem cells are located in the bone marrow. There then follows a series of "relocations" of the stem cells to different organs ( liver, spleen and thymus) within the embryo and fetus. Within the embryo the aorta in the aorta-gonad-mesonephros (AGM) region (para-aortic splanchnopleura), and the vitelline ( yolk sac) and placental arteries are initial sites. Mesoderm both within the embryo ( mesoderm) and outside the mesoderm (extra-embryonic mesoderm). Blood is considered as a form of "liquid conective tissue" consisting of a fluid and cellular component. Initially blood develops within the core of "blood islands" along with blood vessels in mesoderm. Aorta filled with red blood cells ( Carnegie stage 22, Week 8)
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