Introduction
The establishment of blood islands marks the onset of hematopoiesis, which is the process of creating mature blood cells of a distinct lineage from hematopoietic stem cells. Blood development in vertebrates occurs in two stages, the embryonic stage and the adult stage. In this experiment the embryonic stage of hematopoiesis, which initiates circulation, was used. During this time the embryo forms its initial blood cells and forms capillaries with the yolk.
Construction of blood cells occurs through two processes, vasculogenesis and angiogenesis. Vasculogenesis is the formation if blood islands and capillary networks. In the early stage of vasculogenesis, mesoderm cells, the precursors of blood cells and blood vessels, condense into blood islands. The inner most cells of the islands become hematopoietic stem cells, which later become blood cells. The outer cells of the blood islands become angioblasts, which then develop into blood vessels. Next, angioblasts divide and differentiate forming endothelial cells form tubes and connect to form a network of capillaries known as the primary capillary plexus. Angiogenesis, the formation of blood vessels and remodeling of old vessels to complete the circulatory system, occurs next. Ultimately, the blood vessels remodel and mature to for the mature vascular system. Lineage decisions and location of the red, white, and lymphoid cells during development characterize hematopoiesis. For example, within mammalian embryos, the site of primitive hematopoiesis is in the yolk sac blood islands, but definitive hematopoiesis takes place in the fetal liver and then to the bone marrow.
The combined affects of growth factors, which allow cellular proliferation, and transcription factors, which activate lineage specific genes, are the controlling factors of hematopoiesis. The study of cell-cell signaling has helped determine important mechanisms of hematopoietic development. The receptor we chose to manipulate in order to determine its influence in hematopoietic development was Tyrosine Phosphatase Gamma, or Ptpg. Ptpgis a member of a distinct group of phosphatases characterized by the presence of carbonic anhydrase-like domains, fibronectin type III repeats, and a long cystenine-free region in their extracellular domain. Many tyrosine phosphatases have been found in hematopoietic tissues and therefore suggest their direct involvement in blood development.