Embryology Study Documents

‭Embryology‬ ‭ ‬ ➔ ‭Evolution occurs through several stages:‬ ‭The first stage is mesodermal: From mesenchymal cells coming from the mesoderm we have after‬ ‭stimulation by a growth factor such as VEGF of a cell with many potentialities called‬ ‭hemangioblast, this will become an intermediate cell which can give hematopoietic stem cells and‬ ‭cells endothelial. This explains that the first part of this hematopoiesis in the embryo is first‬ ‭intravascular. They constitute the islets of Wolf Expander at 16th day then in the yolk sac on the‬ ‭22nd day and the cells generated intravascularly are essentially erythroblasts and therefore they are‬ ‭nucleated elements. These erythroblasts are large: 250 fenton liters and they are very sensitive to‬ ‭erythropoietin (EPO). This hematopoiesis primitive will decrease towards S5 and then the relay will‬ ‭be taken by hematopoiesis which will be essentially hepatosplenic, this phase will be predominantly‬ ‭between the 3th and the 6th months of intrauterine life. From 6th month we have the beginning of‬ ‭intramedullary hematopoiesis. Sometimes we can see hematopoiesis in the lymph nodes and it stops‬ ‭usually right after birth.‬ ‭❖‬ D ‭ uring the hepato-splenic we have the production of elements of the erythroblastic lineage and‬ ‭granule cells. We have few megakaryocytes. On the other hand, in the medullary phase we have‬ ‭the production of 3 lineages.‬ ‭❖‬ ‭Note: in certain pathological circumstances we can have an insufficiency of bone marrow‬ ‭production in adulthood following irradiation, chemotherapy or marrow pathologies and sometimes‬ ‭we see the reappearance of hepatic or even splenic hematopoiesis because there are circulating‬ ‭hematopoietic stem cells and these will find at the level of these organs a cytokine environment‬ ‭favorable to their proliferation and their differentiation and this has the role of supplementing‬ ‭hematopoiesis‬ ‭deficiency niente, this is what we call myeloid metaplasia (metaplasia = which is the transformation‬ ‭of one structure into another) so we will have a production of blood elements at the level hepatic or‬ ‭splenic. Sometimes there are anecdotal extramedullary hematopoiesis sites: there may be some in‬ ‭the thyroid.‬ ‭ ‬ ➔ ‭The stroma:‬ ‭The process of blood cell multiplication and differentiation occurs in specialized connective tissue‬ ‭that is predominantly cellular and don't the architecture is favorable to cell migration and the‬ ‭diffusion of chemicals such as chemokines. It is a reticulated connective tissue:the same as in‬ ‭certain lymphoid formations such as lymph nodes. We find a framework, cells and significant‬ ‭vascularization. We put in place the different elements of this specialized fabric:‬ ‭Diagram: On the upper part we have the bony trabeculae which define cubicles which communicate‬ ‭with each other. At the bottom we have the bony trabeculae and inside the MOH. We can see empty‬ ‭areas which correspond to fatty tissue and other areas where they are sinusoidal capillaries because‬ ‭in fact it is a very vascularized organ. If we enlarge this area, we will have this: if we remove the‬ ‭MOH what remains is the framework with significant vascularization, a framework made of‬ ‭reticulin fibers, a very hydrated fundamental substance which therefore allows exchanges. As we‬ ‭see in the diagram the vascularization is well represented, it is half the volume of the organ.‬ ‭Vascularization is ensured by arteries penetrating the bone through the feeding hole, they give‬ ‭ramifications central, peripheral.‬ ‭❖‬ T ‭ hese peripheral branches can come into contact with the arterioles coming from the periosteum.‬ ‭The capillaries will extend through venous sinuses which we find against the endosteum and‬ ‭these sinuses will be drained by a straight sinus which is itself drained by a central sinus which‬ ‭is drained by Venules then a vein which will emerge through the feeding hole.‬ ‭❖‬ I‭ t is important to note that the stem cells or Stem Cells arrive through the right sinus close to the‬ ‭central sinus. They will leave the circulatory torrent and there as we have the cells circulating‬ ‭strains are the same thing: they must recognize the expression of certain adhesion molecules on‬ ‭the capillaries so that they can pass into the tissue. The stem cells will go to the level of the‬ ‭endosteum and it is at this level that they are mainly highlighted. Then, when there is‬ ‭differentiation, the mature cells move towards the center, so it is the same migration as in the‬ ‭thymus.‬ ‭❖‬ T ‭ he level of thymus the immature cells penetrated this organ at the level of the corticomedullary‬ ‭junction, migrated to the periphery and when there was differentiation returned towards the‬ ‭corticomedullary junction, there we have approximately the same pattern where the stem cells‬ ‭approach the hematopoietic tissue at the central level, migrate towards the endosteum and when‬ ‭there is maturation it There is migration towards the center of the medullary cavity and then‬ ‭these mature elements can leave the cavity.‬