Ebook Veterinary embryology: Part 2

Part 2 book "Veterinary embryology" includes content: Embryological and post-natal features of haematopoiesis; nervous system, muscular and skeletal systems, urinary system, digestive system, respiratory system, male and female reproductive systems, structures in the head and neck, endocrine system,... and other contents.
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Ebook Veterinary embryology: Part 2 12 Embryological and Post-natal Features of Haematopoiesis Although blood cells have a finite life span in normal influenced by locally-secreted differentiation factors animals, the number of each cell type in the circulation is together with substances produced in developing tissues. usually maintained at a relatively constant level by a Bone marrow haematopoietic stem cells lasting for process of carefully controlled production of mature the lifetime of the animal are capable of giving rise to all cells balanced with the rate at which blood cells leave the blood cell lineages. circulation. In the absence of infectious diseases which can damage both red blood cells and white blood cells Many tissues in mature animals are formed from cells and lead to their premature removal, normal levels of which have a defined life span and are not replaced. In cellular blood components are carefully controlled by contrast, cells of other tissues such as epithelial cells and the removal of ageing cells and their replacement by red blood cells undergo constant replacement when the cells produced either in the bone marrow or by cells former are sloughed and the latter are removed from arising from the bone marrow and maturing in lym- the circulation by the spleen at predetermined times phoid organs. Sustained abnormal elevations of blood related to membrane alterations associated with ageing. cell numbers occur in neoplastic conditions which affect Haematopoiesis is a complex, highly regulated process the bone marrow or lymphoid tissue. At an early stage which ultimately relies on bone marrow stem cells for in embryological development, blood islands which its continuation. Defining characteristics of embryonic form in the splanchnic mesoderm of the yolk sac contain stem cells include the ability to be passaged continu- pluripotent stem cells which can differentiate into myeloid ously, chromosome stability, the ability to form clones or lymphoid progenitor cells. and the expression of a wide range of molecular, bio- chemical and antigenic markers. The ability to differen- A remarkable feature of haematopoiesis, the produc- tiate into various types of tissue sets embryonic stem tion of blood cells, is that every functionally specialised cells apart from other cells in the developing embryo. mature blood cell and a number of other cell types Post-natally, stem cells produce populations which give not present in the circulation, such as macrophages rise not only to more stem cells, but also to cells capable and mast cells, are derived from the same pluripotent of undergoing further development and differentiation mesodermal stem cells. Post-natally, haematopoietic as committed cells. From these committed stem cells, cells develop and mature in the bone marrow on a mesh- progenitor cells capable of differentiating into myeloid work of non-haematopoietic support cells, referred to as and lymphoid lineages arise. In haematopoiesis, there- stromal cells, which include endothelial cells, fibroblasts fore, haematopoietic stem cells can produce progenitors and cells of the monocyte-macrophage series. By pro- of all the myeloid and lymphoid cells required by the viding an appropriate microenvironment, stromal cells animal. Among these cells, red blood cells are concerned influence the growth and differentiation of haemato- with oxygen transportation and binding of carbon dioxide poietic stem cells. A range of growth factors, including for excretion by the lungs, the granulocyte-macrophage colony-stimulating factors, induce the formation of dis- series contribute to innate immune responses, and cells tinct haematopoietic cell lines. The times at which these of the lymphoid series participate in specific cell-mediated pluripotent stem cells move to other sites and the differ- and humoral responses to infectious agents. entiation factors which influence their migratory routes have not yet been established with certainty. Differen- Embryological aspects of haematopoiesis tiation of myeloid and lymphoid cells in the bone marrow and maturation of specialised lymphocytes Haematopoiesis in vertebrates occurs in two successive in organs such as the thymus in domestic mammals, and stages and in two different anatomical sites in the devel- in the cloacal bursa (bursa of Fabricius) in birds, are oping embryo. A transient extra-embryonic phase, which 12 HAEMATOPOIESIS 137 Figure 12.1 Outline of the origin and migration of stem cells from the yolk sac and the aorta–gonad–mesonephros region to the bone marrow in mammals. In the bone marrow, cells of the myeloid lineage differentiate into red blood cells, leukocytes, mast cells and megakaryocytes. Further differentiation can occur after some leukocytes are released into the bloodstream. Cells belonging to the lymphoid lineage include antigen-specific T and B lymphocytes and NK cells which lack antigen specificity. Following appropriate antigenic stimulation, B cells differentiate into plasma cells which secrete antibody. The distribution of cell types in blood and in tissues is indicated. commences in the splanchnic mesoderm of the yolk sac in association with the endoderm form blood islands. with the formation of blood islands, is followed by a The outer cells of these blood islands differentiate into definitive intra-embryonic phase at a site ...