The origin and development


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Slide 1 : Fig 1 Blood island from yolk sac of a rabbit embrio of eight and a half days. M – transformation of endothelial cell into primitive blood cells (erythroblast). Reproduced from Maximow and Bllom (91)
Slide 2 : Fig.2 Blood island in the yolk sac of tenty four day human embrio. Spl. Mes. Is the hematogenic mesoderm. Reproduced from Maximow and Bloom (91)
Slide 3 :   Fig. 3 The sites of origin for blood cells, lymphocytes and macrophages are different. These three types of cells do not originate from a common hematopoietic stem cell in the bone marrow but all of them enter the circulation. The macrophages in the blood stream are the monocytes. The main property of mesodermal derivatives is phagocytosis, while the main property of lymphocytes, which are endodermal derivatives, is secretion and antibody production.
Slide 4 : Fig 4 A new hypothesis for stem cell self-renewal and differentiation. A- sense DNA strand with one structural gene. B- antisense strand. Both DNA strands are blocked by histones (^^^^^). In daughter cell #1 on the template of the parental strand A, a complementary strand b is synthesized, but it is an antisense strand and does not carry the gene. Daughter #1 is replica of the mother cell and depicts the self-renewal of the stem cells. In daughter cell #2 the structural gene is in the newly synthesized sense strand-a, accessible for morphogenes. The complementary mRNA serving as chaperone will read and add the hox protein (^^^^^) to the cDNA. Thus the gene will be unlocked and under activation can start transcription. The two daughter cells are not equal, #2 undergoing differentiation.

 


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