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Thrombopoietin , a class I hematopoietic cytokine , plays critical roles in regulating hematopoietic stem cell numbers and also stimulates the production and differentiation of megakaryocytes , the bone marrow cells that ultimately produce platelets .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
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