Abstract：The let-7 family of microRNAs (miR) which is widely involved in the regulation of development in animal tissues and organs, is an important candidate microRNAs of pig's thyroid development. In order to study the role of let-7 family in the development of Yorkshire pig's (Sus scrofa) thyroid gland, firstly, we stained pig's thyroid at different developmental stages (105 days in the embryonic period, 60, 90, 150 days in the youth period) with hematoxylin-eosin (HE) staining to discover thyroid morphological change; secondly, we used in situ hybridization (ISH) and fluorescence quantitative PCR (qRT-PCR) technique to identify the expression of let-7 microRNAs at different growth stages (105 days in the embryonic period, 60, 90, 150 days in the youth period). The results showed that thyroid follicles gradually became bigger with age; the let-7 family was mainly located in the thyroid follicular epithelium; in the youth period the expression of let-7c, let-7d-3p, let-7e, let-7i rose gradually, while the expression of let-7c, let-7d-3p, let-7e, let-7 increased firstly and then decreased. Based on the above results, we predicted that the expression of let-7 had relation with the expression of thyroid hormone directly or indirectly, thereby playing an important role in thyroid growth. This study provides a new idea for further research on growth and development of pig.
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