Ovarian follicle selection largely depends on the transition of granulosa cells from an undifferentiated to a fully differentiated state, which is accompanied by morphological and functional changes in follicles. The processes and transcriptional regulation of follicles during follicle selection are unclear; we thus used follicles from the prehierarchal to the hierarchal stage to investigate histology, reproductive endocrinology, and transcription. The morphology of follicles changed markedly during follicle selection. The numbers of large white, small yellow, and large yellow follicles (LWF, SYF, and LYF, respectively) were 11.83 ± 2.79, 6.83 ± 2.23, and 1.00, respectively, per ovary. LYF showed thicker granulosa cell layers than those of other prehierarchal follicles. Progesterone concentrations were significantly higher in LYF than that in LWF and SYF. In total, 16,823 genes were positively expressed in LWF, SYF, and LYF. Among follicle types, 1290 differentially expressed genes were enriched regarding cell differentiation, blood vessel morphogenesis, and response to steroid hormones. Candidate genes associated with follicle selection participated in the Wnt signaling pathway, steroid hormone biosynthesis, and the TGF-β signaling pathway. We produced insights into crucial morphological characteristics of transcriptional regulation in follicle development. Our results provide an important basis for revealing the mechanism of follicle selection and potential impact on the poultry industry.

Background: Abdominal fat deposition depends on both the proliferation of preadipocytes and their maturation into adipocytes, which is a well-orchestrated multistep process involving many regulatory molecules. Circular RNAs (circRNAs) have emergingly been implicated in mammalian adipogenesis. However, circRNA-mediated regulation in chicken adipogenesis remains unclear. Our previous circRNA sequencing data identified a differentially expressed novel circRNA, 8:27,886,180|27,889,657, during the adipogenic differentiation of chicken abdominal preadipocytes. This study aimed to investigate the regulatory role of circDOCK7 in the proliferation and adipogenic differentiation of chicken abdominal preadipocytes, and explore its molecular mechanisms of competing endogenous RNA underlying chicken adipogenesis. Results: Our results showed that 8:27,886,180|27,889,657 is an exonic circRNA derived from the head-to-tail splicing of exons 19–22 of the dedicator of cytokinesis 7 (DOCK7) gene, abbreviated as circDOCK7. CircDOCK7 is mainly distributed in the cytoplasm of chicken abdominal preadipocytes and is stable because of its RNase R resistance and longer half-life. CircDOCK7 is significantly upregulated in the abdominal fat tissues of fat chickens compared to lean chickens, and its expression gradually increases during the proliferation and adipogenic differentiation of chicken abdominal preadipocytes. Functionally, the gain- and loss-of-function experiments showed that circDOCK7 promoted proliferation, G0/G1- to S-phase progression, and glucose uptake capacity of chicken abdominal preadipocytes, in parallel with adipogenic differentiation characterized by remarkably increased intracellular lipid droplet accumulation and triglyceride and acetyl coenzyme A content in differentiated chicken abdominal preadipocytes. Mechanistically, a pull-down assay and a dual-luciferase reporter assay confirmed that circDOCK7 interacted with gga-miR-301b-3p, which was identified as an inhibitor of chicken abdominal adipogenesis. Moreover, the ACSL1 gene was demonstrated to be a direct target of gga-miR-301b-3p. Chicken ACSL1 protein is localized in the endoplasmic reticulum and mitochondria of chicken abdominal preadipocytes and acts as an adipogenesis accelerator. Rescue experiments showed that circDOCK7 could counteract the inhibitory effects of gga-miR-301b-3p on ACSL1 mRNA abundance as well as the proliferation and adipogenic differentiation of chicken abdominal preadipocytes. Conclusions: CircDOCK7 serves as a miRNA sponge that directly sequesters gga-miR-301b-3p away from the ACSL1 gene, thus augmenting adipogenesis in chickens. These findings may elucidate a new regulatory mechanism underlying abdominal fat deposition in chickens.