Abstract
Ovary follicular development is a progressive system from the beginning of small cortical follicles to the ovulation of hierarchical follicles. The review was conducted to provide information on the indigenous chickens commonly used for egg production, chicken ovarian follicles morphology and expression of growth differentiation factor 9 (GDF9) gene in ovarian follicles and its relationship with egg production. The research databases used in the study include google scholar, Science Direct, PubMed, JSTOR and Cambridge Core. Google, Yahoo and Baidu search engines were used to search the information. In this study, the papers selected for use were original research articles and reviews to ensure that the information used was from research results. Besides, only recent English papers, 2010–2021, were used. The keywords used to search for articles were chicken ovarian follicles, ovarian morphology and GDF9 gene expression. The documents showed that pre-hierarchical follicles include many small and large white follicles, which are about 2–5mm in diameter and 5 to 6 small yellow follicles (SYF) that are about 5–10mm in diameter. Preovulatory follicles are about five to six in number and above 10mm in diameter, with the sizes from F6 to F1, with F1 as the largest follicle. Further, the studies revealed that GDF9 gene mRNA is expressed in the highest concentration in small yellow follicles and other studies reported that the expression of GDF9 gene has been found in follicles of the primary to preovulatory stages in chickens. This review concludes that the GDF9 gene expression is mainly throughout follicular growth and it stimulates the proliferation of pre-hierarchical granulosa cells. The increased egg production in chickens depends on progressive developmental stages and the growth of ovarian follicles.
Highlights
In the worldwide poultry industry, egg-laying production efficiency is a significant economic trait
Studies were considered for inclusion in the review provided, they dealt with chicken ovarian follicles morphology, Growth differentiation factor 9 (GDF9) gene expression in chicken ovarian follicles and its association with production of eggs as well as studies of chicken egg-laying
The current literature review was conducted to investigate the literature on the expression levels of the GDF9 gene in ovarian follicles of chickens
Summary
In the worldwide poultry industry, egg-laying production efficiency is a significant economic trait. Excellent productivity is measured by the number of ovarian follicles heading for ovulation or atresia and the efficiency of the oviduct to convert the ova into a hard-shelled egg. A follicular hierarchy that is well-organised is important for improved egg laying performance. Growth differentiation factor 9 (GDF9) is one of the transforming growth factor-b (TGF-b) superfamily and plays an exclusive role in female fertility (Huang et al, 2015). GDF9 genes have been found to have an effect in controlling chicken ovarian follicular growth and ovulation, with a great influence in laying hens. GDF9 gene is one factor secreted by the oocyte with an important role in regulating ovarian function in female reproduction, modifying both the cell fate of the somatic granulosa cells and the quality and growing ability of the egg (Lou et al, 2018)
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