Primordial follicle formation represents a critical phase of the initiation of embryonic reproductive organ development, while the primordial follicle transition into primary follicle determines whether oestrus or ovulation will occur in female animals. RCN3, actin, and HNRNPK, while the primordial follicle transformation to primary follicle is usually regulated by MVP and HSPA2. Therefore, our results provide further information for the prospective understanding of the molecular mechanism(s) involved Anxa1 in the regulation of the ovarian follicle development. 1. Introduction Proper gilt development is critical for yielding productive pigs and consequently is essential for sustaining economic efficiency. Conditioning of the ovary can be adjusted to maximize the reproductive lifetime of gilts. Generally, studies on ovary development are buy PETCM focused on primordial follicle formation and transition into primary follicle. Gilt fecundity is determined at the time when the fetal primordial follicle pool is established, while any abnormalities in the formation of primordial follicles can result in infertility . The primary follicles develop from a reserve of primordial follicles generated early in life . Although the total number of primordial follicles is not altered in the offspring, the number of activated cells can determine the lifetime of reproduction . Ovarian dysplasia is usually a disorder characterized by abnormal formation or differentiation of primordial follicles which can lead to failure or early estrus, causing a reduced lifetime reproduction. In the fetal ovaries of pigs, the germ cells migrate in clusters to the gonad  and begin to enter meiosis at day 47 of gestation . The cells in these clusters undergo apoptosis while the cluster cysts are broken down to enclose the immature oocytes and form the primordial follicles . Eventually, each primordial follicle consists of one immature oocyte which is usually surrounded by several somatic granulosa cells. Through a series of activation actions the primordial follicular cells are transformed into primary follicles  and these processes are critical for ovarian development . The process of primordial follicle formation in pigs begins at day 56 of gestation and transformation into primary follicle is first observed at day 90 of gestation (g90) . The heat shock protein (Hsp) is mainly responsible for maintaining appropriate internal environment in the developing ovary. The extensive studies have buy PETCM revealed that buy PETCM an increase in Hsp synthesis may incite oxidative stress that is connected to the risk of reproductive diseases and damage to the ovaries . In addition to Hsp, several other genes have been identified to be involved in the development of the ovary [8C10]. However, the regulatory mechanism(s) leading to ovary and follicle development and the molecular interactions of many genes/proteins involved in these pathways need to be investigated further. 2D-DIGE technology is usually a widely used method which allows the direct comparison of samples with distinct proteomic profiles in order to identify differentially expressed proteins. Moreover, investigation of the fetal ovary proteome in pigs can be crucial for detecting key physiological and biological changes which regulate follicular development. Therefore, the objective of the present study was to compare two distinct follicular developmental stages [at day 55 (g55) and day 90 (g90) of gestation] for detecting differentially abundant proteins and identifying potential markers for primordial follicle formation or transition to primary follicle. Our results offer a new insight towards an understanding of the molecular basis of follicle formation and differentiation. 2. Materials and Methods 2.1. Gilts and Tissue Sample Collection All the experimental procedures performed in this study were approved by the Guide for the Care and Use of Laboratory Animals prepared by the Animal Care and Use Committee of Sichuan Agricultural University. To obtain pregnant animals, similar weight (135.54 0.66?kg) female Yorkshire gilts (= 8), at the 3rd estrus, were mated twice by artificially insemination with the same Yorkshire boar. The diet of the animals was formulated to meet nutrient requirements as recommended by the National Research Council 2012 (NRC 2012), which includes 13.9% of crude protein, 0.69% Lys, 0.96% calcium, and 0.79% phosphorus. After artificial insemination, the pregnant gilts were housed individually and fed 2?kg/day (g0Cg30) and 2.4?kg/day (g31Cg90). All gilts were given ad libitum access to water. Four gilts were selected randomly at g55 to be anaesthetized with an.