The ovarian cortex represents the outer layer of the ovary where follicle development occurs1. Primordial follicles, initially arrested in development, will be activated to become primary, secondary, and then antral or tertiary follicles based on paracrine and gonadotropin inputs1,2,3,4. To better understand physiological processes within the ovary, tissue culture can be used as an in vitro model, thereby allowing for a controlled environment to conduct experiments. Many studies have utilized ovarian tissue culture for research in assisted reproductive technology, fertility preservation, and ovarian cancer5,6,7. Ovarian tissue culture has also served as a model for investigating reproductive toxins that damage the ovarian health and the etiology of reproductive disorders such as Polycystic Ovary Syndrome (PCOS)8,9,10,11. Thus, this culture system is applicable to a wide array of specialties.
In rodents, whole fetal or perinatal gonads have been used in reproductive biology experiments12,13,14,15. However, gonads from larger domestic livestock cannot be cultured as whole organs due to their large size and potential degeneration. Therefore, bovine, and non-human primate ovarian cortex is cut into smaller pieces16,17,18. Many studies have cultured small ovarian cortex pieces to study various growth factor(s) in primordial follicle initiation in domestic livestock and non-human primates1,17,18,19. The use of ovarian cortex culture has also demonstrated primordial follicle initiation in the absence of serum for bovine and primate cortical pieces cultured for 7 days20. Yang and Fortune in 2006 treated fetal ovarian cortex culture medium with a range of testosterone doses over 10 days and observed that the 10-7 M concentration of testosterone increased follicle recruitment, survival, and increased progression of early stage follicles19. In 2007, using ovarian cortex cultures from bovine fetuses (5-8 months of gestation), Yang and Fortune reported a role for Vascular Endothelial Growth Factor A (VEGFA) in the primary to secondary follicle transition21. Furthermore, our laboratory has utilized ovarian cortex cultures to demonstrate how VEGFA isoforms (angiogenic, antiangiogenic, and a combination) may regulate different signal transduction pathways through the Kinase domain receptor (KDR), which is the main signal transduction receptor that VEGFA binds16. This information allowed for a better understanding of how different VEGFA isoforms affect signaling pathways to elicit follicle progression or arrest. Taken together, culturing of ovarian cortex pieces in vitro with different steroids or growth factors can be a valuable assay to determine effects on mechanisms regulating folliculogenesis. Similarly, animals that are developed on different nutritional regimes may have altered ovarian microenvironments, which may promote or inhibit folliculogenesis affecting female reproductive maturity. Thus, our goal in the current manuscript is to report the bovine cortex culture technique and determine whether there are differences in ovarian microenvironments after in vitro culture of bovine cortex from heifers fed either Control or Stair-Step diets collected at 13 months of age as described previously16.
Therefore, our next step was to determine the ovarian microenvironment in these heifers that were developed with different nutritional diets. We evaluated ovarian cortex from heifers fed with either a Stair-Step or Control diet. Controls heifers were offered a maintenance diet of 97.9 g/kg0.75 for 84 days. The Stair-Step diet was initiated at 8 months containing a restricted fed diet of 67.4 g/kg0.75 for 84 days. After the first 84 days, while Control heifers continued to receive 97.9 g/kg0.75, the Stair-Step beef heifers were offered 118.9 g/kg0.75 for another 68 days, after which they were ovariectomized at 13 months of age16 for studying changes in follicular stages and morphology before and after culture. We also assayed for differences in steroids, steroid metabolites, chemokines, and cytokines secreted into cortex media. Steroids and other metabolites were measured to determine if there were any direct effects from treatments conducted in vivo and/or in vitro on tissue viability and productivity. Changes in the ovarian microenvironment prior to and after culture provided a snapshot of the endocrine milieu and folliculogenesis prior to culture and how culture or treatment during culture affects follicle progression or arrest.
Ovaries were collected after ovariectomies were performed at the U.S. Meat Animal Research Center (USMARC) according to their IACUC procedures from Control and Stair-Step heifers at 13 months of age16, cleaned with sterile phosphate buffer saline (PBS) washes with 0.1% antibiotic to remove blood and other contaminants, trimmed excess tissue, and transported to the University of Nebraska-Lincoln (UNL) Reproductive Physiology laboratory UNL at 37°C23. At UNL, ovarian cortex pieces were cut into small square pieces (~0.5-1 mm3; Figure 1) and cultured for 7 days (Figure 2). Histology was conducted on the cortex culture slides prior to and after culture to determine follicles stages16,24 (Figure 3 and Figure 4), and extracellular matrix proteins that may indicate fibrosis (Picro-Sirus Red, PSR; Figure 5). This allowed determination of effect of in vivo nutritional regimes on follicle stages and allowed comparison of 7 days of ovarian cortex on follicle stages and follicle progression. Throughout the culture, the medium was collected and changed daily (approximately 70% of media was collected each day; 250 µL/well) so that either daily hormones/cytokines/chemokines can be assessed or pooled over days to obtain average concentrations. Steroids such as androstenedione (A4) and estrogen (E2) can be pooled over 3 days and assessed through radioimmunoassay (RIA; Figure 6) and pooled over 4 days per animal and assayed via High Performance Liquid Chromatography-Mass Spectrometry (HPLC-MS)24,25 (Table 1). Cytokine arrays were utilized to assess cytokine and chemokine concentrations in ovarian cortex culture medium26 (Table 2). Real-time polymerase chain reaction (RT-PCR) assay plates were conducted to determine gene expression for specific signal transduction pathways as demonstrated previously16. All of the steroid, cytokine, follicle stage and histological markers provide a snapshot of the ovarian microenvironment and clues as to the ability of that microenvironment to promote "normal" or "abnormal" folliculogenesis.