Ageing and chronic disease-related changes in the morphometric characteristics of ovarian follicles in cynomolgus monkeys (<i>Macaca fascicularis</i>)

Abstract STUDY QUESTION How is the localisation of ovarian follicles affected by ageing and chronic diseases? SUMMARY ANSWER Ovarian follicles shift deeper towards the medulla, due to thickening of the tunica albuginea (TA), with ageing and some major common chronic diseases. WHAT IS KNOWN ALREADY The ovary undergoes morphological and functional changes with ageing. The follicular pool follows these changes with alterations in the amount and distribution of residual follicles. Diseases causing a chronic inflammatory process are associated with morphological changes and impaired ovarian function. STUDY DESIGN, SIZE, DURATION We conducted a cross-sectional study, examining 90 ovaries from 90 female monkeys. The samples were collected from April 2018 to March 2019 at Tsukuba Primate Research Center in National Institutes of Biomedical Innovation, Health and Nutrition, Japan. PARTICIPANTS/MATERIALS, SETTING, METHODS Ovarian samples were obtained from cynomolgus monkeys that died from natural causes or were euthanised. Ovarian sections were stained with haematoxylin and eosin (H&E) for histological analyses. In ovarian sections from 64 female macaques aged 0–25 years, a total of 13 743 follicles at different developmental stages (primordial, intermediary, primary, early secondary and late secondary) were assessed to determine the depth of each follicle from the outer surface of the ovarian cortex to the far end of the follicle, by using a digital imaging software. TA thickness was measured as sum of basal membrane and tunica collagen layer for each ovary under H&E staining. To explore the possibility of age-related trends in ovarian morphometric characteristics, samples were divided into four different age groups (0–3 years (pre-menarche), 4–9 years, 10–14 years and 15–20 years). To evaluate the effect of common chronic diseases on ovarian morphometric characteristics, macaques with diabetes mellitus (DM) (n = 10), endometriosis (n = 8) or inflammatory bowel disease (IBD) (n = 8) were compared to age-matched controls without chronic diseases. MAIN RESULTS AND THE ROLE OF CHANCE Ovarian morphometric analysis revealed that the relative location of follicles became deeper in all age groups according to development of follicles (P &lt; 0.05). Total follicle distance from the ovarian surface was increased with ageing (P &lt; 0.05). In a sub-analysis according to developmental stage, only primordial and intermediary follicles were localised deeper with increasing age (P &lt; 0.05). TA thickness was also increased with ageing (P &lt; 0.05). The localisation of the total number of follicles became deeper in ovaries from monkeys with DM, endometriosis or IBD as compared to the control group (P &lt; 0.05). With DM, analysis of follicles distance at almost each developmental stage was significantly deeper compared to controls (P &lt; 0.05) with the exception of early secondary follicles. With endometriosis, follicles at primary and early and late secondary stages were significantly deeper compared to controls (P &lt; 0.05). Also with IBD, follicles at primary and early and late secondary follicles were significantly deeper compared to controls (P &lt; 0.001). The TA was thicker with DM and endometriosis compared to controls (P &lt; 0.05), but not with IBD (P = 0.16). LARGE SCALE DATA NA. LIMITATIONS, REASONS FOR CAUTION Two-dimensional histology was used to assess follicle localisation. The possibility of minimal variations between the measured distance to the actual distance in a spherical structure cannot be excluded. Additionally, the severity of disease was not assessed. WIDER IMPLICATIONS OF THE FINDINGS This study is the first step towards enhancing our understanding of how ageing and chronic diseases affect the relative localisation of dormant and developing follicles. These observations, combined with possible future human studies, may have managerial implications in the field of fertility preservation and other conditions involving ovarian tissue cryopreservation. STUDY FUNDING/COMPETING INTEREST(S) The present work was supported by the Grant-in-Aid for Scientific Research B (19H03801) (to K.K.), Challenging Exploratory Research (18K19624), Japan Agency for Medical Research and Development, Mochida Memorial Foundation for Medical and Pharmaceutical Research, Takeda Science Foundation and Naito Foundation (to K.K.). All authors have no conflicts of interest directly relevant to the content of this article.