Ovarian response to vascular and toxic insults in early life : focus on ovarian reserve and fertility preservation

Abstract

The ovarian reserve is defined as the number of eggs embodied in primordial follicles. Every woman is born with a fixed number of primordial follicles, which are formed during fetal development. Pathological events can affect the ovarian reserve both pre- and postnatally and cause a premature ovarian insufficiency (POI), i.e. a precocious exhaustion of the endocrine and reproductive potential of the ovaries. In prenatal life, intrauterine growth restriction (IUGR) caused by poor maternal nutrition or insufficient placental flow to the fetus, has been associated with alterations in pubertal development and follicle number in animals and humans, with inconclusive evidence. In postnatal life, major iatrogenic causes of POI are chemo- and radiotherapeutic treatments administered to cancer patients. Fertility preservation strategies, such as ovarian tissue cryopreservation (OTC), are of fundamental importance to preserve fertility in these patients. These procedures are still experimental and in continuous evolution as more data are acquired.

In this thesis, we aimed at investigate the impact of IUGR on ovarian follicles and gene expression in neonatal, juvenile and peripubertal rats, using a model of surgically induced placental insufficiency. Secondly, we focused on the effects of chemotherapy exposure on ovarian follicles and stroma in ovarian tissues from cancer patients who underwent OTC for fertility preservation purposes. Additionally, we analyzed the efficacy of fertility preservation criteria in selecting the patients at high risk of infertility, by analysis of ovarian function during follow-up, the fruition rate and the timing of OTC in a large pediatric oncology unit in Finland. Timing of OTC was analyzed in relation to exposure to chemotherapy.

A reduced primordial and total follicle count was found in the ovaries of neonate and juvenile rats. Follicle count normalized at peripubertal ages, alongside with the recovery of body weight. The expression of 24 genes was modified in the rat ovaries in response to the placental insufficiency. We interpreted the gene reprogramming as compensatory, possibly explaining the recovery of follicle number (by a downregulation of the physiological rates of follicle activation and/or depletion). The gene modifications could also have unknown long-term consequences on ovarian function, whose definition was beyond the aims of this work. An increased number of atretic follicles and a reduced size and number of residual intact follicles were evidenced in the ovaries of patients exposed to chemotherapy before OTC. Exposure to chemotherapy also reduced the secretion of steroid hormones in culture and damaged the ovarian stroma, by increasing collagen deposition and apoptosis. Finally, analysis of OTC data from Helsinki Children’s Hospital revealed that patients at high risk of infertility were correctly identified by the selection criteria, despite fruition rate was not optimal. We observed an increased risk of ovarian failure in older patients, which is in line with the age-related decline of the ovarian reserve. We also reported longer time frames between indication to OTC and execution of the procedure in patients with malignant diseases, which was associated to increased exposure to alkylating agents. Delays in the execution of OTC should be limited in order to avoid further damage to the ovarian tissue as a consequence of chemotherapy exposure, especially in older patients.