Survival and growth of isolated pre-antral follicles from human ovarian medulla tissue during long-term 3D culture

Abstract

Study question: Can human pre-antral follicles isolated enzymatically from surplus medulla tissue survive and grow in vitro during long-term 3D culture?

Summary answer: Secondary human follicles can develop to small antral follicles and remain hormonally active in an alginate-encapsulation culture system for more than 30 days.

What is known already: Ovarian tissue cryopreservation followed by transplantation is a promising fertility preservation approach for cancer patients. However, transplantation of cryopreserved tissue to patients may carry the risk of re-implanting malignant cells. Grafting of follicles enzymatically isolated from ovarian tissue or developing a method for follicular culture and maturation in vitro may provide fertility to such patients without the risk of reintroducing the malignancy. However, the growth of pre-antral follicles isolated by enzymatic digestion from medulla tissue during long-term culture has received only little attention.

Study design, size, duration: Two to ten human pre-antral follicles were encapsulated together within an alginate bead and cultured with or without ovarian interstitial tissue for either 7 days or >30 days. Follicles were cultured in either 20% oxygen or 5% oxygen or encapsulated in a lower concentration of alginate together with a lower concentration of FSH in high oxygen.

Participants/materials, setting, methods: A total of 395 pre-antral follicles from 16 cancer patients, aged 9-37 years, were co-cultured for either 7 days or >30 days. A proportion of follicle (64) were removed from culture on Day 7 and assessed for viability using confocal fluorescence microscopy following calcein-AM and ethidium homodimer-1 staining or histology. The remaining follicles (331) were continued in culture for >30 days then assessed for survival and growth. Anti-Müllerian hormone (AMH) and estradiol levels were quantified in the medium.

Main results and the role of chance: An optimized protocol for isolation of intact healthy pre-antral follicles from ovarian medulla was developed. After 7 days of culture, secondary follicles had a significantly higher survival rates compared with primary and primordial follicles (70 versus <38%). Primordial and primary follicles did not develop into the antral follicle stage. In contrast, secondary follicles continued to develop in all culture conditions examined. Based on growth rate and morphology, four distinct cohorts of surviving follicles, 'fast growth', 'slow growth', 'no growth' and 'extruded oocyte' were identified. From Day 1 to Day 30, the mean diameter of follicles increased from 184 ± 35 to 661 ± 120 μm (significant from Day 18), 145 ± 19 to 318 ± 68 μm and 136 ± 15 to 162 ± 25 μm (mean ± SD) in the 'fast growth', 'slow growth' and 'no growth' patterns, respectively. The fast growth follicles also contained a larger diameter oocyte than other follicle groups. From the pre-antral follicle to antral stage, follicles became steroidogenically active and secretion of AMH and estradiol increased. No significant difference between the follicles cultured with or without ovarian interstitial tissue was observed.

Limitations, reasons for caution: The number of surviving follicles at the end of study was low in each of the culture conditions therefore whether there is a benefit with any of the conditions is difficult to ascertain. Multiple pre-antral follicles were cultured within the same alginate bead which may affect the in vitro development of the secondary follicles.

Wider implications of the findings: These findings show that pre-antral follicles, isolated enzymatically from surplus medulla tissue that is normally discarded, possess a developmental potential which may be used to devise safer fertility preservation methods for patients who are at high risk of malignant contamination of their ovarian tissue.

Study funding/competing interests: The Child Cancer Foundation in Denmark (2012-26) and the EU interregional project ReproHigh are thanked for having funded this study; and the Key Program of Medical Science and Technology Innovation of Nanjing Military Area Command in China (14ZX06; 11Z010). They had no role in the study design, collection and analysis of data, data interpretation or in writing the report. The authors have no conflicts of interest to disclose.

Keywords: fertility preservation; follicle culture; human follicle; isolation; ovarian medulla.