Ovarian follicle development in Booroola sheep exhibiting impaired bone morphogenetic protein signalling pathway

Abstract

The role of bone morphogenetic proteins (BMPs) in the regulation of ovarian function has been extensively investigated but the mechanism of regulation is not well understood. The aim of this study was to investigate the effect of mutation in the BMP receptor in Booroola sheep on the number of primordial follicles and rate of follicle recruitment in comparison with that in normal merino sheep in vivo. Whole sheep ovaries at the time of birth, 1.5 and 5 years old were collected and processed for the follicle quantification, using computerised stereological methods and statistical analyses. At birth, the total number of primordial follicles in Booroola sheep was significantly lower than in merino sheep. At 1.5 and 5 years, a reversed pattern in favour of Booroola ewes was seen with significantly more primordial follicles than merino. In parallel, the rate of primordial follicle recruitment to developing cohort was substantially lower in Booroola ewes with only 51 and 66% of primordial follicle consumption at 1.5 and 5 years respectively compared to 92 and 97% in merino ewes. On other hand, the mean numbers of developing primary follicles were smaller in Booroola sheep at the time of birth, yet, Booroola ewes possess more primary follicles than merino at 1.5 years. These findings suggest that attenuation of the intraovarian signalling pathway of BMPs may in fact be a successful means of rationalising follicle consumption, preventing unnecessary loss of follicles from the initial primordial follicle pool, hence increasing reproductive longevity and fertility.

The impact of passive immunisation against BMPRIB and BMP4 on follicle development and ovulation in mice

“…We have supported these in vitro studies using in vivo models namely the Booroola sheep (Ruoss et al 2009) and mice passively immunised against BMP4 (Tanwar et al 2008), both models are characterised by attenuated action of the BMP signalling system, similar to that initially described in Bmp4 lacZneo haplo-insufficient mice (Lawson et al 1999). Our recent report has confirmed that the attenuation of the BMP signalling system by a natural point mutation of BMPRIB in Booroola sheep reduces the rate of primordial follicle recruitment, while increasing primary follicle survival (Ruoss et al 2009). However, it has not yet been determined whether such an effect of BMPs is a sheep species and/or a Booroola strain specific phenomenon or can be applied to other species and different experimental conditions.…”

“…The sections were consecutively labelled, mounted on a glass slide and stained with haematoxylin and eosin for further stereological studies. For this purpose, the stages of follicle development were classified according to previous reports (Myers et al 2004, Ruoss et al 2009). In brief, primordial follicles appeared as a primary oocyte surrounded by a single layer of squamous granulosa cells (Fig.…”

“…The microscope supported by a computerised Stereo Investigator station (Stereo Investigator software version 8, MicroBrightField, Inc., Colchester, MA, USA) was used for the data collection and analysis. The procedure of follicle counting has been fully described previously (Ruoss et al 2009). …”

“…BMPRIB is the main and common receptor used by several intraovarian BMPs such as BMP 2, 4, 7, and 15 . On the other hand, the dominant intraovarian BMP4 exerts a major role in the formation of primordial germ cells (oocytes) (Shimasaki et al 1999, recruitment of primordial to primary follicles in sheep (Ruoss et al 2009) and rat (Nilsson & Skinner 2003), oestradiol production, and inhibition of progesterone secretion and ovulation (Lawson et al 1999, Shimasaki et al 1999, Nilsson & Skinner 2003, Ruoss et al 2009.…”

“…The aim of this study was to investigate the role of the BMP signalling system in the regulation of primordial follicle depletion in an in vivo model of mice passively immunised against BMPRIB and BMP4, using a computer-based stereological method (Braendgaard & Gundersen 1986, Myers et al 2004, Ruoss et al 2009). Specifically, the study aimed to quantify the numbers of primordial and primary follicles and calculate their ratios in different treatment and age groups of mice.…”