Ultrasound Shear Wave Velocity Varies Across Anatomical Region in Ex Vivo Bovine Ovaries

Abstract

The physical properties of the ovarian extracellular matrix (ECM) regulate the function of ovarian cells, specifically the ability of the ovary to maintain a quiescent primordial follicle pool while allowing a subset of follicles to grow and mature in the estrous cycle. Design of a long-term, cycling artificial ovary has been hindered by the limited information regarding the mechanical properties of the ovary. In particular, differences in the mechanical properties of the two ovarian compartments, the cortex and medulla, have never been quantified. Shear wave (SW) ultrasound elastography is an imaging modality that enables assessment of material properties, such as the mechanical properties, based on the velocity of SWs, and visualization of internal anatomy, when coupled with B-mode ultrasound. We used SW ultrasound elastography to assess whole, ex vivo bovine ovaries. We demonstrated, for the first time, a difference in mechanical properties, as inferred from SW velocity, between the cortex and medulla, as measured along the length (cortex: 2.57-0.53 m/s, medulla: 2.87-0.77 m/s, p < 0.0001) and width (cortex: 2.99-0.81 m/s, medulla: 3.24-0.97 m/s, p < 0.05) and that the spatial distribution and magnitude of SW velocity vary between these two anatomical planes. This work contributes to a larger body of literature assessing the mechanical properties of the ovary and related cells and specialized ECMs and will enable the rational design of biomimetic tissue engineered models and durable bioprostheses. Impact Statement Shear wave (SW) ultrasound elastography can be used to simultaneously assess the material properties and tissue structures when accompanied with B-mode ultrasound. We report a quantitative difference in mechanical properties, as inferred from SW velocity, between the cortex and medulla, with SW velocity being 11.4% and 8.4% higher in the medulla than the cortex when measured along the length and width, respectively. This investigation into the spatial and temporal variation in SW velocity in bovine ovaries will encourage and improve design of more biomimetic scaffolds for ovarian tissue engineering.

Mechanobiology of the female reproductive system

“… 196 However, it has been suggested that after activation of primordial follicles, growing follicles migrate from the stiff cortex to soft medulla. 24 However, if the medulla is stiffer than the cortex in human ovaries, as recent studies have shown in bovine 30 and mouse ovaries, 27 durotaxis might be involved in human folliculogenesis. The ovarian wound healing process after ovulation is not clear.…”

“… 28 Large animal models, particularly bovine, equine, and ovine models, have been used to understand ovarian function in women. 29 Gargus et al 30 assessed whole bovine ovaries ex vivo, using shear wave (SW) ultrasound elastography. They showed that the SW velocity was higher in the medulla than the cortex.…”

“…They showed that the SW velocity was higher in the medulla than the cortex. 30 Although laboratory mice are not entirely suitable for investigation of mechanobiology in ovaries, recent studies in both murine and bovine ovaries have provided data contradicting the long‐standing dominant paradigm of a stiff cortex versus Less stiff medulla. 27 , 30 …”

The matrisome contributes to the increased rigidity of the bovine ovarian cortex and provides a source of new bioengineering tools to investigate ovarian biology

“…32 The data here, however, conflicts with two recent studies, one examining ultrasound shear wave velocities in bovine ovaries and another which examined the micromechanical properties of the murine ovary. 65,66 Both studies indicated that the medullary region was more rigid than the cortex with the study by Hopkins, et al concluding that the major driver of rigidity changes in the murine ovary is most closely associated with mature follicles. However, these differences in bovine versus murine ovary rigidity measurements using the same nanoindentation methods could indicate that significant species differences exist in the physical properties and organization of the ovary.…”

“…68 The research by Gargus, et al examined whole bovine ovaries through shear wave velocity. 66 We chose to use nanoindentation methods over other methods that measure viscoelastic properties because nanoindentation can work on the micron scale. However, alterations in shear wave velocity could indicate other changes across compartments associated with differences in ECM architecture and composition, density, size, and shape of follicles and other structures.…”