A Review and Case Study of 3D Imaging Modalities for Female Amniote Reproductive Anatomy

Abstract

Recent advances in non-invasive imaging methods have revitalised the field of comparative anatomy, and reproductive anatomy has been no exception. The reproductive systems of female amniotes present specific challenges, namely their often internal "hidden" anatomy. Quantifying female reproductive systems is crucial to recognising reproductive pathologies, monitoring menstrual cycles, and understanding copulatory mechanics. Here we conduct a review of the application of non-invasive imaging techniques to female amniote reproductive anatomy. We introduce the commonly used imaging modalities of computed tomography (CT) and magnetic resonance imaging (MRI), highlighting their advantages and limitations when applied to female reproductive tissues, and make suggestions for future advances. We also include a case study of micro CT and MRI, along with their associated staining protocols, applied to cadavers of female adult stoats (Mustela erminea). In doing so, we will progress the discussion surrounding the imaging of female reproductive anatomy, whilst also impacting the fields of sexual selection research and comparative anatomy more broadly.

Keywords: Computed tomography; Genitalia; Magnetic resonance imaging; Mustelidae; Staining.

Fig. 1

(A) and (D) Female stoat specimens were bisected and the cavity opened to ensure optimal uptake of iodine solution, internal organization was preserved where possible. (B) Slice data taken of the coronal plane of the unstained specimen in micro CT—specimen displays very little soft tissue contrast, yet skeleton is easily resolvable. Transect line (red) runs between right femur and left femoral head. Plot (inset) identifies gray-scale values extract along transect using the PeakFinder tool (Vischer 2013). Peaks correspond to bone cortices. (C) 3D model of the pelvic region of the unstained specimen—reproductive tract could not be segmented. Scale bar representative of 10 mm. (E) Slice data taken of the coronal plane of the micro CT scan of the stained specimen—iodine stain improves muscle tissue contrast, and also highlights internal organs. Transect plot (inset) highlights “halo” effect on incomplete staining, but does identify smaller localized peaks internally. (F) 3D model of the female reproductive tract, from the uterus to uterine horn and branching oviducts, of the stained specimen—tract was discernible for manual segmentation. Iodine staining causes reduced contrast between bone and soft tissue, however, making skeletal segmentation a challenge. Scale bar representative of 10 mm. Figure key: C—colon; K—kidney; F—femur; VC—vertebral column; P—pelvis; OD—oviduct: UH—uterine horn; UB—uterine body; and C—cervix

Fig. 2

(A) Slice data taken on the transverse plane of the stained specimen in micro CT (Zeiss Versa)—iodine-contrast agent has stained the external margins, and also provided improved contrast enhancement to the internal organs. (B) 3D model of the female reproductive tract, from the uterus to uterine horn and branching oviducts, taken from micro CT (Zeiss Versa) data of a stained specimen. Scale bar representative of 10 mm. Figure key: F—femur; VC—vertebral column; OD—oviduct; UH—uterine horn; and UB—uterine body

Fig. 3

(A) Female stoat specimen was bisected and opened, but internal organization was preserved where possible. (B) Slice data taken on the transverse plane of the unstained specimen in micro CT—blue arrow identifies the internal cavity and area filled with silicone rubber. (C) 3D model of the pelvic bone region and silicone cast in situ in the frontal plane. Scale bar representative of 10 mm. (D) 3D model of the pelvic bone region and silicone cast in situ in the sagittal plane. Scale bar representative of 10 mm. Figure key: OD—oviduct; UB—uterine body; VC—vertebral column; F—femur; V—vagina; and C—cervix

Fig. 4

(A) and (D) Female stoat specimens were bisected, and the cavity opened to ensure optimal uptake of gadolinium-based contrast agent, yet internal organization was preserved where possible. (B) Slice data taken on the coronal plane of the unstained specimen in MRI—specimen displays some soft tissue contrast. Transect line (red) runs between femurs. Plot (inset) identifies gray-scale values extracted along transect using the PeakFinder tool ( Vischer 2013), with several peaks discernible inside the cavity. (C) 3D model of the pelvic region and the reproductive tract of the unstained specimen—ovaries and oviducts were visible for manual segmentation, but tract was not distinguishable below the uterine horn. Scale bar representative of 10 mm. (E) Slice data taken on the coronal plane of the MRI scan of the stained specimen—contrast agent has further illuminated internal architecture, muscle architecture, and bone. Transect plot (inset) displays a smoother variation between high and low gray-scale values. (F) 3D model of the female pelvic region and reproductive tract—ovaries, oviducts, uterus, and upper vagina were visible. Both reproductive tract and bones were manually segmented. Scale bar representative of 10 mm. Figure key: O—ovaries; OD—oviduct; UH—uterine horn; F—femur; P—pelvis; C—colon; VC—vertebral column; UB—uterine body; and V—vagina