A Novel Use of Three-dimensional High-frequency Ultrasonography for Early Pregnancy Characterization in the Mouse

Summary

Mice are widely used to study gestational biology. However, pregnancy termination is required for such studies which precludes longitudinal investigations and necessitates the use of large numbers of animals. Therefore, we describe a non-invasive technique of high-frequency ultrasonography for early detection and monitoring of post-implantation events in the pregnant mouse.

Abstract

High-frequency ultrasonography (HFUS) is a common method to non-invasively monitor the real-time development of the human fetus in utero. The mouse is routinely used as an in vivo model to study embryo implantation and pregnancy progression. Unfortunately, such murine studies require pregnancy interruption to enable follow-up phenotypic analysis. To address this issue, we used three-dimensional (3-D) reconstruction of HFUS imaging data for early detection and characterization of murine embryo implantation sites and their individual developmental progression in utero. Combining HFUS imaging with 3-D reconstruction and modeling, we were able to accurately quantify embryo implantation site number as well as monitor developmental progression in pregnant C57BL6J/129S mice from 5.5 days post coitus (d.p.c.) through to 9.5 d.p.c. with the use of a transducer. Measurements included: number, location, and volume of implantation sites as well as inter-implantation site spacing; embryo viability was assessed by cardiac activity monitoring. In the immediate post-implantation period (5.5 to 8.5 d.p.c.), 3-D reconstruction of the gravid uterus in both mesh and solid overlay format enabled visual representation of the developing pregnancies within each uterine horn. As genetically engineered mice continue to be used to characterize female reproductive phenotypes derived from uterine dysfunction, this method offers a new approach to detect, quantify, and characterize early implantation events in vivo. This novel use of 3-D HFUS imaging demonstrates the ability to successfully detect, visualize, and characterize embryo-implantation sites during early murine pregnancy in a non-invasive manner. The technology offers a significant improvement over current methods, which rely on the interruption of pregnancies for gross tissue and histopathologic characterization. Here we use a video and text format to describe how to successfully perform ultrasounds of early murine pregnancy to generate reliable and reproducible data with reconstruction of the uterine form in mesh and solid 3-D images.

Introduction

Recurrent early pregnancy loss is one of the most common complications after conception and affects approximately 1% of couples trying to conceive^1,2. The underlying mechanisms of early pregnancy loss are varied: from intrinsic embryonic abnormalities and maternal comorbidities to defects in endometrial receptivity^1,3,4. Because of their genetic tractability, mouse models have been widely utilized for investigations of early embryo implantation and pregnancy. Furthermore, the short gestational time of the mouse and th....

Protocol

These studies were conducted in accordance with the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health and animal protocols approved by the Institutional Animal Care and Use Committee (IACUC) of Baylor College of Medicine under protocol number AN-4203.

1. Preparation of the Pregnant Mouse for Ultrasound

1. Timed mating
   1. Place the dam with a proven fertile male mouse overnight beginning after 1700. Separate dam and male by 0700 hours (h), regardless if vaginal plug is present, to ensure accurate dating of pregnancy.
      NOTE: The morning after mating is con....

Results

As demonstrated in Figure 1, the high frequency ultrasound can detect implantation site development beginning as early as the 5.5 d.p.c time point. Using the lighter hyperechoic decidualized endometrium as a marker of implantation sites at 6.5 d.p.c allows the number of implantation sites and spacing of these sites to be quantified. As the pregnancy progresses to 7.5 d.p.c., a darker hypoechoic gestational sac and fetal pole are also easily identifiable.

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Discussion

This novel use of 3-D HFUS imaging demonstrates the ability to successfully detect, visualize, and characterize embryo-implantation sites during early murine pregnancy in a non-invasive manner. The technology offers a significant improvement over current methods, which rely on the interruption of pregnancies for gross tissue and histopathologic characterization. However it should be noted that histological methods would still be considered more optimal when characterization at a more magnified and more cellular level is .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
VisualSonics Vevo 2100 Ultrasound Imaging Platform/Machine	VisualSonics, inc.	VS-11945
Vevo Imaging Station	VisualSonics, inc.	SA-11982
Aquasonic 100 Ultrasound Transmission Gel	Parker	#SKU PLI 01-08
Isoflurane (IsoThesia) 100mL bottle	Henry Shein	#29404
PuraLubenAnimal Ophthalmic Ointment	Dechra	#12920060