Animal Models and Alternatives in Vaginal Research: a Comparative Review

Abstract

While developments in gynecologic health research continue advancing, relatively few groups specifically focus on vaginal tissue research for areas like wound healing, device development, and/or drug toxicity. Currently, there is no standardized animal or tissue model that mimics the full complexity of the human vagina. Certain practical factors such as appropriate size and anatomy, costs, and tissue environment vary across species and moreover fail to emulate all aspects of the human vagina. Thus, investigators are tasked with compromising specific properties of the vaginal environment as it relates to human physiology to suit their particular scientific question. Our review aims to facilitate the appropriate selection of a model aptly addressing a particular study by discussing pertinent vaginal characteristics of conventional animal and tissue models. In this review, we first cover common laboratory animals studied in vaginal research-mouse, rat, rabbit, minipig, and sheep-as well as human, with respect to the estrus cycle and related hormones, basic reproductive anatomy, the composition of vaginal layers, developmental epithelial origin, and microflora. In light of these relevant comparative metrics, we discuss potential selection criteria for choosing an appropriate animal vaginal model. Finally, we allude to the exciting prospects of increasing biomimicry for in vitro applications to provide a framework for investigators to model, interpret, and predict human vaginal health.

Keywords: Estrus; Vaginal microbiome; Vaginal tissue model.

Fig. 1. Vaginal anatomy across species

The anatomical variations of the vagina is schematized with their associated relative size. Although rodent vaginas are much smaller than human vaginas, they similarly possess a unique orifice through which the vagina reaches the external introitus. As with human, the mouse and rat vaginas extend from the uterine body to the external opening dorsal to the urethra. In contrast, the urethra enters the vagina for a single urogenital sinus at the base of the vaginal tract for rabbit, pig, and sheep. Commonly used animal models possess bicornuate uterine horns. The sheep ewe notably does feature a single uterine body similarly to the uterus found in the human reproductive tract. However, the cervical region joining the uterine body to the vaginal cavity differs across species.

Fig. 2. Composition of vaginal tissue layers across species

The histologic features of the vagina is schematized with the vaginal lumen facing up. Rodent epithelium is keratinized although the thickness varies with species as well as across the estrus cycle. The rabbit vagina is unique in that the epithelium is simple columnar and hyper-rugated compared to the other species. Pig and sheep have very similar histologic features, which are similar to human with the exception of epithelium thickness. The subepithelial layer of each species are similar in that they are vascularized, contain fibroblasts, immune cells, and extra cellular matrix, although the relative amount varies with mice and rabbits having less dense matrix. Below the submucosal space is smooth muscle and below that loose connective tissue or adventitia, which attaches the vaginal tissue to the surrounding structures.

Fig. 3. Benchtop 3D printed pelvic model for biomechanical testing

(a) The comprehensive pelvic model incorporates uterine, cervical, and vaginal cavities based on representative adolescent pelvic MRI images. As it is enclosed in acrylic housing, this model can be uniformly pressurized for experimental modeling. (b) Additionally, the vaginal cavity can be isolated as shown here to simulate external pressure applied to it in order to test and validate stent placements or other vaginal medical devices within the model.