Elsevier

Theriogenology

Volume 156, 15 October 2020, Pages 155-161
Theriogenology

Equine endometrial development during late fetal and postnatal periods

https://doi.org/10.1016/j.theriogenology.2020.06.006Get rights and content

Highlights

  • Uterine gland development is essential for fertility.

  • In horses, uterine gland development begins prenatally and is completed postnatally.

  • Gland proliferation occurs later in horses compared to other species.

Abstract

Equine uterine development, including endometrial histogenesis, begins prenatally and is completed postnatally. Little is known about this process in the horse. Uterine tissue was acquired from 38 foals, ranging in developmental age from gestational day (GD) 300 to postnatal day (PND) 180, for assessment of endometrial histogenesis. Patterns of endometrial cell proliferation were evaluated by multispectral imaging of uterine tissue sections stained immunofluorescently for Ki-67. Labeling index (LI, % labeled cells) for Ki-67 was calculated for each endometrial cell compartment (luminal epithelium, glandular epithelium, stroma). Histologically, nascent endometrial glands were present in all pre- and postnatal uterine tissues. Overall, Ki-67 LI increased (P < 0.0001) from the pre-to postnatal periods, and was higher (P < 0.0001) in epithelium as compared to stroma. Postnatally, endometrial Ki-67 LI increased (P < 0.0001) from week 1 to week 24. Our findings confirm that, in contrast to neonatal patterns of uterine development described for domestic ungulates, equine endometrial histogenesis begins prenatally, marked by the appearance of uterine glands as early as GD 300. Epithelial proliferation associated with maturation of the equine endometrium is pronounced by postnatal week 24.

Introduction

The mammalian uterus is derived from the paramesonephric or Müllerian ducts [1]. Uterine organogenesis begins prenatally and, grossly, adult uterine phenotype is determined by the degree of fusion of the paramesonephric ducts, which can be complete, partial or incomplete. Equids (Perissodactyla) exhibit a bicornuate uterus, reflecting a moderate degree of paramesonephric fusion. The result is a “T” shaped organ with relatively short uterine horns (cornua) and a more prominent uterine body [2]. Histologically, the uterine wall is incompletely developed at birth, and endometrial histogenesis is completed postnatally [3].

Despite the importance of normal endometrial development, knowledge of this process in equids is limited. It is known that, in contrast to data reported for domestic ungulates [3], equine endometrial gland genesis begins prenatally, with nascent glandular epithelium present after gestational day 250, and that gland genesis proceeds in the postnatal period [4]. Further, administration of estrogen from postnatal day 1 through 6 months of age simulated precocious uterine gland development [5], indicating that these tissues are, or become, steroid hormone (estrogen) –sensitive during this period. Beyond these observations, little is known about events associated with histogenesis, particularly adenogenesis, of the equine endometrium. The characterization of the development biology of the uterine glands may provide the basis to better understand uterine diseases in adults.

In species where endometrial adenogenesis has been studied intensely, the process involves stromal-epithelial interactions that support tissue remodeling, tissue compartment-specific patterns of cell proliferation, and locally regulated patterns of steroid hormone receptor expression in epithelial and stromal compartments of the developing endometrium [3,[6], [7], [8], [9]]. Patterns defined by these cell behaviors, particularly those associated with cell proliferation and hormone receptor expression, can be used to identify critical periods of tissue sensitivity to developmental endocrine disruption [10]. Such information is completely unknown for the horse, yet critically important given the widespread use of hormonal therapies in the pregnant and lactating broodmare which could potentially interfere with normal endometrial gland development in the perinatal foal.

Current literature devoted to equine endometrial glands is largely focused on the adult mare uterus regarding the presence of receptors, morphological, and pathological appearance [11]. However, it is important not to make assumptions about uterine development during fetal and neonatal life based on what is known about adult animals. Tissues are more malleable in earlier stages of development, and it is probable that juvenile cell types do not behave as they would in the adult endometrium. This may explain how hormones and xenobiotics encountered during perinatal life in other species exert lasting organizational effects on uterine development and fertility in adults [12,13].

The objectives of the present study were to: (1) evaluate equine endometrial histogenesis in uterine tissues obtained from late fetal and postnatal periods using immunohistochemistry, multispectral imaging, and digital image analysis; (2) determine effects of developmental age on patterns of endometrial cell proliferation in glandular epithelium (GE), luminal epithelium(LE), and stroma (ST) as reflected by MKI67 immunostaining.

Section snippets

Tissue sampling

Thirty-nine equine uteri were acquired from one of three sources. Uteri (n = 21) were obtained from normal, healthy animals as part of other unrelated experiments performed at the Maxwell H. Gluck Equine Research Center at the University of Kentucky, Lexington, KY, USA. Animals were euthanized as per Institutional Animal Care and Use Committee guidelines for the respective projects in which animals were enrolled. Additionally, two uteri were obtained from clinical cases at the Auburn University

Results

A summary of observations related to the effects of age on equine uterine histology, from pre-to postnatal periods, is depicted in Table 1. Representative photomicrographs of uterine sections from each age group category are portrayed in Fig. 1. All tissue specimens examined contained all primary tissue layers, including endometrium (stratum compactum [SC] and stratum spongiosum [SS]), myometrium (MYO), and perimetrium (PERI). In the fetal age group (GD 300–337; Fig. 1A), epithelial budding was

Discussion

The present study was designed to better understand uterine development in horses, with emphasis on adenogenesis of endometrial glands starting late in pregnancy and ending at the prepubertal period. Characterization of the normal developmental biology may provide the basis to better understand conditions which may disturb this process. Disruption of the normal developmental program during what Charles Stockard referred to as “critical moments” or periods of development can have lasting

Funding

This work was supported by the Birmingham Racing Commission.

Declaration of competing interest

None.

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  • Cited by (0)

    1

    School of Veterinary Science, The University of Queensland, Gatton, QLD, Australia.

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