Determining the timeline of gonadal and genital differentiation in male and female equine fetuses allows for early detection and intervention in malformations.
Abstract: Sexual differentiation in the equine fetus involves coordinated morphogenetic processes that shape both the gonads and the genital ducts. Although the formation of testes and ovaries has been relatively well documented, the temporal dynamics and morphometric patterns of the mesonephric (Wolffian) and paramesonephric (Müllerian) ducts remain insufficiently characterized. This study aimed to detail the timeline of gonadal and ductal differentiation in male and female equine fetuses during the critical developmental window between 60 and 80 days of gestation. Gross anatomical evaluation, histological analysis, and immunohistochemistry for 17β-hydroxysteroid dehydrogenase (17β-HSD) were performed to assess structural maturation and steroidogenic activity. The mesonephric duct was quantitatively assessed using unbiased stereological methods. Results demonstrated clear sexual dimorphism: males exhibited increased mesonephric duct volume at 80 days, whereas females showed progressive regression of this structure. Notably, this preliminary comparison across two ages appears to quantitatively support prior descriptions of paramesonephric duct regression. Positive 17β-HSD immunolabeling in both sexes indicated active steroidogenesis during this developmental window. By establishing a refined timeline for gonadal and ductal differentiation, this study advances the understanding of normal reproductive tract development and supports early identification of congenital malformations in horses.
© 2026 The Author(s). The Anatomical Record published by Wiley Periodicals LLC on behalf of American Association for Anatomy.
Publication Date: 2026-01-27 PubMed ID: 41589521DOI: 10.1002/ar.70137Google Scholar: Lookup
The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Journal Article
Summary
This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.
Overview
- This study characterizes the timeline and morphological changes of gonadal and genital duct differentiation in male and female horse fetuses between 60 and 80 days of gestation.
- The research highlights differences in development between sexes and identifies active steroid hormone production during this critical period, aiding early detection of reproductive malformations.
Background and Research Purpose
- Sexual differentiation in equine fetuses involves coordinated changes in gonads (testes and ovaries) and the genital ducts (mesonephric/Wolffian and paramesonephric/Müllerian ducts).
- Although gonadal formation has been well studied, the exact timing and morphometric changes of the mesonephric and paramesonephric ducts remain poorly detailed.
- The study aimed to fill this knowledge gap by defining the precise timeline of differentiation and quantifying structural changes in both male and female equine fetuses.
- Understanding these processes has clinical value for early detection and potential intervention in congenital reproductive malformations in horses.
Methods
- Samples were collected from equine fetuses aged between 60 and 80 days of gestation, representing a critical window of sexual differentiation.
- Gross anatomical evaluation was performed to observe overall morphological features of gonads and genital ducts.
- Histological analyses provided microscopic details on tissue structure and maturation.
- Immunohistochemistry targeting 17β-hydroxysteroid dehydrogenase (17β-HSD) was used to identify sites of active steroid hormone synthesis.
- Quantitative assessment of the mesonephric duct was done using unbiased stereological techniques, allowing accurate volume measurement and morphological comparison between sexes and across ages.
Key Findings
- Sexual dimorphism was demonstrated in genital duct development:
- In male fetuses, there was a significant increase in the mesonephric (Wolffian) duct volume by day 80, consistent with its role in male reproductive tract formation.
- In female fetuses, the mesonephric duct showed progressive regression over the same period, aligning with its eventual disappearance as the female reproductive tract develops.
- The study quantitatively supported previous descriptions of paramesonephric (Müllerian) duct regression in males.
- 17β-HSD positive immunolabeling was observed in both male and female gonads, indicating active local steroidogenesis during this differentiation phase.
- This provides insight into the hormonal environment facilitating sexual differentiation.
Significance and Implications
- The refined timeline and morphometric data provide a deeper understanding of normal equine reproductive development from 60 to 80 days of gestation.
- Quantitative assessment methodologies like stereology enhance accuracy and reproducibility in developmental studies.
- Identifying the timing and pattern of duct development and regression informs veterinary diagnostics aimed at early identification of congenital malformations.
- Understanding steroidogenic activity during this window may help elucidate hormonal disruptions underlying developmental abnormalities.
- The study supports improved reproductive management and potential early interventions in equine veterinary medicine.
Cite This Article
APA
de Castro Sasahara TH, Gomes SP, Abdala FCM, Schimming BC, Miglino MA.
(2026).
Determining the timeline of gonadal and genital differentiation in male and female equine fetuses allows for early detection and intervention in malformations.
Anat Rec (Hoboken).
https://doi.org/10.1002/ar.70137 Publication
Researcher Affiliations
- Instituto de Biociências, Departamento de Biologia Estrutural e Funcional, Universidade Estadual Paulista (UNESP), São Paulo, Botucatu, Brazil.
- Faculdade de Medicina Veterinária e Zootecnia, Departamento de Cirurgia, Setor de Anatomia dos Animais Domésticos e Silvestres, Universidade de São Paulo (USP), São Paulo, São Paulo, Brazil.
- Faculdade de Medicina Veterinária e Zootecnia, Departamento de Cirurgia, Setor de Anatomia dos Animais Domésticos e Silvestres, Universidade de São Paulo (USP), São Paulo, São Paulo, Brazil.
- Instituto de Biociências, Departamento de Biologia Estrutural e Funcional, Universidade Estadual Paulista (UNESP), São Paulo, Botucatu, Brazil.
- Postgraduate Program in Animal Health, Production and Environment, University of Marilia (UNIMAR), Marilia, Brazil.
References
This article includes 23 references
- Allen, W. R. (2001). Fetomaternal interactions and influences during equine pregnancy. Reproduction, 121(4), 513–527. https://doi.org/10.1530/rep.0.1210513
- Ball, B. A., Conley, A. J., Grundy, S. A., Sabeur, K., & Liu, I. K. M. (2008). Expression of anti‐Müllerian hormone (AMH) in the equine testis. Theriogenology, 69(5), 624–631. https://doi.org/10.1016/j.theriogenology.2007.11.009
- Barreto, R. S. N., Costa, D. S., Bombonato, P. P., Ambrósio, C. E., & Miglino, M. A. (2018). Reproductive system development in male and female horse embryos and fetuses. Theriogenology, 108, 118–126. https://doi.org/10.1016/j.theriogenology.2017.11.036.
- DeFalco, T., & Capel, B. (2009). Gonad morphogenesis in vertebrates: Divergent means to a convergent end. Annual Review of Cell and Developmental Biology, 25, 457–482. https://doi.org/10.1146/annurev.cellbio.042308.13350
- Douglas, R. H., & Ginther, O. J. (1975). Development of the equine fetus and placenta. Journal of Reproduction and Fertility. Supplement, 23, 503–505.
- Evans, H. E., & Sack, W. O. (1973). Prenatal development of domestic and laboratory mammals: Growth curves, external features and selected references. Anatomia, Histologia, Embryologia, 2, 11–45.
- Franciolli, A. L. R., Cordeiro, B. M., da Fonseca, E. T., Rodrigues, M. N., Sarmento, C. A., Ambrósio, C. E., de Carvalho, A. F., Miglino, M. A., & Silva, L. A. (2011). Characteristics of the equine embryo and fetus from days 15 to 107 of pregnancy. Theriogenology, 76(5), 816–832. https://doi.org/10.1016/j.theriogenology.2011.04.014.
- González‐Angulo, A., Rojas‐Cortés, T., & Merchant‐Larios, H. (1975). Sexual differentiation of the equine embryo and fetus. Journal of Reproduction and Fertility. Supplement, 23, 527–531.
- Gundersen, H. J. G., & Jensen, E. B. (1987). The efficiency of systematic sampling in stereology and its prediction. Journal of Microscopy, 147(3), 229–263.
- Gundersen, H. J. G., Jensen, E. B. V., & Kieu, K. (1999). The efficiency of systematic sampling in stereology—Reconsidered. Journal of Microscopy, 193(3), 199–211.
- Hay, M. F., & Allen, W. R. (1975). An ultrastructural and histochemical study of the interstitial cells in the gonads of the fetal horse. Journal of Reproduction and Fertility. Supplement, 23, 557–561.
- Howard, V., & Reed, M. G. (2004). Unbiased stereology: Three‐dimensional measurement in microscopy (2nd ed.). Garland Science.
- Hyttel, P., Sinowatz, F., & Vejlsted, M. (2012). Veterinary embryology. Elsevier.
- Kierszenbaum, A. L., & Tres, L. L. (2001). Primordial germ cell‐somatic cell partnership: A balancing cell signaling act. Molecular Reproduction and Development, 60(3), 277–280. https://doi.org/10.1002/mrd.1088
- Klattig, J., & Englert, C. (2007). The Müllerian duct: Recent insights into its development and regression. Sexual Development, 1(5), 271–278. https://doi.org/10.1159/000108929
- Klonisch, T., Fowler, P. A., & Hombach‐Klonisch, S. (2004). Molecular and genetic regulation of testis descent and external genitalia development. Developmental Biology, 270(1), 1–18. https://doi.org/10.1016/j.ydbio.2004.02.018
- Merchant‐Larios, H. (1975). Morphogenesis of gonads and ducts in the equine fetus. Journal of Reproduction and Fertility. Supplement, 23, 495–500.
- Moraes, G. D., Miglino, M. A., Silva, F. O. C., et al. (2016). Estudo de ovários fetais equinos: uma abordagem histológica. Pesquisa Veterinária Brasileira, 36(11), 1116–1120. https://doi.org/10.1590/S0100-736X2016001100010
- Mullen, R. D., & Behringer, R. R. (2014). Molecular genetics of Müllerian duct formation, regression and differentiation. Sexual Development, 8(5), 281–296. https://doi.org/10.1159/000364935
- Nyuji, M., Hongo, Y., Yoneda, M., & Nakamura, M. (2020). Transcriptome characterization of BPG axis and expression profiles of ovarian steroidogenesis‐related genes in the Japanese sardine. BMC Genomics, 21, 668. https://doi.org/10.1186/s12864-020-07080-1
- Raeside, J. I., Christie, H. L., Renaud, R. L., & Betteridge, K. J. (2004). Estrogen metabolism in the equine conceptus and endometrium. Biology of Reproduction, 71(4), 1120–1127. https://doi.org/10.1095/biolreprod.104.028712
- Scarlet, D., Handschuh, S., Reichart, U., Podico, G., Ellerbrock, R. E., Demyda‐Peyrás, S., Canisso, I. F., Walter, I., & Aurich, C. (2021). Sexual differentiation and primordial germ cell distribution in the early horse fetus. Animals, 11(8), 2422. https://doi.org/10.3390/ani11082422
- Silveira, J. M., dos Santos, A. C., de Brito, D. C. C., Oliveira, M. F., Conley, A. J., & de Assis Neto, A. C. (2024). Morphohistometric and steroidogenic parameters during testicular and epididymal differentiation in cavy (Galea spixii) fetuses. Reproductive Biology, 24(1), 100829. https://doi.org/10.1016/j.repbio.2023.100829
Citations
This article has been cited 0 times.Use Nutrition Calculator
Check if your horse's diet meets their nutrition requirements with our easy-to-use tool Check your horse's diet with our easy-to-use tool
Talk to a Nutritionist
Discuss your horse's feeding plan with our experts over a free phone consultation Discuss your horse's diet over a phone consultation
Submit Diet Evaluation
Get a customized feeding plan for your horse formulated by our equine nutritionists Get a custom feeding plan formulated by our nutritionists
