Electrolyte distribution and yolk sac morphology in frozen hydrated equine conceptuses during the second week of pregnancy
Laura J. Crews A C , Rudolf O. Waelchli A , Cheng X. Huang B D , Martin J. Canny B D , Margaret E. McCully B E and Keith J. Betteridge A FA Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.
B Cryo-Analytical SEM Facility, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada.
C Present address: University of Minnesota, Veterinary Clinical Sciences Department, College of Veterinary Medicine, 1352 Boyd Ave, St Paul, MN 55108, USA.
D Present address: The Research School of Biological Sciences, Australian National University, Canberra, ACT 0200, Australia.
E Present address: Division of Plant Industry, CSIRO, Canberra, ACT 2601, Australia.
F Corresponding author. Email: kbetter@uoguelph.ca
Reproduction, Fertility and Development 19(7) 804-814 https://doi.org/10.1071/RD07050
Submitted: 14 March 2007 Accepted: 21 May 2007 Published: 8 August 2007
Abstract
To investigate how equine conceptuses expand rapidly despite the hypo-osmolality of their yolk sac fluid, 18 conceptuses, aged 8–12 days and 0.8–10.0 mm in diameter, were examined by cryoscanning electron microscopy and energy dispersive X-ray microanalysis to determine the distribution of Na, Cl and K in their fluids. No osmotic gradient was found between central and peripheral yolk sac fluid. In conceptuses ≥ 6 mm in diameter, the concentrations of both Na and K in the subtrophectodermal compartments were higher than those determined previously in uterine fluid, supporting the concept of osmotic intake of fluid from the uterine environment as far as the compartments. However, electrolyte concentrations in the compartments consistently exceeded those found in the yolk sac, making it likely that ‘uphill’ water transport, rather than a purely osmotic uptake, is involved in yolk sac fluid accumulation. We also speculate that capsule formation could actively contribute to conceptus expansion and thereby to fluid intake.
Additional keywords: blastocyst expansion, cryoscanning electron microscopy, embryonic development, endoderm, energy dispersive X-ray microanalysis.
Acknowledgements
The authors thank the Natural Sciences and Engineering Research Council Canada, the Ontario Ministry of Agriculture, Food and Rural Affairs and the E. P. Taylor Research Foundation for financial support; Lois Betteridge for fabrication of the freezing chamber; Dr Claudia Freyer for provision of unpublished information; and Drs J. I. Raeside, M. A. Hayes, and J. F. Leatherland for their helpful comments on the manuscript.
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