Sperm cryopreservation has become an indispensable tool in biology. Initially, studies were aimed towards the development of efficient freezing protocols in different species that would allow for an efficient storage of semen samples for long periods of time, ensuring its viability. Nowadays, it is widely known that an important individual component exists in the cryoresistance of semen, and efforts are aimed at identifying those sperm characteristics that may allow us to predict this cryoresistance. This knowledge would lead, ultimately, to the design of optimized freezing protocols for the sperm characteristics of each male.
We have evaluated the changes that occur in the sperm head dimensions throughout the cryopreservation process. We have found three different patterns of response, each of one related to a different sperm quality at thawing. We have been able to characterize males based on these patterns. For each male, its pattern remained constant among different ejaculates. This latter would imply that males always respond in the same way to freezing, giving even more importance to this sperm feature.
Changes in the sperm head during cryopreservation process have resulted useful to identify the ability of semen of males for freezing. We suggest that analyses of these response patterns would represent an important tool to characterize the cryoresistance of males when implemented within breeding programs. We also propose follow-up experiments to examine the outcomes of the use of different freezing protocols depending on the pattern of response of males.
Citation: Ramón M, Pérez-Guzmán MD, Jiménez-Rabadán P, Esteso MC, García-Álvarez O, Maroto-Morales A, et al. (2013) Sperm Cell Population Dynamics in Ram Semen during the Cryopreservation Process. PLoS ONE8(3): e59189. https://doi.org/10.1371/journal.pone.0059189
Editor: Stefan Schlatt, University Hospital of Münster, Germany
Received: November 22, 2012; Accepted: February 12, 2013; Published: March 27, 2013
Copyright: © 2013 Ramón et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by the Education and Science Council of Castilla-La Mancha (PCC08-0105-8042). Manuel Ramón was supported by the DOC-INIA program. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Understanding the switching of phenotypes in response to environmental changes is at the forefront of biological research in diverse areas of study . One of these areas is focused on the study of cell response to low temperatures. Cell cryopreservation has become an indispensable tool in biology. Biological materials can be safely kept and used after a very long period of time. For the particular case of spermatozoa, cryopreservation is used in livestock management and in the conservation of wild and domestic species, as a complementary tool for managing live animals and preserving their genetic diversity.
Several studies have been conducted to understand the changes that occur during cryopreservation of spermatozoa and develop a protocol to ensure a successful storage (see  for references and general discussion). These studies have focused on different traits such as sperm motility and morphometry, integrity of sperm membranes or DNA status among others –. As a result of these studies, standard cryopreservation protocols have been developed for many species, both domestic and wild.
One important problem for standardizing sperm cryopreservation protocols is that spermatozoa from different individuals exhibit significant different responses to the same freezing treatment , –. Males can be classified into “bad” and “good” freezers depending on the ability of their spermatozoa to resist to the cryopreservation process. Many studies have b