Biological Resource Banks represent a potentially valuable tool for species conservation. It is, however, necessary to understand the species-specific cryopreservation process and its consequences for spermatozoa to aid in the development of assisted reproduction as a future conservation tool. The aim of this study was to assess the in vitro functionality of white rhinoceros Cerathoterium simum epididymal spermatozoa both before and after cryopreservation. Testes from a harvested white rhino bull were removed and transported at 5°C to the laboratory within 4h. The cauda epididymis was dissected out and flushed with 2mL of Tris-citrate egg yolk extender (fraction A, Biladyl, Minitüb, Germany). A 0.1mL aliquot was removed for analysis and the balance (9mL; 2mL fraction A+7mL sperm sample) mixed with an additional 27.2mL of Tris-citrate egg yolk with glycerol (fraction B, Bidadyl). The extended sample was allowed to cool to 4°C over a 6-h period before an additional 29.2mL of cooled fraction B were added (final sperm concentration=150×106mL−1). Sperm samples were loaded into 0.25-mL straws and frozen over LN2 vapor (4cm for 20min) for later assessment. Sperm straws were thawed by placing the straws in water at 37°C for 30s. Pre-freeze and post-thaw evaluations were carried out in the same manner. Media used included: HEPES for washing (20mM HEPES, 355mM sucrose, 10mM glucose, 2.5mM KOH) and HEPES saline (197mM NaCl, instead of sucrose). An aliquot was diluted with HEPES (washing) and centrifuged for 5min at 600×g; the pellet was resuspended in HEPES saline. Sperm motility (total motility %, TM;; and progressive motility %, PM) was assessed using phase contrast microscopy (×200; 37°C). Sperm plasma membrane status was assessed using the fluorescent dye, propidium iodide (50ngmL−1 in HEPES saline;; 10min, RT). Percentage of cells with plasma membranes intact (unstained;; PMI) was recorded. Mitochondrial status was assessed with the fluorescent dye, JC-1 (7.5μM in HEPES saline;; 30min, 37°C). The % of cells with an orange-stained midpiece was recorded (active mitochondria;; MIT). Resilience to hypoosmotic shock (HOS test) was assessed by diluting a sample in 100mOsm/kg HEPES saline (1:20; 15min, RT). An aliquot was stained with PI to assess plasma membrane status (HOSPMI), and the rest was fixed with formaldehyde, and % coiled tails (positive endosmosis;; HOST) was estimated using phase contrast microscopy (×400). Evaluations of PMI, MIT and HOSPMI were performed using fluorescence microscopy (×400, 450–490nm excitation filter). The results indicated that quality was good pre-freezing (TM: 60%; PMI: 86%; MIT: 100%), except for a PM value of 15%. After thawing, although there was a drop in TM (30%), there was no decrease in PM (20%). Our in vitro functional assessment indicated a loss of quality between the pre-freeze and post-thaw evaluations, but PMI and MIT maintained their pre-thaw levels (60% and 72%, respectively). The HOS test, which indicates plasma membrane integrity, decreased from the pre-freeze level (91%) to a post-thaw value of 70%. HOSTPMI was 72% pre-freeze, and decreased to 54% post-thaw. In conclusion, epididymal spermatozoa from the white rhino may retain its functionality after cryopreservation in a commerically available cryo-extender (Bidadyl). The use of assisted reproduction techniques could someday play a role in the management and conservation of the white rhinoceros and related species.
Correlation between the motility of frozen-thawed epididymal spermatozoa and the outcome of intracytoplasmic sperm injection