|Procedure||Sperm collection and cryopreservation|
|Posted On||12/29/2010 12:07 PM|
|Science Writer||Nora G. Smart|
|Reagents and Solutions|
Table1: Listing of all used chemicals with related supplier informations
Table 2: Listing of all used equipment and materials with related supplier information
Sperm collection from male mice and sperm cryopreservation
Method is adapted from references (1-4).
Sperm collection for cryopreservation is performed in the cryoprotectant medium (CPM) directly. For other purposes than cryopreservation (e.g. IVF), sperm collection is performed following the described method but other collection media must be used. Photographs of the dissection procedure of cauda epididymis and vas deferens (Figure 1) for collecting sperm are presented in Figure 2. The cryoprotectant solution, containing 18% D+ raffinose and 3% skim milk in embryo-graded water, is prepared according to a method described by Nakagata (5). After dissolving the raffinose completely at 37°C using a 15 mL Falcon tube, milk powder is added and vortexed until the emulsion becomes homogenous, transferred to 2 mL tubes and centrifuged at 13’000 rcf (11’500 rpm) for 15 minutes. Supernatants are pooled and filter sterilized (0.45 μm). If necessary, the CPM can be stored at + 4°C for five days or at – 20°C for up to three months.
A three to six-month old male is humanely euthanized by cervical dislocation, placed on its back on absorbent paper and the abdomen is generously sprayed with 70% EtOH for disinfection (Figure 2 a, b). With the 1st pair of dissecting forceps a small fold of skin is lifted in the center of the abdomen level with the top of the legs and a small cut is made with scissors (4.5”) to open the abdominal cavity (c), the skin is pulled in opposite directions towards head and tail until the abdomen is completely exposed (d). Using the 2nd pair of forceps a small fold of the peritoneum is lifted and a small cut using 4” scissors is made to allow air to enter the abdominal cavity (e, f). The peritoneum is cut open to expose the body cavity, using caution not to cut any of the internal organs and the coils of the gut are pushed towards the head exposing the testicular fat pad (g). Pulling the testicular fat pad towards the head exposes the attached testis, vas deferens and cauda epididymis (h). The muscle tissue adjacent to the cauda epididymis is cut apart (i, j). The vas deferens is cut near the bladder end (k) and gently peeled away from the body cavity, leaving behind the major blood vessel and fat (l, m). A last cut is made just below the cauda epididymis (n). These steps are repeated for the second cauda epididymis and vas deferens.
The dissected cauda epididymis and vas deferens are placed in a 60 x 15 mm collection dish with 1 mL CPM prewarmed at 37°C. The sperm is released by slicing the cauda epidymis 5 to 7 times and “walking“ down the vas deferens using two insulin syringes or forceps (o – r). After allowing the sperm to disperse for approximately 10 to 15 minutes at 37°C the tissues are removed.
Sperm cryopreservation is either performed using nunc cryotubes or plastic semen straws. In both cases a container with a lid and enough LN2 to produce a stable vapor phase (the temperature approximately – 120°C) has to be set-up prior to freezing. Additionally an open rack to hold the cryotubes or straws in the vapor phase of LN2 is needed.
For freezing sperm in cryotubes, after the collected sperm (~ 1 mL in CPM) is incubated 10 to 15 minutes at 37 C and tissues are removed, 100 μL aliquots are distributed into nunc cryotubes tubes using wide-bore pipette tips and slow pipetting to avoid shearing forces. The closed tubes are immediately placed into the rack resting in the LN2 vapor. Enough space is left between the tubes to ensure a full exposure to LN2 vapor. After 10 minutes in the vapor phase with a descending cooling rate of approximately – 20°C to – 40°C per minute the sperm is directly transferred into the liquid nitrogen.
For the cryopreservation of sperm using plastic semen straws (2.5 mL, 133 mm), 100 μL of the collected sperm in CPM are aspirated with a Monoject syringe applied to the labeled end of the straw. Both ends of the straw are sealed with Critoseal and placed into the rack resting in the LN2 vapor. After 10 minutes the straws are directly immersed in the LN2 tank for long-term storage.
Sperm thawing for in vitro fertilization
Sperm frozen in cryotubes is rapidly thawed by transferring the tube into a 37.5°C water bath for approximately 2 minutes until all ice crystals are melted (visual check). Subsequently, the sample is centrifuges at 735 x g (max. 2000 rpm) for 4 minutes. The supernatant (CPA) is discarded and replaced with 50 μL pre-equilibrated HTF/BSA and gently mixed by tapping the tube. The resuspended sperm sample is incubated for 10 minutes at 37°C to allow for minimal “swim up”. A maximal 40 μL aliquot of the sperm is taken for IVF. The 10 μL remainder is utilized for concentration and motility analysis.
Evaluation of sperm concentration and motility
Concentration and motility of sperm are determined using a Neubauer-improved haemocytometer. For counting fresh collected sperm (1 mL per male) a dilution of 1:30 for hybrid mice and 1:20 for inbred is made with FHM using 10μL of sperm. Frozen-thawed and resuspended samples are diluted in a ratio of 1:10 only. The numbers of total and motile sperm are assessed twice per sample by counting sperm cells within the 25 group squares (total central large square with an area of 1 mm2 and a depth of 0.1 mm resulting in a volume of 0.1 mm3 equivalent to 0.1 µL) of the hemocytometer. To ensure adequate accuracy of the method, at least 200 to 400 spermatozoa should be counted within one large square; in the case of uneven distribution of the sperm cells within the counting chamber, two ore more of the corner large squares are counted and the total amount of cells is divided by the number of analyzed fields. By convention, sperm concentration is expressed as the number of spermatozoa per milliliter. Motility is defined as any movement of the sperm head (% moving), and progressive motility as the count of those spermatozoa that move in a forward, linear direction at a speed of 50 μm per second (automated sperm counter).
1. Sztein, J.M., Farley, J., Young, A. and Mobraaten, L.E. (1997) Motility of cryopreserved mouse spermatozoa affected by temperature of collection and rate of thawing. Cryobiology 35, 46-52.
2. Anon. (2006): Cryopreservation of mouse germplasm. The Jackson Laboratory, Course book
3. Anon. (2006): Cryopreservation of mouse germplasm. The Jackson Laboratory, Lab manual p. 7