day 30. When sperm were collected 3–9 days after insulation and examined immediately, their motility and morphology were similar to pre‐insulation values [30]. Compared to semen collected prior to insulation, following freezing, thawing, and incubation at 37 °C for three hours [30], there were significant reductions in the proportion of progressively motile sperm (46 vs 31%, respectively) and the proportion of sperm with intact acrosomes (73 vs 63%). Freezing plus post‐thaw incubation manifested changes that had occurred in sperm that were in the epididymis at the time of scrotal insulation.
In another study [32], scrotal insulation (four days) and dexamethasone treatment (20 mg/day for seven days) were used as models of testicular heating and stress, respectively. Some bulls seemed predisposed to produce sperm with a specific abnormality. Pyriform heads, nuclear vacuoles, microcephalic sperm, and abnormal DNA condensation were more common in insulated than dexamethasone‐treated bulls. Conversely, dexamethasone treatment resulted in an earlier and more severe effect on epididymal sperm, an earlier and greater increase in distal midpiece reflexes, and an earlier increase in proximal and distal droplets. Overall, types of defective sperm and the time of their detection were similar between treatments.
Insulation of the Scrotal Neck
The scrotal neck of five bulls was insulated for seven days (days 1–8) as a model of bulls with excessive body condition (which typically have considerable fat in the scrotal neck). Sperm within the epididymis or at the acrosome phase during insulation appeared to be most affected [33]. Insulated bulls had twice as many sperm with midpiece defects and four times as many with droplets on day 5, fewer normal sperm and three times as many with midpiece defects and droplets on day 8, fewer normal sperm on days 15 and 18, and more sperm with head defects on days 18 and 21. Semen quality in insulated bulls had nearly returned to pre‐insulation values by day 35. In a second experiment [33], scrotal subcutaneous temperature increased 2.0, 1.5, and 0.5 °C at the top, middle, and bottom of the testis, respectively, and intratesticular temperature was 0.9 °C higher at the corresponding three locations 48 hours after scrotal neck insulation compared to before insulation. Clearly, the scrotal neck is an important site of heat loss.
Increased Epididymal Temperature
In most mammals, the cauda epididymis is cooler than the testes [34], facilitating its sperm storage function. Increasing cauda temperature disrupts absorptive and secretory functions, changes the composition (ions and proteins) of the cauda fluid, and increases (approximately threefold) the rate of sperm passage through the cauda [34]. Consequently, the number of sperm in the first ejaculate declines, with an even more dramatic decline in sperm number in successive ejaculates. In addition, increased temperature seems to hasten sperm maturation [34].
Effects of Increased Temperature on Testicular Cells
Although heating seems to affect Sertoli and Leydig cell function, germ cells are the most sensitive [35]. All stages of spermatogenesis are susceptible, with degree of damage related to the extent and duration of the increased temperature [35]. Spermatocytes in meiotic prophase are killed by heat, whereas sperm that are more mature usually have metabolic and structural abnormalities [36]. Heating testes usually decreases the proportion of progressively motile and live sperm, and increases the incidence of morphologically abnormal sperm, especially those with defective heads [37]. Increased testicular temperature caused a lack of chromatin protamination and subtle changes in head shape of bull sperm [38]. Despite considerable variation among bulls in the nature and proportion of defective sperm, order of appearance of specific defects is relatively consistent [31, 32]. Unless spermatogonia are affected, the interval from cessation of heating to restoration of normal sperm in the ejaculate corresponds to the interval from the beginning of differentiation to ejaculation [37]. Following scrotal insulation in bulls, blastocyst rate (in vitro system) was more sensitive than cleavage rate [39]. Even though sperm morphology has returned to normal, their utilization may result in decreased fertilization rates and an increased incidence of embryonic death [40].
Summary
When scrotal/testicular temperature is increased (regardless of the cause), sperm morphology is generally unaffected initially (for an interval corresponding to epididymal transit time) but subsequently declines. In some studies [29, 33], sperm in the epididymis at the time of scrotal heating were morphologically abnormal when collected soon after heating. In another study [30], changes in these sperm were manifest only after they were frozen, thawed, and incubated. Sperm morphology usually returns to pretreatment values within approximately six weeks after the thermal insult. However, a prolonged and/or severe increase in testicular temperature will increase the interval for recovery. In general, the decrease in semen quality following increased testicular temperature is related to severity and duration of the thermal insult.
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