X-Y chromosome univalency in the testes of hyperthermic mice. II: Light and electron microscopic analysis of synaptonemal complexes

Hyperthermia-induced X-Y dissociation has been observed in diakinesis-metaphase I sper-matocytes but not in pachytene spermatocytes, which have been implicated as highly susceptible to heat stress. To determine X-Y dissociation in pachytene spermatocytes and to compare levels of dissociation between pachytene and diakinesis-metaphase I spermatocytes male ICR mice were exposed to 35°C ± 0.07°C and 65% ± 0.14% relative humidity for 2 or 4 days. Control mice were housed at 24°C ± 0.04°C and 43% ± 0.58% relative humidity. Mice were killed 0, 3, 5, 8, or 10 days after stress and the testes processed for meiotic chromosome display at diakinesis-metaphase I and synaptonemal complex display at pachynema. Twenty-five to thirty cells per mouse at both stages of meiosis were observed with light microscopy, and pachytene spreads with transmission electron microscopy to determine heat-stress effects on synaptonemal complex structure. Statistical analyses revealed significant linear increases in X-Y dissociation with prolonged exposure to heat at pachynema and diakinesis-metaphase I. Levels of pachytene dissociation were one-half the levels of dissociation at diakinesis-metaphase I. The resolvable structure of the lateral elements of the synaptonemal complex was not affected by heat stress.