Abstract: Objective To observe dynamically the location and time of attachment and invasion of Toxoplasma gondii tachyzoites to murine intestinal mucosa by chromogenic in situ hybridization targeting SAG2 mRNA. Methods Thirty 7 to 8 week old BALB/c mice were randomly divided into experiment group（24 mice）and control group（6 mice）. Each animal in the experiment group was given 2×104 tachyzoites of RH stain in 0.2 ml PBS by intragastric administration and that in the control group was given 0.2 ml PBS. Four mice in the experiment group and one in the control group were sacrificed at 15 min，30 min，1 h，2 h，4 h and 8 h after infection，respectively，and paraffin sections of duodenum，jejunum and ileum were prepared to perform the in situ hybridization with Dig labeled oligonucleotide probe complementary to SAG2 mRNA of T. gondii. Results Tachyzoites were found on the striated border of small intestine epithelial cells （absorptive cells，goblet cells and endocrine cells），in or between two absorptive cells or in the lamina propria. At 15 min-2 h after infection，there was significant difference in the number of attachment on jejunum and ileum （P<0.05）；the number of invasion in jejunum was significantly higher than in duodenum and ileum at minute 15 and 30 after infection （P＜0.05）. Following the lapse of time，the number of attaching tachyzoites gradually reduced，whereas the number of invading tachyzoites gradually increased. Compared with 15 min after infection，for all the intestinal sections，the number of attachment significantly reduced at 8 h after infection（P<0.05），in contrast，the number of invasion significantly increased at 4 h and 8 h after infection （P<0.05）. Between 4 h and 8 h after infection，a significant increase in the number of invasion was showed in jejunum and ileum （P<0.05）. Conclusion Although the cell selectivity of attachment has not been observed，the location selectivity of invasion is present，jejunum is more susceptible to the tachyzoite invasion.

Key words: RNA in situ hybridization, Toxoplasma gondii, Tachyzoite, Intestinal mucosa, Epithelial cell