Electron microscope immunocytochemical localization of cyclooxygenase-1 and -2 in pseudopregnant rat corpus luteum during luteolysis.

Prostaglandins converted from arachidonic acid by cyclooxygenases play an important regulatory role in regression of the corpus luteum. To reveal luteal distribution of cyclooxygenase isoforms during luteolysis, an electron microscope immunocytochemical study was performed. Cyclooxygenase-1 and -2 were found both in luteal steroid-producing and interstitial cells on days 13, 15 and 18 of the adult pseudopregnant rat. Cyclooxygenase-2 immunolabelling was predominantly seen in non-luteal cells. The two enzymes were localized in similar fashion to the plasma membrane, rough and smooth endoplasmic reticulum, lipid bodies and mitochondria, but differently in the nuclear compartment. Cyclooxygenase-1 labelling was found only in the perinuclear region, while cyclooxygenase-2 was localized to the nuclear envelope, region of condensed heterochromatin as well as at the perimeter of the heterochromatin. Nuclear residence may indicate additional roles for cyclooxygenase-2 in regulating gene expression. Identification of both enzymes on lipid bodies suggests that these inclusions may be involved in luteal prostanoid production.