Lipids in the mouse uterus during early pregnancy

Lipids in the uterine endometrium of mice during early pregnancy were investigated by histochemical techniques. Three groups of animals were studied: (1) on days 1-8 inclusive of the first pregnancy, (2)on days 1-9 after mating at the post-partum oestrus and suckling seven or eight young from their first pregnancy, (3) on days 1-6 after delivery of the first litter but with no access to males. Day 1 was that on which a seminal plug appeared in the vagina or (group 3) after birth of a litter during the previous night. In post-partum mice, only the areas between the former implanation sites were studied. Epithelial lipids are sparse during the first 24 h. On day 2 and early day 3 in nulliparous mice, and for a longer period in post-partum mice, the presence of lipid in epithelium and stroma seems mainly a degenerative phenomenon associated with cell breakdown and consequent tissue disruption. During late day 3 and on day 4 the increase in lipid droplets giving staining reactions characteristic for triglycerides may represent the storage in this form of fatty acids no longer needed for phospholipid synthesis or as energy sources, once the intense mitotic proliferation of these cells dies down. Triglycerides also predominated in the necks of the glands, but in the deeper parts, the sparse droplets often gave staining reactions for acidic lipids. In differentiating decidual cells during day 5 and early day 6 in nulliparous mice, histochemical reactions suggest that fatty acids accumulate. Some of these are probably utilized in synthesis of the increasing amounts of phospholipids which were particularly prminent later on day 6 and during day 7, while others appeared to be temporarily stored during that period as triglycerides The phased deposition and removal of triglyceride proceeds centrifugally through the decidua and is slightly in advance of glycogen deposition and removal in the same cell; its time-course correlated well with that which other workers have described from chemical assay. Histochemical tests appeared to reveal activity of a lipase-esterase (fatty acid ester hydrolase) which strengthened and spread through the decidua in parallel with the disappearance of lipid droplets. In lactating mice, as long as the blastocysts remained in diapause, epithelial lipid resembled that on day 4; in the stromal cells, lipid droplets were plentiful in areas close to the former placental sites but were few or absent elsewhere. Once the delayed implantation had started no abnormalities were detected. Alterations in lipids are discussed in the context of the known changes in the levels of ovarian hormones. Lipids within uterine macrophages are also discussed.