A histological study of corpora lutea from superovulated beef heifers

There is great variability between animals in the number of viable embryos produced following different superovulation regimens. It is not clear if all the follicles that ovulate produce healthy oocytes and form normal corpora lutea (CL) following superovulation. The objective of this study was to assess and compare CL from heifers undergoing three superovulatory regimes with CL from unstimulated heifers on the basis of morphology and morphometric analysis of luteal cells. Beef heifers were superovulated using either: (a) 24 mg porcine follicle stimulating hormone (pFSH) given twice daily over a 4 day period in decreasing doses commencing on day 10 of the oestrous cycle; (b) a single injection of 2000 IU pregnant mare serum gonadotrophin (PMSG) given on day 10 of the cycle; (c) as in (b) but followed by 2000 IU anti-PMSG (IgG to neutralise endogenous PMSG) at the time of the first insemination which was 12-18 h after the onset of oestrus (n = 33 per treatment). Luteolysis was induced 48 h after initial gonadotrophin administration and CL were collected on day 7 of the subsequent cycle and from ten unstimulated heifers (controls) at the same stage of the oestrous cycle. CL morphology was studied at light and electron microscopy levels, Morphometric analysis was performed on luteal cells. Subcellular morphology was similar in heifers from all groups. However, CL from superovulated heifers had more connective tissue than CL from control heifers; the connective tissue content of CL in the anti-PMSG-treated group was particularly marked. Both large and small luteal cells in the heifers receiving anti-PMSG had significantly smaller (P < 0.001) area and sphere volume than similar cells from CL of heifers in the other groups.