Scanning electron microscopy of corpus luteum formation in the golden hamster.

SEM observations of corpus luteum formation in the golden hamster reveal considerable cellular migration occurring in both the newly forming luteal mass and the remaining surface epithelium. During the first 24 h of the 4-day cycle, cells in the luteal mass become aligned into cords which obliterate the crater-like rupture. Individual cords lose their distinct surface profiles as the luteal mass forms a mushroom-like cap, the edges of which overhang surface epithelial cells (SEC) bordering the rupture site. The luteal mass continues to be remodeled as the edges slowly recede during the next 3 days of the cycle, and the follicle assumes a more spherical shape with a slightly sunken apex as regression begins. Covering of the rupture site by the SEC is a slow process which involves two mechanisms. Most of the mass is gradually covered by finger-like columns of SEC which migrate from the collar of cells surrounding the luteal mass to slowly surround, then cover small spherical or oval regions of the slightly sunken mass. In addition, individual SEC migrate out of the collar and move up onto the mass. The entire covering process appears to be random and poorly coordinated until very late in the 4-day cycle. This is attributed to changing surface properties of the luteal mass during the cycle.