An ultrastructural study of the fibromuscular capsule of the corpus luteum in mice.

The presence and distribution of contractile cells in mature corpora lutea (CL) of pregnant and normal cycling mice were studied by transmission (TEM) and scanning (SEM) electron microscopy. In the connective tissue surrounding the CL, SEM observations made after maceration in alkali, revealed long, flat, spindle-shaped elements arranged parallel to each other, forming a continuous capsule. TEM of these elements revealed ultrastructural features of mature smooth muscle cells (SMC), as well as structures intermediate between fibroblasts, fibrocytes and SMC. Discrete bundles of SMC were seen to exit the external capsule of the CL and to penetrate the luteal tissue. The extensive branches of these bundles of SMC appeared to make contact with numerous vessels. Therefore, three-dimensional reconstruction, based on TEM and SEM analysis, revealed that the SMC of the CL were organized such that they form a wide fibromuscular skeleton around and within the luteal mass. It is possible that pulsatile or tonic contraction of this structure may help to regulate the secretory activity of the luteal tissue by modifying the pressure inside the CL.