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The distribution of actin and myosin in the rat ovary at different stages of postnatal development was studied by examination of cryostat sections treated with antibodies to chicken smooth muscle (gizzard) myosin or to chicken (pectoral muscle) actin and subsequently with fluoresceinated goat anti-rabbit gamma-globulins. Staining with either antiserum revealed several layers of intensely fluorescent elongated cells within the theca externa, forming a coherent band around the larger Graafian follicles. In smaller follicles, this band of fluorescent cells was incomplete, and ovaries of immature (6-day-old) rats were devoid of strongly fluorescent cells. Corpora lutea contained only scattered fluorescent cells at their circumference. Sections of mature ovaries incubated with antibodies raised against striated (pigeon pectoral) muscle myosin generated no significant fluorescence. Large oocytes stained with either anti-actin or anti-smooth muscle myosin showed a thin fluorescent band just beneath the zona pellucida, suggesting that actin and myosin are associated with the oolemma. The distribution of the two antigens in serial sections of follicles coincided, suggesting that the same cells contained both actin and myosin. It is suggested that follicular growth and maturation is attended by the development of a smooth muscle layer in the theca and that contraction of this layer in response to catecholamines and/or prostaglandins may play a role in the extrusion of the oocyte. The role of contractile elements in the oocyte remains to be elucidated.
The distribution of actin and myosin in the rat ovary at different stages of postnatal development was studied by examination of cryostat sections treated with antibodies to chicken smooth muscle (gizzard) myosin or to chicken (pectoral muscle) actin and subsequently with fluoresceinated goat anti-rabbit gamma-globulins. Staining with either antiserum revealed several layers of intensely fluorescent elongated cells within the theca externa, forming a coherent band around the larger Graafian follicles. In smaller follicles, this band of fluorescent cells was incomplete, and ovaries of immature (6-day-old) rats were devoid of strongly fluorescent cells. Corpora lutea contained only scattered fluorescent cells at their circumference. Sections of mature ovaries incubated with antibodies raised against striated (pigeon pectoral) muscle myosin generated no significant fluorescence. Large oocytes stained with either anti-actin or anti-smooth muscle myosin showed a thin fluorescent band just beneath the zona pellucida, suggesting that actin and myosin are associated with the oolemma. The distribution of the two antigens in serial sections of follicles coincided, suggesting that the same cells contained both actin and myosin. It is suggested that follicular growth and maturation is attended by the development of a smooth muscle layer in the theca and that contraction of this layer in response to catecholamines and/or prostaglandins may play a role in the extrusion of the oocyte. The role of contractile elements in the oocyte remains to be elucidated.
The role of contractile cells in mammalian ovulation is uncertain. In this study, we examined the morphology and distribution of cells within the theca externa of hamster follicles at various times during ovulation. Cells with all the ultrastructural features of smooth muscle (SMC) were found only in the basal hemisphere of the follicle. In contrast, the theca externa in the top half of the follicle was composed of fibroblasts. We next examined living hamster follicles during in vitro ovulation for morphological evidence of follicular contraction. The following changes in follicle shape were observed: (1) The base of the follicle moved apically; (2) follicles, which were initially spherical, became taller and thinner; and (3) after rupture, the apical follicle wall collapsed onto the surface of the ovary. To analyze the cause of these changes, sections of fixed follicles were examined by light and electron microscopy. During the final minutes before rupture, a V-shaped constriction formed in the base of the follicle. This constriction continued to narrow and by the time of rupture, it obliterated the basal part of the antrum. We concluded that the apical movement of the base of the follicle seen in living ovaries corresponds to the formation of the basal constriction seen in fixed sections. To determine if follicular SMC were involved in formation of the constriction, the ultrastructure of SMC was examined before, during, and after the constriction formed. The morphology of the SMC changed from the characteristic of relaxed or stretched SMC to that of contracted SMC when the constriction began to form. No other type of cell in the follicle wall showed these changes in morphology. We conclude that the formation of this constriction, and possibly the increase in height of preovulatory follicles and the collapse of the follicle wall after rupture, are due to contraction of SMC in the theca externa in the basal hemisphere of the follicle. This is the first morphological demonstration that follicular SMC contract prior to rupture of the follicle. The significance of these observations in mammalian ovulation is discussed.
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