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Integrated transmission and scanning electron microscopic (TEM and SEM)techniques have provided the first detailed description of the ultrastructuralfeatures of the bovine cumulus-corona (CC) cells surrounding oocytes at thetime of final maturation, zygotes and early cleaving embryos (2/4 to6/8 blas-tomeres). TEM revealed the presence of rough endoplasmicreticulum and Golgi complexes in the cyto-plasm of CC cells surroundingimmature, mature and fertilized eggs, and also revealed an increasing amountof smooth endoplasmic reticulation membranes, lipid droplets and mitochondriawith villiform and/or tubular cristae in the cytoplasm of CC cells duringmaturation and fertilization of the oocyte. In addition, a loss ofcell-to-cell junctions between CC cells was evident. TEM also demonstratedthat a few residual CC cells were still associated with early embryos and thatthese cells showed rather degenerative or apoptotic patterns, the latterpattern also observed on cells associated with fertilized eggs. SEM revealedthat the complex of CC cells of immature oocytes was compact with narrowintercellular spaces, which progres-sively enlarged in size around matureoocytes. This phenomenon is mostly due to the production of abun-dantextracellular matrix. Immature CC cell complexes possessed characteristic longand filiform microvilli whereas the surface of CC cells surrounding matureoocytes showed numerous blebs and occasional large cytoplasmic protrusions aswell as microvilli. Zygotes and early embryos were covered with a fewpolyhe-dral CC cells possessing scarce and short microvilli and a large amountof pleomorphic blebs. This study demonstrated a precocious luteinizationoccurring in bovine CC cells at ovulation until zygote segmenta-tion, and thisprocess was associated with a progressive apoptotic mechanism that ended inthe complete denudation of the zona pellucida covering the early embryo. Thepresence of CC cells around the maturing oocyte and fertilized egg could haveimportant functions related to the microenvironmental requirements of ovummaturation as well as facilitating activities related to fertilization. |
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