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In the sheep ovary, primordial follicles are formed as an oocyte surrounded by either a single layer of flattened granulosa cells (type 1) or a mixture of flattened and cuboidal granulosa cells (type 1a). Booroola sheep have a mutation in the growth factor receptor, activin-like kinase receptor 6 (ALK6) which is expressed in oocytes of follicles at the type 1 stage of development. In Booroola ewes homozygous for the ALK6 mutation (BB), oocytes undergo precocious maturation that appears to be initiated during the preantral growth phase. The aim of this study was to quantify the ultrastructural features of oocytes of ovarian follicles at the types 1/1a, 2 and 3 stages of development, from BB and wild-type (++) ewes. Ovaries from 6 ++ and 5 BB 4 week old ewe lambs were processed for both light microscopy (LM) and electron microscopy (EM). LM and stereological methods were used to estimate the mean volume of the oocytes of each follicular type and genotype. EM images and point counting or linear intercept counting were used to estimate the volume of smooth endoplasmic reticulum (SER), Golgi, mitochondria, vesicles, lipid, ribosomes, zona pellucida (ZP) and cortical granules (CG), and the surface area of the outer and inner mictochondrial membranes, microvilli and cell junctions. Oocytes of type 1/1a follicles of BB animals had a greater diameter than that in ++ animals (BB: 29.76 ± 0.58 μm vs ++: 27.05 ± 0.30 μm; P

A Quantitative Ultrastructural Study of Oocytes During the Early Stages of Ovarian Follicular Development in Booroola and Wild-Type Sheep