Effect of co-culture with theca interna on nuclear maturation of horse oocytes with low meiotic competence, and subsequent fusion and activation rates after nuclear transfer Young-Ho Choi a , Taeyoung Shin a , Charley C. Love a , Cindy Johnson a , Dickson D. Varner b , Mark E. Westhusin a , Katrin Hinrichs a,* a Departments of Veterinary Physiology and Pharmacology, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843-4466, USA b Departments of Large Animal Medicine and Surgery, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843-4466, USA Received 20 November 2000; accepted 30 April 2001 Abstract We conducted this study to examine whether or not co-culture with theca cells improves the maturationrateofhorseoocyteswithcompactcumuliandtoevaluatethecytoplasmiccompetenceof oocytes after maturation by assessing fusion, activation and cleavage rates after nuclear transfer. We collected oocytes by scraping follicles from slaughterhouse-derived ovaries and classi®ed them as having an expanded or a compact cumulus. Expanded oocytes were matured in M199 supplemented with10%FBSand5 mU/mlFSHfor24h;compactoocyteswereculturedinthesamemedium,orthey were co-cultured in the same medium with theca interna explants, for 24 or 42h. Oocytes were held withorwithout10 mg/mlcytochalasinB,beforewashingandmicromanipulation,andtheywerefused with donor ®broblasts by electrical pulse. Fused oocytes were activated with Ca ionophore/cyclohex- imide, cultured for 5 days, and stained with Hoechst to assess nuclear development. We considered oocyteswithanenlargednucleus,orhavingcleavagewithmultiplenuclei,tobeactivated.Therewas no signi®cant difference in overall maturation rate between compact oocytes cultured with theca and compact controls. When these two groups were combined, there was a signi®cant increase in the proportion of oocytes in MII between 24 and 42 h P < 0:05). Expanded oocytes had a signi®cantly higherrateofmaturationthandidcompactoocytes64%versus25±30%; P < 0:001).Therewereno signi®cant differences in rates of successful enucleation, fusion, activation or cleavage between compactcontrolandcompact  thecaoocytes,norbetweencompactandexpandedoocytes;however, Theriogenology 57 2002) 1005±1011 * Corresponding author. Tel.: 1-979-862-1338; fax: 1-979-845-6544. E-mail address: khinrichs@cvm.tamu.edu K. Hinrichs). 0093-691X/02/$ ± see front matter # 2002 Elsevier Science Inc. All rights reserved. PII:S0093-691X01)00708-7