Chicken coagulation factor XIIIA is produced by the theca externa and stabilizes the ovarian follicular wall

Development of the follicle in egg-laying species such as the chicken is re gulated by systemic factors as well as by-the highly orchestrated interplay of differentially expressed genes within this organ. Differential mRNA dis play analysis of defined phases of follicle development resulted in the cha racterization of coagulation factor XIIIA. It is expressed and produced by cells of the theca externa in a highly regulated manner during distinct gro wth phases of the follicle. Transcripts for factor XIIIA are already detect able at the beginning of follicle development,and peak at the end of phase 2. Protein levels, however, still increase during phase 3, peak shortly aft er ovulation, and persist until the postovulatory tissue is completely reso rbed. Factor XIIIA is secreted asa monomer into the extracellular matrix of the theca-externa and is not associated with factor XIIIB as is the case i n plasma. Our data suggest that, due to its transglutaminase activity, fact or XIIIA stabilizes the follicular wall by cross-linking matrix components. Thus, coagulation factor XIIIA might play a key role in coping with-the ma ssive mechanical stress exerted by the large amount of yolk accumulating du ring the rapid growth phase of the oocyte.