The PG synthesis can be stimulated by GlcAT-I gene delivery (Venkatesan et al., 2004). Previous studies reported that keratan and dermatan PG are involved in the early stage of eggshell formation and in the control of crystal growth (Arias et al., 1993; Nys et al., 2001). Furthermore, a strong correlation was found between eggshell breaking strength and glycosaminoglycans (GAG) in the eggshell membrane (Young et al., 2007). We hypothesize that dietary Mn supplementation may improve eggshell quality and ultrastructure by enhancing the synthesis of GAG in eggshell glands.