Hyaluronan (HA) is a major constituent molecule in most extracellular matrices and is synthesized by Hyaluronan synthase (Has). In the present study, we examined expression patterns of Has1, -2, -3 mRNA in developing mouse molar and incisor tooth germs from embryonic day (E) 11.5 to postnatal day (P) 7, focusing on Hertwig's epithelial root sheath (HERS) and the apical bud in particular. Has1 mRNA expression was not detected in all tooth germs examined. Has2 mRNA was expressed in the surrounding mesenchyme from E12.0 to 18.0 in both molar and incisor tooth germs, but disappeared after birth. Meanwhile, Has3 mRNA was exclusively expressed within the enamel organ, especially in the inner enamel epithelium (IEE), stellate reticulum (SR), and stratum intermedium (SI) until the early bell stage at E16.0. Has3 mRNA disappeared as IEE differentiated into differentiating ameloblasts (dABs), but remained in SI until the root developmental stage of the molar tooth germ at P7. Has3 mRNA was also expressed in HERS until P7. In incisors, Has3 mRNA was expressed in the apical bud, especially in the transit-amplifying (TA) cell region from E16.0 to P7, and in the papillary layer (PL) adjacent to the mature enamel. These gene expression patterns suggested that Has3 is the main control factor for prenatal and postnatal HA synthesis of the tooth germ, and may in part regulate crown and root formation of the tooth germ, maintenance of stem cell niches in the apical bud as well as mineral transport in PL.