Catalyzes the addition of GlcNAc or GlcUA monosaccharides to the nascent hyaluronan polymer. Therefore, it is essential to hyaluronan synthesis a major component of most extracellular matrices that has a structural role in tissues architectures and regulates cell adhesion, migration and differentiation. Also able to catalyze the synthesis of chito-oligosaccharide depending on the substrate. [hyaluronan](n) + UDP-N-acetyl-alpha-D-glucosamine = H(+) + N-acetyl-beta-D-glucosaminyl-(1->4)-[hyaluronan](n) + UDP N-acetyl-beta-D-glucosaminyl-(1->4)-[hyaluronan](n) + UDP-alpha-D-glucuronate = [hyaluronan](n+1) + H(+) + UDP Glycan biosynthesis; hyaluronan biosynthesis. Expression moves as a gradient through the embryo. The mRNA is first expressed in the animal region of the blastula, and by early gastrula is found everywhere except in the outer layer of the dorsal blastopore lip. By mid-gastrula, protein is present in the inner ectodermal layer and the endoderm, then disappears from dorsal ectoderm as the neural plate is induced and later decays in a dorsoventral direction. Last expressed in ventral regions of the gut at the tailbud stage (at protein level). Expressed during a short window of embryogenesis. Expression of mRNA begins just after mid-blastula, peaks at late gastrula, and declines by the end of neurulation. Protein expression follows that of the mRNA with a time lag of approximately 2 hours: accumulates through gastrula and early neurula stages and peaks at about stage 18 (mid-neurula), then decays in a non-uniform manner, persisting about 12 to 18 hr longer than the RNA (at protein level). Belongs to the NodC/HAS family.