Distribution of collagen birefringence observed in coronal (A,C) and diagonal (B,D) sections at various depths within lamina propria of human (A,B) and rhesus monkey (C,D) vocal folds. Human samples had pattern very different from rhesus monkey samples in terms of birefringence level throughout thickness; ie, rhesus monkey samples had high birefringence level closer to epithelial layer, whereas in human samples, high birefringence was mostly at region closer to muscle. TA — thyroarytenoid muscle; LP — lamina propria; E — epithelial layer; d — normalized depth of LP; d=0 — E/LP boundary; d=1 — TA/LP boundary.