Basal lamina heparan sulphate proteoglycan is involved in otic placode invagination in chick embryos

Formation of the otocyst from the otic placode appears to differ from invag ination of other cup-shaped organ primordia. It is known that the cellular cytoskeleton plays a limited role in otic placode invagination, whilst the extracellular matrix underlying the otic primordium intervenes in the foldi ng process. In this study we have analysed the role of the basal lamina hep aran sulphate proteoglycan in otic primordium invagination. At 10 H.H, stag e, heparan sulphate proteoglycan immunomarking begins to appear on the otic placode basal lamina, increasing noticeably at 13 H.H. stage, coinciding w ith maximum folding of the otic epithelium, and is still present at later s tages. Enzyme degradation of heparan sulphate proteoglycan in the otic prim ordium basal lamina, by means of microinjection with heparinase III prior t o folding, significantly disrupts invagination of the otic placode, which r emains practically flat, with a significant reduction in the depth of the o tic pit and an increase in the diameter of the otic opening. The immunocyto chemistry analysis revealed a notable depletion of basal lamina heparan sul phate proteoglycan in the otic primordia microinjected with heparinase, wit h no statistically significant differences observed in the volume or rate o f cell proliferation in the otic epithelium relative to the control, which suggests that heparan sulphate proteoglycan disruption does not interfere w ith the epithelial growth. In addition, a study of apoptosis distribution b y the TUNEL method confirmed that treatment with heparinase does not cause interference with cell survival in the otic epithelium. Our findings suppor t the theory that otic primordium invagination may be regulated, at least i n part, by the basal: lamina components, which might contribute towards anc horing the otic epithelium to adjacent structures.