Fig. 1: TEM micrographs of ultramicrotomy of non-stained, naturally occurring spongin fiber of Hippospongia communis demosponge origin.
From: Discovery of mammalian collagens I and III within ancient poriferan biopolymer spongin
a Assembly of about 120 nm diameter nanofibrils as typically found in collagen type I. b Zoom displays individual triple helices (yellow arrows). c, d Further enlargement reveals a triple helix of 1.5 nm diameter of collagen type I with a characteristic helix periodicity of 2.86 nm along the chain (red arrows). d Inset Fast Fourier transform (FFT) of the zoomed region with discrete reflection (red circle) indicating 2.86 nm spacing. e Fourier-filtered high-resolution displays a collagen type I fibril (orange lines). f Collagen triple helices (white arrows) with 1.5 nm diameter within the spongin are visualized (orange lines). Also, their individual subunits, the so-called alpha chains with 0.85 nm diameter, are resolved (white arrow) (see Supplementary Figs. 2 and 3). The measured spacings were confirmed by repeating the measurements at least at 3 different regions of the sample.
