Glycoproteins and skin-core structure in Nephila clavipes spider silk observed by light and electron microscopy

Microscopical imaging of natural, unstressed draglines or of untreated bulk samples showed two types or threads with diameters of either approximately 1-2 microm or 4-5 microm, which could be identified as products of the minor or major ampullate glands. The threads had a circular profile in serial cross sections and are surrounded by a thin outer layer of a different material within the section. Such fibrillar configurations were also found in untreated threads or in the same serial sections of transmission electron microscopy (TEM) samples by means of the special technique of laser scanning microscopy. In TEM slides, numerous cavities with the same circular profile were detectable, and the length of these cavities is variable from 40-300 nm. The threads are oriented parallel and twisted around themselves to construct a double thread. In the interface between the two single threads, bridge-like structures are prominent. The single untreated thread consists of cylindrical fibers with a diameter of approximately 1-1.5 microm. Apparently more than eight fibers are within a thread and each fiber is composed of a great number of fibrils with a diameter of about 150 nm. The surface of threads is coated with a characteristic layer approximately 150-250 nm thick that contains glycoproteins. These were demonstrated for the first time by labeling with concanavalin A lectin-gold complex and are dependent on the diameter and length of the thread. The same substances could also be detected inside the single thread. The skin can be removed completely or partially by mechanical treatment, or by washing with phosphate-buffered saline or trypsin.