Desmosome assembly may be induced in simple epithelial (MDBK and MDCK) cells maintained in low calcium medium (LCM: [Ca2+] less than 0.05 mM) by raising [Ca2+] to that of standard culture medium (SM: [Ca2+] = 1.8 mM). Here it is shown that if cells in SM are simply returned to LCM, their desmosomes split in the intercellular region within 15 min and the desmosomal halves are internalized within 30 min. This is the first time that desmosome splitting has been shown to occur in response to a reduction in [Ca2+] rather than Ca2+ chelation. Fluorescent antibody staining shows that the desmosomal glycoproteins as well as the plaque constituents are internalized, although a pool of the glycoproteins known as desmocollins remains at the cell surface, apparently unassociated with other desmosomal components. Desmosomal halves that have been recently internalized in response to LCM treatment do not return to the cell surface to participate in new desmosome formation. MDCK cells are able to form new desmosomes rapidly (15–30 min) while old desmosomes continue to be internalized. The desmosomes of MDBK cells remain sensitive to splitting and internalization in response to reduction in [Ca2+] for up to 14 days of culture in SM. In contrast, the desmosomes of MDCK cells become resistant to reduction in [Ca2+], as well as Ca2+ chelation by EGTA, after 4–5 days in SM. When treated with LCM or EGTA, MDCK cells with ‘stabilized’ desmosomes partially separate but remain attached to each other at some points. Regions of attachment stain brightly with anti-desmosomal antibodies and are characterized by ‘giant’ desmosomes, up to 4 micron long, roughly 20 times larger than those formed in cells in SM. These giant desmosomes may form by lateral fusion of small desmosomes.

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