STRUCTURE AND TURNOVER OF JUNCTIONAL COMPLEXES BETWEEN PRINCIPAL CELLS OF THE RAT EPIDIDYMIS

The epididymal junctional complex between adjacent principal cells is composed of apically located gap, adherens and tight junctions. Tight junctions between adjacent epithelial cells lead to the formation of t he blood-epididymal barrier. The objectives of this study were to exam ine the structure of the epididymal junctional complex in the differen t regions of the epididymis and to review the regulation of epithelial cadherin in the rat epididymis. Changes in the structure of the junct ional complex, at the level of the electron microscope, were evident w hen comparing the initial segment to other regions of the epididymis. In the initial segment, the tight junction spanned a considerable leng th of the apical plasma membrane but had few desmosomes. In the other regions of the epididymis, the span of merging plasma membranes was co nsiderably reduced, but in these regions, numerous desmosomes were pre sent in the apical region. Several examples of what appeared to be a l oss of portions of the plasma membrane of adjacent principal cells wer e evident along the entire epididymis. Such images as the invagination of a portion of the lateral plasma membrane of one principal cell int o another, constriction of the invaginated area and eventual detachmen t leading to the formation of annular junctions suggest that there is a turnover of plasma membranes. The formation of cellular junctions in volves the interactions of cell adhesion proteins followed by the addi tion of junctional proteins which assemble into tight and gap junction s. Epithelial cadherin (E-Cad), a calcium-dependent cell adhesion prot ein, was localized to the principal cells of the epididymis. Immunocyt ochemistry at the level of the electron microscope showed that E-Cad w as present between the lateral plasma membranes of adjacent principal cells, both in the region of the junctional complex and in the deeper lying areas. E-Cad was also present in annular junctions located in cl ose proximity to the junctional complex, indicating that these structu res were related to the plasma membrane. E-Cad mRNA levels are regulat ed during postnatal epididymal development. In the caput-corpus epidid ymidis, E-Cad mRNA concentrations increase to peak at 42 days of age. This is well correlated with the conversion of testosterone to dihydro testosterone in the epididymis. In the cauda epididymidis, however, E- Cad mRNA concentrations do not increase as a function of age, indicati ng that this protein is regulated in a segment-specific manner. In add ition, the longitudinal distribution of E-Cad mRNA along the epididymi s differs when comparing young rats to 42-day-old rats, when serum lev els of androgens are high. In the adult, the pattern of E-Cad mRNA con centrations along the epididymis appears to be dependent on serum andr ogen levels. A dose-dependent maintenance of E-Cad mRNA concentrations was observed throughout the epididymis of orchidectomized rats after replacement with testosterone. Together, these data indicate that the epididymal junctional complex is a dynamic structure which varies alon g the epididymis; this complex may be renewing itself. The regulation of cell adhesion proteins such as E-Cad offers a mechanism whereby and rogens may regulate the cell-cell interaction and junctional formation in the rat epididymis. (C) 1995 Wiley-Liss, Inc.