Shrinkage of motor axons following systemic exposure to inorganic mercury.

Systemically-administered inorganic mercury localizes to motor neurons, but it is not known if mercury injures these neurons. We therefore looked for signs of damage to the motor and sensory neurons of mice that had been exposed to inorganic mercury. Young adult mice were injected intraperitoneally with either 1 or 2 microg/g of mercuric chloride and perfused 1 or 30 weeks later. The cellular distribution of mercury in the spinal cord was examined with silver nitrate autometallography. The numbers and sizes of myelinated axons in the L5 anterior and posterior roots were quantitated using an image analysis program. Mercury was found throughout the cytoplasm of motor neuron cell bodies after 1 week and in paranuclear aggregations after 30 weeks. Thirty weeks after exposure to either 1 or 2 microg/g of mercury, fewer large myelinated axons were seen in mercury-injected groups than in controls, though total numbers of myelinated axons did not differ between groups. A slight increase in numbers of small axons was seen in the posterior roots of mice exposed to 1 microg/g of mercury. In conclusion, inorganic mercury remains within mouse neurons for prolonged periods and causes a reduction in the size of myelinated axons in the anterior root and to a lesser extent the posterior spinal root. Inorganic mercury within motor neurons therefore appears to behave as a slowly-acting neurotoxin that shrinks motor axons.

Pan-cadherin concentrates apically in uterine epithelial cells during uterine closure in the rat.

The ubiquitous cadherin probe, pan-cadherin was used to study changes in the distribution of cadherins in the plasma membrane of rat uterine epithelial cells during early pregnancy when the uterine lumen closes down. A major reorganisation of cadherin expression was observed to occur in uterine epithelial cells between days 1 and 6 of pregnancy with a clear shift in molecular distribution from a basal location on day 1 to a distinctly and almost exclusively apical distribution by day 6 of pregnancy when the blastocyst is attached. We suggest these results indicate participation of this molecule in closure of the uterine lumen, a phenomenon well-known to occur in the rat uterus during early pregnancy and in which apical surfaces of opposing uterine epithelial cells cohere.

Closure of the uterine lumen and the plasma membrane transformation do not require blastocyst implantation.

Ultrastructural changes in the plasma membrane of uterine epithelial cells in the pseudopregnant rat were examined to determine if these changes resemble those found during normal pregnancy and also to examine if the well-known membrane alterations of early pregnancy are intrinsic to uterine epithelial cells. Changes in the surface contours of uterine epithelial cells from the afternoon of day 6 to the morning of day 9 of pseudopregnancy were similar to those present after attachment in normal pregnancy although somewhat delayed. The presence of short, irregular microvilli was seen from as early as day 7 of pseudopregnancy, with regular microvilli returning to the epithelial surface by days 8-9 of pseudopregnancy but to a slightly lesser extent as compared to normal pregnancy. Furthermore, observations made on the afternoon of day 6 to the morning of day 7 of pseudopregnancy showed that the uterine lumen was closed down and that complete membrane flattening between opposing uterine epithelial cells was seen all along the uterus in the absence of a blastocyst. These observations establish that the "plasma membrane transformation" does not depend on blastocyst implantation.

The plasma membrane transformation does not last: microvilli return to the apical plasma membrane of uterine epithelial cells after the period of uterine receptivity.

Transmission electron microscopy was used to investigate morphological changes in surface ultrastructure of rat uterine luminal epithelial cells and to determine how quickly and to what extent microvilli return to the apical surface after the period of uterine receptivity for blastocyst attachment. Major, observable, differences in the surface morphology of uterine epithelial cells from six groups of pregnant rats were found. On the afternoon of day 6 of pregnancy, the apical surface is typically flattened and consists mostly of irregular projections, but by the morning of day 7, short, irregular microvilli are already evident among the remaining irregular projections giving the cell surface a 'taller' profile. By the afternoon of day 7, the microvilli have increased in height and frequency. This trend continues until by day 9 of pregnancy, the apical surface bears microvilli that are long, thin, and comparatively regularly distributed. These ultrastructural alterations demonstrate that the plasma membrane transformation of early pregnancy which is essential for uterine receptivity for blastocyst attachment, is transient and is followed by the return of regular microvilli.