Neural plate microvillus lengthening in rat embryos grown in various concentrations of glucose and further studies of the mechanism
Authors: Shepard Thomas H. , Park Hyoung Woo 
Published: June 2005

Glucose is an important cellular nutrient, and in the early embryo, which is dependent mostly on anaerobic glycolysis, it is even more essential. Based on tissue culture cells in which glucose utilization has become membrane‐limited, a concept has been developed that the tip of the microvilli is the entrance compartment for glucose and that the shaft sets up a diffusion barrier. An increase in length of the microvillus is associated with decreased entry of phosphorylated hexose into the cells. Our previous findings of lengthening of the microvilli of the neural plate cells after 40 min exposure to glucose at room temperature have been extended to a 17 hr whole embryo culture system. In cultures where the final concentration of glucose was 20 and 24 mg/dl there was embryonic death. In those cultures ending with 29–137 mg/dl of glucose the embryos developed normally. Those grown in dialyzed serum supplemented with B vitamins and glucose grew equally as well as those in whole rat serum. Somite numbers attained did not change with increasing glucose concentration but a modest increase in micromoles of glucose used per embryo was found, suggesting the presence of another source of energy at lower glucose concentrations. The average glucose utilization per gram of protein per hour was 844 μmol in these day 9. 5–10 embryos and this compares to 733 μmol previously found using uniformly labeled14C glucose on day 10.3. Lactate production averaged 85% of the glucose utilized. Pyruvate did not support growth in the absence of glucose. Lengthening of the microvilli was studied using scanning electron microscopy (SEM) and an association between increasing glucose concentration and lengthening (with matting) of the microvilli was shown after whole embryo culture and after 40 min exposure at room temperature. In whole culture the microvilli were whort in cultures ending with glucose concentrations below 30 mg/dl, partially matted at 36–81 mg/dl, and fully matted at 78–137 mg/dl. Elongation of microvilli was found in the mouse during the early somite neural plate stage, but the microvilli were more sparse and the cells contained a single cilium. These later two differences might explain the increased sensitivity to hypoglycemia in the mouse. These studies were done after exposure to glucose at 150 mg/dl for 40 min at room temperature. Also at room temperature, day 9. 5 rat neural epithelial microvilli lengthened when exposed to glucose. Studies for 40 min at room temperature were carried out to investigate the mechanism of lengthening. L‐glucose, fructose, galactose, and pyruvate, which are not transported and phosphorylated, did not cause microvillar lengthening. 2‐Deoxyglucose, which is transported and phosphorylated but does not enter the glycolytic pathway, caused microvillar lengthening. Cytochalasin D, which interferes with actin polymerization, caused marked shortening and ballooning of the microvilli. The elongation of microvilli exposed to glucose at room temperature was reversed by a 40 min exposure to glucose‐free Hanks′ balanced salt solution. © 1994 Wiley‐Liss, Inc.