问题 问答题 简答题

上皮细胞的基底面常有哪些特化结构?其基本功能是什么?

答案

参考答案:

常有基膜、质膜内褶和半桥粒三种特化结构。

基膜:是位于上皮基底面与其深面结缔组织之间的一层薄膜。电镜下可分为两层,由上而下分别为基板和网板。基膜由上皮和其下方的结缔组织共同产生,是两者进行物质交换的选择性透过膜,并有支持、连接作用,对上皮细胞的增殖、分化、迁移等也有重要作用。

质膜内褶:是细胞基底面的细胞膜向胞质内凹陷而形成的一些微小皱褶,皱褶之间的胞质中富含线粒体。其生物学意义是扩大了细胞基底部的表面积,有利于水和电解质的迅速转运。

半桥粒:是上皮细胞基底面与其下方基膜间的半个桥粒样结构,可将上皮细胞牢固地连接在基膜上。

判断题
单项选择题

The day was star-crossed: Friday the 13th in the month of October, on the eve of the second looming anniversary of a devastating market crash. "I’m telling you, psychology is really funny. People get crazy in situations like that," said portfolio strategist Elaine Garzarelli. Last week Friday the 13th lived up to its frightful reputation. After drifting lower at a sleepy pace for most of the day, the Dow Jones industrial average abruptly lurched into a hair-raising sky dive in the final hour of trading.

The Bush Administration moved swiftly to avert any sense of crisis after the market closed. Declared Treasury Secretary Nicholas Brady: "It’s important to recognize that today’s stock market decline doesn’t signal any fundamental change in the condition of the economy. The economy remains well balanced, and the outlook is for continued moderate growth." But Massachusetts Democrat Edward Markey, who chairs a House subcommittee on telecommunications and finance, vowed to hold hearings this week on the stock market slide. Said he: "This is the second heart attack. My hope is that before we have the inevitable third heart attack, we pay attention to these problems."

Experts found no shortage of culprits to blame for the latest shipwreck. A series of downbeat realizations converged on Friday, ranging from signs of a new burst of inflation to sagging corporate profits to troubles in the junk-bond market that has fueled major takeovers. The singular event that shook investors was the faltering of a $6.75 billion labor management buyout of UAL, the parent company of United Airlines, the second largest U. S. carrier.

On one point most thoughtful Wall Streeters agreed: the market had reached such dizzying heights that a correction of some sort seemed almost inevitable. Propelled by favorable economic news and a wave of multibillion-dollar takeovers, stocks had soared more than 1,000 points since the 1987 crash. But by last August some Wall streeters were clearly worried.

The heaviest blow to the market came Friday afternoon. In a three-paragraph statement, UAL said a labor-management group headed by Chairman Stephen Wolf had failed to get enough financing to acquire United. Several banks had apparently balked at the deal, which was to be partly financed through junk bonds. The take-over group said it would submit a revised bid "in the near term,’ but the announcement stunned investors who had come to view the United deal as the latest sure thing in the 1980s buyout binge. Said John Downey, a trader at the Chicago Board Options Exchange: "The airline stocks have looked like attractive takeover targets. But with the United deal in trouble, everyone started to wonder what other deals might not go through.

The main reason for the latest crisis in stock market as pointed out by the writer lies in()

A.the perception of inflation

B. the collapse of confidence

C. the failure of a buyout deal

D. the correction of a market