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Difficulty: MediumProcedural Steps

To prepare a biological specimen for transmission electron microscopy (TEM), a precise sequence of preparation steps must be followed to preserve the sample's ultrastructure. First, the tissue is subjected to primary fixation, typically using a buffered glutaraldehyde solution, which cross-links proteins and stabilizes the cellular architecture. Subsequently, the sample is washed in a buffer to remove excess fixative before undergoing secondary fixation with osmium tetroxide. This crucial secondary step imparts electron density to cellular lipids, rendering them visible under the microscope's electron beam. Following this, the specimen must be dehydrated. Because the vacuum chamber of a TEM requires a completely dry environment, water in the tissue is gradually replaced by passing the specimen through a graded series of ethanol solutions, ending with absolute ethanol. Once dehydration is complete, the specimen is infiltrated with a liquid epoxy resin. The resin gradually displaces the ethanol over several hours. Finally, the infiltrated specimen is placed in a capsule and heated in an oven to polymerize the resin, creating a hard plastic block that can be sliced into ultra-thin sections using an ultramicrotome. Slicing the specimen prior to complete polymerization would result in the tearing and destruction of the soft tissue.

According to the passage, if a researcher fails to complete the dehydration step before introducing the liquid epoxy resin, which of the following is the most direct consequence during the imaging process?

  1. A
    The cellular lipids would fail to gain the necessary electron density required to make them visible under the electron beam.
  2. The remaining water in the specimen would prevent it from being successfully imaged under the vacuum conditions of the microscope's chamber.Answer
  3. C
    The primary glutaraldehyde solution would fail to stabilize the cellular architecture by cross-linking proteins.
  4. D
    The specimen would tear and be destroyed during the slicing phase because the resin failed to polymerize in the oven.

Answer

The remaining water in the specimen would prevent it from being successfully imaged under the vacuum conditions of the microscope's chamber.
The passage states that the vacuum chamber of a TEM requires a completely dry environment, and dehydration replaces the water in the tissue with ethanol. If dehydration is omitted, the remaining water would directly violate the dry environment requirement of the microscope's vacuum chamber, preventing proper imaging.

Step-by-Step Solution

1
Identify the step being altered or omitted in the question stem.
The question asks about the consequence of failing to complete the dehydration step.
To determine the consequence, we must locate where dehydration is discussed in the passage.
2
Locate the description of the dehydration step in the text and identify its purpose.
The passage states that dehydration is required because the vacuum chamber of a TEM requires a completely dry environment, and water is replaced by ethanol.
This links the dehydration step directly to the requirement of a dry environment in the microscope's vacuum chamber.
3
Evaluate the consequences of omitting this step and match them with the options.
Failing to dehydrate leaves water in the specimen, which violates the dry vacuum environment needed for imaging.
This confirms that the option identifying the failure of the vacuum conditions is correct, while other options describe the consequences of altering different steps in the procedure.

Key Concept

Understanding the chronological sequence and functional dependencies in a scientific procedure.
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Topics

Procedural Steps

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