AQA A Level Biology复习笔记6.3.3 Examining Skeletal Muscle Under a Microscope

Examining Skeletal Muscle Under a Microscope

 

  • Many biological structures are too small to be seen by the naked eye
  • Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied
  • For example, the movement of chromosomes during mitosis can be observed using a microscope

How optical microscopes work

  • Light is directed through the thin layer of biological material that is supported on a glass slide
  • This light is focused through several lenses so that an image is visible through the eyepiece
  • The magnifying power of the microscope can be increased by rotating the higher power objective lens into place

Apparatus

  • The key components of an optical microscope are:
    • The eyepiece lens
    • The objective lenses
    • The stage
    • The light source
    • The coarse and fine focus

     

  • Other tools used:
    • Forceps
    • Scissors
    • Scalpel
    • Coverslip
    • Slides
    • Pipette

     

 

Optical-microscopeImage showing all the components of an optical microscope

 

Method

  • Preparing a slide using a liquid specimen:
    • Add a few drops of the sample to the slide using a pipette
    • Cover the liquid/smear with a coverslip and gently press down to remove air bubbles
    • Wear gloves to ensure there is no cross-contamination of foreign cells

     

  • Preparing a slide using a solid specimen:
    • Use scissors to cut a small sample of the tissue
    • Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide (using a scalpel or forceps)
    • Some tissue samples need be treated with chemicals to kill/make the tissue rigid
    • Gently place a coverslip on top and press down to remove any air bubbles
    • A stain may be required to make the structures visible depending on the type of tissue being examined
    • Take care when using sharp objects and wear gloves to prevent the stain from dying your skin

     

  • When using an optical microscope always start with the low power objective lens:
    • It is easier to find what you are looking for in the field of view
    • This helps to prevent damage to the lens or coverslip incase the stage has been raised too high

     

  • Preventing the dehydration of tissue:
    • The thin layers of material placed on slides can dry up rapidly
    • Adding a drop of water to the specimen (beneath the coverslip) can prevent the cells from being damaged by dehydration

     

  • Unclear or blurry images:
    • Switch to the lower power objective lens and try using the coarse focus to get a clearer image
    • Consider whether the specimen sample is thin enough for light to pass through to see the structures clearly
    • There could be cross-contamination with foreign cells or bodies

     

  • Using a graticule to take measurements of cells:
    • A graticule is a small disc that has an engraved ruler
    • It can be placed into the eyepiece of a microscope to act as a ruler in the field of view
    • As a graticule has no fixed units it must be calibrated for the objective lens that is in use. This is done by using a scale engraved on a microscope slide (a stage micrometer)
    • By using the two scales together the number of micrometers each graticule unit is worth can be worked out
    • After this is known the graticule can be used as a ruler in the field of view

     

Graticule-Calibration

The stage micrometer scale is used to find out how many micrometers each graticule unit represents

 

Limitations

  • The size of cells or structures of tissues may appear inconsistent in different specimen slides
    • Cell structures are 3D and the different tissue samples will have been cut at different planes resulting in this inconsistencies when viewed on a 2D slide

     

  • Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that can not be seen
  • The treatment of specimens when preparing slides could alter the structure of cells

Skeletal muscle under the microscope

  • It can be very difficult to make out the features of skeletal muscle fibres using an optical microscope
  • Banding is visible, this is why it is referred to as striated muscle

Optical-microscope-skeletal-muscle

The dark bands produce a characteristic striped appearance

 

  • Electron microscopes are often used to see muscle fibres in more detail
  • They reveal the structure of myofibrils

 

Mammalian-Muscle-Under-The-Microscope-1Mammalian-Muscle-Under-The-Microscope-2The detailed structures of the muscle fibres are visible due to the much stronger magnification of the electron microscope

 

 

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