Biology 225- Animal Development Lab

Biology 225- Animal Development Lab - Biology 225 Lab...

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Biology 225 Lab – Animal Development Learning objectives: By the end of this laboratory exercise you should be able to: 1. Use a compound light microscope properly. 2. Describe the first stage of embryonic development – cleavage – in both the starfish and the frog. 3. Use models and microscope slides to gain a general understanding of how the vertebrate body plan is established during embryonic development. Background: Microscopy Individual cells typically are small and cannot be seen with the unaided eye. In order to view them, microscopes must be employed. Two important characteristics – magnification and resolution – are used to assess the limits of a microscope. Magnification The compound light microscope is the type of microscope used commonly in teaching labs. The word compound refers to the fact that this type of microscope contains two magnifying lens systems. One lens is called the ocular lens or eyepiece (Figure 1). The eyepiece lens is fixed and, for our microscopes, magnifies an object 10-fold or 10X. The second lens is called the objective lens. Compound light microscopes usually have 3 or 4 objectives located on a rotating nosepiece, but, when examining a specimen, only one can be used at a time. The most common objectives are 4X, 10X, 40X or 45X, and 100X. The 100X objective is an oil immersion lens. When this objective is used, a drop of immersion oil is placed on the slide before the objective is swung into operating position, and as a result a column of oil spans the gap between the top of the coverslip and the lens. The total magnification of an object can be determined by multiplying the eyepiece magnification by the magnification of the objective lens currently being used. For an object being examined, magnification is the ratio of apparent size to actual size. Using a compound light microscope, one can achieve a much higher magnification than using a simple (single lens) microscope. The maximum magnification of a typical compound light microscope, like the ones we will use in lab, is 1000X, although some compound light microscopes can have a maximum magnification of 2000X. Beyond the maximum magnification of a microscope, two individual objects start to blur together and cannot be easily distinguished as two objects. Resolution The distance between two separate objects that can still be distinguished as separate is called the resolution , or resolving power , of the microscope. Even though more lenses could be added to a light microscope to further increase magnification, the effective magnification limit is related to resolution; above the effective limit the image is still magnified, but it is fuzzy. When comparing microscopes, a microscope with a lower resolution will be able to distinguish between objects that are closer together. Thus, the typical electron microscope with a resolution of 0.003 μ m is a more powerful microscope than the typical compound light microscope with a resolution of 0.2 μ m. Coupled with its higher maximum total magnification (100,000X), an electron microscope can be used to examine specimens at much greater detail.
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