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  Microtomy and hand sectioning, Biology tutorial

Introduction to Microtomes:

Mechanical cutting of plant/animal materials are completed by the help of microtomes and procedure is usually called as microtomy. Mircrotomes can be separated in four essential groups:

i) Rocking

ii) Rotary

iii) Sledge (sliding)

iv) Freezing

All microtomes, though, have few things in common; all have means of holding the block, chuck; all have knife past which chuck can be moved to cut block, and all have device by which chuck can be advanced micron by micron, or in few cases by sub-micron steps, towards blade. Blade in all types is angled with respect to chuck, and that angle may be diverse.

1. Rocking Microtomes:

Commonest rocking microtome is known as Cambridge Rocker. Mechanism of microtome is mounted on the heavy cast iron base that helps to decrease vibrations by giving the inertial sink and therefore keeping all fixed points really fixed.

Block for sectioning is fixed to chuck that is rocked up and down about pivot by action of pawl lever on arm. On upward stroke pawl itself is involved and moves chuck forward by an amount that can be diverse between two and twenty microns in two micron steps, by means of screw. For several purposes this is insignificant but it would not do for the embryological specimen. The rocking microtome is usually best for cutting sections thicker than six microns.

2. Rotary Microtomes:

Rotary Microtomes are larger and heavier instruments even more greatly damped to stop vibrations and are certainly able to make thinner sections than rocking variety. They are much more expensive.

3. Sledge Microtomes:

Sledge microtome is the devise that may weigh as much as 50 kilograms, and comprises of heavy base in which are machined flat tracks on which chuck sledge slides. Chuck holds block with the surface to be cut uppermost. Chuck slides past extremely heavy blade that is held horizontally on mountings that are bolted to, or drawn from, base. Different devices may be joined to sledge microtomes among which are chuck stages for sectioning of small complete animals.

4. Freezing Microtome:

The only other kind of microtome of interest here is freezing microtome. This tool generally has the wedge blade and instrument is strange as blade is moved past fixed chuck rather than other way around. Fresh tissue to be sliced is placed on hollow chuck, and carbon dioxide gas passed quickly through chuck under pressure from cylinder. Expanding CO2 is sprayed on tissue from above, and on blade to cool it. Cold blade is passed by frozen tissue and single sections removed with the cooled section lifter or forceps to be floated in water. Such microtomes are at times mounted in chilled cabinets to make working with them easier. It is now probable to cut frozen sections by freezer aerosols instead of CO2 and freezing chucks are prepared that are electrically cooled by Peltier cells. Microtomes of all types must be cleaned of wax after use, perhaps by using the little xylene or less harmful solvent, and any parts responsible to rust must be wiped with the little thin oil. Action must be examined and lubricated where essential and any soiled paintwork cleaned

Microtome knives:

There are 3 basic cross sectional shapes of knives, wedge, plano-concave and double concave. Plane wedge knife is usually, though not entirely utilized for cutting frozen sections, and plano-concave knives are frequently used for sectioning soft materials like celloidin embedded tissues. Botanical sectioning razors are usually plano-convex.

Blades are planned to undertake particular jobs, but microtomists are fundamentally experimentalists at heart and frequently use them for other objectives than those for which they were planned. Weighty hunk of stainless steel that is microtome blade has been prepared with precision of Swiss watch and for section cutting the edge must look perfect when observed under microscope at approx 50 times.

Hand cut sections (hand sectioning):

Few tissue like plant roots, stems, cartilage in animals are hard enough to be held in hand directly or sustained in matrix while cutting the section. Traditional supporting matrix for young stems and roots and leaves which are soft is pith. The objective is to get thin slices of specimen, if possible only one cell thick. Very skilled workers may be capable to cut whole section to correct thickness. Single hollow ground botanical razor is perfect tool for cutting sections. Though, it is hard to sharpen blade sufficiently. The satisfactory replacement is single-edged disposable razor blade. Length and size of botanical razor makes it simpler to use than small disposable blades.

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