A histological slide is a rectangular glass slide on which cells or very thin slices of tissues and organs were placed. These slices are called histological sections or tissue sections. For protection, the sections are covered with a thin sheet of glass that is glued on the slide, called the coverslip.
Tissue or organ slices for observation under an optical microscope measure usually 2 to 10 µm in thickness.
The most usual measurment used in Histology: the µm (micrometer). It measures 1 thousandth of a millimeter = 1 millionth of a meter.
Permanent histological slides are usually made to last many years. To obtain a permanent slide, isolated cells or fragments of tissues and organs have to undergo a procedure called fixation , to maintain their structure and some of its chemical composition during the further stages of the whole procedure. Fixation is carried out by chemical means (e.g. with formaldehyde) or physical means (e.g. by freezing).
Following their fixation, small fragments of tissues and organs are submitted to a sequence of proceedings to allow them to be cut in a device, the microtome in order to obtain histological sections. The sections as well as isolated cells are placed on glass slides in order to be studied with a light microscope.
Cells as well as tissue and organ sections are almost always colorless and need to be stained to allow them to be observed under a light microscope. After the staining procedure the cells or sections are covered with a glass coverslip for their protection.
Glass slides are also used to observe bacteria, protozoa, fungi and other microscopic objects.
Liver. Staining: hematoxylin and eosin (HE). Medium magnification.