The slices are called histological sections or tissue sections. For their protection, they are covered with a thin sheet of glass, the coverslip, that is glued over the slide.
The most usual dimension used in Histology: the µm (micrometer). It measures 1 thousandth of a millimeter = 1 millionth of a meter. Tissue or organ slices for observation under an optical microscope measure usually 2 to 10 µm in thickness.
Permanent histological slides are made to last many years. To obtain a permanent histological slide: isolated cells or fragments of tissues and organs undergo several procedures that begin with their fixation . Fixation maintains their structure and some of their chemical composition during the further stages of the whole process. It is carried out by chemical means (e.g. with formaldehyde) or by physical means (e.g. by freezing the specimens).
Following fixation, the small fragments of tissues and organs undergo a sequence of steps to allow them to be cut in a device, the microtome, to obtain histological sections. To be studied with a light microscope the sections as well as isolated cells are placed on glass slides.
Cells as well as tissue and organ sections are almost always colorless and need to be stained to allow their observation with a microscope. After the staining procedure they are covered with a coverslip for their protection.
Liver. Staining: hematoxylin and eosin (HE). Medium magnification.