There are two major techniques for taking skin biopsies. These are a punch biopsy technique and ellipse/excision technique. The latter is usually for lesions larger than skin punches and also if possible, for vesicular diseases because using a skin punch tends to break the thin-walled vesicles. This would include pemphigus foliaceous.
More generalised skin diseases are usually sampled using a biopsy punch. These come in different sizes – there are 2 mm, 4 mm, 6 mm and 8 mm in use in the veterinary field. 2 mm are of extremely limited value because of the small size as they will only usually sample one adnexal unit (hair follicle) but can be of some value for inflammatory diseases although still limited due to the size.
4 mm is better but the majority that we see are 6 mm which usually gives us at least four adnexal units and in most cases 6 mm biopsies do not require a stitch. 8 mm biopsies are even better but require a stitch. For some sites in the body such as nasal planum small biopsy punches may be the only option.
When biopsying haired skin, it is very important to take biopsies along the same plane as the hair growth. In order to achieve this, it is recommended to use an indelible marker and draw across the biopsy site in the direction of the hair growth. We can then trim the biopsy in half and cut sections from the flat trimmed surface. If the biopsies are taken along the line of the hair follicle, we get sections of the hair follicles in longitudinal section. This allows us good examination of the follicle and is necessary to look at the stages of follicle growth. If the biopsies are not taken along the line of the follicle/hair shaft we have sections, then of a number of different follicles in partial transverse section. It is then difficult to look at follicle growth because we only have part of it in any section.
There are advocates for transverse sectioning of follicles, which we do occasionally, but we can only do this if we have a sufficient number of biopsies also for longitudinal sections. Most biopsies and most illustrations that you will see in textbooks are of longitudinal sections through a follicle, not transverse.
As with all pathology the larger the sample the better and the more numerous the samples also the better. We can process up to four punch biopsies for the standard histology charge.
If you require culture of a biopsy, this must not be put into formalin, but for all other situations the biopsies can be preserved in formalin. Immunostaining can be undertaken for most of the common markers on formalinised paraffin embedded tissue which is the usual process for the majority of samples for diagnostic pathology.
Source: Companion Animal, March 2021, Volume 26, Issue 4