Dermatopathology

Preparation of specimen CME

Created 2008.

Learning objectives

  • Describe the preparation of a skin specimen for histopathology
  • Explain potential errors in pathological diagnosis

Introduction

Skin biopsies are taken for the following reasons:

  • Tmake or confirm a clinical diagnosis
  • Tremove a skin lesion
  • Research

Dermatopathology is necessary tmake or confirm a clinical diagnosis and determine whether margins of excision are adequate.

After a piece of skin is removed the specimen is sent ta surgical pathology (histology) laboratory. A general histopathologist or a specialist dermatopathologist examines processed routine sections under a light microscope. In some cases special stains or other tests are required for diagnosis. A report is then issued tthe referring practitioner.

Skin biopsy specimens

Specimens of skin removed for pathological examination ideally should contain a representative sample of epidermis, dermis and subcutaneous tissue. The skin biopsy may be one of the following types: :

Excision biopsy A skin lesion is completely cut out for diagnosis and treatment
Incisional biopsy A segment of the lesion is removed for diagnosis only
Shave / tangential biopsy A horizontal section of the skin is removed for diagnosis or treatment
Punch biopsy A standard round specimen 2-6 mm in diameter for diagnosis
Curettings Fragments of tissue are removed using a ring or spoon curette for treatment
Fine Needle Aspiration A needle is inserted intthe lesion and cells are aspirated for direct examination.

The sample should be transported in fixative (usually 10% neutral buffered formalin); the volume should be at least ten times that of the tissue. Make sure the pot is properly sealed. Label it with the patient's full name and a second identifier (date of birth and/or National Health Index number), the site of the biopsy and the date.

A request form should accompany specimens tthe pathology laboratory. The form should include patient details including age and sex, a description of the specimen type, exact site of the biopsy, and clinical details including as a minimum the suspected diagnosis, differential diagnosis and brief clinical history, including duration of the lesion and any treatment applied.

In the laboratory

Fixation
The purpose of fixation is tpreserve the skin specimen indefinitely in a life-like state. For normal histological sections, the specimen of skin is immediately placed in formol saline (formalin). It remains in this preservative for a minimum of 24 hours prior tprocessing by a histotechnologist.

Formalin fixes the tissue by forming cross-links between lysine residues in the proteins, which does not alter their structure. It is buffered with phosphate tprevent acidity provoked by tissue hypoxia, maintaining a pH of 7. It penetrates effectively intthe tissue but rather slowly, depending on its thickness; tissues can be fixed faster if they are warmed.

If nfixative is available at the time of biopsy, the specimen should be kept moist with saline and transported tthe laboratory as soon as possible, contacting the lab in advance tconfirm that there will be someone available treceive it.

Tissue processing
The technologist assigns the specimen a unique accession number, examines it and describes the gross appearance of the specimen. If malignancy is suspected, the edges of the specimen may be marked with ink tidentify excision margins. All or parts of the specimen are placed intone or more small plastic cassettes which hold and identify the tissue while it is being processed, and which acts as the backing for the final paraffin block. Initially, the cassettes are placed inta fixative.

Paraffin blocks are then prepared tallow the tissue tbe cut intthin microscopic sections (3-5 microns). Using an automated processor, the water is removed from the tissue by alcohol dehydration. The alcohol is cleared using xylene and then the tissue is ready tbe embedded in paraffin wax. The technician removes the tissue from its cassette, aligns it carefully in a mould and pours hot paraffin wax over it. As the wax cools, it solidifies tform the block ready for sectioning.

Sectioning
The embedded tissue is cut intsections using a microtome, which moves the block across a very sharp knife every 3 t5 microns. The sections are then floated on a warm water bath, picked up on a glass microscope slide and dried in a warm oven.

Preparation of the specimen

Staining
By another automated process, the paraffin is removed from the sections by sequentially immersing in xylene, then alcohol and then water. The slide is then stained routinely using haematoxylin and eosin (H and E). This is an automated process in larger laboratories, but some centres stain manually. In some situations, other special stains may be used.

  • Haematoxylin stains include a variety of metal cations that result in varying hues (blue). It is a basic dye that stains the nucleic acids of the cell nucleus and is usually partially removed by an acid-alcohol solution.
  • Eosin stains are acidic and dye cytoplasmic components of the cell red.

Mounting
The stained slide is processed again through water, alcohol and xylene. A resin is applied tglue a cover slip or plastic film in place over the section. The slide is now ready for the pathologist texamine and report the microscopic description, the diagnosis and comments.

Potential errors

If the tissue is not processed carefully, various artefacts may be present on the slide.

Fine black spots Formalin-haem pigment if the formalin used is toacidic (unbuffered).
Holes Tearing by microtome because of insufficient tissue dehydration or the presence of hard material such as calcium or sutures.
Bubbles Mounting media tothin or contaminated clearing agents.
Carry over artefacts and floaters Tissue from another specimen is carried over on the sectioning knife, or has dropped ontthe slide in the water bath because of inadequately cleaning between cases

Mislabelling specimens or lack of labelling is another potential error, although blocks are usually routinely given twmeans of identification (e.g. name and lab number) tprevent this.

Laboratory safety

  • The chemicals used in tissue processing are potentially toxic.
  • The laboratory should have good ventilation.
  • Chemicals must be correctly labelled and stored.
  • A materials safety data sheet should be available.
  • Hazardous wastes must be safely disposed of.
  • Occupational hazards should be identified and every precaution taken treduce danger.

Activity

Arrange tvisit your local medical laboratory and ask tbe shown the techniques described above.

 

Acknowledgements

Developed in collaboration with the University of Auckland Goodfellow Unit in 2007.

Authors: Hon A/Prof Amanda Oakley, Dermatologist, and Dr Paul Newman, Dermatopathologist, Hamilton, New Zealand, 2008.

Images have been sourced from the following:

  • Hon Assoc Prof Amanda Oakley
  • The Department of Dermatology, Health Waikato
  • Prof Raimo Suhonen (Finland)
  • Arthur Ellis (medical artist)

 goodfellow unit logo

 

Related Information

References:

On DermNet NZ:

Information for patients

Other websites:

Books about skin diseases:

See the DermNet NZ bookstore