Chapter 1 The big picture

Pathology is unusual in that it can be viewed as a core basic medical science and as a range of clinical specialties. Pathology as a basic academic subject is the study and understanding of disease processes. A thorough knowledge of pathology, therefore, underpins the study of clinical medicine. Pathology as a clinical specialty is the use of scientific methods to investigate the causes and effects of disease within the body—in particular, the structural and functional changes that occur in cells and tissues in response to injuries, abnormal stimuli or genetic abnormalities, and their consequences for the organism as a whole. Clinicians within almost every branch of medicine interact with clinical pathology services; consequently, it is very important that the student understands the role of pathology specialties in patient management, whether or not a career in pathology is being considered!

The aim of this book is to instil a broad working knowledge of pathology as relevant to current clinical medicine. The book is not intended as a fully comprehensive reference text—there are several other volumes that perform this function admirably—but as a ‘checklist’ of the most important concepts within pathology that facilitate intellectual links between disease processes at the cellular and clinical levels.

This section sets the scene by defining the term pathology and describing the roles of this subject both as a basic medical science and as a clinical speciality. It is followed by the ‘High return facts’—a checklist of the most important pathology-based concepts. Each one of these is expanded into more detail in Section three, which uses a double-page format with simple figures wherever possible to facilitate the learning process.

WHAT IS PATHOLOGY?

The word ‘pathology’ is derived from the Greek pathos, suffering, and logos, study. It uses scientific methods to investigate the causes and effects of disease within the body, in particular the structural and functional changes that occur in cells and tissues in response to injuries, abnormal stimuli or genetic abnormalities, and their consequences for the organism as a whole.

As a clinical subject pathology grew up

in the postmortem dissecting room where autopsies revealed the morphological changes associated with disease (morbid anatomy); the word autopsy comes from the Greek word for eyewitness and can be translated as ‘seeing for oneself’

in side-rooms adjacent to hospital wards, in which doctors, nurses and medical students could carry out simple diagnostic tests.

As the number and complexity of tests grew, a centralized laboratory was set up in each hospital with dedicated technical staff. The work of the hospital laboratory continued to expand until it required the attention of a full-time specialist: the pathologist.

In the UK, pathology is nowadays divided into a number of disciplines, each with its own specialist practitioners:

cellular pathology (histopathology): structural changes in diseased tissues are examined macroscopically or microscopically; this is the focus of the remainder of this book and the use of the term ‘pathology’ from this section on refers mainly to cellular pathology

clinical chemistry or chemical pathology: metabolic disturbances are investigated, mainly by changes in the concentration of substances in body fluids

haematology: diseases of the blood and bone marrow

microbiology: detection and characterization of bacteria, viruses, fungi and parasites

immunology: abnormalities of the immune system

medical genetics: the inheritance of disease.

Pathology has a central place in modern medical practice. On the one hand, it represents the conceptual framework by which healthcare professionals organize their understanding of disease processes. Advances in clinical and basic science research are constantly modifying this understanding. On the other hand, laboratory tests are a vital part of the diagnosis of a wide range of diseases. Thus, pathology represents the link between the patient and the laboratory.

LABORATORY TECHNIQUES WITHIN CELLULAR PATHOLOGY

Macroscopic and microscopic assessment of specimens

Most small biopsies obtained with a biopsy needle (e.g. needle biopsies from breast or prostate) or at endoscopy (e.g. gastric, colonic and bronchial biopsies) may be ‘processed’ in their entirety into paraffin wax tissue blocks (see below). However, larger specimens (e.g. from a small ellipse of skin to an entire organ) do not need to be processed in their totality in order to reach a diagnosis. With these larger specimens, accurate diagnosis relies on naked eye (macroscopic) examination of specimens, with recording of the most important anatomical and pathological features and careful choice of tissue samples for processing and microscopic assessment.

Tissue processing

The formalin-preserved tissue from which thin sections are to be made for microscopic examination is placed into small plastic containers termed ‘processing cassettes’. Each cassette can contain tissue up to approximately 20 mm × 20 mm in area and up to 4 mm in thickness. Using an automated machine that usually works during the night, these tissue fragments are dehydrated and then impregnated with paraffin wax before being manually orientated correctly within a solid block of wax (embedding). This imparts stiffness to the tissues and enables very thin sections, typically 4–5 μm, to be cut manually from the tissue blocks and placed on to glass microscope slides. These thin sections are then dewaxed prior to staining and a thin glass cover slip is placed over the section to protect the delicate tissue. The glass slides with attached sections are then ready for viewing under a light microscope. Glass microscope slides and tissue blocks effectively form part of patients’ medical records and are kept for many years within archived hospital stores in case further examination is required.

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