Histopathology: A Whistle-Stop Tour of GOSH Pathology

Histopathology is the study of changes in tissue, which can have immunological, metabolic, oncological, or infection-based causes. We deal with a pediatric population – from fetal to about 16 years of age – and there are nuances to the pathology of those patients that you wouldn’t necessarily see in an adult. To give you an example – consider the development of a kidney. A fetus may have a kidney, but because it has not completely finished its growth phase, there will be differences in its appearance to that of an adult kidney. To the average pathologist, it might look unusual or even pathological – but actually, for patients at that stage of development, it is the norm. Another classic example is the thymus; in a fetus or newborn, there’s a prominent thymus to permit T cell maturation – but in adults, the thymus is atrophied or even completely absent, because it’s no longer required. If you have a specialty service, you need specialty support. And there are a number of subtleties that our pathologists need to be aware of to best serve a specialty population like ours.

In the Department of Histopathology here at GOSH, we take in two types of samples: neuropathology and “surgical pathology” (almost anything other than neurological). In short, we accept everything – brain, heart, lung, renal, tumor biopsies of all sorts… Because we are a tertiary referral center, our patients will have seen a general practitioner; the GP will have referred them to a hospital; the hospital will have undertaken its own investigations and then, if the issue is too obscure or too complex, the patient comes to us. In other words, we have to try to make a diagnosis where others could not – likely because they didn’t have the same level of exposure to the condition or tumor, or to pediatric pathology as a whole.

In histopathology, we use a machine called a microtome to cut thin section of tissue. We then use these slices to visualize the cells that make up that tissue so that we can understand and report on a disease process. Unlike many other hospitals, when a sample comes to us, we use almost 100 percent of what we get; because our patients are so young, our samples are often very small. Much of the material we receive is fresh, not fixed, because of the investigations we undertake. If an adult hospital receives a tumor biopsy (for instance, from a breast tumor or prostate core), they put it into formalin, the formalin goes off with the sample in it, it’s given its individual accession number, popped into a processor, cut, stained, and the resulting images interpreted. When we receive a tumor biopsy, we take a portion of that fresh tissue, immediately produce imprints (by dabbing the tissue onto a slide so that cells adhere to the glass), and send the slides for cytogenetic testing. If the sample comes from the brain, we may also do a brain smear, which involves producing a monolayer of cells on a slide for interpretation – something that we turn around very quickly so that we can offer surgical guidance. Are tumor cells present in the sample? Is it an aggressive tumor type? What form of resection or other treatment would be best?

We then subdivide the remaining material into pieces and freeze some of them prospectively, in case a future test is able to give us an answer that our current ones cannot. Many of the tumors we see are rare – some come into the laboratory only once every few years – so we always try to retain some material for future investigations. Every sample is a new challenge, so we prepare ourselves to run every test that is currently available – and those that may become available one day…

Finally, we fix the remaining tissue to run what adult pathologists might consider more “routine” histopathology. The gold standard for looking at tissue is, of course, hematoxylin and eosin staining – a pattern that is recognized by all trained pathologists. Beyond that, we also do special stains. For instance, we have a special stain for acid-fast bacilli (like those that cause tuberculosis). We can tell you whether or not a patient has the disease – but what we can’t tell you is whether or not that pathogen exhibits drug resistance. And that’s when we need to communicate with microbiology, who can add the next piece of the puzzle and tell us (and the clinician looking after the patient) which antibiotics are most likely to be effective.

Neurometabolic disorders are a good example of our approach to testing. When we receive a skin or muscle biopsy to test for metabolic disorders, we conduct a batch of tests; most of those must be performed on fresh or snap-frozen tissue because the techniques we use are dependent upon the enzymes present. As soon as you take that material away from the body, it begins to break down. So we perform enzyme histochemistry, which indicates whether or not an enzyme of interest is present in the tissue. We send some material to other laboratories for biochemistry or other analyses (such as enzymology). We section some material and send it for electron microscopy. Why? Because no one test can stand in isolation. Microscopy might reveal enlarged mitochondria or storage granules in the tissue – but why are the mitochondria enlarged? What’s in those storage granules? We need enzyme histochemistry to reveal the actual biochemical deficiency. But what causes that deficiency? Is it a genetic condition? Our colleagues in the genetics department can tell us that – either by finding the genes for a known disorder, or by examining the entire exome or genome to identify a new genetic condition. It’s clear that none of us can do our jobs without the others; we all have to work together to figure out what is going on.

We are also involved with evaluating “new” histopathology techniques, such as micro-CT for visualizing small specimens, mass spectroscopy to evidence the presence of particular proteins within formalin-fixed, paraffin-embedded (FFPE) material, and DNA sequencing of historical archival material (see “Historical investigations”).

Historical investigations

The histopathology department here at GOSH has records going back to post-mortems in the 1850s and archived material dating from about 1900. It’s all FFPE (Formalin-Fixed Paraffin-Embedded), which means it’s stable and safe indefinitely at room temperature. Recently, we went back to the archival material, cut fixed sections, removed the wax, and put it through mass spectrometry to see what proteins were present – and what anomalies existed in them (1).

Because our tissue archive is so huge, we can look back and see how disease has changed through time. So if you look through our records in 1900, a large number of children were affected by infectious diseases, such as tuberculosis – and, with the advent of antibiotics, we see a huge reduction in the number of children affected by infectious disease. And as children then began to live longer, cancer and chronic illnesses became more prominent.

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