Comparative hematology

Dear reader,

Naked-eye inspection of blood at phlebotomy was practiced far back to ancient times. Invention of the compound microscope in the Netherlands around 1590 by Hans and Zacharias Jansen made the examination of the content of blood possible. In 1658, the Dutch naturalist, Jan Swammerdam (1637–1680) was the first person to observe red blood cells under a microscope. Another Dutch microscopist, Antoni van Leeuwenhoek (1632–1723), who was an acquaintance of Swammerdam, described the size and shape of ‘red corpuscles’ and rendered the first illustration of them in 1695. In addition, physicians in France and England made landmark discoveries in the mid-1800s that established the foundations of a new field in medicine: hematology (Hajdu 2003; Hoffmann 2013). It was not until the second half of the nineteenth century that hematological investigations were performed on a regular basis in medicine. This became possible after a number of significant achievements had been made (Hoffmann 2013). As there is no formal definition of what belongs to laboratory hematology and what not, included in the definition are subjects like blood cell counting and differentiation, erythrocyte sedimentation rate, cellular analysis in bone marrow and body fluids, coagulation, fibrinolysis, platelet function, blood grouping and blood transfusion (Hoffmann 2013).

For some reason, the Veterinary Quarterly received some very interesting manuscripts on hematology recently. This series started with a report on hairy cell-like leukemia in a nine-year-old Friesian mare (Achten-Weiler et al. 2016) illustrating a morphology not identified in veterinary medicine before and of interest in comparison to human hematology. Furthermore, the Veterinary Quarterly published a report on presumptive granulocytic leukemia in an eight-year-old eastern hognose snake (Heterodon platirhinos) (Rousselet et al. 2017). Last but not least, the most recent publication on whip-like heterophil projections in a gopher tortoise (Gopherus polyphemus) needs to be mentioned addressing very interesting leukocyte morphology as well (Stacy et al. 2017). Dr Stacy and co-workers described a new distinct type of abnormal heterophil (most abundant granulocyte in birds and reptiles) morphology being the first report of heterophil projections in the literature resembling heterophil extracellular traps (HETs) described in chicken heterophils. Neutrophils in general function in three primary capacities: generation of oxidative bursts, release of granules and formation of neutrophil extracellular traps (NETs) (Selders et al. 2017). Dr Stacy and co-workers state that, although probably not true HETs, the heterophil projections described in the gopher tortoise may represent heterophil activation, possibly even as a step in the process of HET formation. As such, it is a fine example of comparative hematology and it will hopefully initiate more research to further determine the cause and clinical significance of the presence of heterophil projections in blood films from (reptile) patients.

These publications demonstrate clearly that examining cells visually by experienced (veterinary) hematologists in peripheral blood smears using 100× to 1000× light microscopy might reveal fascinating results even more than 350 years after the first observation of red blood cells under a microscope. As a consequence, veterinary hematology remains a challenging field.

Disclosure statement

No potential conflict of interest was reported by the authors.