Osler's role in defining the third corpuscle, or “blood plates”

Abstract

As a young microscopist in 1874, William Osler was probably the first physician to recognize platelets in blood as a single unit and their coalescence when blood was shed. He expounded on the earlier observations of Schultze, who in 1865 noted abundant, irregular masses of colorless globules in normal blood that were almost certainly platelets. The impact of Osler's initial observations were diminished by an unclear relation between this third blood particle and bacteria. Initial observers of this blood element were handicapped by the inadequacy of microscopes, the lack of anticoagulants and blood stains, and the tendency of platelets to clump and undergo morphologic change when blood is shed.

Hayem in 1877 provided a firm histologic basis for platelets, albeit he felt they were the origin of red cells and referred to them as “hematoblasts.” Bizzozero in 1882 introduced the term “blood plates” and documented their importance in blood coagulation and the formation of thrombus. William Osler also studied platelets in the 1870s and contributed several original articles on the topic. In 1886, he gave the Cartwright Lectures in New York, a series of three lectures reviewing his initial observations on the presence of “blood plates” in blood vessels.

Yet, in 1892, when Osler published the first edition of the Principles and Practice of Medicine—a single-authored, >1000 page textbook that would become the major reference source in medicine for several decades—platelets or “blood plates” were only mentioned twice (1). In the section on anemia, Osler noted that blood plates were “either absent or very scanty” in pernicious anemia, not a surprising observation given the pancytopenia associated with that disease. In his discussion of leukemia, Osler noted, “The blood plates exist in variable numbers, they may be remarkably abundant.” Although this observation was noted in a paragraph discussing acute lymphocytic leukemia, it is likely such patients with abundant platelets had chronic myeloid leukemia or some other myeloproliferative disorder where thrombocytosis is readily observed. The scant mention of “blood plates” in Osler's text is somewhat surprising given his prior interest in this element of the blood. However, there was considerable uncertainty regarding the physiology of platelets at the time Osler's textbook was published. There was also the difficulty of enumerating platelets without standard anticoagulants. Finally, the origin of platelet production was unknown, as it would be an additional 14 years before Homer Wright in 1906 using his famous stain would clarify that platelets were produced by megakaryocytes in the marrow (2).

Platelets were initially recognized about the middle of the 19th century, whereas red cells had been identified in the 17th century and leukocytes in the 18th century. It is probable that platelets could not have been seen before 1830 when Lister devised lens combinations for the microscope that reduced aberration sufficiently for a 2-micron cell to be identified. In addition, platelets were not only small but adhered to injured vessels, foreign surfaces, or themselves with resultant morphologic distortion. Pioneer investigators were therefore handicapped by the poor resolution of microscopes, lack of blood stains, and inadequate anticoagulants.

Around 1842 several physicians shared credit for the earliest descriptions of platelets as a morphologic entity. The French physician Albert Donne noted that there were three types of particles in blood: the erythrocytes and leukocytes were clearly defined but the third more elusive to identify. He described “globular masses” that are the “product of chyle and show incessant diversity in the blood” (3). Yet, because Donne had no illustrations and didn't define his methodology, the reader must take his observations on faith. The first drawing of a platelet was probably by the British physician George Gulliver, although he did not associate these particles with fibrin formation (4). The British physician William Addison noted that the “liquor sanguinis contained a great number of extremely minute molecules or granules, varying in size, the largest being at least eight or ten times less than the colorless corpuscles” (5). He also associated these granules with fibrin formation: “Whilst examining these minute bodies, I observed the coagulation of the fibrin commence.” Figure 1 is Addison's drawing of a platelet/fibrin clot. It should be noted that Addison believed that leukocytes ruptured, discharging granules that become these minute particles. Hence, he did not believe they were independent cellular elements.

Figure 1.

Addison's drawing of a platelet/fibrin clot in 1842. Reproduced from reference 5.

About 20 years later, in 1863, Lionel Beale, a British pathologist, postulated various bizarre theories concerning platelets, suggesting that they were minute particles of germinal matter that grew into leukocytes or red cells, with their rapid death producing fibrin. However, one of his illustrations clearly shows platelets in a fibrin network (6). In 1865, Max Schultze evolved a technique to help keep blood warm on the microscope stage during periods of study. He noted the coalescence of colorless bodies into “granular masses,” possibly products of corpuscular degeneration (7). These preliminary studies set the stage for William Osler, who made some important observations about “blood plates” as a young 24-year-old microscopist in 1873. Osler was one of three investigators, the others being Hayem and Bizzozero, who working independently over an 8-year interval would provide observations that would establish a foundation for platelet biology. Still, historians noted there would be false starts and unsteady steps along the way (8).

After graduating from McGill Medical School, Osler arrived in England to study in Burdon Sanderson's physiology lab at University College (Figure 2). It is of historical interest that it would be Sanderson whom Osler would ultimately succeed in 1905 as the Regius Professor of Medicine at Oxford. Working with Schaefer in Sanderson's laboratory, Osler noted globoid bodies in his own blood and that of animals. In 1873 he published an article coauthored with Schaefer in a German periodical mistakenly suggesting that these bodies could be a peculiar type of bacteria, even though they were present in normal blood and associated with fibrin formation (9).

Figure 2.

Osler during his postgraduate stay in London. Reproduced with permission from the Secretary-Treasurer of the US and Canadian Academy of Pathology.

In his follow-up article in 1874 entitled “An Account of Certain Organisms Occurring in the Liquor Sanguinis” (10), Osler gave his original description of platelets: “Careful investigation of the blood proves that in addition to the usual elements, there exist pale granular masses, which on closer inspection present a corpuscular appearance. In size they vary greatly from half or quarter that of a white blood-corpuscle, to enormous masses… . They have a compact solid look … while in specimens examined without any reagents the filaments of fibrin adhere to them.” In this article Osler also credited Schultze's observations of “granular masses.” He examined blood in mesenteric and subcutaneous vessels of rats. These blood vessels contained individual pale, round disks showing no tendency to adhere to one another but readily coalesced when blood was shed. He concluded that it was untenable that these particles were due to leukocyte degeneration, as proposed by Addison and others. In this follow-up article, he concluded that nothing can be said of their nature or relation to bacteria. Figure 3 shows drawings by Osler, first published in 1874, showing individual platelets in a blood vessel. Clearly, he considered platelets to be distinct cellular elements.

Figure 3.

Osler's diagram of platelets in a blood vessel. Reproduced from reference 10.

The second major investigator at this time was the French physician Georges Hayem. Starting in 1877, Hayem's reports also helped establish that platelets were a distinct cellular entity (11). He prepared a diluting and fixing medium for blood cells that aided his studies. He accurately counted platelets and noted their role in coagulation. Unfortunately, Hayem stubbornly maintained for many years that platelets as “haematoblasts” were an early stage in erythrocyte development.

Julius Bizzozero of Turin, who first noted that the marrow was the site of red cell production in 1868, published a monograph in 1882 introducing the term blood plates or “Plättchen” (12). With the terms “blutplättchen” and “petite plaques” as labels for the tiny corpuscles, his terminology is likely the source for the contemporary nomenclature of “platelet.” His studies were done on mesenteric vessels of live animals, whereas Osler's work was on excised tissue. He popularized the concept that this new cell represented an independent cell line with the specialized function of hemostasis or arresting the flow of blood. He also noted that hemostasis and blood coagulation were not synonymous. Much to Osler's chagrin, Bizzozero's writings ignored Osler's earlier reports on these elements. However, Osler's usage of the terms “bacteria” and “organism” in reference to platelets in his earlier studies raised the possibility of contamination.

Six years before publishing the Principles and Practice of Medicine, Osler gave a series of three Cartwright Lectures in New York, reviewing his own work and that of others on blood platelets, coagulation, and thrombosis (13). The first lecture was titled “The Blood Plaque or Third Corpuscle.” He discussed platelet morphology and number, reviewed the formation of the granular masses of Schultze, and speculated concerning their origin. He also discussed the role of these “plaques” in disease and referred to his own observations. He stated that these plaques were increased in all chronic wasting diseases, possibly the first description of reactive thrombocytosis. Osler also noted that they may be elevated in some cases of leukemia and Hodgkin's disease but may be scanty with profound anemia.

In the second lecture he discussed degeneration and regeneration of the corpuscles and noted: “This it is which makes the blood such a puzzle, for the corpuscles, so far as observation goes, neither die nor are born in the circulating fluid but appear to enter it as perfect elements and are removed from it before they are changed as to be no longer recognizable.”

In the last lecture, “The Relation of the Corpuscles to Coagulation and Thrombosis,” he reviewed his work and that of Bizzozero and others, indicating that platelets, not leukocytes, were the initial cellular element of thrombosis. Also, blood plaques “are the elements which first settle on the edges of a wounded vessel and form the basis for thrombosis.” He noted that white thrombi are composed almost entirely of blood plaques. Figure 4 shows the section of a femoral artery of a dog illustrated in the text of this lecture. Note the aggregation of blood plaques in enormous numbers about the intima in contrast to the clot. Figure 5 shows that white thrombi are composed almost entirely of blood plaques in the abdominal aorta of a woman who died of stomach cancer.

Figure 4.

Section of the femoral artery of a dog. Aggregation of blood plaques occurs in enormous numbers about the intima. The clot is composed chiefly of red corpuscles. From 1886 Cartwright lecture; reproduced from reference 13.

Figure 5.

White thrombi composed almost entirely of blood plaques in the abdominal aorta of a patient who died of stomach cancer. From 1886 Cartwright lecture; reproduced from reference 13.

The Cartwright Lectures were the last of Osler's experimental observations on platelets, although he did participate in a 1905 symposium at Johns Hopkins University before leaving for England. That conference was the source of his often-quoted response to Dr. George Kemp's observation that platelets at high altitudes contained hemoglobin. Osler quipped that he had seen many platelets, but never one that blushed (14).

Clearly, the role of platelets in thrombus formation was identified before the association of decreased platelets with bleeding. Paul Werlof, court physician to King George II of England, described a young woman's acute and reversible purpura in 1775 (15). This disease, likely immune thrombocytopenic purpura, was known as Werlof's disease at the time of Osler. It was over 100 years after Werlof's initial description before Hayem established in 1891, 1 year before Osler's textbook, that a decreased number of platelets was the basis for this disease (16).

In conclusion, some historians have proposed that Osler's most significant original scientific contribution was his key observation that platelets were a normal and independent component of blood linked to thrombus formation (17). Yet, little attention has been given by historians concerning these observations, including only 1 page in Cushing's 1371-page biography (18). Of course, Osler himself barely mentioned “blood plates” in his first edition of his textbook, likely because of limited information on the origin of these cells, the inaccuracy of routine platelet quantitation, and especially the poorly understood role of platelets in the pathophysiology of disease at the time.