"A crystalline fluid filled with red globules" – Antoni van Leeuwenhoek’s Observations of RBCs

Antoni van Leeuwenhoek’s letters are essential documents in the field of science and historical microscopy.
Round, red globules floating in a crystalline fluid – Antoni van Leeuwenhoek’s observations of red blood cells and hemocytes
Round, red globules floating in a crystalline fluid – Antoni van Leeuwenhoek’s observations of red blood cells and hemocytesCDC PHIL

Antoni Van Leeuwenhoek was a microbiologist and a microscopist. Born in 1632, Antoni is known as the 'Father of Microbiology'. He is of Dutch origin. The idea of creating microscopes was due to his interest in checking the quality of threads from the textile industries. During his lifetime, Antoni made over 500 tiny microscopes. Later, he explored blood cells using his microscopes and defined their characteristic features. Therefore, making significant contributions to the field of medicine.

Antoni van Leeuwenhoek exchanged a lot of letters with his fellow researchers. These letters are essential documents in the field of science and historical microscopy to this day. He has described structures and objects that are typically hidden within specific scientific disciplines and theories. Leeuwenhoek often returned back to subjects and samples discussed in earlier letters to clear many questions and queries about any particular topics. Sometimes, it is due to his curiosity or perhaps because of an unresolved question that has persisted from some of the previous letters. At other times, one or more of his correspondents and colleagues who followed his work had asked him for more information and details . Among his many areas of interest and likes include circulation of fluid — in plants and animals — and mainly the structures found in the blood of creatures, including mammals, fish, birds, amphibians, and arthropods attracted most of his attention.

Leeuwenhoek did his best to explain, sometimes through the use of metaphor, how the red blood cells — his blood globules or those of warm-blooded creatures — looked to him. He also dissected several arthropods and noticed the hemocytes in their hemolymph. Leeuwenhoek saw objects beyond his understanding but noted their peculiarities, uniqueness and moved on to other matters. He has described his observations so thoroughly and so well that there is almost no doubt to the reader about what Leeuwenhoek has observed and what he has concluded. In other cases, room for interpretation and investigation still remains. The editors of the letters come to the reader’s points with comments on what Leeuwenhoek has reported and observed.

Between 1673 and 1712, Leeuwenhoek’s certain book included at least thirty-five topics and well known information of blood globules and hemocytes in insects and crustaceans, structures that he described as having no color and “less transparent than the fluid in which they were present.”

According to some books which go on to say that…the tiny constituents of the blood, the so-called red globules or corpuscles were first seen which are normally globular in shape and become sickle cell shaped in Sickle Cell Anemia , Also other cells which mainly provide immunity and act to provide host defense mechanisms called lymphocytes were also seen first by Leuwenhoek.

Round, red globules floating in a crystalline fluid – Antoni van Leeuwenhoek’s observations of red blood cells and hemocytes
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In a letter dated 5 April 1674 to Constantijn Huygens, Leeuwenhoek wrote about what he observed at his house , he saw blood from his hand which showed red globules and too present in a watery or so called whey like fluid , but after careful observations again he observed that the red globules or the red cells he saw were not in a watery fluid but in a crystalline fluid.

On 24 April, Leeuwenhoek wrote to Huygens again. He shared that “four or five globules” which he observed “ are less colored than the others” and he also added that the globules “can only pass in one thickness and singly through some veins only ,” perhaps being his first observation of red blood cells passing through capillaries or arterioles feeding the capillaries. As we know that capillaries provide nutrients and oxygen to cells in different parts of the body. When the cells pass through these tiny vessels, “one cannot think that the color of these red cells will remain red instead there is a change in color and also size but they then regain their normal size and shape back and it is astonishing to see each red cell one by one , because each cell is different in its own way.” The red blood cells has Hemoglobin which is a very important molecule for oxygen transport. Hemoglobin is made up of 2 molecules Heme and Globin.

Leeuwenhoek spoke on blood color and how it changes, the relative heaviness of blood globules compared to the serum in which they are flowing and passing through, and color differences of blood cells within the diameter of a vein and how when they flow through tiny blood vessels their size and shape changes. Leeuwenhoek then made nine numbered observations, the first of which caused him to answer Boyle’s question directly. Robert Boyle had asked him certain questions regarding the blood cells and its physiology. The second response is related to Leeuwenhoek’s methods for blood sample which he observed and he compared the size of the blood globules to grain of sand and included a further clarification that the globules are “white and colorless” as single cells and there are 2 main types of blood cells which are then further subdivided into many tiny blood cells also.

Blood cells
Blood cellsCDC PHIL

"The small red globules in the blood, formerly spoken of are heavier than the crystalline liquor in which they are carried, because soon after that the blood is let out of the veins, those globules by little and little subside towards the bottom; and being made up of soft fluid corpuscles, and many lying upon one another, they do unite themselves close together, and by this close conjunction the blood that is under the surface alters its color, and becomes dark-red or blackish, as I have observed several times: of which I take the reason to be, (with submission to better judgments) that the air cannot move every way round about the globules, and hits as 'twere against a close darkish body. Touching the florid red color of the surface of the blood exposed to the air, that comes, in my opinion, from hence, that the uppermost globules are not pressed, and therefore retain their nature, and the globules subjacent to the uppermost lye close together, by reason of which close conjunction the light cannot penetrate through them, but is reflected, and so gives a greater light to, and about, the uppermost globules, than they had before the union of the inferior globules; and [consequently it is this] that makes [their red color brighter and more florid].", an excerpt from Leeuwenhoek’s letter. (AN/ScienceDirect)

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Round, red globules floating in a crystalline fluid – Antoni van Leeuwenhoek’s observations of red blood cells and hemocytes
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