Granulocytes are leukocytes that contain cytoplasmic granules visible under light microscopy. Based on the staining affinity for acidic (e.g., eosin) or basic (hematoxylin, methylene blue) dyes of the specific (characteristic) granules of each type of granulocyte, they are classified as:
- Neutrophils / heterophils.
- Eosinophils (or acidophils).
- Basophils.
In addition to the specific granules, granulocytes contain a second population of lysosomal-type granules, less abundant, called primary granules (also called azurophilic granules), which are similar in all granulocytes.
They constitute the most abundant leukocytes in the blood (representing 60% - 70% of the total leukocytes in humans) and are found in all vertebrates.
The term neutrophil granulocyte is traditionally used for this type of leukocyte in humans and laboratory rodents, as their specific granules stain weakly with acidic or basic dyes. But in other vertebrates, the granules of this leukocyte population show a wide variability in their acidophilic or basophilic staining. Therefore, they were termed heterophils.
However, the use of neutrophil granulocyte has now been generalized for all vertebrates, with the use of heterophil being less common. Additionally, regardless of the species and class of vertebrate, these granulocytes exhibit similar structural characteristics and functional activities. Therefore, in this atlas, we will use the term neutrophil to refer to this type of leukocyte.
In blood smears, neutrophils are characterized by their globular shape (8-12 µm in diameter) and multilobed nucleus. The degree of nuclear lobulation varies, generally increasing with the maturity of the cell, so that forms with a band nucleus (horseshoe-shaped) and polymorphonuclear forms with a high degree of lobulation can be found in the blood.
The cytoplasm of neutrophils is weakly acidophilic and finely granular in appearance, due to the presence of abundant specific granules, small and heterogeneous in their affinity for acidic or basic dyes. Thus, they stain weakly in typical neutrophils of humans and other mammals, while in teleosts, they are acidophilic.
Ultrastructurally, the specific granules of neutrophils are characterized by their oval or elongated shape, moderate electron density, and heterogeneous content. In tetrapods, their longitudinal axis usually measures between 0.1 to 0.2 µm, while in teleosts and cyclostomes, they are larger (up to 0.5 µm in length).
The primary granules of neutrophils have a lysosomal appearance, rounded and high electron density. In tetrapods, they are larger (approximately 0.5 µm) than the specific granules, while in teleosts and cyclostomes, they are smaller.
The cytoplasm of neutrophils contains Golgi apparatus dictyosomes, a moderate number of rough endoplasmic reticulum cisternae, and mitochondria. Additionally, they often contain lipid droplets.
Both primary and specific granules are formed from vesicles of Golgi apparatus dictyosomes, with clear content. As the maturation progresses of specific granules, their electron density increases, and they acquire the elongated shape.
Circulating neutrophils are short-lived cells, with an average lifespan in blood of hours, but they are highly motile cells capable of leaving the blood and migrating through tissues.
An important part of the circulating neutrophil population remains adhered to the endothelium of blood vessels (marginalized population), but when necessary, they can mobilize rapidly in response to chemotactic and alarm factors (tissue damage, presence of pathogens).
Neutrophils have a high phagocytic capacity for small particles, such as viruses, bacteria, microscopic parasites, cellular debris, etc., hence they are known as microphages.
Their specific granules contain a variety of microbicidal products, such as lysozyme, lactoferrin, and enzymatic activities myeloperoxidase, which generate oxidative radicals (H2O2, O-, NO-), and alkaline phosphatase. The primary granules, of lysosomal type, contain microbicidal cationic peptides and hydrolytic enzymes.
In addition to intracellular microbicidal activity, neutrophils can exocytose their granules, thereby releasing their microbicidal products that act in the intercellular medium.
Eosinophils are granulocytes that contain distinct cytoplasmic granules that stain intensely with eosin and, in general, with acidic dyes, hence they are also known as acidophilic granulocytes.
Eosinophils have a very similar morphology in all vertebrates in which they have been described, although their percentage varies between species (around 2% - 5% of total leukocytes). They have a rounded shape, measuring 10 to 14 µm in diameter, and a generally bilobed nucleus, with abundant strongly acidophilic specific granules, about 0.5 µm in diameter, which at optical microscopy occupy almost the entirety of the cytoplasm.
Ultrastructurally, the specific granules of eosinophils are electron-dense and in most vertebrates contain crystal bodies or bars. In amphibians, specific granules are elongated, while in teleosts, they are rounded.
Eosinophils have the ability to exocytose the content of specific granules, so it is common to find emptied granules, in the form of vesicles, with heterogeneous content of low electron density.
Eosinophils also contain primary granules, of lysosomal type, Golgi apparatus, rough endoplasmic reticulum cisterns, and tubules of the smooth endoplasmic reticulum.
They are highly motile cells, migrating to tissues and moving by means of pseudopods. Their cell surface may be covered with abundant microvilli.
The specific granules of mammalian eosinophils contain strongly cationic microbicidal proteins, such as major basic protein (MBP) and arginine-rich cationic protein, which account for their high affinity for eosin. They also contain enzymatic activities myeloperoxidase, histaminase, and arylsulfatase.
Eosinophils have a microbicidal and antiparasitic defense function, mediated by the cationic proteins of specific granules, which they can exocytose, and by the generation of oxidative radicals (H2O2, O-).
They do not have phagocytic capacity, but actively endocytose antigen-antibody complexes and degrade them, preventing tissue damage from the accumulation of these complexes.
Additionally, eosinophils are involved in the generation and control of inflammatory and hypersensitivity responses, during which their numbers increase in the blood and in hematopoietic and immune system organs.
In mammals, the initiation of these responses is mediated by their surface receptors for immunoglobulin E (IgE). The binding of these IgEs to their antigen can trigger the exocytosis of granule contents.
On the other hand, histaminase and arylsulfatase activities degrade proinflammatory mediators (histamine, serotonin), aiding in the control of this response.
Basophil granulocytes are the least common leukocytes (0.5% of total leukocytes in blood). They have not been found in all vertebrates, probably due to their low frequency, and are believed to be absent in the blood of some mammals such as mice, rats, and cats.
The morphological characteristics of basophils are similar in most vertebrates in which they have been described. They are identified in blood smears by specific granules that are strongly basophilic, which when abundant can partially obscure the nucleus. Basophils are large (8 to 12 µm in diameter) and have a lightly lobulated nucleus.
The size of specific granules varies among different classes and species of vertebrates, from approximately 0.2 µm to over 1 µm in diameter, but always much larger than that of primary granules. Similarly, the content of the granules varies, in some species with a granular appearance and moderate electron density, while in others it is homogeneous and of medium to high electron density. Frequently, lamellar structures associated with specific granules of basophils are found.
Developing granules of differentiating basophils, such as in metamyelocytes, show lower electron density and heterogeneous content than granules of mature basophils.
Characteristically, basophils show abundant smooth endoplasmic reticulum tubules, in addition to some rough cisterns, and mitochondria.
Basophils are involved in inflammatory responses, increasing their numbers during these processes. Their specific granules contain numerous proinflammatory and vasoactive mediators, such as histamine, serotonin, heparin, chemotactic factors for other leukocytes, as well as myeloperoxidase that generates oxidative radicals.
Additionally, they synthesize various proinflammatory, vasoactive, and smooth muscle contractile mediators without prior storage. These factors belong to the eicosanoid family (prostaglandins, thromboxanes, leukotrienes), whose synthesis involves the smooth endoplasmic reticulum, as well as histamine and serotonin.
Similar to eosinophils, basophils have the ability to endocytose antigen-antibody complexes and present surface receptors for IgE. The binding between IgE anchored to the cell surface and its antigen can induce rapid degranulation of basophils and release of their proinflammatory and vasoactive factors.
Although basophils share some structural and functional characteristics with mast cells residing in connective tissue, they are distinct cell populations and, although they differentiate from the same precursor cells, mast cells have a different development pathway.
Granulopoiesis involves the formation of granulocytes through the differentiation of committed hematopoietic cells from the population of common myeloid progenitors, which give rise to myeloblasts.
Myeloblasts are mitotic cells, with euchromatic nucleus and basophilic cytoplasm. Ultrastructurally, myeloblasts present an undifferentiated appearance, prominent nucleoli, and scant development of cytoplasmic organelles. They may present primary granules, of lysosomal type.
From myeloblasts, granulopoiesis involves the synthesis of primary and specific granules of each series and a succession of structural and functional changes, defining stages called, in order of decreasing differentiation, promyelocytes, myelocytes, metamyelocytes, and granulocytes.
In the process of granulopoiesis, promyelocytes differentiate into granulocytes of the neutrophil, eosinophil, and basophil series. Promyelocytes may show initiation of nuclear lobulation and present greater development of organelles, mainly small dictyosomes and rough endoplasmic reticulum cisternae.
The progression of granulopoiesis involves further development of rough endoplasmic reticulum and Golgi apparatus, from which granules are formed. By the myelocyte stage, specific granules are recognizable.
Subsequently, there is an increase in heterochromatin accompanied by a higher degree of nuclear lobulation and disappearance of nucleoli, along with a decrease in cell size.
Myeloblasts, promyelocytes, and myelocytes are located in the cellular cords of hematopoietic tissues. Starting from the metamyelocyte stage, granulopoietic cells acquire motility and can migrate into the blood, but usually only do so when they reach the granulocyte stage. However, during a demand for granulocytes, such as during an infectious process, immature forms of these cells, usually recognizable by their lower nuclear lobulation (e.g., band neutrophils), migrate into the blood.