Page 155 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Blood and haemopoiesis (sanguis et haemocytopoesis) 137

Table 7.1 Physiological values for the number, diameter and lifespan of mature erythrocytes and haematocrit
VetBooks.ir Species Number (10 /μl) Diameter (μm) Lifespan (days) Haematocrit
in various domestic mammal species.
6
Horse
6 (5–7)
Ox 7.5 (6–9) 5.5 140–150 42
5.7
35–40
50–60
Sheep 10 (8–13) 5.1 110–120 32–38
Goat 14 (13–17) 4.1 125 34
Pig 6.5 (5–8) 6.1 65 41.5
Dog 6.8 (5.5–8) 7.3 107–122 45.5
Cat 7.5 (7.2–10) 5.7 68–77 37–40

Leucocytes can be divided into two broad categories,
granulocytes and agranulocytes (mononuclear leuco-
cytes). Based on the staining properties of their granules,
granulocytes are further divided into neutrophils, eosino-
phils and basophils. Agranulocytes include lymphocytes
and monocytes.
Based on their principal functions, leucocytes can be
categorised broadly as those involved in innate immunity
(phagocytosis, inflammation, allergy; neutrophils, eosin-
ophils, basophils, monocytes/macrophages) and those
involved in adaptive immune responses (lymphocytes,
monocytes/macrophages).
Specific surface molecules of different leucocytes
7.3 Scanning electron micrograph of canine erythrocytes. form the basis of a further means of classification,
referred to as the cluster of differentiation (CD) system of

hypertonic conditions, they release fluid, becoming thorny nomenclature.
in appearance (crenated).
The lifespan of erythrocytes also exhibits species Granulocytes (granulocytus)
variation (Table 7.1). Breakdown of erythrocytes occurs In contrast to lymphocytes and monocytes, granulocytes
primarily in the spleen. There, red blood cells pass from are polymorphonuclear cells (though this term is some-
vessels known as terminal capillaries into expanded sinuses times reserved for neutrophils). In functional terms, these
(red pulp). Changes in the surface membrane of senescent can be considered as microphages. They exhibit amoeboid
erythrocytes allow these cells to be recognised by cells of motility and the capacity to move actively from the circu-
the mononuclear phagocyte system (MPS). Aged erythro- lating blood through the wall of blood vessels (diapedesis).
cytes are digested by macrophages. The porphyrin scaffold By releasing lytic enzymes, these cells can contribute to
of the haem molecule, from which iron has been released, innate defence both within the circulatory system and in
is converted into biliverdin and bilirubin. Ferritin (24–36% the interstitial tissue. They play a significant role in inflam-
iron) liberated from the haem molecule is stored as hae- matory processes. Granulocytes are also released from
mosiderin in macrophages and is used in the bone marrow the bone marrow to participate in localised cell-mediated
for haemoglobin synthesis. Erythrocyte degradation also responses.
takes place to a limited extent at other sites such as the liver
(Kupffer cells) and bone marrow. Granulocyte formation (granulopoiesis)
Granulopoiesis (myelopoiesis, granulocytopoiesis)
White blood cells (leucocytes) refers to the formation and differentiation of polymor-
Leucocytes perform multiple roles in defending the body phonuclear leucocytes (granulocytes) (Figure 7.4). The
against foreign substances. Upon gaining access to the pluripotent stem cells of the bone marrow, the hae-
body, foreign materials can be eliminated by either non- mopoietic stem cells, divide repeatedly to give rise to
specific processes (phagocytosis) or targeted immune myeloblasts which serve as the common progenitor cell
responses carried out by lymphoid cells. of all types of granulocyte. Myeloblasts (diameter 15
The complexity of these defence mechanisms is μm) have a round, euchromatic nucleus. The wide rim
reflected in the morphology of the white blood cells. of cytoplasm contains numerous organelles, particularly

Vet Histology.indb 137 16/07/2019 14:58