Page 175 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Immune system and lymphatic organs (organa Iymphopoetica) 157

and 8.15). The subserosa consists of a vascular loose con-
The flow of lymph is therefore opposite to that in
VetBooks.ir other domestic mammalian species. Lymphoid follicles nective tissue layer interfused with elastic fibres and a
dense connective tissue network with embedded smooth
are grouped in the medullary region of the lymph node;
muscle fibres.
the outer region contains mainly macrophages and plasma
cells. The sinuses and cords are relatively poorly developed. The amount and arrangement of smooth muscle varies
with species. Flattened to rounded trabeculae, containing
collagen fibres, elastic fibres and smooth muscle, extend
Spleen (lien) from the inner layer of the capsule into the parenchyma
In domestic mammals, the spleen is the largest organised (splenic pulp), incompletely subdividing the organ (Figure
form of lymphoid tissue in the body. In contrast to other 8.15). Larger connective tissue septa provide passage for
lymphatic organs, it is interconnected with the blood vas- arteries and veins. The fibro-elastic and muscular elements
cular system. Accordingly, the spleen performs several of the spleen are important for the role of the spleen as
specialised functions that ‘cleanse’ the circulating blood a blood reservoir. Active contraction, induced by sympa-
and its cellular components. thetic nerve fibres, leads to the rapid ejection of blood.
The principal functions of the spleen include destruc-
tion and removal of aging erythrocytes and filtration Splenic parenchyma and blood vessels
of blood plasma by activated macrophages. In domestic The splenic parenchyma consists of a three-dimensional
mammals, the spleen also serves as a blood reservoir (stor- network of reticular cells and fibres enclosing lymphatic
age spleen), in contrast to the primary immunoprotective tissue within its spaces (Figures 8.12 to 8.15). It is divided
function performed by the spleen in rodents and humans into two components:
(defence spleen). Storage of blood by the spleen is particu-
larly pronounced in carnivores and horses. Up to a third · white pulp (pulpa alba) and
of the circulating blood and platelets may accumulate · red pulp (pulpa rubra).
here. Through its role in regulating blood circulation, the
spleen contributes in a broad sense to thermoregulation. The white pulp comprises the lymphatic tissue of the
During fetal development, the spleen is a site of hae- spleen, including lymphoid follicles (Malpighian cor-
mopoiesis. This function is maintained for some weeks puscles) and periarteriolar lymphoid sheaths (PALS).
postnatally in the foal and calf. In adults, the spleen is a site These are arranged around arteries of the white pulp, also
of terminal erythrocyte differentiation. referred to as central arteries (Figure 8.15). Splenic follicles
The spleen also participates in immune responses. are spherical accumulations of lymphocytes and reticular
Splenic lymphoid tissue is involved in cell-mediated and tissue. Their structure and function are similar to those of
humoral immunity. Monocytes are transformed into other lymphoid follicles.
active macrophages in the spleen (part of the mononuclear A particular feature of the spleen is the marginal zone
phagocyte system). located between the white and red pulp. In this region
The presence of the spleen is not essential for life. there is a close association between the peripheral blood
Following its removal (splenectomy), the functions of the vessels of the white pulp and the sinuses of the red pulp
spleen are taken over by the bone marrow, in particular, (and their associated macrophages and immunologi-
and by the liver and lymph nodes. cally active marginal reticular cells). Innate and adaptive
immune responses occur particularly in this region of the
Structure of the spleen spleen.
The spleen is covered by a capsule, the outer layer of which The red pulp contains splenic cords composed of
(tunica serosa) forms part of the peritoneum (Figures 8.12 a loose meshwork of reticular cells and fibres enclosing

8.12 Spleen (young
cat). Lymphoid follicles
predominate in the white
pulp of this specimen and
are distributed throughout
the spleen. The red pulp
consists of splenic cords
(reticular tissue, blood cells,
macrophages) and blood
sinuses. Haematoxylin and
eosin stain (x10).

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