Page 82 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

64 Veterinary Histology of Domestic Mammals and Birds

Mast cells are large cells (20–30 μm) found in loose con- Lymphocytes are small spherical cells (8 μm). Their
VetBooks.ir nective tissue, particularly around blood vessels (Figure strongly basophilic nucleus is surrounded by a thin band
3.3). They are typically ovoid with dense, ellipsoid nuclei. of cytoplasm that appears pale under light microscopy.
Medium (10–18 μm) to large (15–25 μm) lympho-
The cytoplasm encloses numerous basophilic metachro-
matic granules containing substances including histamine, cytes are also present in connective tissue. Lymphocytes
heparin and chemotactic factors. In addition, mast cells are found in epithelia, the blood and lymphatic vascular
synthesise prostaglandins, leukotrienes and platelet- systems, in subepithelial tissue and in lymphatic organs.
stimulating factors through which they influence the They are involved primarily in acquired cell-mediated
diameter and permeability of vessels. The surface of and humoral immunity. Pluripotent haemopoietic cells
mast cells expresses specific receptors for immunoglobu- originating from the bone marrow differentiate into lym-
lin E. Under certain circumstances, antigenic substances phocytic progenitor cells that pass to the thymus, where
bind with these receptors, triggering the sudden release they differentiate further and reach a state of immuno-
of chemical mediators from the cell (allergic reaction). A competence (T lymphocytes). These are the agents of
distinction is made between mucosal and connective tissue cell-mediated immunity. Other lymphocytic stem cells
mast cells. Mast cells have a long lifespan, surviving weeks differentiate in the bone marrow, Peyer’s patches and ton-
to months. sils (in mammals) or the bursa of Fabricius (birds) into
Chondroblasts, osteoblasts and adipocytes are described immunocompetent B lymphocytes, which are responsible
below. Other cell types sometimes included in the resi- for the humoral immune response (see Chapter 7, ‘Blood
dent cell population of connective tissue are reticular cells and haemopoiesis’ and Chapter 8, ‘Immune system and
(see below) and pericytes (see Chapter 6, ‘Circulatory lymphatic organs’).
system’). Plasma cells are ovoid basophilic cells (20 μm)
derived from B lymphocytes. The heterochromatin is
Transient cells concentrated in peripheral clumps, giving the nucleus a
Transient (wandering) cells are found primarily in loose ‘cartwheel’ appearance. Plasma cells synthesise antibod-
connective tissue, in which the fibre content is relatively ies (immunoglobulins). This is reflected in the abundant
low. These cells continuously undergo active migration endoplasmic reticulum and numerous free ribosomes
from the blood and lymphatic vascular systems into the within the cytoplasm. In the presence of chronic infec-
extravascular space, where they pass through spaces in the tion, the endoplasmic reticulum may contain acidophilic
interstitium. They are capable of amoeboid motility, which inclusions (Russell bodies).
some use to re-enter the circulatory system. Transient cells Plasma cells are routinely found in subepithelial con-
include: nective tissue of the gastric and intestinal mucosa, in
germinal centres within lymphatic organs and in excretory
· lymphocytes, glands. They possess limited amoeboid motility and most
· plasma cells, have a lifespan of 10–30 days.
· monocytes and Monocytes are predominantly ovoid cells (20 μm) with
· granulocytes. an indented, kidney-shaped heterochromatin-rich nucleus
(Figure 3.4). The abundant cytoplasm contains large

3.3 Tissue mast cell with numerous granules and adja- 3.4 Monocyte in loose connective tissue (x6000).
cent lymphocytes (x3000).

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