Aughey E., Frye F.L. Comparative veterinary histology with clinical correlates

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Blood

4.31

Clinical correlates

Disorders of the blood can be attributed to

abnormalities in production of blood cells from

the bone marrow or other central blood-form-

ing organs or to abnormal loss or consumption

of these cells. Anaemia, the deficiency of oxy-

gen-carrying erythrocytes, can be regenerative

(i.e. the bone marrow is able to respond by

releasing new cells, including immature cells

into the circulation) or non-regenerative (in

which this response does not occur). A blood

film that illustrates haemolytic anaemia in a dog

is shown in 4.31. In this regenerative anaemia

there is variation in cell size (anisocytosis) with

red cell precursors, such as the large, stippled

reticulocyte (1) and nucleated normoblast (2),

4.31 Haemolytic (regenerative) anaemia in a dog. Note the variation in

appearance of the red cells. Reticulocytes, nucleated normoblasts and

spherocytes are present. Giemsa. ×200.

visible. Some of the red cells have small, dense

dots (Howell–Jolly bodies), which represent

remnants of nuclear chromatin and are charac-

teristic of regenerative anaemias. Spherocytes,

red cells that have lost their biconcave shape

and become globular due to immunological

attack, are present (3). There is an increased

neutrophil count with some immature neu-

trophils (reactive neutrophilia with a left shift),

which typically accompanies haemolysis. These

findings are diagnostic of autoimmune

haemolytic anaemia, the most common type of

anaemia in the dog.

In immune-mediated anaemia, erythrocyte

destruction follows the attachment of antibody

to the cell membrane. In most cases the aetiology

is unknown.

Comparative Veterinary Histology with Clinical Correlates

4.32

4.33

4.33 Megakaryocytic myelosis in the liver of a cat. Note the megakaryocyte

with the multilobed nuclei in the cellular infiltrate. H & E. ×255.

4.32 Acute lymphoblastic leukaemia (dog) showing large numbers of

malignant lymphoblasts. Giemsa. ×255.

The blood film in 4.32 was prepared from a

dog with acute lymphoblastic leukaemia. Very

high numbers of malignant lymphoblasts, which

are larger than normal lymphocytes and have

bluer cytoplasm, are seen. In some cases of

leukaemia, although the bone marrow is invaded

by neoplastic haemopoietic cells, no abnormal

cells are released into the peripheral blood. Such

cases may be termed aleukaemic leukaemias.

Non-regenerative anaemia is a common finding

with many leukaemias.

The liver of a cat in which the sinusoids (vas-

cular channels) are heavily infiltrated by large

neoplastic megakaryocytes with multilobed

nuclei is shown in 4.33. This is megakaryocytic

myelosis, a rare, chronic disease of dogs and

cats characterized clinically by bleeding and

thrombosis of the ear and tail tips.

4.34 Head of the humerus (domestic hen). (1) Outer layer of eosinophilic

cortical bone. (2) The spiculated medullary bone is basophilic. (3) The marrow

cavity is lined by osteoblasts. H & E. ×62.5.

4.34

4.35 Head of the humerus (domestic hen). (1) The osteocytes in the

eosinophilic cortical bone form an osteon. (2) Osteocytes in the open lacunae

of the basophilic medullary bone. (3) Marrow cavity lined by osteoblasts.

H & E. ×125.

4.35

Avian bone

A unique feature of avian bone in egg-producing

birds is the accumulation of spiculated bone in the

medullary cavity, under the combined influence of

oestrogens and androgens. This medullary bone

is particularly labile and the stored calcium is uti-

lized in the formation of the calcareous egg shell.

The medullary bone is basophilic and decreases

during shell deposition and increases at other

times. The basophilia is caused by a change in the

density of the matrix and in the glycosaminogly-

cans that are present. During resorption, osteo-

clasts are active; during deposition, osteoblasts are

prominent on the surface of the bone spicules

(4.34 and 4.35).

Blood

Reptilian, amphibian and fish

bone and bone marrow

Reptiles do not possess pneumatized bone such as

that found in birds capable of flight. The box-like

carapace and plastron of chelonians are composed

of specialized bone that must be both strong and

relatively lightweight in order for this bone to pro-

tect efficiently the delicate internal structures.

Compressional stresses applied to the dorsal and

ventral surfaces are distributed widely and are then

borne upon buttress-like vertical supporting pillars

of bone that are at each end of the ‘bridge’ that joins

the carapace to the plastron on each side. The

strength of their shells is further enhanced by the

curved shape and by additional internal struts that

distribute compressive forces so that they are not

concentrated onto a single focus. The shell is com-

posed of parallel layers of inner and outer tables

of compact bone with an intervening layer of

spongy cancellous bone characterized by spaces

filled with bone marrow. In form and function the

bony shell resembles the calvaria that covers the

brain case of a mammalian skull.

The bone of amphibians and reptiles contains

numerous sites of haemopoiesis; long bones (see

3.29), ribs, skull and mandibles are locations in

which active blood cell formation normally occurs.

During severe blood loss and a few other condi-

tions, sites other than bone marrow are recruited

for extramedullary haemopoiesis; liver, spleen and

kidney are then most often involved.

In fish, the major organ of haemopoietic activity

is the cranial pole of each kidney (see Chapter 9),

with lesser amounts occurring in other extra-

medullary sites during times of severe anaemia,

infection or stress.

The presence of numerous megakaryocytes in

the splenic red pulp is a common finding in the

healthy house mouse, Mus musculus, whereas

extramedullary haemopoiesis is usually considered

an abnormal finding in most other animals (4.36).

4.36 Extramedullary haemopoiesis in the spleen of a domestic mouse (Mus

musculus). Note the multinucleated megakaryocytes which are normal in

murine splenic tissue. H & E. ×125.

4.36

Comparative Veterinary Histology with Clinical Correlates

Contractility, a fundamental property of cytoplasm,

is developed to a highly specialized degree in mus-

cle tissue. The elongated muscle cell is commonly

referred to as a muscle fibre, the plasma membrane

as the sarcolemma and the cytoplasm as the sar-

coplasm. There are basically two types of muscle:

smooth, visceral or involuntary muscle; and striated

muscle, which is further subdivided into skeletal

voluntary muscle and cardiac involuntary muscle.

Muscle contraction depends upon the proteins

actin and myosin in the sarcoplasm. In skeletal and

cardiac muscle the longitudinal arrangement of

these proteins is aligned in register to give cross-

striations, which are absent from smooth muscle.

Muscle types

Smooth muscle

Smooth muscle cells are elongated fusiform fibres

(2–50 +m long) with a single, centrally located

nucleus with several nucleoli. These fibres may

occur singly, as in the lamina propria of intestinal

villi, but are more commonly arranged in sheets or

layers in the walls of a wide range of tubes of the

alimentary, urogenital, respiratory and cardiovas-

cular systems. The fibres are packed in a staggered

fashion with the thickest nucleated portion of one

fibre juxtaposed to the thin tapered end of an

adjoining one. Individual smooth muscle fibres are

supported by reticular fibres, with collagen and

elastic fibres forming a supporting framework of

connective tissue carrying blood vessels and nerves.

Smooth muscle is under involuntary control; it is

innervated by the autonomic nervous system

(5.1–5.3).

5. MUSCLE

5.1 Smooth muscle. Uterus (cat). (1) Nucleus.

(2) Sarcoplasm. H & E. ×100.

5.1

5.2 Smooth muscle. Uterus (cat). (1) Nucleus.

(2) Sarcoplasm. (3) Longitudinal fibres. (4) Transverse

fibres. H & E. ×200.

5.2

5.3 Smooth muscle. Duodenum (dog). (1) Fusiform

smooth muscle fibre. H & E. ×200.

5.3

Striated (skeletal) muscle

Skeletal muscle fibres are multinucleated giant cells

that range in length from a few millimetres to sev-

eral centimetres. The nuclei lie immediately beneath

the sarcolemma, and the myofibrils give both a lon-

gitudinal and a cross-striation arrangement. The

contractile myofilaments are arranged in alternat-

ing isotropic I bands and anisotropic A bands. This

imparts the cross-striated effect.

Skeletal muscle fibres are bound into large bun-

dles by an outer connective tissue investment, the

epimysium. This dips into the muscle and invests

bundles of muscle fibres (fascicles) in connective

Comparative Veterinary Histology with Clinical Correlates

5.4 Striated muscle. Tongue (ox).

(1) Transverse section (TS) muscle

fibres. (2) Longitudinal section (LS)

muscle fibres. Masson’s trichrome.

×125.

5.4

5.5 Striated muscle. Tongue (cat).

(1) Nucleus. (2) Sarcoplasm.

(3) Endomysium. H & E. ×200.

5.5

5.6 LS striated muscle. Tongue (cat).

(1) Longitudinal striated muscle.

(2) Nuclei. (3) Endomysium stained

green. Masson’s trichrome. ×200.

5.6

tissue, the perimysium. This in turn invests each

muscle fibre in a vascular loose connective tissue,

the endomysium. A fine network of reticular fibres

lies against the sarcolemma. The collagen fibres of

the tendon extend into the epimysium and allow

muscular contraction to effect movement (5.4–5.9).

Every skeletal muscle fibre receives an axon termi-

nal from a motor neuron at the myoneural junction,

the motor endplate (see 13.27). Muscle spindles,

which are attenuated skeletal muscle fibres, act as

stretch receptors that are innervated both by motor

and by sensory nerve terminals.

5.7 TS striated muscle. Tongue (cat).

(1) Transverse section of muscle

fibres. (2) Connective tissue

perimysium. Masson’s trichrome.

×62.5.

5.7

5.8 Striated muscle. Tongue (ox).

Muscle fibres showing cross-striations.

Masson’s trichrome. ×625.

5.8

5.9 Tendon/muscle junction (dog).

(1) Collagen fibres of the tendon.

(2) Striated muscle fibres. H & E.

×200.

5.9

Muscle

Cardiac muscle (myocardium)

Cardiac muscle is exclusive to the myocardium, the

muscular wall of the heart. The fibres are smaller

than skeletal muscle fibres and branch repeatedly

(5.10). The nucleus lies in the centre of the fibre

(occasionally two nuclei are present) and the fibres

have strong areas of attachment at the intercalated

disc. These are visible as a dark cross-striation at

the end of one fibre and the beginning of the next,

and confer structural integrity on the heart muscle

to allow contraction to spread throughout the

myocardium (5.11 and 5.12). These are represented

at the ultrastructural level by tight junctions and

gap junctions. The cardiac conducting system is

composed of several specialized muscle fibres in the

sinoatrial and atrioventricular nodes. In the sinoa-

trial node (5.13, 5.14), small cardiac muscle fibres,

which are low in myofilaments, have an intrinsic

ability to contract at a species-specific rate and act

as the pacemaker for cardiac muscle contraction.

The atrial wave of depolarization converges on the

second node, the atrioventricular node, from where

specialized large muscle fibres spread throughout

the ventricular muscle and initiate contraction.

These specialized conducting fibres are binucleate

and the nucleus lies in a clear area of sarcoplasm

(5.15).

The various types of muscle tissue can be dis-

cerned, even in invertebrates low on the phyloge-

netic scale. For example, the striated muscle fibres

of a spider are structurally similar to those found

in mammals and the multiple heart-like pumping

chambers that circulate the haemolymph and

Comparative Veterinary Histology with Clinical Correlates

5.10 Cardiac muscle (horse).

(1) Nucleus of the muscle fibre.

(2) Sarcoplasm. H &.E. ×125.

5.10

5.11 Cardiac muscle (horse).

(1) Cardiac muscle fibre. (2) Vascular

connective tissue of the endomysium.

(3) Intercalated disc. Heidenhain’s

iron haematoxylin. ×625.

5.11

5.13 Cardiac muscle (horse). (1) Cardiac muscle.

(2) Purkinje’s fibres. H & E. ×62.5.

haemolymphocytes through the coelom of earth-

worms are formed from myocardium.

As all fish and amphibians and most reptiles (all

except the crocodilians) possess a three-chambered

heart with paired atria and a single ventricle, there

are some structural differences between the hearts

of these animals. For example, a ridge of

myocardium helps direct the flow of blood through

the ventricle so as to reduce the mixing of oxy-

genated and deoxygenated blood in non-crocodil-

ian reptiles. Blood flow through twin aortic arches

and atrioventricular valves is controlled by typical

heart valve leaflets composed of myxoid connective

tissue that are covered by a thin endothelial lining

in all vertebrates.

5.12 Cardiac muscle (horse). (1) Cardiac muscle fibre.

(2) Vascular connective tissue of the endomysium.

(3) Intercalated disc. Heidenhain’s iron haematoxylin.

×200.

5.12

5.14. Cardiac muscle (horse). (1) Cardiac muscle.

(2) Purkinje’s fibres. (3) Loose vascular connective tissue.

H & E. ×125.

5.14

5.13

5.15 Cardiac muscle (horse).

(1) Endocardium. (2) Impulse-

conducting fibres (Purkinje’s fibres).

(3) Muscle fibre. H & E. ×125.

5.15

Muscle

5.175.16

Clinical correlates

Myopathies, primary disorders of muscle struc-

ture, may be congenital, metabolic or inflamma-

tory. Viral, bacterial, fungal, parasitic, protozoal

and metazoal agents can be implicated in infec-

tive myopathies.

Where there is frank inflammation the disease

would be termed a myositis. In eosinophilic

myositis, a condition that affects the masticatory

muscles of dogs, in particular in the German

Shepherd breed (5.16), the affected muscle is dif-

fusely infiltrated by numerous eosinophils

accompanied by lesser numbers of lymphocytes

and other inflammatory cells. There is muscle

degeneration and atrophy. Production of abnor-

mal antibodies which attack these muscles is

believed to initiate the process, which then

becomes dominated by a cellular response.

Progressive destruction of these muscles leads to

fixation of the jaws.

Other non-neoplastic conditions that affect

the muscle may be loosely divided into neu-

ropathies which result from disturbance of

innervation and myasthenic conditions of the

motor end plate. Muscle atrophy (reduction in

cross-sectional area of the muscle fibres) will

tend to occur.

Under certain circumstances, all types of mus-

cle can be vulnerable to pathological deposition

of mineral salts. This may be induced by condi-

tions characterized by persistent hypercalcaemia

(see 6.28) or at sites of previous injury.

Primary neoplasms of muscle are quite rare

with malignant tumours (rhabdomyosarcoma,

5.17) outnumbering benign ones (rhabdomyoma).

Muscle can also be affected by neoplasms of asso-

ciated connective tissue origin.

Comparative Veterinary Histology with Clinical Correlates

5.17 Rhabdomyosarcoma in a 10-year-old male

cat. Large, rather pleomorphic, elongate to strap-

like cells invade and replace skeletal muscle. These

tumours, although uncommon, are highly malignant.

H & E. ×100.

5.16 Eosinophilic myositis. Masticatory muscles from a

dog. Phosphotungstic acid haematoxylin. ×125.