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

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241

14.38 Inner ear. Macula (cat).

(1) Columnar epithelium of the

macula consists of neuroepithelial

cells and supporting cells continuous

with the lining epithelium of the

vestibule. (2) Otoliths. (3) Connective

tissue. H & E. ×160.

14.38

14.39 Inner ear. Cochlea (cat).

(1) Dorsal scala vestibuli. (2) Middle

cochlear duct. (3) Ventral scala

tympani. (4) Spiral ganglion.

H & E. ×7.5.

14.39

14.40 Inner ear. Spiral organ (of

Corti) (cat). Located on the floor of

the cochlear duct. (1) Osseous spiral.

(2) Spiral ganglion. (3) Basilar

membrane. (4) Sensory and

supporting cells. H & E. ×160.

14.40

Special Senses

14.41

Reptilian and amphibian

ears

The external pinna and ceruminous glands are

absent. Some terrestrial frogs and toads that uti-

lize audible calls in their courtship and territorial

behaviour have acute hearing. Many amphibians

possess external tympanic membranes; others lack

them entirely. However, some aquatic amphibians

have lateral line systems with which they detect

water-transmitted vibratory signals and hydrosta-

tic pressure changes (see 14.50).

Crocodilians are capable of hearing air-transmit-

ted sounds. They have slit-like auditory openings that

can be closed when they submerge themselves.

Most lizards have ears with tympanic membranes

that are located close to the integumentary surface

Avian ear

The external ear lacks a pinna and ceruminous

glands. The auditory ossicles are fused to form a

cartilaginous rod, the columella, which extends

from the tympanic membrane to the oval window.

The membranous labyrinth is essentially similar to

that of the mammal (14.42). The saccule, however,

differs in that it contains two maculae. The

cochlear duct also possesses a terminal expansion

that is peculiar to birds, the lagena, and is sepa-

rated from the dorsal scala vestibuli by the tegmen-

tum vasculosum, a thin connective tissue

membrane integrated with a highly folded vascu-

lar epithelium.

242

Comparative Veterinary Histology with Clinical Correlates

14.42 Avian inner ear. (1) Spiral ganglion. (2) Osseous

labyrinth. (3) Membraneous labyrinth. (4) Neuroepithelial

sensory cells. H.& E. ×125.

14.42

Clinical correlates

Diseases of the ear in domestic animals are prob-

ably not commonly investigated by pathologists,

although the three broad categories otitis

externa, otitis media and defects of hearing may

be considered. Otitis externa (14.41), or inflam-

mation of the external ear canal, may be a prob-

lem restricted to the waxy integument of this site

or it may be part of a widespread skin condition.

Usually only chronic cases of this very common

condition, in which the severe thickening of the

walls of the ear canal may raise clinical suspicion

of tumour development, are submitted for histo-

pathological examination.

14.41 Otitis externa (dog) The surface epithelium

(centre) is hyperplastic and there is dermal fibrosis. The

patchy appearance of the dermis is caused by a mixed,

mainly mononuclear, inflammatory infiltrate. The

sebaceous glands are hyperplastic and there is cystic

dilatation of the ceruminous glands, some of which are

seen filled with eosinophilic cerumen. H & E. ×20.

of the skull. In some species the tympanic membrane

is flush with the skin surface. In others it lies within

a shallow depression or deeper auditory meatus.

Snakes, chelonians and the tuatara lack external

auditory structures. The philosophical question of

whether their sense of ‘hearing’ is limited to substrate-

transmitted vibrations is a topic of spirited debate.

The inner ear is responsible for the spacial and

postural orientation of the animal within its envi-

ronment. The morphological and histological fea-

tures of the amphibian and reptilian inner ear are

sufficently similar to the mammalian and avian ear

that further discussion is not warranted here.

243

Organ of smell

(olfactory organ)

The nose, in addition to its role in the respiratory

system, also functions as the olfactory organ. In the

nasal cavity, brown–yellow olfactory epithelium

occurs together with pinkish respiratory epithelium.

It is a pseudostratified columnar epithelium with

olfactory (sensory) cells, basal cells and support-

ing cells. Tubular mucoserous glands (Bowman’s)

lie in the lamina propria and secrete onto the sur-

face through simple ducts lined with cuboidal cells

(14.43 and 14.44).

14.43 Olfactory organ (horse).

(1) The olfactory epithelium is

pseudostratified ciliated columnar

with many rows of nuclei; the more

superficial are of the sustentacular

cells; the deeper nuclei are of the

olfactory nerve cells. (2) Vascular

connective tissue with mucous

glands. H & E. ×160.

14.43

14.44 Olfactory organ (horse). The

mucous cells stain deep blue. Alcian

blue. ×100.

14.44

Special Senses

Other specialized

sense organs

Besides sight, hearing, taste, touch and the per-

ception of pain, heat and cold, many of the lower

vertebrates possess highly specialized organs that

augment their awareness of their external en-

vironment.

Parietal eye

In addition to their paired lateral eyes, many lizards

and the primitive tuatara (Sphenodon punctatus)

have a parietal eye. This photosensitive organ con-

sists of a scale-like and cell-poor cornea, a cellular

lens, a central chamber filled with clear fluid and a

few macrophages, and a cup-shaped pigmented

visual epithelium that is analagous to the retina and

choroid (14.45). The parietal eye is partially

responsible for regulating basking and other

thermoregulating behavioural activities, and it can

warn of the approach of potential predators whose

shadows are detected by the upward directed eye-

like structure.

Facial and labial pit organs

Rattlesnakes, water moccasins, copperhead snakes

and other pit vipers possess paired facial pit organs

(14.46) with which they sense very small differences

in the background thermal environment. This abil-

ity to discriminate slight temperature variations aids

in prey detection both before and after the prey

animals have been envenomated.

Non-venomous snakes of the family Boidae

(boas, pythons and anacondas that are ambush

predators) possess labial pit organs (14.47), which

help them locate warm-blooded prey. These

branched pit-like depressions are lined with lightly

keratinized squamous epithelium through which

numerous dendritic sensory neurons penetrate.

Vomeronasal organ

Most snakes and lizards have well-developed

vomeronasal (Jacobson’s) organs (14.48), which

assist in sampling and discriminating chemosensory

stimulatory particles such as prey-related scent or

pheromones. The vomeronasal organ consists of a

mushroom-shaped rounded column with a carti-

laginous core. This column is surrounded by a cup-

shaped spherical cavity. The luminal surfaces of the

column and cavity are covered with ciliated colum-

nar or pseudostratified columnar epithelium

through which myriad numbers of tiny dendritic

nerves pass between adjacent cell membranes. These

nerve endings project out into the lumen (14.49).

Lateral line organ

Most fish and many amphibians (particularly

aquatic frogs, newts and salamanders) have lateral

line systems (14.50) that are sensitive to slight

changes in hydrostatic pressure and water-borne

vibration. Tall cuboidal to fully columnar cells form

vibration- and pressure-sensitive neuromasts that

receive and transmit impulses from the aquatic envi-

ronment to the animal’s central nervous system.

244

Comparative Veterinary Histology with Clinical Correlates

14.45 Whole mount section of the

parietal eye of a green iguana (Iguana

iguana). At the top of the image is a

relatively thick, but avascular, cornea

(1). A cellular lens (2), composed of tall

columnar cells packed closely and parallel

to each other, lies beneath the cornea.

The lumen (3) of the central chamber

contains clear fluid and is surrounded at

the sides and back by heavily pigmented

photosensitive retinal cells (4) and

unpigmented ganglion cells (5). A giant

melanin-packed macrophage (6) can be

seen at the bottom of the capsule. The

parietal nerve exits the rear of the

parietal eye and courses through the

parietal foramen. A thin connective tissue

capsule (7) envelops the parietal eye

where it is surrounded by calvarial bone.

H & E. ×125.

14.45

14.46 Facial pit organ of a western diamondback

rattlesnake (Crotalus atrox) is surrounded on its inner

surfaces by a cup-shaped bony depression. A thin, lightly

keratinized diaphragm-like membrane (1) divides the

posterior of the pit chambers into two compartments.

Within the membrane are embedded numerous dendritic

neuron endings. H & E. ×20.

245

14.46

14.47 Labial pit organ of a reticulated python (Python

reticulatus). These infrared superficial depressions over

much of the external surface of upper lips of some boas

and pythons consist of parallel branched or unbranched

shallow passages lined by a thin layer of lightly

keratinized squamous epithelium. A rich network of dark

staining, fine dendritic nerve endings penetrate to just

beneath the epithelium. Bodian’s silver stain. ×125.

14.47

14.48 Whole mount sagittal section of the vomeronasal

(Jacobson’s) organ of a small skink (Scinella lateralis). The

raised mushroom-shaped protruberance is supported by a

core of hyaline cartilage and is surrounded by a narrow

cavity the surfaces of which are covered on all sides by

ciliated simple columnar epithelium. H & E. ×125.

14.48

14.49 Section through the superficial surface of the

vomeronasal organ of a green iguana (Iguana iguana).

Note the myriad number of thin dendritic nerve endings

that course between and penetrate the epithelium to the

lumenal surface. Bodian’s silver stain. ×250.

14.49

14.50 Cross-section of the dermis and the pressure-

sensitive lateral line of an aquatic salamander (Amphiuma

tridactyla). The clear central cavity is lined with large,

plump columnar glandular secretory cells. H & E. ×250.

14.50

Special Senses

Electric organ

Some teleost fish, such as the electric eel and elec-

tric catfish [and at least one family of elasmo-

branch ray, such as the torpedo (Torpedo spp.)],

possess specialized muscles arranged into discrete

electric organs (electroplax) that produce and

detect powerful electric impulses. Paired electric

lobes on the medulla oblongata are the motor cen-

tres for the integration of electroplax activity.

When these specialized muscular organs suddenly

discharge their electrical potential, prey fish and

predators are subjected to pulses of high amper-

age electrical current that can be incapacitating or

fatal. Some electric eels produce pulses of direct

current that measure 600 V.

Swim bladder

Most but not all teleost fish possess a specialized

elongated gas- (or oil-) filled organ: the swim blad-

der. This is a major hydrostatic organ that helps

these fish maintain their buoyancy and orientation

within a column of water. In some bottom-feeding

species the swim bladder is much reduced or may

be absent. In fast swimming fish it is elongated and,

thus, enhances streamlining. The swim bladder is

formed from thin sheets of dense fibrocollagenous

connective tissue laid at acute angles to each other.

This arrangement aids in maintaining its shape and

reducing deformation. In some fish, skeletal mus-

cles insert into its outermost surface. In other species

the swim bladder is only attached to the body wall

along its dorsal surface. The swim bladder is lined

by a thin, much flattened, non-keratinized squa-

mous epithelium.

Several bacterial, viral and protozoan diseases

are characterized by inflammation of the swim blad-

der. Thus, when performing a necropsy on a fish,

it is important to examine this organ for haemor-

rhage(s), oedematous thickening, discolouration or

other abnormality.

246

Comparative Veterinary Histology with Clinical Correlates

The lymphatic system has a dual function: the lym-

phatic vessels drain interstitial tissue, returning fluid to

the bloodstream; and lymphoid tissue produces phago-

cytes and the immunologically competent cells, which

are the body’s most important defence mechanism

against invasion by pathogens. Lymphoid tissue is

widely distributed in the body and comprises: lymph

nodes and associated lymphatic vessels; spleen; thy-

mus; local aggregations of lymphoid tissue in the

mucosa of the digestive, respiratory and urogenital

tracts; and any local tissue aggregation of lymphocytes.

Lymphoid tissue consists predominantly of lym-

phocytes. These and a variable number of plasma

cells, macrophages and other cells are supported by

a delicate network of reticular fibres that fill the

spaces between the trabeculae. Diffuse lymphatic tis-

sue and lymphatic nodules are the components of

most lymphatic organs, and also appear in the con-

nective tissue of the digestive, respiratory, urinary and

247

15. LYMPHATIC SYSTEM

15.1 Lymph node (dog). (1) Connective tissue capsule.

Cortex with (2) lymphatic follicles and (3) paracortex.

Masson’s trichrome. ×20.

15.1

15.2 Lymph node (sheep). (1) Connective tissue capsule.

(2) Connective tissue trabeculae. (3) Lymphatic tissue.

H & E. ×62.5.

15.2

15.3 Lymph node (cat). The reticular fibres form a black

network and support the cells of the lymphatic tissue.

Gordon and Sweet. ×100.

15.3

reproductive organs, among other locations. The for-

mer is characterized by a moderate concentration of

scattered lymphocytes; the latter comprises an aggre-

gation of mostly small, densely packed lymphocytes.

Lymph nodes

A lymph node contains diffuse and nodular lym-

phatic tissue and lymphatic sinuses that are organized

into a cortical and medullary region. A capsule of

connective tissue sends fine vascular fibrous trabec-

ulae into the substance of the node. Afferent lym-

phatic vessels enter the capsule and drain into the

subcapsular sinus. From there the lymph drains into

a labyrinth of sinuses extending along the trabecu-

lae, eventually emptying into the efferent lymphatics

in the hilus. In the cortex, circular aggregations of

lymphocytes form follicles or nodules (15.1–15.4).

15.4 Lymph node (dog). (1) Connective tissue capsule

with some fat cells. (2) Subcapsular sinus. (3) Cortical

follicle. (4) Parafollicular tissue. (5) Sinusoid. H & E. ×125.

15.4

248

15.7 Lymph node. Paracortex (pig). (1) Connective tissue

capsule. (2) Connective tissue trabeculae. (3) Subcapsular

sinus. (4) Thymus-derived T lymphocytes. (5) Sinusoid.

Alcian blue. ×125.

15.7

15.5 Lymph node. Cortex (dog). A single follicle is

present. The central zone is the pale staining reactive

germinal centre. There is an outer rim of closely packed

small lymphocytes. H & E. ×125.

15.5

15.6 Lymph node. Cortex (dog). The germinal centre

is composed of lymphoblasts, reticular cells and

macrophages. H & E. ×125.

15.6

The primary follicle is a solid packed, evenly distrib-

uted mass of cells. The secondary follicle has an outer

rim of densely packed small lymphocytes and a pale

staining, loosely packed germinal centre, with a mixed

population of lymphoblasts, dendritic reticular cells

and macrophages (15.1 and 15.4<15.6). The sec-

ondary follicle responds actively to antigen stimulus,

and B lymphocytes (originating in the bone marrow)

are present, as are macrophages. Thymus-derived T

lymphocytes between the follicles form the paracor-

tex (15.7).

In the medulla there is a looser aggregation of cells.

Sinusoidal spaces are lined with endothelial cells on

a reticular framework with attached macrophages.

Rosette-like clusters of lymphocytes, plasma cells,

macrophages and leucocytes form the medullary

cords (15.8 and 15.9). The sinuses are fenestrated and

lymphocytes and macrophages have free access.

Monoclonal antibodies are used to identify sub-

sets of lymphocytes and other cells in lymphoid tis-

sue. The cell surface glycoproteins CD4 and CD8 are

expressed on exclusive populations of mature T lym-

phocytes in the parafollicular and deep cortex of

lymph nodes (15.10–15.12). The anti-CD21 mono-

clonal antibody recognizes the follicular dendritic

cells and the cell processes (15.13). Haemolymph

nodes, dark red aggregations of lymphoid tissue the

function of which is unknown, are present in rumi-

nants. The sinusoids are filled with blood instead of

lymph (15.14).

15.8 Lymph node. Medulla (dog). (1) Loose aggregation

of lymphoid tissue. (2) Open meshwork of sinusoids.

H & E. ×250.

15.8

Comparative Veterinary Histology with Clinical Correlates

15.12

15.12 Lymph node. Cortex. Anti CD8/CD4 monoclonal

antibody. This subset of positive cells are rarely present in

the follicle germinal centre. Avidin/biotin method. ×100.

249

15.9 Lymph node. Medulla (dog). (1) Sinusoid lined by

macrophages. Also, clusters of small lymphocytes

(arrowed). Alcian blue. ×125.

15.9

15.10 Lymph node (cat). Anti-CD4 monoclonal antibody.

The avidin/biotin method detects an exclusive population of

T lymphocytes in the parafollicular and deep cortex of the

lymph node; brown reaction. Avidin/biotin method. ×62.5.

15.10

15.11 Lymph node (cat). Anti-CD4 monoclonal antibody.

The avidin/biotin method detects an exclusive population of

T lymphocytes in the parafollicular and deep cortex of the

lymph node; brown reaction. Avidin/biotin method. ×125.

15.11

15.13 Lymph node. Cortex. Anti-C.D.21 monoclonal

antibody. This method recognizes the follicular dendritic

cells and the cell processes; stained red. Avidin/biotin

method. ×62.5.

15.13

15.14 Haemal lymph node (ox). (1) Connective tissue

capsule. (2) Blood filled sinusoids. H & E. ×20.

15.14

Lymphatic System

250

15.15 Spleen (horse).

(1) Fibromuscular capsule.

(2) Fibromuscular trabeculae.

(3) Splenic pulp. H & E. ×12.5.

15.15

15.16 Spleen (horse). (1) White pulp.

(2) Red pulp. (3) Trabecula. H & E.

×62.5.

15.16

Spleen

The spleen, the largest organ in the lymphatic sys-

tem, is usually situated in the cranial part of the

abdominal cavity on the left of the stomach (in

ruminants on the left lateral wall of the reticulum).

It contains the largest collection of reticulo-

endothelial cells in the body. It has no afferent lym-

phatics. The capsule consists of smooth muscle,

collagen and elastic fibres with fibrocytes, and

extends into the gland to form the supporting

framework for the parenchyma, which is divided

into red and white splenic pulp (15.15).

White pulp contains lymphatic follicles (which

may be primary or secondary), together with dense

accumulations of T lymphocytes arranged around

arteries to form the periarterial lymphatic sheaths.

Splenic corpuscles, arterioles with a cuff of T lym-

phocytes, occupy an eccentric position in the white

pulp (15.16 and 15.17). The arteriolar branches leave

the white pulp and enter the red pulp as straight peni-

cilli. Some acquire a coat of reticular fibres and

become ellipsoids (well developed in the cat) and

empty into the splenic sinusoids of the red pulp

(15.18). These sinusoids are wide channels lined with

endothelial cells with gaps occupied by macrophages.

Foreign material is recognized and removed as part

of the immune response; senescent erythrocytes are

also removed from the circulation (15.19).

Red pulp, so called because of the large number

of erythrocytes it contains in the reticular frame-

work, is a loose arrangement of blood filled, fen-

estrated sinusoids, opening into venules and

draining into the splenic vein to leave at the hilus.

The region between the red and white pulp is the

marginal zone and is phagocytic (15.20). The spleen

thus functions as part of the immune system and

part of the mononuclear phagocyte system.

Comparative Veterinary Histology with Clinical Correlates