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

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13.15 Spinal cord (dog). (1) Neuron.

(2) Nuclei of protoplasmic astrocytes.

H & E. ×125.

13.15

13.16 Spinal cord (dog). (1) Neuron.

(2) Nuclei of protoplasmic astrocytes.

H & E. ×125.

13.16

13.17 Spinal cord (dog). (1) Neuron. (2) Nuclei of

protoplasmic astrocytes. (3) Fibrous astrocytes. H & E.

×125.

13.17

13.18 Corpus callosum (dog). Oligodendrocyte nuclei in

orderly columns (arrowed) providing the neurilemmal

sheath in the central nervous system. H & E. ×125.

13.18

Nervous System

13.21

The cerebral and cerebellar cortices are highly

specialized folded areas of outer grey matter with

layers of neurons of various sizes and supporting

cells, as well as a central core of white matter

(13.20–13.25).

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Comparative Veterinary Histology with Clinical Correlates

13.19 Microglial cells in the central nervous system (cat). H & E. ×256.

13.19

13.20 Cerebellum (cat). (1) Outer cellular layer of grey

matter and (2) inner fibrous layer of white matter. H & E.

×25.

13.20

13.21 Cerebellum (cat). (1) Inner granular layer of small

neurons. (2) Middle Purkinje layer of large neurons.

(3) Outer molecular layer with few neurons and many

branching processes. H & E. ×100.

lined by a continuous endothelium. There is a well-

developed basal lamina that, with the protoplas-

mic astrocyte processes, forms the formidable

blood<brain barrier. Microglia are the phagocytic,

scavenging cells of the CNS (13.19).

13.24 Cerebellum (cat). Each folium has (1) a central core

of white matter, the nerve fibres and supporting cells,

and (2) a surface cellular layer of grey matter. Cajal’s

silver. ×62.5.

13.24

223

13.22 Cerebellum (sheep).

(1) Granular layer. (2) Purkinje cell

layer. (3) Molecular layer. H & E. ×125.

13.22

13.23 Cerebellum (dog). Large

Purkinje cells are arrowed. H & E.

×250.

13.23

13.25 Cerebral cortex (cat). (1) Meninges. (2) White

matter. (3) Grey matter. H & E. ×100.

13.25

Nervous System

224

13.26 Cerebral cortex (cat). Cellular grey matter.

H & E. ×25.

13.26

13.27 Motor end plate in striated

muscle (dog). (1) Striated muscle

fibre. (2) Terminal nerve fibre. Cajal’s

silver. ×250.

13.27

13.28 Pacinian corpuscle. Bladder

(dog). (1) Pacinian corpuscle.

(2) Loose areolar connective tissue.

H & E. ×20.

13.28

Nerve endings and receptors are specialized ter-

minal parts of axons or dendrites. They include

motor end plates, pressure receptors (such as

Pacinian corpuscles) and taste buds (13.26–13.31).

Nerve tissue is so characteristic that it is readily

identified, even in invertebrates.

Comparative Veterinary Histology with Clinical Correlates

225

13.29 Lamellar (Pacinian) corpuscle

in a serous gland (dog).

H & E. ×20.

13.29

13.30 Circumvallate papilla. Tongue

(cow). Taste buds are arrowed.

Masson’s trichrome. ×160.

13.30

13.31 Cat skin to illustrate nerve

endings stained black with Cajal’s

silver. ×250.

13.31

Nervous System

13.34

13.33

13.32

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Comparative Veterinary Histology with Clinical Correlates

13.34 Bovine spongiform encephalopathy (BSE, cow).

This section is taken from the midbrain of a cow with

BSE. The histological changes are similar to those found

in scrapie, with vacuolation of neuronal cell bodies in

target areas of the brain, which include in particular the

medullary nuclei. Some vacuolation of the neuropil is

present in this section. The severity is variable between

cases. Special precautions, including treating the tissues

with formic acid to inactivate the BSE agent, are

employed when processing these tissues. H & E. ×125.

from almost every type of cell within the nervous

system have been recognized. The functional dis-

tinction between benign and malignant neo-

plasms within the CNS is not as significant as in

other systems, as any space-occupying lesion,

even the most biologically benign, has serious

consequences. Even non-neoplastic lesions, such

as equine cholesteatoma (13.32) in which a mass

of cholesterol crystals surrounded by inflamma-

tory cells located in the choroid plexuses can pro-

duce space-occupying effects, in particular

hydrocephalus caused by obstruction of ventric-

ular drainage.

Transmissible spongiform encephalopathies

(TSE) are an unusual group of infectious central

nervous disorders, although the nature of the

infective agent, which differs from conventional

agents like bacteria and viruses, is not precisely

defined. The agent is believed to enter via the

oral route and proliferate in lymphoid tissues and

in the intestine, but no effects are observed until

it reaches target areas of the CNS. The sheep

TSE, scrapie (13.33), has been known for a very

long time. This disease produces no significant

gross pathology, although emaciation and self

trauma are common, and provokes no immune

response. The cattle variant, bovine spongiform

encephalopathy (BSE; 13.34) was recognized

more recently and produces similar histological

changes to those seen in scrapie in sheep.

Clinical correlates

The CNS is affected by a variety of congenital

disorders that occur with relatively high fre-

quency, possibly because of the susceptibility of

this complex system to teratogenic insult.

Inherited CNS disease is also recognized and a

heritable basis is suspected for conditions such

as idiopathic epilepsy.

Inflammatory, neoplastic, metabolic and par-

asitic disorders are all recognized in the CNS,

with certain specific entities appearing in partic-

ular species. For example, feline parvovirus,

which is trophic for dividing cells, attacks the

external germinal layer of the cerebellum in kit-

tens when infection occurs before or shortly after

birth. Primary CNS neoplasms are not uncom-

mon in the dog and cat and tumours that arise

13.32 Cholesteatoma in the brain of a horse. Note the

numerous clefts that previously held cholesterol deposits

lost during the histological processing. H & E. ×20.

13.33 Scrapie (sheep). This is a section of midbrain

from a sheep with scrapie. The characteristic large

intraneuronal vacuoles and diffuse vacuolar change

in the neuropil can be seen. No stainable material is

present in the vacuoles. H & E. ×125.

Eye

The eye is the organ of vision and comprises the eye-

ball or globe of the eye and the optic nerve. It is pro-

tected by the eyelids and lacrimal apparatus.

Eyelids

The eyelids are movable folds of skin in front of the

eyeball protecting and lubricating the surface of the

eye. The outer surface is covered with stratified

squamous epithelium, tactile hairs, sebaceous

glands (glands of Zeiss) and sweat glands (glands

of Möll; 14.1). The inner surface is lined with the

palpebral conjunctiva, a thin transparent mucous

membrane. The bulbar conjunctiva is continuous

with the surface of the cornea at the limbus. The

epithelium may be stratified columnar or stratified

squamous, with goblet cells. Between the dermis of

the skin and the lamina propria of the conjunctiva

is a plate of dense connective tissue: the tarsal plate

is surrounded by the multilobular tarsal glands

(14.2). The nictitating membrane (third eyelid) is

situated at the medial angle of the eye. It is a semi-

227

14. SPECIAL SENSES

14.1 Eyelid (horse). Outer skin layer

of the eyelid with a tactile hair

(arrowed). H & E. ×7.5.

14.1

14.2 Eyelid (horse). (1) The

conjunctiva, stratified columnar

epithelium with mucus-secreting

cells. (2) Sebaceous glands in the

lamina propria. (3) Tarsal plate.

H & E. ×25.

14.2

circular fold of conjunctiva enclosing a plate of car-

tilage (hyaline in ruminants and dog; elastic in the

horse, pig and cat; 14.3–14.5).

Lacrimal apparatus

The lacrimal glands are tubuloacinar: serous in the

cat, seromucous in the horse, ruminant, dog and pig.

(In the horse and ruminant they are predominantly

serous, and in the pig predominantly mucous.)

Lymph nodules are seen in the lamina propria (14.6

and 14.7). The pig and ox also have a deep gland

(Harderin gland) of the nictitating membrane.

Development of the eye

The eyes are recognizable in the early embryo as lat-

eral diverticulae of the diencephalon. As the diver-

ticulum approaches the surface ectoderm, it

invaginates to form the optic cup. The inner layer

becomes the light-sensitive retina; the outer layer

becomes the retinal pigment, pigmented ciliary and

anterior iris epithelium. The surface ectoderm over-

lying the optic cup thickens and invaginates to form

the lens. The reconstituted ectoderm and the local

mesenchyme form the cornea; the surrounding

mesoderm provides the connective tissue, blood ves-

sels and ocular muscles (14.8 and 4.9).

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14.3 Nictitating membrane (membrana nictitans; horse).

(1) Anterior surface is covered by conjunctiva. (2) Elastic

cartilage plate in a central core of connective tissue.

(3) Posterior plate covered by conjunctiva. H & E. ×7.5.

14.3

14.4 Nictitating membrane (horse). (1) Anterior

conjunctival surface. (2) Elastic cartilage plate.

(3) Posterior conjunctival surface. (4) Lamina propria.

H & E. ×7.5.

14.4

14.5 Nictitating membrane (dog). (1) Hyaline cartilage.

(2) Seromucus-secreting glands in the lamina propria.

H & E. ×50.

14.5

Comparative Veterinary Histology with Clinical Correlates

Special Senses

14.8 Developing eye in a 35-day

cat embryo. (1) Surface ectoderm.

(2) Developing lens. Optic cup has a

thick inner layer (3) and a thin outer

layer (4) with the pigment cells.

H & E. ×7.5.

14.8

14.9 Developing eye in a 35-day cat

embryo. (1) The lens fibres almost fill

the lens vescicle. (2) Loose vascular

mesenchyme. (3) Retina. H & E. ×25.

14.9

14.7 Nictitating membrane, posterior surface (dog).

(1) Lymphoid cells in the lamina propria. H & E. ×125.

14.7

14.6 Nictitating membrane (pig). The mucus-secreting

cells in the outer epithelium are stained pale pink.

H/PAS. ×100.

14.6

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Structure of the eyeball

The eyeball (14.10) is spheroid and composed of

a lens and a wall that is divided into three layers:

an outer fibrous tunic (corneoscleral layer); a mid-

dle vascular tunic (uvea); and an inner retinal

tunic, which consists of a 10-layered photosensi-

tive retina and a bilayered non-photosensitive por-

tion that covers the ciliary body and posterior

surface of the iris.

The eye contains three fluid-filled regions: the

anterior chamber, bordered by the cornea, iris and

lens; the posterior chamber, located between the iris,

lens, zonular fibres and ciliary processes; and the

cavity of the vitreous humour, which lies behind the

lens (14.10). The lens is a biconvex transparent body

composed of epithelial cells within a homogeneous

outer capsule. The anterior epithelial cells are

cuboidal at the pole, and become elongated, pris-

matic and arranged in meridional rows at the equa-

tor where they form lens fibres (14.11). The aque-

ous humour circulates continuously, and is secreted

and absorbed by the blood vessels in the sclera at the

corneoscleral junction.

The corneoscleral layer is divided into a trans-

parent anterior segment, the cornea, and an opaque

posterior segment, the sclera. The avascular cornea

is transparent, covered with non-keratinized strat-

ified squamous epithelium resting on a basement

membrane (Bowman’s). The underlying stroma, the

substantia propria, consists of thin lamellae of col-

lagenous fibres and flattened fibroblasts running

parallel to the corneal surface in a mucoid ground

substance (14.12 and 14.13). The caudal limiting

membrane (Descemet’s) separates the connective tis-

sue of the substantia propria from the simple squa-

mous corneal endothelium (also called posterior

epithelium; 14.14).

The opaque sclera consists of interlacing bundles

of white fibrous tissue with a few elastic fibres. The

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Comparative Veterinary Histology with Clinical Correlates

14.10 Diagram of the eye. (1) Cornea. (2) Anterior

chamber. (3) Iris. (4) Lens. (5) Posterior chamber.

(6) Ciliary body. (7) Ciliary processes. (8) Sclera.

(9) Choroid. (10) Retina. (11) Optic papilla.

(12) Optic nerve. (13) Vitreous humour.

14.10

14.11 Lens (dog). (1) Anterior epithelium. (2) Nuclei of

the lens fibres. H & E. ×62.5.

14.11