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

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12.84

12.83

211

12.82 TS eggshell with the

membranes removed. (1) Gaseous

pore. (2) Cuticle. (3) Multiple layers

of calcite containing organic matrix.

(4) Mammillary layer. Scanning

electron micrograph. ×160.

12.82

12.83 Mammillary layer of the

eggshell. This is the organic

component. The individual structural

units are the mammillae arranged as

(1) mammillary caps and

(2) mammillary cones. Scanning

electron micrograph. ×320.

12.84 Vagina (bird). A

pseudostratified columnar epithelium

lines the vagina; small tubular glands

(sperm-host glands) are found in the

lamina propria (arrowed). H & E.

×62.5.

Female Reproductive System

12.86

Reptilian, amphibian and

fish female reproductive

system

Paired ovaries are typical in fish, amphibians and rep-

tiles, and the histology of oogenesis is similar to that

in mammals (12.85 and 12.86). The ovaries are com-

posed of germinative, stromal, vascular and nervous

tissue. The ova begin their development as oogonia

that are mitotically derived from successive genera-

tions of oocytes. Diploid primary oocytes then

undergo meiotic division to become primary oocytes,

and primary polar bodies that are discarded. Another

reduction division yields haploid ova, and secondary

polar bodies that also are discarded.

The ovum is encircled by a cell membrane, a vari-

ably narrow zona pellucida and a layer of follicle

cells. During vitellogenesis, the yolk is added. This

process occurs after a variable period of time after

ovulation. Although the ova of most amphibians are

uninuclear, some species are known to produce mult-

inucleated ova. However, before fertilization all of

the nuclei, except one, become inactivated. Reptile

ova typically have a uninuclear ovum. However, bin-

ucleated ova are produced occasionally and, after

being fertilized, may yield twin embryos.

After ovulation, the corpora lutea, then the cor-

pora albicans and finally the corpora atretans

replace the ovulated follicles.

The appearance of reptilian oviducts varies

depending on the species and whether the female

is egg laying (oviparous) or live bearing (ovivivip-

arous or viviparous). However, they are readily rec-

ognizable. The histological features of the tubular

oviduct changes with each segment as it courses dis-

tally from the infundibulum. It is lined by ciliated,

often mucus-secreting, glandular columnar epithe-

lium for at least part of its length. The thin walls of

the oviducts contain alveolar or tubuloalveolar

glands that secrete albumin and shell substrate onto

the yolked egg as it descends. The cuboidal to

columnar epithelial cells comprising these glands

tend to be characterized by their distinctive

eosinophilic cytoplasmic granularity (12.87 and

12.88). The caudal oviduct of many reptiles also

contains straight or branched crypt-like depressions

that are surrounded by cuboidal epithelium with

eosinophilic granular cytoplasm (12.89). In many

species these glandular crypts serve as spermathe-

cae in which spermatozoa are nourished and stored

for prolonged periods of time. The oviducts of

viviparous and oviviviparous reptiles are thick-

walled, muscular and vascular, and contain glands

with secretions that help nourish the developing

embryo(s). They often exhibit marked plaiting,

which facilitates their distension during gravidity or

pregnancy. These modified oviducts are called the

‘uterus’ by some authorities.

The embryos of some viviparous reptiles develop

a primitive vascular placenta. In the lizard (Xantusia

vigilis) it is disc-shaped; in others it is more diffuse.

Although it was long thought that embryonic devel-

opment did not occur in shelled eggs until they were

deposited and exposed to atmospheric oxygen, it

has been demonstrated that significant embryonic

development can occur before egg deposition in

some species.

212

Comparative Veterinary Histology with Clinical Correlates

12.85 Ovary and proximal oviduct (fimbrium) of a desert

tortoise (Xerobates agassizi). H & E. ×25.

12.85

12.86 Corpus luteum (arrowed) of a leopard gecko

(Eublepharis macularius). (1) Granulosa lutein cells.

(2) Primordial follicles. (3) Oocyte cytoplasm. (4) Tunica

albuginea. (5) Theca externa. (6) Stroma. H & E. ×62.5.

213

Female Reproductive System

12.88

12.87 and 12.88 Shell gland portion

of the oviduct of a Pacific pond turtle

(Clemmys marmorata). The luminal

lining epithelium is pseudostratified

columnar and contains numerous

goblet cells. Subjacent to the mucosa

are lobules of shell-secreting glands

with characteristic eosinophilic

granule-containing cells. H & E. ×127.

12.87

12.89 Caudal oviduct from a sexually

mature female green iguana (Iguana

iguana). This portion of the oviduct is

characterized by numerous shallow

crypts that connect with small lobules

of glandular epithelial cells. These

crypts are thought to be sites of

sperm storage and nourishment that

sustain spermatozoa for prolonged

periods that may exceed 4–6 years in

some reptiles. H & E. ×62.5.

12.89

12.90

12.92

12.91

Clinical correlates

Ovarian and oviductal (or ‘uterine’) inflammation

and infections are less common in fish, amphib-

ians and reptiles than in mammals. Rupture of

yolked eggs with subsequent leakage of yolk into

the coelomic cavity is, however, a relatively fre-

quent reproductive disorder in oviparous reptiles.

Once lipid has gained entry into the vascular sys-

tem, it is disseminated widely in the form of yolk-

lipid emboli throughout the body (12.90 and

12.91).

Ovarian neoplasms have been observed in fish

and amphibians, but are less prevalent than in

mammals, birds and reptiles. As with mammals,

relatively common reptilian ovarian neoplasms are

granulosa cell tumours (see 12.68), luteal tumours

and thecal cell tumours (see 12.69). Dys-

germinoma (see 12.70) and teratoma (12.92)

occur less often. The teratoma is distinguished

by usually having tissue from all three germ lay-

ers present: hair (or scales), bone, cartilage,

teeth, muscle (of all three types), glandular tis-

sue, nerve and brain-like structures, and so on.

214

Comparative Veterinary Histology with Clinical Correlates

12.92 Ovarian teratoma from a

green iguana (Iguana iguana).

This section illustrates (1) two

masses of cartilage, (2) a duct-like

structure, (3) an aggregate of

thyroid-like follicles filled with

pink staining protein resembling

colloid, and (4) nervous tissue.

H & E. ×62.5.

12.91 Yolk serocoelomitis in a green iguana (Iguana

iguana). The smooth muscle tunic of the viscus organ

is covered with yolk, and an inflammatory exudate

is comprised of macrophages with engulfed lipid.

H & E. ×125.

12.90 Egg-yolk serocoelomitis from an iguana. The

intense exudative reaction to yolk characterized by

numerous histiocytic macrophages with engulfed yolk

lipid. H & E. ×62.5.

Two properties of cytoplasm are typified by nervous

tissue: irritability – the ability to react; and conduc-

tivity – the ability to transmit the elicited response.

The nervous system is subdivided into the cen-

tral nervous system (CNS), which comprises the

brain and spinal cord, and the peripheral nervous

system (PNS), which covers all other nervous tissue

and acts to interconnect all the tissues of the body

with the CNS. The nervous system is derived from

a specialized region of surface ectoderm along the

dorsal midline of the flat embryonic disc. The ecto-

dermal cells thicken to become neural ectoderm.

During flexion, the neural folds form and the lips

fuse to become the neural tube. This is incorporated

into the developing embryo to form the brain and

spinal cord. A separate population of neural ecto-

dermal cells remains separate from the

neural tube. This is the neural crest and

forms the PNS.

The basic cellular unit of nervous tis-

sue is the neuron. This comprises a large

cell body (from 0.4 to 12 +m in diame-

ter) with a single nucleus and a promi-

nent nucleolus. Basophilic granules

(Nissl’s granules) are present in the cyto-

plasm and represent rough endoplasmic reticulum

(13.1). The function of the neuron is to receive and

transmit impulses. The dendrites are branching

processes at the receiving end, and the axon is the

single, long process for onward transmission of the

impulse. Information passed along a chain of neu-

rons is transmitted from axon to dendrite at a spe-

cialized junction: the synapse.

Multipolar neurons are by far the most common

type. The single axon arises from one pole at a gran-

ule-free area of the neuron body: the axon hillock.

One or two dendrites arise from the opposite pole

and branch extensively. Examples of these neurons

are found in the CNS, in the cerebral and cerebel-

lar cortex, and in the spinal cord (13.2). Bipolar

neurons have one axon and one main dentrite, with

215

13. NERVOUS SYSTEM

13.1 Motor neuron. Trigeminal nerve

(dog). (1) Nucleus and nucleolus (arrow).

(2) Basophilic granules (Nissl’s granules).

(3) Axon hillock. (4) Neuroglial cells.

H & E. ×200.

13.1

13.2 Multipolar neuron. Cerebellar cortex

(dog). (1) Cell body. (2) Branching processes.

Golgi’s silver. ×125.

13.2

216

13.3 Longitudinal section (LS)

peripheral nerve (cat). (1) Axon.

(2) Neurilemmal sheath. (3) Node.

Osmic acid. ×200.

13.3

13.4 Transverse section (TS)

peripheral nerve (dog). (1) Bundles

of axons cut in transverse section,

fascicles. (2) Epineurium.

(3) Perineurium. (4) Endoneurium.

Masson’s trichrome. ×25.

13.4

little branching, and are found in the retina.

Pseudounipolar neurons are so-called because a sin-

gle process arises from the cell body and divides into

two. These cells are found in the dorsal root gan-

glia and in the sensory ganglia of the cranial nerves.

The axon with its covering myelin sheath, the

neurilemma, constitutes the nerve fibre. The neurilem-

mal sheath is composed of individual cells that form

a continuous investment from the origin of the axon

almost to the peripheral termination (13.3). These

cells produce a lamellar system of membranes with

a high fat content. In the PNS the neurilemmal sheath

is produced by neurolemmocytes, and in the CNS by

neuroglial cells: oligodendrocytes. The degree of the

investment varies; heavily invested fibres are known

as myelinated and minimally invested fibres as non-

myelinated. The axon has no sheath at the peripheral

nerve endings; these are naked fibres. The individual

sheaths divide the nerve fibre into segments. Where

adjoining cells meet, a node (of Ranvier) is formed so

that each internodal segment represents an investing

neurilemmal cell. The high fat content of the sheath

means that fat stains are best used to demonstrate the

myelinated fibres (13.3).

Peripheral nervous

system

The PNS is derived from the neural crest and is

composed of nerves and ganglia. Nerves are com-

posed of nerve fibres and ganglia are localized

groups of nerve cells. The individual fibres are

gathered into bundles or fascicles by a connective

tissue sheath, the epineurium, and form an

anatomical nerve. The epineurium extends into the

Comparative Veterinary Histology with Clinical Correlates

217

13.5 LS peripheral nerve (dog).

(1) Epineurium with fat cells.

(2) Nuclei of the neurilemmal cells.

(3) Axons cut longitudinally; note

the wavy appearance. H & E. ×62.5.

13.5

13.7 TS laryngeal nerve (horse).

(1) Perineurium with blood vessels.

(2) Endoneurium. (3) Axons. Toluidine

blue. ×62.5.

13.7

13.6 LS peripheral nerve (dog).

(1) Epineurium. (2) Axons.

(3) Neurilemmal cell nuclei.

H & E. ×125.

13.6

nerve and subdivides the fascicles with a vascular

connective tissue investment: the perineurium.

This in turn surrounds each nerve fibre with fine

vascular connective tissue: the endoneurium

(13.4–13.7). In the dorsal root ganglion the neu-

rons are peripherally arranged, with the nerve

Nervous System

218

13.8 Dorsal root ganglion (cat).

(1) Neuron bodies. (2) Nerve fibres

and supporting neuroglial cells.

Masson’s trichrome. ×160.

13.8

13.9 Dorsal root ganglion (cat).

(1) Neuron body. (2) Satellite cells;

these are supporting neuroglial cells.

Masson’s trichrome. ×160.

13.9

13.10 Sympathetic ganglion (dog).

(1) Neuron body with eccentric

nucleus (arrow) and satellite cells

(arrowhead). H & E. ×125.

13.10

fibres in the centre of the ganglion (13.8). The

large neuron bodies are surrounded and supported

by satellite cells (13.9). In the sympathetic ganglion

the neurons are evenly distributed, the cell bodies

are smaller and the nucleus is eccentrically placed

in the cell (13.10). Parasympathetic ganglia are

small groups of neurons within the terminal tissue

(13.11).

Comparative Veterinary Histology with Clinical Correlates

219

13.11 Parasympathetic ganglion.

Stomach (pig). (1) Neuron body.

(2) Nerve fibres and supporting

neuroglial cells. H & E. ×250.

13.11

Nervous System

Central nervous system

The brain and spinal cord are enveloped in connec-

tive tissue membranes, the meninges. The outer

dense membrane is the dura mater; the middle fine

cobweb-like membrane is the arachnoid; and the

inner, very vascular, areolar membrane is the pia

mater. The arachnoid and pia mater are continuous

and called the leptomeninges (13.12). The central

fluid filled canal and the ventricles of the brain are

lined with a columnar epithelium: the ependyma.

The tela choroidea lies in the thin roof plate of the

third and fourth ventricles. Its surface is covered

with modified ependymal cells that secrete

13.12 TS spinal cord (cat). (1) Dura

mater. (2) Arachnoid. (3) Pia mater.

(4) Spinal canal lined by ependyma.

H & E. ×7.5.

13.12

cerebrospinal fluid (13.13). The pia mater coats the

entire surface of the CNS, extends into the fissures

and sulci and is separated from the ependyma only

by the basal lamina.

In the CNS, collections of functionally related

neurons are called nuclei and collections of nerve

fibres are called tracts. The fibres have an investing

sheath cell: the oligodendrocyte. The presence of fat

gives a white glistening appearance and the fibres

are called white matter. By contrast, the cell bodies

appear grey and form the grey matter (13.14).

Neuroglial cells are the supporting cells of the

CNS, taking the place of the connective tissue of

other systems. Oligodendrocytes invest the axon,

provide the myelin sheath and act as supporting

satellite cells. Astrocytes are stellate cells with long

processes. In fibrous astrocytes the processes are

thin with minimal cytoplasm, whereas in proto-

plasmic astrocytes they are thick and fleshy

(13.15–13.18). The cell processes extend to the

blood vessels in the pia mater, acting as anchors

and transferring nutrients. The blood vessels are

220

Comparative Veterinary Histology with Clinical Correlates

13.14 TS spinal cord (cat). (1) Black

areas are the fat stained white

matter. (2) Pink stained areas are the

cellular grey matter. Osmic acid. ×7.5.

13.14

13.13 TS choroid plexus in the

fourth ventricle (cat). (1) Ependyma.

(2) Capillaries. H & E. ×160.

13.13