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

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3.29 Scapulohumeral articulation of

a small skink, Scincella lateralis.

(1) Both the humeral head (lower

right) and the scapula (upper left)

contain cancellous spaces filled with

bone marrow. (2) Articular cartilage.

H & E. ×25.

3.30 Hyaline cartilage. Costal

cartilage (dog). (1) Hyaline cartilage.

(2) Chondrocytes. (3) Perichondrium.

H & E. ×100.

3.30

3.29

3.31 Hyaline cartilage. Costal

cartilage (dog). (1) Chondrocyte in a

lacuna. (2) Extracellular matrix. H & E.

×250.

3.31

Connective Tissue

3.35 Fibrocartilage (ox).

(1) Chondrocyte. (2) Collagen fibres

arranged as dense regular connective

tissue. H & E. ×125.

3.35

3.36 Fibrocartilage (ox).

(1) Chondrocyte. (2) Collagen fibres

arranged as dense regular connective

tissue. Gomori’s trichrome. ×160.

3.36

3.37 Fibrocartilage (ox).

(1) Chondrocyte. (2) Collagen fibres.

Gomori’s trichrome. ×250.

3.37

Connective Tissue

with bone marrow (3.38 and 3.39). Compact bone

is a solid continuous mass in which the spaces are

only visible with the aid of a microscope (3.40).

Bones are covered with a specialized connective

tissue, the periosteum, the inner layer of which is

osteogenic (capable of laying down new bone; 3.40).

Spaces in bone, like the marrow cavity (3.38–3.40)

and the canal system (3.40–3.42), are lined with a

single layer of osteogenic cells, the endosteum. The

characteristic feature of all bone tissue is the arrange-

ment of mineralized bone matrix in layers, the

lamellae. Small lacunae present in the lamellae are

occupied by a single bone cell, the osteocyte. Tubular

passages, the canaliculi, radiate from each lacuna

and link up with canaliculi from adjacent lacunae

to create an extensive system of interconnecting

canals. This arrangement is clearly defined in

compact bone where the lamellae are arranged

concentrically around a longitudinal canal to form

an osteon (Haversian System; 3.41–3.43). The

central canal of the osteon carries blood vessels,

3.38 Spiculated bone (dog).

(1) Periosteum. (2) Bone spicules with

osteocytes in the lacunae (arrowed).

(3) Bone marrow filled spaces lined by

osteoblasts (arrowhead). H & E. ×100.

3.38

3.39 Spiculated bone (dog).

(1) Periosteum. (2) Bone spicules with

osteocytes (arrowed). (3) Osteoblasts

(arrowhead) on the free surface of the

bone. H & E. ×160.

3.39

Bone

Bone is a rigid form of connective tissue composed

of cells embedded in an intercellular matrix of col-

lagen fibres, glycosaminoglycans and calcium phos-

phate deposited as hydroxyapatite crystals. Bone

provides the framework of the body and serves as

a lever for muscle action, as protection for viscera,

as a haemopoietic organ, and as a reservoir of body

stores of calcium and phosphorus. Small bones are

found in soft tissues to provide extra rigidity – the

os penis in the dog and ossa cordis in the ox. Bone

is a living tissue that is supplied with blood vessels

and nerves, and is constantly changing in response

to body stresses and circumstances.

There are two forms of bone: cancellous (spongy,

medullary) and compact (cortical, dense). All bones

have both cancellous and compact forms of bone

deposition. Cancellous bone consists of irregular

interconnecting bars, the trabeculae, forming a

three-dimensional network of lined spaces filled

Comparative Veterinary Histology with Clinical Correlates

3.40 Compact bone (dog). (1) Compact bone

of the diaphysis of the femur. (2) Periosteum.

(3) Endosteum. (4) Marrow with a high

proportion of fat cells. H & E. ×20.

3.40

3.41 Compact bone (dog). (1) Compact bone with osteocytes in

lacunae arranged in lamellae with a central canal carrying blood

vessels and nerves. The canal is lined by endosteum. (2) Periosteum.

(3) Endosteum of the marrow cavity. H & E. ×62.5.

3.41

3.42 Compact bone (dog). (1) The osteon

fills the field. The central canal is lined by

endosteum and carries blood vessels and

nerves. (2) Osteocytes in lacunae in the

circumferential lamellae. (3) Periosteum.

H & E. ×100.

3.42

3.43 Compact bone (dog). This is a high power view of Fig. 3.42.

H & E. ×250.

3.43

Connective Tissue

cell in bone marrow. The cell migrates to the

developing tissue. The ruffled (brush) border is the

undulating mobile cell membrane.

Osteoblasts are present on the inner surface of

the periosteum and endosteum, and cover the

surface of bone spicules in an active osteogenic area

(3.38, 3.39 and 3.46). The cell has a central or

slightly eccentric nucleus with chromatin granules,

and the cytoplasm stains deeply with haematoxylin

because of the concentration of organelles. Long

cytoplasmic processes extend out from the osteoblast

and the matrix is deposited around them. This

provides a fine canalicular network and allows

nutrients to pass to the bone cells. Unlike cartilage

there is no diffusion in bone. Once the bone has been

deposited the trapped cells (osteocytes) function to

maintain the bone and retain contact with the blood

vessels through the canalicular system.

3.44 Developing periosteal bone

(foal). (1) Periosteum. (2) Bone

spicules. (3) Osteogenic tissue fills

the spaces between the spicules.

(4) Endosteum. (5) Marrow. H & E.

×62.5.

3.44

3.45 Developing periosteal bone

(foal). (1) Periosteum. (2) Bone

spicules. (3) Osteogenic tissue fills

the spaces between the spicules.

(4) Endosteum. (5) Marrow. H & E.

×125.

3.45

branches of the perpendicular canals (of Volkmann).

These are part of the main blood supply to the bone

and link the endosteal and periosteal surfaces. The

lamellae may be regular circular rings as in the

osteon, surround the shaft of the bone as circum-

ferential lamellae, or fill in the angular spaces

between lamellae as interstitial lamellae. They are

often the result of bone remodelling. Bone contains

a vast continuous network of canals that are essential

for the nutrition of the bone cell in the lacuna, the

osteocyte (3.41–3.45). The cell body lies in the lacuna

and extends in long processes into the canaliculi to

contact similar processes from adjacent osteocytes.

Bone is a living tissue and is constantly being

remodelled; osteoclasts are multinucleate non-

dividing cells that are found in resorption bays

(Howship’s lacunae) at the site of bone remodelling

(3.46). Osteoclasts are derived from a progenitor

Comparative Veterinary Histology with Clinical Correlates

Connective Tissue

Endochondral ossification

In the embryo, mesenchymal cells at predetermined

sites differentiate into chondroblasts and lay down

cartilage models of the long bones (3.47). Later in

gestation, the mesenchyme surrounding the carti-

lage model becomes very vascular and the cartilage

3.46 Compact bone (cat). Osteoclast (arrowed).

(1) Osteocytes in lacunae. (2) Osteoblasts on the free

surface of the bone. H & E. ×250.

3.46

3.47 Whole mount section of the left forelimb of a small

viviparous yucca night lizard (Xantusia vigilis). Note the

transition between the blue staining cartilaginous ends

and the mineralized diaphyseal compact bone. H & E. ×5.

3.47

3.48 Endochondral ossification (cat). (1) Zone of

resting cartilage. (2) Zone of proliferating cartilage.

(3) Zone of hypertrophied chondrocytes.

(4) Basophilic calcified cartilage with freshly

deposited eosinophilic bone. (5) Osteogenic tissue.

H & E. ×62.5.

3.48

3.49 Endochondral ossification (cat). (1) Zone of

hypertrophied chondrocytes. (2) Calcified cartilage

with bone deposits. (3) Osteogenic tissue. H & E.

×160.

3.49

matrix is calcified. Nutrients are unable to diffuse

through this calcified cartilage and the cells die. Cell

death is followed by breakdown of the matrix.

Vascular mesenchymal tissue moves in, differenti-

ates into osteogenic tissue and deposits bone on the

remains of the calcified cartilage (3.48 and 3.49).

Growth of a long bone

Bones are able to grow in width and length and

do so until the adult size is reached. Thereafter,

bone continues to remodel as circumstances

demand. Growth in width is appositional from the

osteogenic inner layer of the periosteum and new

bone is formed locally, the periosteal collar (3.44).

This ability to deposit bone is utilized in bone

grafts. Growth in length is accomplished at the dia-

physeal/epiphyseal junction at the persistent layer

of the epiphysis. Cartilage grows interstitially on the

epiphyseal side and dies on the diaphyseal side of

the plate. Osteogenic tissue invades and deposits

bone, the epiphyses are pushed apart and the bone

grows in length (3.50).

3.51 Section of femur from a green

iguana (Iguana iguana) with

metabolic bone disease. Note the

multinucleated osteoclasts (arrowed)

that have been removing ossified

tissue and fibrous connective tissue

that has replaced the cancellous

bone. H & E. ×250.

3.51

3.50 Epiphyseal plate (cat).

(1) Epiphysis. (2) Spiculated bone.

(3) Growth area of endochondral

ossification. H & E. ×25.

3.50

This is an ossification centre. The cartilage is

replaced by bone beginning at the centre of the dia-

physis and extending to the epiphysis. The epiph-

ysis has a separate centre of ossification and a plate

of cartilage persists, the epiphyseal plate; this sep-

arates the epiphysis from the diaphysis (3.50). The

perichondrium becomes the periosteum.

Multinucleated osteoclasts remove mineralized

osseous matrix (3.52). In metabolic bone disease

(fibrous osteodystrophy, secondary nutritional

hyperparathyroidism, ‘rubber jaw’, ‘renal rickets’),

which may be caused by an improper dietary

calcium : phosphorus ratio, or in some cases of

severe chronic renal disease the osteopenic bone is

replaced by fibrous connective tissue (3.51).

Comparative Veterinary Histology with Clinical Correlates

3.53

3.52

which are continuous with the more central can-

cellous bone. The surrounding mesenchyme con-

denses to become the periosteum; the inner layer is

osteogenic (see 3.44). The osteoblasts on the inner

surface lining the spicules of cancellous bone also

retain their osteogenic ability.

3.52 Osteogenic osteosarcoma

(dog) showing tumour bone

formation. H & E. ×20.

Intramembranous ossification

Small bones and flat bones develop directly from

mesenchymal cells; these differentiate into osteo-

blasts. Bone is laid down as a network of bony

spicules that are gradually enlarged by the deposi-

tion of new bone lamellae. The spaces are filled in

and compact bone is formed in the outer plates

Clinical correlates

Although a variety of benign and malignant

tumours of various origins arise in the skeleton,

osteosarcomas, a malignant mesenchymal

tumour in which the neoplastic cells produce

tumour osteoid or bone, account for approxi-

mately 80% of canine and 50% of feline skele-

tal neoplasms. Osteosarcomas can be subdivided

according to their location within bone or histo-

logical appearance. An osteogenic, or bone pro-

ducing, osteosarcoma that has multifocal

formation of well mineralized new bone is shown

in 3.52. This was taken from an 8-year-old St

Bernard dog. An example of a giant cell osteosar-

coma (3.53) from a German Shepherd dog shows

numerous multinucleate syncytia formed by

fusion of the neoplastic cells.

3.53 Giant cell osteosarcoma

(dog). Note the numerous

multinucleate cells. H & E. ×250.

Connective Tissue

3.54

Comparative Veterinary Histology with Clinical Correlates

3.54 Plasmacytic arthritis in the

dog. The synovial membrane is

infiltrated by plasma cells. H & E.

×125.

Arthritis is a general term for inflammatory

disease of the joints. There are many causes and

many types of arthritis but often a similar pattern

of reaction and joint damage may be observed. A

synovial biopsy from an adult German Shepherd

dog in which the thickened synovial membrane is

invaded by large numbers of plasma cells is

shown in 3.54. The predominance of this cell type

suggests an immunological basis for the disease

in this case.