040 - Bone Histology

7/31/2019 040 - Bone Histology

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1. Milk teeth that are already erupted, or in the process of eruption2. Unerupted adult teeth at an early stage of formation3. Gingiva (gum)4. Alveolar bone of jaw

Gum and alveolar bone, kitten jaw1. Gum - thick stratified squamous

epithelium, with keratinized surface layer2. Underlying dermal / connective tissue

layer; highly cellular, containing numerousblood vessels

3. Alveolar bone; black arrowheadsindicate surfaces where active bone

formation by osteoblasts (layer of smalldark purple cells) is occurring

The bone matrix itself is stained pink.

The micrograph illustrates the dynamic cellular activity in this immature bone:

The lower margin of the alveolar bone isbeing eroded by osteoclasts, thespecialised multinucleate bone-resorbingcells (black arrows) to make way for the

erupting tooth. Close by, however, bone is being deposited

by rows ofosteoblasts, the specialisedbone-forming cells (white arrows), onsurfaces facing away from the tooth.

These dramatic cellular events probablyoccur as a result of upward pressure fromthe erupting tooth; similar events occurwhen orthodontists use the constantpressure provided by tooth braces to moveteeth through the aveolar bone of juvenile

or adult jaws.


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This rapidly changing, immature bone is known as "woven bone"; it is widelydistributed in growing animals (and humans!) and also occurs in the fracturecallus that knits together broken bones in adults.

This type of bone is essentially temporary; it is replaced eventually bylamellar bone, the adult form of bone.

This is woven bone (rapidly produced, temporary bone)o Note the lack of internal structure to this woven bone matrix, and the

haphazard arrangement of osteocytes embedded in it (which are large &round)

Contrast with the micrographs of lamellar bone

1. Concave resorption pits (Howships lacunae) in bone matrix, excavated bylarge, multinucleated osteoclasts (stained purple)

2. Row ofosteoblasts laying down new bone matrix (called "osteoid").o The newly synthesised matrix essentially consists of tangle type 1

collage fibreso Osteoid is initially unmineralised (narrow, pale layer between cells and

pink-stained matrix)

3. Some osteoblasts become buried in the accumulating bone matrix they aresynthesising and differentiate into osteocytes;

o Form an interconnecting network (of canaliculi), and are present

throughout all living bone

4. Many small blood vessels are present in the stroma around the bone;osteoblast and osteoclast precursor cells are also present here.


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Demineralised section of entire adult human finger bone

N.B. There are no epiphyseal growth plates present in adult bone!1. Cortical bone (aka compact bone)2. Articular cartilage3. Tendon4. Marrow cavity, with bony trabeculae at epiphyses (subchondral bone at the end

of the long bones is always trabecular)

1. Articular cartilage (hyaline cartilage)2. Trabecular bone3. Marrow cavity, mainly filled with

adipocytes

1. Lamellar cortical bone.

This apperance is the result ofsuccessive regular layers of bone matrixbeing deposited such that the orientation ofthe type 1 collagen fibres is different ineach layer.

The arrangement is analagous to plywood,and confers great strength / fractureresistance to the bone.

Osteocytes (white arrowheads) tend to be

sparesely & regularly distributed in adultbone (they are small and flattened)


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2. Fibrous periosteum - containing type 1 collagen fibres (unmineralised),fibroblasts, osteoblast precursors, etc.

3. Marrow containing adipocytes and a fewhaemopoietic cells.

High power view of periosteal surface, showingremodelling activity1. Cavity (filled with small cells) excavated by large

osteoclasto Multinucleated osteoclast (delineated by

small white arrowheads)

Articular cartilage and subchondral bone1. Matrix ofarticular (hyaline) cartilage

(smooth articular surface indicated by blackarrows)

2. Mineralised articular cartilage3. Subchondral trabecular bone, surrounded

mainly by adipocytes, with a few haemopoieticcells.With advancing age, the amount of yellowfatty marrow increases in adult bone.

Articular cartilage and subchondral bone

This image illustrates very clearly thetransition from unmineralised articularcartilage to mineralised articular cartilageto bone

The wavy line separating mineralised fromunmineralised cartilage is known as the"tidemark zone"; this advances towards

the articular surface with increasing age.1. Matrix ofunmineralised articular

(hyaline) cartilage2. Matrix ofmineralised articular (hyaline)

cartilage3. Matrix ofsubchondral bone - note typical

adult lamellar structure

Chondrocytes, arranged in small clusters,in both unmineralised and mineralisedcartilage

Osteocytes in bone


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Periosteal surface of adult bone1. Blood vessel channels (contrast with avascular cartilage)2. Periosteal bone apposition - note layer of osteoblasts which lines the bone

(underneath arrowheads)3. Lamellar cortical bone4. Fibrous periosteum5. Complex of blood vessels (with nerves?) in mixed fibrous / fatty connective

tissue

Transverse section of cortical bone fromhuman finger

Note this is cortical bine so must be thediaphysis of the finger (the epiphysis wouldinstead show trabecular subchondral bone).

The bone was not demineralised or stained

The section clearly illustrates the complex(Haversian) remodelling activity in adultcortical bone.

Cortical bone consists of longditudinal units osteons (Haversian)

Each osteon consists ofconcentric lamellaesurrounding a central blood vessel

1,2,3,4: Osteonal (Haversian) systems.

Some of the channels have blood residues present (blood vessels run in the

centre of these channels). Systems 1 and 2 can be seen to be partly obliterating systems 3 and 4 - the

result of remodelling. Therefore systems 1 and 2 are more recent - evidencedalso by their larger lumens - not yet fully filled with concentric bone layers.

The whitearrowheads indicatethe regularlyspacedosteocytes that


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interconnect with each other (black, spidery projections) and ramify throughoutliving bone

This network of cells enables contact between the central blood vessel channeland other part of the osteon.

It is thought that this arrangement may function in sensing & responding to

mechanical strain.(Contrast with chondrocytes in cartilage which do not interconnect)

Osteoporosis:

Left: Low power scanning electron microscope image showing a sawn sectiondown through the body of the third lumbar vertebra of a normal 30 year-oldwoman.

Vertebral body = trabecular bone

The marrow has been removed and no cells are visible - only the "naked" bonesurfaces. Thick, regular, interconnecting plates of trabecular bone are evident.

In engineering terms, this is a strong, rigid structure that can withstand thestresses and strains encountered in normal life.

Right: Low power scanning electron microscope image of a sawn section throughthe body of the third lumbar vertebra of a 71 year-old woman.

Osteoporosis is evident in this image.

Contrast with the bone from the 30 year old: the thick, interconnecting platesof trabecular bone have been eroded to thin, spindly rods in the 71 year old.

This is a mechanically inadequate structure that could fracture with minimal

trauma (eg sitting down with a bump). Spinal osteoporosis leads to compression fractures, height loss and the classic

"dowager's hump" often seen in elderly women.

Osteoporotic bone:

The severe erosion of the vertical bone rodin the foreground can be seen to haveresulted from osteoclastic action - thesurface is covered with resorption pits.


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If this process had continued, this trabecular element would have been lostcompletely.

Most osteoporosis is thought to result from osteoclast over-activity(osteoclastic resorbtion > osteoblastic formation)

The swelling at the top of the vertical bone rod could possibly be part of amicrofracture callus from an earlier trauma.

SELF ASSESMENT QUIZ

7) Resting line: between newly formed bone, and

older, resting bone.

1) Youngest2) Middle3) Oldest

4) = Haversian canal

1) Osteocyte

2) Canaliculi


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3) Lamellae

YOUNGER Haversian osteons of cortical tissue:

Have larger central lumens

Are obliterating older osteons.

CORE KNOWLEDGE

Give brief explanations of the following terms (a few words only foreach):

(a) osteoblast

bone-forming cell

(b)osteocyte

ex-osteoblast embedded in matrix -communication via canaliculi

(c) osteoclast

bone-resorbing (destroying) cell, usually multinucleate

(d)osteoid

newly deposited, unmineralised bone matrix

(e) lamellar bone

highly organised bone, deposited in discrete layers, each with differing collagenfibre

orientation (like plywood)

(f) woven bone

rapidly produced, immature bone with random collagen fibre organisation;replaced by

lamellar bone

(g)trabecular (cancellous) bone

3-d meshwork of bone; open, sponge-like arrangement (but not soft!), eg invertebral


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bodies, ends of long bones

(h)cortical bone

dense bone, eg in shafts of long bones

(i) Haversian system (osteon)

concentric layers of bone around a blood vessel (bulk of cortical bone) -run parallelto

long axis of bone

(j) bone remodelling

co-ordinated activity of osteoclasts and osteoblasts to replace or modify areas of

bone

that might be damaged, redundant or weak

2. What is intramembranous ossification?

(see CORE KNOWLEDGE: CARTILAGE for comparison with endochondralossification)

Intramembranous ossification: de novo bone formation by osteoblasts -eg

mandible and vault of skull

3. Describe the key structural components of bone matrix

Organic component: type 1 collagen fibres (>90%); balance is mixture of smallproteins including osteocalcin, osteopontin, osteonectin, plus growth factorsincluding TGF, IGFs.

Inorganic component: hydroxyapatite -Ca10(PO4)6(OH)2

4. What is osteoporosis? Who gets it and why?

Bone loss due to imbalance of resorption/formation; remaining bone normal butmechanically inadequate fractures. Seen at high turnover sites (trabecular bone)-vertebrae & femoral neck. Age-related, common in post-menopausal women(


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6. Which hormones are thought to be important in regulating boneturnover?

Parathyroid hormone (PTH) - turnover1,25-dihydroxyvitamin D - turnover

(calcitonin) - OC activity

gonadal steroids: estrogens, androgens

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040 - Bone Histology