Ivyanatomy.com

Chapter 7

Osteology is the science of bones

Bones are a living dynamic tissue: constantly remodeled through life

Functions of Bone:

• Protection – brain is protected by the skull & heart and lungs by the thoracic cage

• Support & Movement – muscles attach to the skeleton

• Calcium and Phosphate storage

• Blood Cell Production – blood cells are produced by red bone marrow

Red Bone Marrow – site of blood cell production

• Locations of Red Bone Marrow:

• Infant – all bones are filled with red bone marrow

• Adult – red bone marrow is limited to

• flat bones (sternum, ribs, skull, hips) and

• irregular bones (vertebrae)

Yellow Bone Marrow – Adipose tissue

The cells associated with bones include:

• Osteocytes = cells that maintain bone.

o Osteocytes reside within chambers, called lacunae

• Osteoblasts = cells that deposit new bone. Once mature, osteoblasts

become osteocytes.

• Osteoclasts = cells that dissolve bone.

• Osteoclasts originate from white blood cells

• and they secrete an acid that dissolves the inorganic salts of bone.

The extracellular matrix of bones is composed of

• hydroxyapatite – a calcium phosphate salt that provides the hardness of bones

• collagen fibers – provides bone with some pliability

1. Long bones = elongated diaphysis

• Humerus radius ulna metacarpals phalanges

• Femur tibia fibula metatarsals phalanges

2. Short Bones = cube-shaped

• carpals

• tarsals

3. Flat Bones = plate-like

• Sternum, ribs, scapula

• parietal and frontal bones

4. Irregular bones = variety of shapes

• vertebrae

• mandible

• maxilla

• ethmoid bone

• sphenoid bone

5. sesamoid (or round) bone = develops

within tendons

• patella

Parts of a long bone

Epiphysis = expanded end of bone

• Filled with spongy (cancellous) bone

• Proximal epiphysis & distal epiphysis

• Sites of articulation (joint)

Diaphysis = shaft of long bone

• Lined with compact bone

Articular cartilage = covers epiphyses

• Composed of Hyaline cartilage

Epiphyseal line = remnant of bone growth (epiphyseal plate)

Parts of a long bone

Medullary Cavity = chamber within diaphysis

• Filled with bone marrow, blood vessels and nerves

• Yellow Bone Marrow in adults• Red Bone Marrow in children

Endosteum = Membrane that lines medullary cavity

• Contains dense connective tissue

• Blood vessels and nerves• osteoblasts

Periosteum = Tough membrane covering the bone

• Composed of Dense Connective Tissue

• Blood Vessels, Nerves, Osteoblasts

• Continuous with tendons and ligaments

Parts of a long bone

Compact bone

• Lines the Diaphysis and a thin layer

surrounds the epiphyses

• Composed of osteons

Spongy bone

• Fills the epiphyses and a thin layer

lines the medullary cavity

• Trabiculae = thin bony plates

• Osteocytes lie within trabiculae

Compact Bone

Osteon = Structural & functional unit of compact bone

Lamella = concentric rings of bone around a central canal

Central Canal (Haversian Canal)

• Contains blood supply and nerve

• Aligned parallel to diaphysis

Lacunae = bony chamber that contains an osteocyte

Osteocyte = maintains the bone

Canaliculi = canals connecting osteocytes to the central canal

• Canaliculi are filled with cellular processes

• Pathway for nutrient and waste diffusion

Osteon continued

Perforating Canal (Volkmann’s Canal)

• conveys blood from periosteum

towards the central canal

• Aligned perpendicular to diaphysis

Compact bone is composed of osteons cemented together by bone matrix.

Bone Development and Growth

Parts of the skeletal system begin to develop during the first

few weeks of prenatal development

Bone formation = ossification

Bones replace existing connective tissue in one of two ways:

As intramembranous bones

As endchondral bones

Intramembranous Ossification

Intramembranous Ossification

Forms broad, flat bones of the skull

Formed by replacing layers of embryonic connective

tissue (mesenchyme) with bone

Osteoblasts within mesenchyme

deposit bony matrix in all directions

Osteoblasts become osteocytes

once surrounded by bone

Intramembranous Ossification

Intramembranous ossification follows four steps.

(a) Mesenchymal cells group into clusters, and ossification centers form.

(b) Secreted osteoid traps osteoblasts, which then become osteocytes.

(c) Trabecular matrix and periosteum form.

(d) Compact bone develops superficial to the trabecular bone, and crowded blood vessels condense into red marrow.

Endochondral Bones

Endochondral Bones

Most of the bones in the skeleton are endochondral

Bone formation begins with a hyaline cartilage model

Cartilage decomposes and is replaced by bone.

Endochondral Ossification

1. Hyaline cartilage forms model of

future bone

2. Cartilage degenerates and

periosteum surrounds bone

3. Osteoblasts from periosteum invade

the degenerating tissue

4. Osteoblasts beneath periosteum

form compact bone at diaphysis =

primary ossification center

5. Later, Osteoblasts form spongy bone

at epiphyses = secondary ossification

center

Endochondral Ossification

Intramembranous ossification follows four steps.

(a)Hyaline cartilage forms model of future bone

(b)Cartilage degenerates and periosteum forms around

bone

(c) Primary ossification center forms compact bone within

the diaphysis

(e) Secondary ossification centers form spongy bone within

the epiphyses

(f) Some cartilage remains at the epiphyseal plates & as

articular cartilage at the end of bones

Endochondral Ossification

Two areas of endochondral bone retain

cartilage after ossification.

1. Articular cartilage

• surrounds the epiphyses for joints

2. Epiphyseal plates

• retain cartilage for bone growth

Growth at the Epiphyseal Plate

Epiphyseal Plate

• Band of hyaline cartilage that remains

between the two ossification centers

• Bone growth continues at epiphyseal

plates until adulthood.

• New cartilage is added towards the epiphysis

and cartilage is ossified towards diaphysis

• Once the epiphyseal plates ossify the

bones can no longer be lengthened

4 Layers (zones) of growth at

epiphyseal Plate

1. Zone of resting cartilage (reserve zone)

• Cartilage cells near epiphysis

• Do not participate in bone growth

• Anchor epiphyseal plate to epiphysis

2. Zone of proliferating cartilage

• Young chondrocytes undergoing

mitosis

• Adds new cartilage to plate

(b)

4 Layers (zones) of growth at

epiphyseal Plate

3. Zone of hypertrophic cartilage

• Older cells enlarge and thicken the

epiphyseal plate

• Osteoblasts invade and calcify the

cartilaginous matrix.

4. Zone of calcified cartilage

• Dead cells & calcium matrix

Ossified bone (zone of ossification)• Osteoclasts dissolve and phagocytize the matrix• Osteoblasts invade the region and deposit new bone.

Age Occurrence

3rd month prenatal development

Ossification begins in long bones4

4th month prenatal development

Most primary ossification centers have appeared in diaphysis of long bones.

Birth to 5 years Secondary ossification centers appear in the epiphyses of long bones.

5 to 12 years (females)

5 to 14 years (males)

Ossification rapidly spreads from the ossification centers

15 to 18 years (females)

17 to 20 years (males)

Bones of the upper limbs and scapulae completely ossified.

16 to 21 years (females)

18 to 23 years (males)

Bones of the lower limbs and hip bones completely ossified

21 to 23 years (females)

23 to 25 years (males)

Bones of the sternum, clavicles, and vertebrae completely ossified.

By 23 (females)

By 25 (males)

Nearly all bones completely ossified.

Bone Remodeling

Calcium is constantly exchanged between the blood and bone.

Bone resorption = Osteoclasts breakdown bone releasing

calcium into the blood.

• Bone resorption occurs when blood [Ca2+] is low and it’s

stimulated by parathyroid hormone (PTH).

Bone deposition = Osteoblasts deposit new bone from

calcium in the blood stream.

• Bone deposition occurs when blood [Ca2+] is high and it’s

stimulated by the hormone calcitonin.

Vitamin D – promotes Ca2+ absorption in small intestine

• Vitamin D deficiency = softened and deformed bones

• Osteomalacia in adults

• Rickets in children

Vitamin A – balances bone resorption and deposition

• Vitamin A deficiency = retards bone development

Vitamin C – is required for collagen synthesis.

• Vitamin C deficiency = results in fragile bones

Nutrients that effect bone homeostasis

Growth Hormone (GH)

• Secreted from pituitary gland

• Promotes bone growth at epiphyseal plates

Pituitary Gigantism

over secretion of GH during childhood

Pituitary Dwarfism

insufficient GH during childhood

Acromegaly

• Over secretion of GH as an adult

• Occurs after epiphyseal plates have sealed

• Enlargement of hands, feet, nose

Hormones that affect bone homeostasis

Calcitonin

• Secreted from thyroid gland

• Promotes bone deposition

• Stimulates Osteoblast activity

• Inhibits Osteoclast activity

Parathyroid Hormone (PTH)

• Secreted from parathyroid glands

• Promotes bone resorption

• Stimulates Osteoclast activity

• Inhibits Osteoblast activity

Hormones that affect bone homeostasis

Thyroid Gland

Blood CalciumNormal Range

Increased blood calcium detected by cells in

thyroid gland

Thyroid Gland secretes Calcitonin

Osteoblasts deposit calcium into bones

Blood calcium returns to normal

Blood calcium level increases

Blood calcium level decreases

Decreased blood calcium detected by cells in parathyroid

gland

Parathyroid glands secrete PTH

Osteoclasts resorb boneto release Calcium into

the blood

Sex Hormones (testosterone & estrogen)

• Promotes long bone growth at puberty

• Sex hormones also stimulate ossification at

epiphyseal plates*

*Estrogen has a stronger effect than testosterone on

bone ossification, which is why ossification of the

epiphyseal plates occurs earlier in development in

females than in males.

Hormones that affect bone homeostasis

Impacts of Exercise and Sunlight on Bone Homeostasis

Exercise – especially resistance or strength exercise strengthens bones. Muscles pull on bones, and bones thicken in response.

Sunlight – UV rays promote the release of Vitamin D from skin. Circulating vitamin D is activated in the liver, then it promotes the absorption of Calcium from the intestines.

Greenstick fracture

One side of the bone bends,

the other side breaks.

Most common in children,

because their bones are

more pliable than adults.

Fissured fracture

Longitudinal break on the bone.

Transverse fracture

Complete break perpendicular

to the diaphysis

Oblique fracture

Complete break at any other

angle not perpendicular to

diaphysis

Comminuted fracture

Results in several bony

fragments

Spiral fracture

Results from twisting

the bone

(a) Hematoma Formation Blood vessels rupture causing severe bleeding.The blood clots, forming a hematoma

(b) Cartilaginous Callus Formation

Fibroblasts deposit a mass of fibrocartilage

(c) Bony Callus Formation

Osteoblasts replace the cartilaginous callus with a bony callus

(d) Bone RemodelingOsteoclasts and

macrophages remove excess bone and debris

Repair of a fracture

Osteopenia “low bone mass”

• Progresses towards osteoporosis

Osteoporosis “porous bone”

• Bones develop spaces and canals

• Bones are fragile and easily broken

• Common in menopausal women

(from the low estrogen levels)

Over time, osteoclasts outnumber osteoblasts, and

more bone is resorbed than can be deposited. Bone

mass decreases as a result.

Healthy Bone Osteoporosis Bone

Ways to delay or prevent osteoporosis:

1. Exercise daily.

2. Consume enough calcium and

vitamin D every day.

3. Do not smoke.

Attribution

• Skeleton illustration Pixabay CC0 Public Domain https://pixabay.com/p-30160/?no_redirect• Classification of bone by shape By BruceBlaus (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia

Commons https://upload.wikimedia.org/wikipedia/commons/7/77/Blausen_0229_ClassificationofBones.png• Parts of a long bone By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

https://upload.wikimedia.org/wikipedia/commons/2/23/603_Anatomy_of_Long_Bone.jpg• Diagram of Compact Bone By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

https://upload.wikimedia.org/wikipedia/commons/5/58/624_Diagram_of_Compact_Bone-new.jpg• Diagram of Osteon By BDB [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons

https://upload.wikimedia.org/wikipedia/commons/7/75/Transverse_Section_Of_Bone.png• Intramembranous ossification By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

https://upload.wikimedia.org/wikipedia/commons/a/a9/611_Intramembraneous_Ossification.jpg• Endochondral Ossification By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

https://upload.wikimedia.org/wikipedia/commons/9/97/608_Endochrondal_Ossification.jpg• Epiphyseal Plate By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

https://upload.wikimedia.org/wikipedia/commons/1/15/622_Longitudinal_Bone_Growth.jpg• Bone Remodeling By Cancer Research UK (Original email from CRUK) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)],

via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/d/dd/Diagram_showing_bone_remodelling_Fig_CRUK_112.svg

• Types of Fractures By OpenStax College [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/3/35/612_Types_of_Fractures.jpg

• Repair of Bone Fracture By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/1/12/613_Stages_of_Fracture_Repair.jpg

• Normal and Degraded Bone By Gtirouflet (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/8/8e/Bone_normal_and_degraded_micro_structure.jpg