Biology And Bone Repair

Types of Bone
•Lamellar Bone
–Collagen fibers arranged in parallel layers
–Normal adult bone
•Woven Bone (non-lamellar)
–Randomly oriented collagen fibers
–In adults, seen at sites of fracture healing, tendon or ligament attachment and in
pathological conditions

Lamellar Bone
•Cortical bone:
–Comprised of osteons (Haversian systems)
–Osteons communicate with medullary cavity by Volkmann’s canals

Haversian System
•Osteon with central haversian canal containing
–Cells
–Vessels
–Nerves
•Volkmann’s canal
–Connects osteons

Lamellar Bone
•Cancellous bone (trabecular or spongy bone)
–Bony struts (trabeculae) that are oriented in direction of the greatest stress

Woven Bone
•Coarse with random orientation
•Weaker than lamellar bone
•Normally remodeled to lamellar bone

Bone Composition
•Cells
–Osteocytes
–Osteoblasts
–Osteoclasts

•Extracellular Matrix
–Organic (35%)
•Collagen (type I) 90%
•Osteocalcin, osteonectin, proteoglycans, glycosaminoglycans, lipids (ground substance)
–Inorganic (65%)
•Primarily hydroxyapatite Ca5(PO4)3(OH)2

Osteoblasts
•Derived from mesenchymal stem cells
•Line the surface of the bone and produce osteoid
•Immediate precursor is fibroblast-like preosteoblasts

Osteocytes
•Osteoblasts surrounded by bone matrix
–trapped in lacunae
•Function poorly understood
–regulating bone metabolism in response to stress and strain

Osteocyte Network
•Osteocyte lacunae are connected by canaliculi
•Osteocytes are interconnected by long cell processes that project through the canaliculi
•Preosteoblasts also have connections via canaliculi with the osteocytes
•Network probably facilitates response of bone to mechanical and chemical factors

Osteoclasts
•Derived from hematopoietic stem cells (monocyte precursor cells)
•Multinucleated cells whose function is bone resorption
•Reside in bone resorption pits (Howship’s lacunae)
•Parathyroid hormone stimulates receptors on osteoblasts that activate osteoclastic bone
resorption

Components of Bone Formation
•Cortex
•Periosteum
•Bone marrow
•Soft tissue

Prerequisites for Bone Healing
•Adequate blood supply
•Adequate mechanical stability

Mechanisms of Bone Formation
•Cutting Cones
•Intramembranous Bone Formation
•Endochondral Bone Formation

Cutting Cones
•Primarily a mechanism to remodel bone
•Osteoclasts at the front of the cutting cone remove bone
•Trailing osteoblasts lay down new bone

Intramembranous (Periosteal) Bone Formation
•Mechanism by which a long bone grows in width
•Osteoblasts differentiate directly from preosteoblasts and lay down seams of osteoid
•Does NOT involve cartilage anlage

Endochondral Bone Formation
•Mechanism by which a long bone grows in length
•Osteoblasts line a cartilage precursor
•The chondrocytes hypertrophy, degenerate and calcify (area of low oxygen tension)
•Vascular invasion of the cartilage occurs followed by ossification (increasing oxygen
tension)

Blood Supply
•Long bones have three blood supplies
–Nutrient artery (intramedullary)
–Periosteal vessels
–Metaphyseal vessels

Nutrient Artery
•Normally the major blood supply for the diaphyseal cortex (80 to 85%)
•Enters the long bone via a nutrient foramen
•Forms medullary arteries up and down the bone

Periosteal Vessels
•Arise from the capillary-rich periosteum
•Supply outer 15 to 20% of cortex normally
•Capable of supplying a much greater proportion of the cortex in the event of injury to
the medullary blood supply

Metaphyseal Vessels
•Arise from periarticular vessels
•Penetrate the thin cortex in the metaphyseal region and anastomose with the medullary
blood supply

Vascular Response in Fracture Repair
•Fracture stimulates the release of growth factors that promote angiogenesis and
vasodilation
•Blood flow is increased substantially to the fracture site
–Peaks at two weeks after fracture

Mechanical Stability
•Early stability promotes revascularization
•After first month, loading and interfragmentary motion promotes greater callus
formation
•Mechanical load and small displacements at the fracture site stimulate healing
•Inadequate stabilization may result in excessive deformation at the fracture site
interrupting tissue differentiation to bone (soft callus)
•Over-stabilization, however, reduces periosteal bone formation (hard callus)

Stages of Fracture Healing
•Inflammation
•Repair
•Remodeling

Inflammation
•Tissue disruption results in hematoma at the fracture site
•Local vessels thrombose causing bony necrosis at the edges of the fracture
•Increased capillary permeability results in a local inflammatory milieu
–Osteoinductive growth factors stimulate the proliferation and differentiation of
mesenchymal stem cells

Repair
•Periosteal callus forms along the periphery of the fracture site
–Intramembranous ossification initiated by preosteoblasts
•Intramedullary callus forms in the center of the fracture site
–Endochondral ossification at the site of the fracture hematoma
•Chemical and mechanical factors stimulate callus formation and mineralization

Remodeling
•Woven bone is gradually converted to lamellar bone
•Medullary cavity is reconstituted
•Bone is restructured in response to stress and strain (Wolff’s Law)

Mechanisms for Bone Healing
•Direct (primary) bone healing
•Indirect (secondary) bone healing

Direct Bone Healing
•Mechanism of bone healing seen when there is no motion at the fracture site (i.e. rigid internal fixation)
•Does not involve formation of fracture callus
•Osteoblasts originate from endothelial and perivascular cells
•A cutting cone is formed that crosses the fracture site
•Osteoblasts lay down lamellar bone behind the osteoclasts forming a secondary osteon
•Gradually the fracture is healed by the formation of numerous secondary osteons
•A slow process – months to years

Components of Direct Bone Healing
•Contact Healing
–Direct contact between the fracture ends allows healing to be with lamellar bone
immediately
•Gap Healing
–Gaps less than 200-500 microns are primarily filled with woven bone that is
subsequently remodeled into lamellar bone
–Larger gaps are healed by indirect bone healing (partially filled with fibrous
tissue that undergoes secondary ossification)

Indirect Bone Healing
•Mechanism for healing in fractures that are not rigidly fixed.
•Bridging periosteal (soft) callus and medullary (hard) callus re-establish
structural continuity
•Callus subsequently undergoes endochondral ossification
•Process fairly rapid - weeks

Local Regulation of Bone Healing
•Growth factors
•Cytokines
•Prostaglandins/Leukotrienes
•Hormones
•Growth factor antagonists

Growth Factors
•Transforming growth factor
•Bone morphogenetic proteins
•Fibroblast growth factors
•Platelet-derived growth factors
•Insulin-like growth factors

Transforming Growth Factor
•Superfamily of growth factors (~34 members)
•Act on serine/threonine kinase cell wall receptors
•Promotes proliferation and differentiation of mesenchymal precursors for
osteoblasts, osteoclasts and chondrocytes
•Stimulates both endochondral and intramembranous bone formation
–Induces synthesis of cartilage-specific proteoglycans and type II collagen
–Stimulates collagen synthesis by osteoblasts

Bone Morphogenetic Proteins
•Osteoinductive proteins initially isolated from demineralized bone matrix
–Proven by bone formation in heterotopic muscle pouch
•Induce cell differentiation
–BMP-3 (osteogenin) is an extremely potent inducer of mesenchymal tissue
differentiation into bone
•Promote endochondral ossification
–BMP-2 and BMP-7 induce endochondral bone formation in segmental defects
•Regulate extracellular matrix production
–BMP-1 is an enzyme that cleaves the carboxy termini of procollagens I, II and
III

Bone Morphogenetic Proteins
•These are included in the TGF-β family
–Except BMP-1
•BMP2-7,9 are osteoinductive
•BMP2,6, & 9 may be the most potent in osteoblastic differentiation
•Work through the intracellular Smad pathway
•Follow a dose/response ratio

BMP Antagonists
•May have important role in bone formation
•Noggin
–Extra-cellular inhibitor
–Competes with BMP-2 for receptors

BMP Future Directions
•BMP-2
–Increased fusion rate in spinal fusion
•BMP-7 equally effective as ICBG in nonunions
•Must be applied locally because of rapid systemic clearance
•? Effectiveness in acute fractures
•? Increased wound healing in open injuries
•Protein therapy vs. gene therapy

Fibroblast Growth Factors
•Both acidic (FGF-1) and basic (FGF-2) forms
•Increase proliferation of chondrocytes and osteoblasts
•Enhance callus formation
•FGF-2 stimulates angiogenesis

Platelet-Derived Growth Factor
•A dimer of the products of two genes, PDGF-A and PDGF-B
–PDGF-BB and PDGF-AB are the predominant forms found in the circulation
•Stimulates bone cell growth
•Mitogen for cells of mesenchymal origin
•Increases type I collagen synthesis by increasing the number of osteoblasts
•PDGF-BB stimulates bone resorption by increasing the number of osteoclasts

Insulin-like Growth Factor
•Two types: IGF-I and IGF-II
–Synthesized by multiple tissues
–IGF-I production in the liver is stimulated by Growth Hormone
•Stimulates bone collagen and matrix synthesis
•Stimulates replication of osteoblasts
•Inhibits bone collagen degradation

Cytokines
•Interleukin-1,-4,-6,-11, macrophage and granulocyte/macrophage (GM) colony-
stimulating factors (CSFs) and Tumor Necrosis Factor
•Stimulate bone resorption
–IL-1 is the most potent
•IL-1 and IL-6 synthesis is decreased by estrogen
–May be mechanism for post-menopausal bone resorption
•Peak during 1st 24 hours then again during remodeling
•Regulate endochondral bone formation

Prostaglandins / Leukotrienes
•Effect on bone resorption is species dependent and their overall effects in humans
unknown
•Prostaglandins of the E series
–Stimulate osteoblastic bone formation
–Inhibit activity of isolated osteoclasts
•Leukotrienes
–Stimulate osteoblastic bone formation
–Enhance the capacity of isolated osteoclasts to form resorption pits

Hormones
•Estrogen
–Stimulates fracture healing through receptor mediated mechanism
–Modulates release of a specific inhibitor of IL-1
•Thyroid hormones
–Thyroxine and triiodothyronine stimulate osteoclastic bone resorption
•Glucocorticoids
–Inhibit calcium absorption from the gut causing increased PTH and therefore
increased osteoclastic bone resorption
•Parathyroid Hormone
–Intermittent exposure stimulates
•Osteoblasts
•Increased bone formation
•Growth Hormone
–Mediated through IGF-1 (Somatomedin-C)
–Increases callus formation and fracture strength

Vascular Factors
•Metalloproteinases
–Degrade cartilage and bones to allow invasion of vessels
•Angiogenic factors
–Vascular-endothelial growth factors
•Mediate neo-angiogenesis & endothelial-cell specific mitogens
–Angiopoietin (1&2)
•Regulate formation of larger vessels and branches

Local Anatomic Factors That Influence Fracture Healing
•Soft tissue injury
•Interruption of local blood supply
•Interposition of soft tissue at fracture site
•Bone death caused by radiation, thermal or chemical burns or infection

Systemic Factors That Decrease Fracture Healing
•Malnutrition
–Causes reduced activity and proliferation of osteochondral cells
–Decreased callus formation
•Smoking
–Cigarette smoke inhibits osteoblasts
–Nicotine causes vasoconstriction diminishing blood flow at fracture site
•Diabetes Mellitus
–Associated with collagen defects including decreased collagen content, defective cross-
linking and alterations in collagen sub-type ratios

Electromagnetic Field
•In vitro bone deformation produces piezoelectric currents and streaming potentials
•Electromagnetic (EM) devices are based on Wolff’s Law that bone responds to
mechanical stress: Exogenous EM fields may simulate mechanical loading and
stimulate bone growth and repair
•Clinical efficacy very controversial

Types of EM Devices
•Microamperes
•Direct electrical current
•Capacitively coupled electric fields
•Pulsed electromagnetic fields (PEMF)

PEMF
•Approved by the FDA for the treatment of non-unions
•Efficacy of bone stimulation appears to be frequency dependant
–Extremely low frequency (ELF) sinusoidal electric fields in the physiologic
range are most effective (15 to 30 Hz range)
–Specifically, PEMF signals in the 20 to 30 Hz range (postural muscle activity)
appear more effective than those below 10 Hz (walking)

Ultrasound
•Low-intensity ultrasound is approved by the FDA for stimulating healing of fresh
fractures
•Modulates signal transduction, increases gene expression, increases blood flow,
enhances bone remodeling and increases callus torsional strength in animal models
•Human clinical trials show a decreased time of healing in fresh fractures
•Has also been shown to decrease the healing time in smokers potentially reversing the ill
effects of smoking

Summary
•Fracture healing is influenced by many variables including mechanical stability,
electrical environment, biochemical factors and blood flow
•Our ability to enhance fracture healing will increase as we better understand the
interaction between these variables

Mechanical effect on Fx healing --> three routes:

Mechanical effect on Fx healing -- three routes:
1. Inter-cortical bridging (primary cortical union)
--by normal cortical remodeling under rigid fixation.
2. External callus bridging
--by new bone arising from the periosteum and the soft
tissues surrounding the fracture.
3. Intramedullary bridging
--by endosteal callus.

Fracture Healing

There are three distinct phases of fracture healing: 1) inflammation, 2) reparation, and 3) remodeling.

The first phase, inflammation, occurs immediately following the bone fracture. At that time, a hematoma or blood clot occurs at the fracture site. This hematoma provides two important factors important for fracture healing. First, the hematoma provides a small amount of mechanical stability to the fracture site. Second, and perhaps more importantly, the hematoma brings osteoblast, and chondrocyte precursors to the fracture site in large numbers that can begin to differentiate into osteoblasts and chondrocytes to begin producing matrix.

In addition, macrophages and osteoclasts come into the site to remove damaged and necrotic tissue. Also, since bone fracture usually involves disruption of the periosteum surrounding the bone, more precursor cells from the periosteum will be introduced into the fracture site. The will begin the process of making a fracture callus through the general process of osteogenesis, laying down bone on soft tissue. Both types of osteogenesis, intramembranous and endochondral ossification may be occuring at the fracture site. The resulting proliferation of woven bone tissue will produce a fracture callus, bridging the fracture gap.

The second step in the biology of fracture healing is he reparation phase. In this phase, the processes of osteogenesis continue and a fracture callus bridges the fracture site.

The bone again can be produced through intramembranous ossification, endochondral ossification or both. It is at this stage of fracture healing that external mechanical stimuli can have the greatest affect on fracture healing. This is because mechanical stability is crucial at this stage of fracture healing.

Although it is not necessary to completely immoblize the fracture, and there is some debate about the need for small motion at the fracture site, it is definitely clear that too much motion will lead to a non-union. A non-union is the healing of a fracture site with soft tissue instead of bone. The desire to prevent non-unions is the reason that different types of fracture fixation devices are used in clinical practice.

The healed bony callus is formed of woven bone and primary bone. At this point, it consists of a large bony bridge connecting the two bones. The base material of the callus typically will have lower strength and stiffness than mature lamellar bone. It is the large mass of bone in the callus that gives the construct its strength. To reduce the callus mass while maintaining mechanical integrity the callus must be remodeled to produce the lamellar bone. During the remodeling period, the large fracture callus is reduced to become the size of the bone at the fracture site. The woven/primary bone is replaced with secondary lamellar bone. This process may take months or even up to a year or more in adults.

The steps of fracture healing may be summarized as follows:
Phase I
1. Bleeding and fracture hematoma forms2. Inflammation3. Next 2-3 Days, granulation tissue formation4. Osteogenic Cells invade tissue and lay down osteoid
Phase II
5. At 3 weeks a soft callus forms consisting of osteoid and cartilage6. Hard tissue callus forms in 6 - 12 weeks7. Clinical union of bone ends occurs in 12 - 16 weeks
Phase III
8. Remodeling of united fracture

Bone

Bones are rigid organ that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Because bones come in a variety of shapes and have a complex internal and external structure, they are lightweight, yet strong and hard, in addition to fulfilling their many other functions.

One of the types of tissues that makes up bone is the mineralized osseous tissue, also called bone tissue, that gives it rigidity and honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum and periosteum, nerves, blood vessels and cartilage. There are 206 bones in the adult body, and about 300 bones in a infants body.

Functions
Bones have eight main functions:

1. Protection — Bones can serve to protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs.
   Shape — Bones provide a frame to keep the body supported.
   
2. Blood production — The marrow, located within the medullary cavity of long bones and the interstices of cancellous bone, produces blood cells in a process called haematopoiesis.
   
3. Mineral storage — Bones act as reserves of minerals important for the body, most notably calcium and phosphorus.
   
4. Movement — Bones, skeletal muscles, tendons, ligaments and joints function together to generate and transfer forces so that individual body parts or the whole body can be manipulated in three-dimensional space. The interaction between bone and muscle is studied in biomechanics.
   
5. Acid-base balance — Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts.
   
6. Detoxification — Bone tissues can also store heavy metals and other foreign elements, removing them from the blood and reducing their effects on other tissues. These can later be gradually released for excretion.[citation needed]
   
7. Sound transduction — Bones are important in the mechanical aspect of hearing.
   

Characteristics

The primary tissue of bone, osseous tissue, is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxylapatite (this is the osseous tissue that gives bones their rigidity).

It has relatively high compressive strength but poor tensile strength, meaning it resists pushing forces well, but not pulling forces. While bone is essentially brittle, it does have a significant degree of elasticity contributed chiefly by collagen. All bones consist of living cells embedded in the mineralised organic matrix that makes up the osseous tissue.

Macrostructure
Bone is not a uniformly solid material, but rather has some spaces between its hard elements.

Compact bone

The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps and spaces. This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult skeleton. Compact bone may also be referred to as dense bone or cortical bone.

Trabecular bone

Filling the interior of the organ is the trabecular bone tissue (an open cell porous network also called cancellous or spongy bone) which is comprised of a network of rod- and plate-like elements that make the overall organ lighter and allowing room for blood vessels and marrow. Trabecular bone accounts for the remaining 20% of total bone mass, but has nearly ten times the surface area of compact bone.

Cellular structure

There are several types of cells constituting the bone;

• Osteoblasts are mononucleate bone-forming cells which descend from osteoprogenitor cells. They are located on the surface of osteoid seams and make a protein mixture known as osteoid, which mineralizes to become bone. Osteoid is primarily composed of Type I collagen. Osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself. They robustly produce alkaline phosphatase, an enzyme that has a role in the mineralisation of bone, as well as many matrix proteins. Osteoblasts are the immature bone cells.
  
• Bone lining cells are essentially inactive osteoblasts. They cover all of the available bone surface and function as a barrier for certain ions.
  
• Osteocytes originate from osteoblasts which have migrated into and become trapped and surrounded by bone matrix which they themselves produce. The spaces which they occupy are known as lacunae. Osteocytes have many processes which reach out to meet osteoblasts probably for the purposes of communication. Their functions include to varying degrees: formation of bone, matrix maintenance and calcium homeostasis. They possibly act as mechano-sensory receptors—regulating the bone's response to stress. They are mature bone cells.
  
• Osteoclasts are the cells responsible for bone resorption (remodeling of bone to reduce its volume). Osteoclasts are large, multinucleated cells located on bone surfaces in what are called Howship's lacunae or resorption pits. These lacunae, or resorption pits, are left behind after the breakdown of bone and often present as scalloped surfaces. Because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with engulfment strategies similar to circulating macrophages. Osteoclasts mature and/or migrate to discrete bone surfaces. Upon arrival, active enzymes, such as tartrate resistant acid phosphatase, are secreted against the mineral substrate

Molecular structure

Matrix
The matrix is the major constituent of bone, surrounding the cells. It has inorganic and organic parts.

Inorganic
The inorganic is mainly crystalline mineral salts and calcium, which is present in the form of hydroxyapatite. The matrix is initially laid down as unmineralized osteoid (manufactured by osteoblasts). Mineralisation involves osteoblasts secreting vesicles containing alkaline phosphatase. This cleaves the phosphate groups and acts as the foci for calcium and phosphate deposition. The vesicles then rupture and act as a centre for crystals to grow on.

Organic
The organic part of matrix is mainly Type I collagen. This is made intracellularly as tropocollagen and then exported. It then associates into fibrils. Also making up the organic part of matrix include various growth factors, the functions of which are not fully known. Other factors present include glycosaminoglycans, osteocalcin, osteonectin, bone sialo protein and Cell Attachment Factor. One of the main things that distinguishes the matrix of a bone from that of another cell is that the matrix in bone is hard.

Woven or lamellar

Collagen fibres of woven bone

Bone is first deposited as woven bone, in a disorganized structure with a high proportion of osteocytes in young and in healing injuries. Woven bone is weaker, with a small number of randomly oriented collagen fibers, but forms quickly. It is replaced by lamellar bone, which is highly organized in concentric sheets with a low proportion of osteocytes.

Lamellar bone is stronger and filled with many collagen fibers parallel to other fibers in the same layer. The fibers run in opposite directions in alternating layers, much like plywood, assisting in the bone's ability to resist torsion forces. After a break, woven bone quickly forms and is gradually replaced by slow-growing lamellar bone on pre-existing calcified hyaline cartilage through a process known as "bony substitution."

Five types of bones

There are five types of bones in the human body: long, short, flat, irregular and sesamoid.

1. Long bones are longer than they are wide, consisting of a long shaft (the diaphysis) plus two articular (joint) surfaces, called epiphyses. They are comprised mostly of compact bone, but are generally thick enough to contain considerable spongy bone and marrow in the hollow centre (the medullary cavity). Most bones of the limbs (including the three bones of the fingers) are long bones, except for the kneecap (patella), and the carpal, metacarpal, tarsal and metatarsal bones of the wrist and ankle. The classification refers to shape rather than the size.


2. Short bones are roughly cube-shaped, and have only a thin layer of compact bone surrounding a spongy interior. The bones of the wrist and ankle are short bones, as are the sesamoid bones
   
3. Flat bones are thin and generally curved, with two parallel layers of compact bones sandwiching a layer of spongy bone. Most of the bones of the skull are flat bones, as is the sternum.


4. Irregular bones do not fit into the above categories. They consist of thin layers of compact bone surrounding a spongy interior. As implied by the name, their shapes are irregular and complicated. The bones of the spine and hips are irregular bones.
   
5. Sesamoid bones are bones embedded in tendons. Since they act to hold the tendon further away from the joint, the angle of the tendon is increased and thus the force of the muscle is increased. Examples of sesamoid bones are the patella and the pisiform

Formation

The formation of bone during the fetal stage of development occurs by two methods: intramembranous and endochondral ossification.

Intramembranous ossification
Intramembranous ossification mainly occurs during formation of the flat bones of the skull; the bone is formed from mesenchyme tissue. The steps in intramembranous ossification are:

1. Development of ossification center
2. Calcification
3. Formation of trabeculae
4. Development of periosteum
5. Endochondral ossification

Endochondrial ossification
Endochondral ossification, on the other hand, occurs in long bones, such as limbs; the bone is formed from cartilage. The steps in endochondral ossification are:

1. Development of cartilage model
2. Growth of cartilage model
3. Development of the primary ossification center
4. Development of medullary cavity
5. Development of the secondary ossification center
6. Formation of articular cartilage and epiphyseal plate

Endochondral ossification begins with points in the cartilage called "primary ossification centers." They mostly appear during fetal development, though a few short bones begin their primary ossification after birth.

They are responsible for the formation of the diaphyses of long bones, short bones and certain parts of irregular bones. Secondary ossification occurs after birth, and forms the epiphyses of long bones and the extremities of irregular and flat bones.

The diaphysis and both epiphyses of a long bone are separated by a growing zone of cartilage (the epiphyseal plate). When the child reaches skeletal maturity (18 to 25 years of age), all of the cartilage is replaced by bone, fusing the diaphysis and both epiphyses together (epiphyseal closure).

Bone marrow
There are two types of bone marrow, yellow and red, most commonly seen is red Bone marrow can be found in almost any bone that holds cancellous tissue. In newborns, all such bones are filled exclusively with red marrow (or hemopoietic marrow), but as the child ages it is mostly replaced by yellow, or fatty marrow. In adults, red marrow is mostly found in the flat bones of the skull, the ribs, the vertebrae and pelvic bones.

Remodeling
Remodeling or bone turnover is the process of resorption followed by replacement of bone with little change in shape and occurs throughout a person's life. Osteoblasts and osteoclasts, coupled together via paracrine cell signalling, are referred to as bone remodeling units.

Purpose
The purpose of remodeling is to regulate calcium homeostasis, repair micro-damaged bones (from everyday stress) but also to shape and sculpture the skeleton during growth.

Calcium balance
The process of bone resorption by the osteoclasts releases stored calcium into the systemic circulation and is an important process in regulating calcium balance. As bone formation actively fixes circulating calcium in its mineral form, removing it from the bloodstream, resorption actively unfixes it thereby increasing circulating calcium levels. These processes occur in tandem at site-specific locations.

Repair
Repeated stress, such as weight-bearing exercise or bone healing, results in the bone thickening at the points of maximum stress (Wolff's law). It has been hypothesized that this is a result of bone's piezoelectric properties, which cause bone to generate small electrical potentials under stress.[citation needed]

Osteology

The study of bones and teeth is referred to as osteology. It is frequently used in anthropology, archeology and forensic science for a variety of tasks. This can include determining the nutritional, health, age or injury status of the individual the bones were taken from. Preparing fleshed bones for these types of studies can involve maceration - boiling fleshed bones to remove large particles, then hand-cleaning.

Typically anthropologists and archeologists study bone tools made by Homo sapiens and Homo neanderthalensis. Bones can serve a number of uses such as projectile points or artistic pigments, and can be made from endoskeletal or external bones such as antler or tusk.
Alternatives to bony endoskeletons

There are several evolutionary alternatives to mammilary bone; though they have some similar functions, they are not completely functionally analogous to bone.

• Exoskeletons offer support, protection and levers for movement similar to endoskeletal bone. Different types of exoskeletons include shells, carapaces (consisting of calcium compounds or silica) and chitinous exoskeletons.


• A true endoskeleton (that is, protective tissue derived from mesoderm) is also present in Echinoderms. Porifera (sponges) possess simple endoskeletons that consist of calcareous or siliceous spicules and a spongin fiber network.

Exposed bone
Bone penetrating the skin and being exposed to the outside can be both a natural process in some animals, and due to injury:

• A deer's antlers are composed of bone
• The extinct predatory fish Dunkleosteus, instead of teeth, had sharp edges of hard exposed bone along its jaws
• A compound fracture occurs when the edges of a broken bone punctures the skin
• Though not strictly speaking exposed, a bird's beak is primarily bone covered in a layer of keratin