Bone remodelling

Endochondral ossification

1. Cartilage formation- mesenchymal cells divide and differentiate into chondroblasts. They then secrete cartilage and become embedded in lacunae within matrix. 2. Vascular invasion& longitudinal growth- a ring of woven bone is formed in midshaft. Osteoclasts then allow vascular and invasion of woven bone and cartilage, and the growth plate forms.

Development v remodeling v differentiation

Development- bone cells need to differentiate in order to lay down mineralized skeleton. Growth- skeleton is modeled to produce adult shape and size. Adult skeleton- continually remodeled according to metabolic and mechanical requirements. Pathology- too much or little bone. All affected by factors that control differentiation and activity of bone cells.

Osteoblast differentiation

Mesenchymal stem cell-osteoblast-osteocyte

RANK signaling in osteoclasts

RANKL binds to RANK if it does not bind to OPG - decoy receptor. RANK then activates TRAF6- a receptor associated factor which then activates numerous second messengers such as ERK and Src, which then activate transcription factors.

Bone development

The formation of bone is called "ossification". During the fetal stage of development this occurs by two processes, Intramembranous ossification and endochondral ossification- happens in flat bones e.g. skull, scapula, mandible, ileum. Intramembranous ossification involves the creation of bone from connective tissue, whereas in the process of endochondral ossification bone is created from cartilage- happens in long bones- tibia, femur, humerus.

Factors controlling osteoblast differentiation

3 genes are known to be essential for osteoblast differentiation (i.e. development of bony skeleton), -Ihh, Cbfa1, Osx. Ihh is a member of the hedgehog family of growth factors. Ihh knockouts result in disorganized growth plate, no osteoblasts at sites of endochondral ossification- Die at birth therefore may be delayed endochondral ossification. Also osteoblasts present in intramembraneous bones. Cbfa1 & Osx- Mice lacking either of these have cartilaginous skeletons and osteoblasts are completely absent.

Briefly describe bone structure

Bone is mostly a matrix. Osseous tissue is the primary tissue of bone and is relatively hard and lightweight. It is mostly made up of calcium phosphate. Bone has a layered structure. The hard outer layer is composed of cortical bone also called compact bone. Filling the interior of the bone is the cancellous bone also known as trabecular or spongy bone tissue.[4] It is an open cell porous network. The shaft of the bone is the diaphysis, the rounded head at the end of each shaft is the epiphysis. The metaphysis is the wide portion between the epiphysis and the narrow diphysis.

Modeling and remodeling

Bone remodeling (or bone metabolism) is a lifelong process where mature bone tissue is removed from the skeleton (a process called bone resorption) and new bone tissue is formed (a process called ossification or new bone formation). This is done via the ARF sequence. Bone modeling is changes in shape and diameter of bone resulting from browth.

Intramembranous ossification

Intramembranous ossification mainly occurs during formation of the flat bones of the skull but also the mandible, maxilla, and clavicles; the bone is formed from connective tissue such as mesenchyme tissue rather than from cartilage. The steps in intramembranous ossification are: 1.Development of ossification center 2.Calcification 3.Formation of trabeculae 4.Development of periosteum. Firstly mesenchymal cells divide and differentiate into osteoblasts. The osteoblasts then form woven bone with irregular collagen with irregular collagen orientation, large and numerous osteocytes and patchy calcification. Blood vessels become trapped between trabeculae will form bone marrow. Woven bone is then remodeled and replaced by lamella bone.

Secreted factors controlling osteoclast differentiation

M-CSF. Knockout mice are osteopetrotic. OPG- osteoprotegerin (osteoclast inhibitory factor). Discovered in 1997 in genomic screen for novel secreted factors. Overexpression causes osteopetrosis (no mature osteoclasts). OPG knockouts cause osteoporosis. OPG has soluble ligand RANKL.

Outline the function of bones.

Mechanical- bone provides a rigid skeleton that protects vital organs and facilitates locomotion. Calcium- homeostasis- calcium is stored in bone matrix and released from bone low blood calcium conc. Bone marrow is responsible for red blood cell production in a process known as hematopoiesis.

Cells of bone

Osteoblasts are involved in the creation and mineralization of bone tissue. They are located on the surface of osteoid seams and make a protein mixture known as osteoid, which mineralizes to become bone. Osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself. Osteocytes are mostly inactive osteoblasts. Osteocytes originate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that they themselves produced. Osteoclasts are the cells responsible for bone resorption, thus they break down bone. New bone is then formed by the osteoblasts. Bone is constantly remodelled by the resorption of osteoclasts and created by osteoblasts.[7] Osteoclasts are large cells with multiple nuclei located on bone surfaces in what are called Howship's lacunae (or resorption pits).

Genes controlling osteoclast differentiation

PU.1 interacts with c-fms to develop myeloid precursor cell to osteoclast precursor or monocyte. NFkB then acts on osteoclast precursor along with c-fos and mi to develop it into a functional osteoclast. Cathepsin K then acts on functional osteoclast.

Transcription factors controlling osteoclast differentiation

PU.1- regulates c-fms receptor (receptor for M-CSF). Knockout mice for PU.1 are osteopetrotic- that is they have no macrophage and no osteoclasts. C-fos is a component of AP-1 TF. Knockout mice are also osteopetrotic. NFkB is a dimer composed of proteins containing 'Rel' domain (p50 & p52). Knockouts are osteopetrotic. Mi- knockoutsare osteopetrotic

RANKL

RANKL is expressed on cell surface of osteoblast progenitor and secreted in bone mincroenvironment. Systemic administration in vivo of RANK increases bone resorption. In vitro causes increase in osteoclast number and activity. Knockout results in osteopetrosis.

Bone remodeling cycle

Reversal- mononuclear cells prepare bone surface for new osteoblasts to begin building bone. Next is formation- osteoblasts form a matrix to replace resorbed bone with new bone. After this is resting- a prolonged resting period follows until a new remodeling cycle begins. Next is resorption- osteoclasts break down bone mineral and matrix, creating an erosion cavity.

Osteoclast differentiation

The initial stage is the pluripotent mononuclear precursor. This arises in bone marrow and circulates in blood. M-CSF then helps it develop into a mononuclear cell. This fuses at bone surface and with the help of appropriate signaling chemicals develops into a preosteoclast. The preosteoclast multinucleates and develops into a mature osteoclast which polarizes on bone surface and resorbs bone.

Haversian system

The osteon or haversian system is the fundamental functional unit of much compact bone.Each osteon consists of concentric layers, or lamellae, of compact bone tissue that surround a central canal, the haversian canal. The haversian canal contains the bone's nerve and blood supplies. The boundary of an osteon is the cement line.