32 Bone Formation

Embryonic and Fetal Bone Formation

During fetal development, bone tissue is created through intramembranous ossification and endochondral ossification.

Fetal Development

Embryonic/fetal development proceeds from rostral (nose and mouth area) to caudal (posterior). The skull and vertebral column are produced by intramembranous ossification. As development proceeds down the body axis, the long bones of the arms and legs are produced by endochondral ossification.

Intramembranous ossification is one of the two essential processes during fetal development of the mammalian skeletal system. It is the process by which bone tissue is created.

Unlike the other process of bone creation— endochondral ossification—intramembranous ossification does not involve cartilage. It is also an essential process during the natural healing of bone fractures and the rudimentary formation of the bones of the head.

The first step in the process is the formation of bone spicules (aggregates of bony matrix) that eventually fuse with each other and become trabeculae. The periosteum is formed and bone growth continues at the surface of trabeculae.

Much like spicules, the increasing growth of trabeculae result in interconnection, and this network is called woven bone. Eventually, woven bone is replaced by lamellar bone.

Embryonic mesenchymal cells (MSC) condense into layers of vascularized primitive connective tissue. Certain mesenchymal cells group together, usually near or around blood vessels, and differentiate into osteogenic cells that deposit bone matrix constitutively. Separate mesenchymal cells differentiate into osteoblasts, which line up along the surface of the spicule and secrete more osteoid, increasing the size of the spicule.

When osteoblasts become trapped in the matrix that they secrete, they differentiate into osteocytes. Osteoblasts continue to line up on the surface, which increases their size. As growth continues, trabeculae become interconnected and woven bone is formed.

The primary center of ossification is the area where bone growth occurs between the periosteum and the bone.

Osteons are units or principal structures of compact bone. During the formation of bone spicules, cytoplasmic processes from osteoblasts interconnect. This becomes the canaliculi of osteons.

Since bone spicules tend to form around blood vessels, the perivascular space is greatly reduced as the bone continues to grow. When replacement with compact bone occurs, this blood vessel becomes the central canal of the osteon.

Endochondral Ossification

Endochondral ossification is the other essential bone creation process during fetal development of the mammalian skeletal system. Unlike intramembranous ossification, cartilage is present during endochondral ossification. It is also an essential process during the rudimentary formation of long bones, the growth of the length of long bones, and the natural healing of bone fractures.

The first site of ossification occurs in the primary center of ossification, which is in the middle of diaphysis (shaft). The perichondrium becomes the periosteum. The periosteum contains a layer of undifferentiated cells (osteoprogenitor cells) that later become osteoblasts.

The osteoblasts secrete osteoid against the shaft of the cartilage model (appositional growth). This serves as support for the new bone. Chondrocytes in the primary center of ossification begin to grow (hypertrophy). They stop secreting collagen and other proteoglycans and begin secreting alkaline phosphatase, an enzyme essential for mineral deposition. Then calcification of the matrix occurs.

Postnatal Bone Growth

Secondary ossification occurs after birth at the epiphyses of long bones and continues until skeletal maturity.

Postnatal Ossification

Secondary ossification occurs after birth. It 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 a 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). This process involves replacing the hyaline cartilage, initially present at the epiphyseal region, with active osteoblasts that deposit bone structural proteins.

The Role of Osteoblasts

Osteoblasts are mononucleate cells that are responsible for bone formation. In essence, osteoblasts are specialized fibroblasts that, in addition to fibroblastic products, express bone sialoprotein and osteocalcin.

Osteoblasts produce a matrix of osteoid that is composed mainly of Type I collagen. Osteoblasts are also responsible for the mineralization of this matrix. Minerals required for mineralization and related processes include zinc, copper, and sodium.

Bone is a dynamic tissue that is constantly being reshaped by osteoblasts and osteoclasts. Osteoblasts produce bone matrix and mineral, and osteoclasts break down the tissue. The number of osteoblasts tends to decrease with age, affecting the balance of the formation and resorption in the bone tissue, and potentially leading to osteoporosis.

During postnatal bone formation, endochondral ossification initiates bone deposition by first generating a structural framework at the ends of long bones, within which the osteoblasts can synthesize a new bone matrix.

Cartilage to Bone

1. Zone of reserve cartilage: This region is farthest from the marrow cavity and consists of hyaline cartilage that does not actively transform into bone. Quiescent chondrocytes are found here.
2. Zone of cell proliferation: Closer to the marrow cavity, chondrocytes in this region multiply and arrange themselves into longitudinal columns of flattened lacunae.
3. Zone of cell hypertrophy: They stop dividing and begin to hypertrophy (enlarge). The walls of the matrix between the lacunae become very thin.
4. Zone of calcification: The region where the cartilagenous matrix begins to calcify.
   Minerals are deposited in the matrix between the columns of lacunae, but are not the permanent bone mineral deposits. This acts as a temporary support for the cartilage that would otherwise be weakened due to the breakdown of the lacunae.
5. Zone of bone deposition (ossification): The walls between the lacunae break down and the chondrocytes die. Each column then becomes a longitudinal channel that is immediately invaded by blood vessels and marrow from the marrow cavity. Osteoblasts begin depositing concentric lamellae of matrix, while osteoclasts dissolve the temporarily calcified cartilage.

The growth in the diameter of bones around the diaphysis occurs through the deposition of bone beneath the periosteum. Osteoclasts in the interior cavity continue to degrade bone until its ultimate thickness is achieved. At this point the rate of formation on the outside and degradation from the inside is constant. This process is termed appositional growth.

Bone Remodeling

Bone remodeling or bone turnover is the process of resorption followed by replacement of bone and occurs throughout a person’s life.

Remodeling or bone turnover is the process of resorption followed by the replacement of bone with limited change in shape; this process occurs throughout a person’s life. Repeated stress, such as weight-bearing exercise or bone healing, results in the bone thickening at the points of maximum stress.

It has been hypothesized that this is a result of bone’s piezoelectric properties that cause bone to generate small electrical potentials under stress. Osteoblasts and osteoclasts, coupled together via paracrine cell signaling, are referred to as bone remodeling units. The purpose of remodeling is to regulate calcium homeostasis, repair micro-damaged bones (from everyday stress), and to shape and sculpture the skeleton during growth.

Bone volume is determined by the rates of bone formation and bone resorption. The action of osteoblasts and osteoclasts are controlled by a number of chemical factors that either promote or inhibit the activity of the bone remodeling cells, controlling the rate at which bone is made, destroyed, or changed in shape. The cells also use paracrine signalling to control the activity of each other.

Role of Growth Factors

Recent research has suggested that certain growth factors may work to locally alter bone formation by increasing osteoblast activity. Numerous bone-derived growth factors have been isolated and classified via bone cultures. These factors include insulin-like growth factors I and II, transforming growth factor beta, fibroblast growth factor, platelet-derived growth factor, and bone morphogenetic proteins.

• Insulin-like
  growth factors protect cartilage cells, and are associated with the activation of osteocytes.
• The transforming
  growth factor beta superfamily includes bone morphogenic proteins involved in
  osteogenesis.
• Fibroblast
  growth factor activates various cells of the bone marrow including osteoclasts
  and osteoblasts.
• Platelet-derived
  growth factor has been found to enhance bone collagen degradation.

Evidence suggests that bone cells produce growth factors for extracellular storage in the bone matrix. The release of these growth factors from the bone matrix could cause the proliferation of osteoblast precursors. Essentially, bone growth factors may act as potential determinants of local bone formation.

Research has suggested that trabecular bone volume in postmenopausal osteoporosis may be determined by the relationship between the total bone forming surface and the percent of surface resorption.

Clinical Note: Osteoporosis means porous bone, which is caused by an over-reaction to osteoclastic bone resorption, and makes bones quite fragile for the elderly. Falls are dangerous for the elderly because they are more likely to break a bone. Hip fractures are especially troublesome as they result in a long recovery period during which complications that may lead to death are quite common.

Bone Repair

Bone fractures are repaired through physiological processes in the periosteum via chrondroblasts and osteoblasts.

Bone healing, or fracture healing, is a proliferative physiological process in which the body facilitates the repair of a bone fracture. Generally, bone fracture treatment consists of a doctor reducing (pushing) dislocated bones back into place via relocation with or without anesthetic, stabilizing their position, and then waiting for the bone’s natural healing process to occur.

While immobilization and surgery may facilitate healing, a fracture ultimately heals through physiological processes. The healing process is mainly determined by the periosteum (the connective tissue membrane covering the bone).

The periosteum is one source of precursor cells that develop into the chondroblasts and osteoblasts that are essential to heal bone. The bone marrow (when present), endosteum, small blood vessels, and fibroblasts are other sources of precursor cells.

Days after a fracture, the cells of the periosteum replicate and transform. The periosteal cells proximal (closest) to the fracture gap develop into chondroblasts that form hyaline cartilage.

The periosteal cells distal to (further from) the fracture gap develop into osteoblasts that form woven bone. The fibroblasts within the granulation tissue develop into chondroblasts that also form hyaline cartilage.

These two new tissues grow in size until they unite with their counterparts from other parts of the fracture. These processes culminate in a new mass of heterogeneous tissue that is known as the fracture callus.

Eventually, the fracture gap is bridged by the hyaline cartilage and woven bone, restoring some of its original strength.

The next phase is the replacement of the hyaline cartilage and woven bone with lamellar bone. The replacement process is known as endochondral ossification with respect to the hyaline cartilage and bony substitution with respect to the woven bone.

Substitution of the woven bone with lamellar bone precedes the substitution of the hyaline cartilage with lamellar bone. The lamellar bone begins forming soon after the collagen matrix of either tissue becomes mineralized. At this point, the mineralized matrix is penetrated by channels, each containing a microvessel and numerous osteoblasts.

The osteoblasts form new lamellar bone upon the recently exposed surface of the mineralized matrix. This new lamellar bone is in the form of trabecular bone. Eventually, all of the woven bone and cartilage of the original fracture callus is replaced by trabecular bone, restoring most of the bone’s original strength.

The remodeling process continues with substitution of the trabecular bone with compact bone. The trabecular bone is first resorbed by osteoclasts, creating a shallow resorption pit known as Howship’s lacuna, and then osteoblasts deposit compact bone within the resorption pit.

Eventually, the fracture callus is remodeled into a new shape that closely duplicates the bone’s original shape and strength. The remodeling phase takes three to five years depending on factors such as age or general condition.

When the humerus in the upper arm is fractured and properly set, bone healing can repair the bone. However, if the bone is not set or improperly set, the chondroblasts and osteoblasts will still try to heal the bone but will be unable to return the bone to full proper functioning.