Anatomy and physiology- 7.3- Bone development

Orderly remodeling of bone

It depends on a precise balance between deposition and resorption, between osteoblasts and osteoclasts. If one process outpaces the other, or both processes occur too rapidly, various bone deformities, developmental abnormalities, and other disorders occur, such as osteitis deformans, osteogenesis imperfecta (brittle bone disease), and osteoporosis.

Ossification

It does not end at birth, but continues throughout life with the growth and remodeling of bones.

Appositional Growth

It occurs by intramembranous ossification at the one surface. Osteoblasts in the inner layer of periosteum deposit osteoid tissue on the bone surface, calcify it, and become trapped in it as osteocytes. They lay down matrix in layers parallel to the surface, not in cylindrical osteon like those in the deeper in the bone. This produces circumferential lamellae. As a bone increases in diameter, its marrow cavity also widens. This is achieved by osteoclasts of the endosteum dissolving tissue on the inner bone surface. Thus flat bones develop by intramembranous ossification alone.

Intramembranous Ossification importance

It plays an important role in the lifelong thickening, strengthening, and remodeling of the long bones. It is the method of depositing new tissue on the bone surface even past the age where the bones can no longer grow in length.

Wolff's law of bone

It states that the architecture of a bone is determined by the mechanical stresses placed upon it, and the bone thereby adapts to withstand those stresses. Wolff's law is a fine example of the complementary of form and function, showing that the form of a bone is shaped by its functional experience. In figure 7.5 we see that the trabeculae of spongy bone have developed along lines of stress placed on the femur. Wolff observed that these stress lines were very similar to the ones that engineers knew of in mechanical cranes.

Long bones and stress

Long bones of the limbs are thickest at about midshaft, because this is where they are subjected to the greatest stress.

The epiphyseal plate

It consists of typical hyaline cartilage in the middle, with transitional zone on each side where cartilage is being replaced by bone.

Appositional Growth

growth in width; This is the deposition of new tissue at the surface.

Bone grow in which two directions

1. Length 2. Width

Steps of intramembranous ossification 1-2

1. Mesenchyme first condenses into a soft sheet of tissue permeated with blood vessels. Mesenchymal cells line up along the blood vessels, become osteoblasts, and secrete a soft collagenous osteoid tissue (prebone) in the direction away from the vessel. Osteoid tissue resembles bone but is not yet calcified. 2. Calcium phosphate and other minerals crystallize on the collagen fibers of the osteoid tissue and harden the matrix. Continued osteoid deposition and mineralization squeeze the blood vessels and future bone marrow into narrower and narrower spaces. As osteoblasts become trapped in their own hardening matrix, they become osteocytes.

The histology structure of the metaphysics. Steps in the conversion of cartilage to bone.

1. Zone of reserve cartilage. This region, farthest from the marrow cavity, consists of typical hyaline cartilage with resting chondrocytes, not yet showing any sign of transformation into bone. 2. Zone of cell proliferation. A little closer to the marrow cavity, chondrocytes multiply and arrange themselves into longitudinal columns of flattened lacunae. 3. Zone of cell hypertrophy. Next, the chondrocytes cease to multiply and begin to hypertrophy (enlarge), much like they do in the primary ossification center of the fetus. The walls of matrix between lacunae become very thin. 4. Zone of calcification. Minerals are deposited in the matrix between the columns of lacunae and calcify the cartilage. (These are not the permanent mineral deposits of bone, but only a temporary support for the cartilage that would otherwise soon be weakened by the breakdown of the enlarged lacunae. 5. Zone of bone deposition. Within each column, the walls between the lacunae break down and the chondrocytes die. This converts each column into a longitudinal channel (clear spaces in the figure), which is immediately invaded by blood vessels and marrow from the marrow cavity. Osteoblasts line up along the walls of these channels and begin depositing concentric lamellae of matrix, while osteoclasts dissolve the temporarily calcified cartilage.

Steps of intramembranous ossification 3-4

3. While the foregoing processes are occurring, more mesenchyme adjacent to the developing bone condenses and forms a fibrous periosteum on each surface. The spongy bone becomes a honeycomb of slender calcified trabeculae. 4. At the surfaces, osteoblasts beneath the periosteum deposit layers of bone, fill in the spaces between trabeculae, and create a zone of compact on each side as well as thicken the bone overall. This process gives rise the structure of a flat cranial bone, a layer of spongy bone between two layers of compact bone.

Bone remodeling

Bones are continually remodeled throughout life by the absorption of old bone and deposition if new. This process repairs 10% of the skeletal tissue per year. It repairs microfractures, releases minerals into the blood, and reshapes bones in response to use and disuse. Bone remodeling comes about through the collaborative action of osteoblasts and osteoclasts.

steps of endochondral ossification 4-6

4. The secondary ossification center hollows out by the same process as the diaphysis, generating a secondary marrow cavity in the epiphysis. This cavity expands outward from the center, in all directions. At the same time of birth, the bone typically looks like step 4. In bones with two secondary ossification centers, one center lags behind the other, so at one end while chondrocytes growth has just begun at the other. 5. During infancy and childhood, the epiphyses fill with spongy bone. Cartilage is then limited to the articular cartilage covering each joint surface, and to an epiphyseal plate. The plate persists through childhood and adolescence and serves as a growth zone for bone elongation. 6. By the late teens to early twenties, all remaining cartilage in the epiphyseal plate is generally consumed and the gap between the epiphysis and diaphysis closes. The primary and secondary marrow cavities then unite into a single cavity.

Wolff's law

Bones gives evidence of a person's sex, race, height, weight, work or exercise habits, nutritional status and medical history.

cartilage of epiphyseal plate

All cartilage of the epiphyseal plate is depleted. The primary and secondary marrow cavities now unite into one cavity. The junctional region where they meet is filled with spongy bone, and the site of the original epiphyseal plate is marked with a line of slightly denser spongy bone called the epiphyseal line. Often a delicate ridge on the bone surface marks the location of this line. When the epiphyseal plate is depleted, we say that the epiphyses have "closed" because no gap between the epiphysis and diaphysis is visible on an X-ray. Once the epiphyses have all closed in the lower limbs, a person can grow no taller. The epiphyseal plates close at different ages in different bones and in different regions of the same bone.

Anthropologist and the skeletal system

Anthropologist who study ancient skeletal remains use evidence of this sort to help distinguish between members of different social classes, such as aristocrats from laborers.

Bone density and athletes

Bones have a greater density and mass in athletes and people engaged in heavy manual labor than they do in sedentary people.

Bone elongation

Bone growth in length: From infancy through adolescence, an epiphyseal plate is present at one or both ends of a long bone, at the junction between the diaphysis a epiphysis. Epiphyseal plate is a region of transition from cartilage to bone, and functions as a growth zone where the bones elongate. Growth here is responsible for a person's increase in height.

Bone age

Bone maturation; correlates well with stage of pubertal development. The state of closure in various bones of a subadult skeleton is often used in forensic science to estimate the individual's bone age at death.

Bone widening and thickening

Bones also grow in diameter and thickness. This involves a process called appositional growth.

Long bones develop by

By a combination of intramembranous and endochondral methods.

Ways in which cartilage can enlarge

Cartilage can enlarge; 1. Interstitial growth 2. Appositional growth

Growth in height

Chondrocyte multiplication in zone 2 and hypertrophy in zone 2 continually push the zone of reserve cartilage (1) toward the ends of the bone, so the bone elongates. In the lower limbs this process causes a person to grow in height while bones of the upper limbs grow proportionately. Bone elongation is only a result of cartilage growth. Cartilage growth from within, by the multiplication of chondrocytes and deposition of new matrix in the interior, is called interstitial growth.

Steps in Endochondral ossification 1-2

Having only one epiphyseal bone (growth center). Many other bones develop in more complex ways, having and epiphyseal plate at both ends or multiple plates at each end, but the basic process is the same. 1. Mesenchyme develops into a body of hyaline cartilage, covered with a fibrous perichondrium, in the location of a future bone. For a time, the perichondrium produces chondrocytes and the cartilage model grows in thickness. 2. In a primary ossification center near the middle of this cartilage, chondrocytes begin to inflate and die, while the thin walls between them calcify. The perichondrium stops producing chondrocytes and begins producing osteoblasts. These deposit a thin collar of bone around the middle of the cartilage model. The former perichondrium is now considered to be a periosteum. As chondrocytes in the middle of the model die, their lacunae merge into a single cavity.

Bone remodeling (osteoblasts and osteoclasts)

If a bone is little used, osteoclasts remove matrix and get rid of unnecessary mass. If a bone is heavily used, or a stress is consistently applied to a particular region of a bone, osteoblasts deposit new osseous tissue and thicken the bone.

Osteoblasts lining

Osteoblasts lining the aforementioned channels deposit layer after layer of bone matrix, so the channel grows narrower and narrower. These layers become the concentric lamellae of an osteon. Finally only a slender channel persists, the central canal of a new osteon. Osteoblasts trapped in the matrix become osteocytes.

Steps in Endochondral ossification 3

Osteoclasts arrive in the blood and digest calcified tissue in the shaft, hollowing it out and creating the primary marrow cavity. Osteoclasts also arrive and deposit layers of bone lining the cavity, thickening the shaft. As the bony collar under the periosteum thicken and elongates, a wave of cartilage death progresses toward each end of the bone. Osteoclasts in the marrow cavity follow this wave, dissolving calcified cartilage remnants and enlarging the marrow cavity of the diaphysis. The region of transition from cartilage to bone at each end of the primary marrow cavity is called a metaphysis. Soon, chondrocyte enlargement and death occur in the epiphysis of the model as well. creating a secondary ossification center. In the metacarpal bones this occurs in only one epiphysis. In longer bones of the arms, forearms, legs, and thighs, it occurs at both ends.

Ways in which bone can enlarge

Osteocytes embedded in calcified matrix have little room to spare for the deposition of more matrix internally. Bone is therefore limited to appositional growth.

Zone 5

The process of bone deposition creates a region of spongy bone at the end of the marrow cavity facing the metaphysis. This spongy bone remains for life, although with extensive lifelong remodeling. But around the perimeter of the marrow cavity, continuing ossification converts this spongy bone to compact bone.

Two methods of ossification

These both begin with embryonic mesenchyme. 1. Intramembranous 2. Endochondral

Endochondral Ossification

This is a process in which a bone develops from a preexisting model composed of hyaline cartilage. It begins around the sixth week of fetal development and continues into a person's 20s.

Epiphyseal plate

This is a thin wall of cartilage separating the primary and secondary marrow cavities at one or both ends of the bone.

Circumferential lamellae

This is layers of bone.

Metaphysis

This is the transitional zone, facing the marrow cavity.

Intramembranous Ossification

This produces the flat bones of the skull and most of the clavicle (collarbone).

Ossification or osteogenesis

This term to describe the formation of bone.

Bones developed from endochondral ossification

Vertebrae, ribs, sternum, scapula, pelvic girdle, and bones of the limbs.

Dwarfism

condition of being abnormally small The most common form of dwarfism results from a failure of cartilage growth in the long bones.