Bone Formation and Development

Bone Formation and Development

Cartilage Template, Intramembranous Ossification, Endochondral Ossification, Longitudinal Growth, Appositional Growth, Bone Modeling

__________.

1. Bone is a replacement tissue; that is, it uses a model tissue on which to lay down its mineral matrix. For skeletal development, the most common template is cartilage.

2. During fetal development, a framework is laid down that determines where bones will form.

3. This framework is a flexible, semi-solid matrix produced by chondroblasts and consists of hyaluronic acid, chondroitin sulfate, collagen fibers, and water.

4. As the matrix surrounds and isolates chondroblasts, they are called chondrocytes.

5. Unlike most connective tissues, cartilage is avascular, meaning that it has no blood vessels supplying nutrients and removing metabolic wastes.

6. All of these functions are carried on by diffusion through the matrix. This is why damaged cartilage does not repair itself as readily as most tissues do.

Cartilage Template

____________.

1. Formation of bones without a cartilage model. Typical in flat bones, mandible, clavicles, and patella. Begins approximately 8 weeks after fertilization.

2. Compact and spongy bone develops directly from sheets of mesenchymal (undifferentiated) connective tissue.

3. Steps of intramembranous ossification:

a. The process begins when mesenchymal cells in the embryonic skeleton gather together and begin to differentiate into specialized cells. Some of these cells will differentiate into capillaries, while others will become osteogenic cells and then osteoblasts. Although they will ultimately be spread out by the formation of bone tissue, early osteoblasts appear in a cluster called an ossification center.

b. The osteoblasts secrete osteoid, uncalcified matrix, which calcifies (hardens) within a few days as mineral salts are deposited on it, thereby entrapping the osteoblasts within. Once entrapped, the osteoblasts become osteocytes

c. Osteoid (unmineralized bone matrix) secreted around the capillaries results in a trabecular matrix, while osteoblasts on the surface of the spongy bone become the periosteum. The periosteum then creates a protective layer of compact bone superficial to the trabecular bone. The trabecular bone crowds nearby blood vessels, which eventually condense into red marrow.

d. At birth, the skull and clavicles are not fully ossified nor are the sutures of the skull closed. This allows the skull and shoulders to deform during passage through the birth canal. The last bones to ossify via intramembranous ossification are the flat bones of the face, which reach their adult size at the end of the adolescent growth spurt.

Intramembranous Ossification

__________.

1. Formation of most bones using a hyaline cartilage model. In endochondral ossification, bone develops by replacing hyaline cartilage.

2. Approximately 6 weeks after fertilization, some of the mesenchymal cells differentiate into chondrocytes that form the cartilaginous skeletal precursor of the bones. Soon after, the perichondrium, a membrane that covers the cartilage, appears

Endochondral Ossification

Steps of endochondral ossification:

Cavitation of hyaline shaft, Invasion of internal cavities, Formation of the Medullary cavity, Formation of epiphyses

__________: (picture b in the diagram)

i. Chondrocytes within the shaft hypertrophy (enlarge) and the surrounding matrix begins to calcify.

ii. The impermeable matrix causes chondrocytes to die from lack of nutrients leaving the matrix that starts to deteriorate (cavitate).

iii. Blood vessels grow around the edges of the cartilage. These enlarging spaces eventually combine to become the medullary cavity.

Cavitation of hyaline shaft

________: (picture c in the diagram)

i. Blood vessels invade the resulting spaces, not only enlarging the cavities but also carrying osteogenic cells with them, many of which will become osteoblasts. This area within the shaft of hyaline cartilage is called the primary ossification center.

i. Migrating with the blood vessels are fibroblasts (which differentiate into osteoblasts), lymph vessels, nerve fibers, red marrow elements.  Collectively, these are called the periosteal bud.

ii. The osteoblasts secrete osteoid around remaining fragments of hyaline, forming trabeculae, or spongy bone.

Invasion of internal cavities

__________: (pictures c & d in the diagram)

i. As the primary ossification center enlarges, osteoclasts break down newly formed spongy bone and opens up a medullary cavity in the center of the diaphysis.

ii. The osseous tissue of the outer shaft becomes thicker forming compact bone.

Formation of the Medullary cavity

__________: (picture f in the diagram)

i. Secondary ossification centers appear in the area atthe opposite ends of the bone.  The cartilage in the epiphyses calcifies and deteriorates, forming cavities that allow entry of a periosteal bud.

ii. Soon the epiphyses are filled with spongy bone.  The spongy bone is NOT broken down during the remodeling process.

iii. Cartilage remains at epiphyseal (growth) plate and at joint surface as articular cartilage.

Formation of epiphyses

How Bones Grow in Length. _________ (length)

Longitudinal Growth

Hyaline cartilage cells form tall columns at the ___________(or growth plate). On the epiphyseal side of the epiphyseal plate, cartilage is formed. On the diaphyseal side, cartilage is ossified, and the diaphysis grows in length. The reserve zone is the region closest to the epiphyseal end of the plate and contains small chondrocytes within the matrix. These chondrocytes do not participate in bone growth but secure the epiphyseal plate to the osseous tissue of the epiphysis.

epiphyseal plate

The __________ is the next layer toward the diaphysis and contains stacks of slightly larger chondrocytes. It quickly makes new chondrocytes (via mitosis) to replace those that die at the diaphyseal end of the plate.

proliferative zone

Chondrocytes in the next layer, the ________, are older and larger than those in the proliferative zone. The more mature cells are situated closer to the diaphyseal end of the plate. Older chondrocytes closer to the shaft enlarge, die, and the surrounding cartilage matrix deteriorates. The longitudinal growth of bone is a result of cellular division in the proliferative zone and the maturation of cells in the zone of maturation and hypertrophy.

zone of maturation and hypertrophy

Most of the chondrocytes in the zone of calcified matrix, the zone closest to the diaphysis, are dead because the matrix around them has calcified. Capillaries and osteoblasts from the diaphysis penetrate this zone, and the osteoblasts secrete bone tissue on the remaining calcified cartilage. Thus, the zone of calcified matrix connects the epiphyseal plate to the diaphysis. A bone grows in length when osseous tissue is added to the diaphysis. The deterioration leaves spicules of calcified cartilage.

zone of calcified matrix

Osteoblasts in the _______ then ossify the cartilage spicules, forming spongy bone.

medullary cavity

The Bones continue to grow in length until early adulthood. When the chondrocytes in the _________ cease their proliferation and bone replaces the cartilage, longitudinal growth stops. All that remains of the epiphyseal plate is the ________ This typically occurs in the person’s early twenties and as a result the person stops growing in height.  

epiphyseal plate; epiphyseal line

How Bones Grow in Diameter. ________ (width)

Appositional Growth

Osteoprogenitor cells beneath the ______ differentiate into osteoblasts and form new osteons on the external bone surface.

periosteum

While bone is being added to the outer surface through appositional growth, osteoclasts are removing and recycling lamellae at the inner surface.  As a result, the medullary cavity gradually enlarges as the bone increases in diameter. This process is called _______.

modeling

________ - the process in which matrix is resorbed on one surface of a bone and deposited on another.

Bone Modeling

Modeling primarily takes place during a bone’s ______.

growth

However, in adult life, bone undergoes _______, in which resorption of old or damaged bone takes place on the same surface where osteoblasts lay new bone to replace that which is resorbed. Injury, exercise, and other activities lead to ______.

remodeling