This results in their death and the disintegration of the surrounding cartilage. Blood vessels invade the resulting spaces, not only enlarging the cavities but also carrying osteogenic cells with them, many of which will become osteoblasts. These enlarging spaces eventually combine to become the medullary cavity. As the cartilage grows, capillaries penetrate it. This penetration initiates the transformation of the perichondrium into the bone-producing periosteum.
Here, the osteoblasts form a periosteal collar of compact bone around the cartilage of the diaphysis. By the second or third month of fetal life, bone cell development and ossification ramps up and creates the primary ossification center , a region deep in the periosteal collar where ossification begins Figure c.
By the time the fetal skeleton is fully formed, cartilage only remains at the joint surface as articular cartilage and between the diaphysis and epiphysis as the epiphyseal plate, the latter of which is responsible for the longitudinal growth of bones.
After birth, this same sequence of events matrix mineralization, death of chondrocytes, invasion of blood vessels from the periosteum, and seeding with osteogenic cells that become osteoblasts occurs in the epiphyseal regions, and each of these centers of activity is referred to as a secondary ossification center Figure e.
The epiphyseal plate is the area of growth in a long bone. It is a layer of hyaline cartilage where ossification occurs in immature bones. 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 epiphyseal plate is composed of four zones of cells and activity Figure. 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. The proliferative zone is the next layer toward the diaphysis and contains stacks of slightly larger chondrocytes.
It makes new chondrocytes via mitosis to replace those that die at the diaphyseal end of the plate. Chondrocytes in the next layer, the zone of maturation and hypertrophy , are older and larger than those in the proliferative zone. The more mature cells are situated closer to the diaphyseal end of the plate. 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.
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. Bones continue to grow in length until early adulthood.
The rate of growth is controlled by hormones, which will be discussed later. When the chondrocytes in the epiphyseal plate cease their proliferation and bone replaces the cartilage, longitudinal growth stops. All that remains of the epiphyseal plate is the epiphyseal line Figure. How Bones Grow in Diameter While bones are increasing in length, they are also increasing in diameter; growth in diameter can continue even after longitudinal growth ceases.
This is called appositional growth. Osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts, via intramembranous ossification, produce new bone tissue beneath the periosteum. The erosion of old bone along the medullary cavity and the deposition of new bone beneath the periosteum not only increase the diameter of the diaphysis but also increase the diameter of the medullary cavity. This process is called modeling. The process in which matrix is resorbed on one surface of a bone and deposited on another is known as bone modeling.
However, in adult life, bone undergoes remodeling , 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. Those influences are discussed later in the chapter, but even without injury or exercise, about 5 to 10 percent of the skeleton is remodeled annually just by destroying old bone and renewing it with fresh bone. Skeletal System Osteogenesis imperfecta OI is a genetic disease in which bones do not form properly and therefore are fragile and break easily.
It is also called brittle bone disease. The disease is present from birth and affects a person throughout life.
The severity of the disease can range from mild to severe. Those with the most severe forms of the disease sustain many more fractures than those with a mild form.
Frequent and multiple fractures typically lead to bone deformities and short stature. Bowing of the long bones and curvature of the spine are also common in people afflicted with OI. Curvature of the spine makes breathing difficult because the lungs are compressed. Because collagen is such an important structural protein in many parts of the body, people with OI may also experience fragile skin, weak muscles, loose joints, easy bruising, frequent nosebleeds, brittle teeth, blue sclera, and hearing loss.
There is no known cure for OI. Calcium plays a significant role in the body and is required for muscle contraction, nerve conduction, cell division and blood coagulation.
Serum calcium levels are tightly regulated by two hormones, which work antagonistically to maintain homoeostasis. Calcitonin facilitates the deposition of calcium to bone, lowering the serum levels, whereas the parathyroid hormone stimulates the release of calcium from bone, raising the serum calcium levels.
Osteoclasts are large multinucleated cells typically found at sites where there is active bone growth, repair or remodelling, such as around the periosteum, within the endosteum and in the removal of calluses formed during fracture healing Waugh and Grant, The osteoclast cell membrane has numerous folds that face the surface of the bone and osteoclasts break down bone tissue by secreting lysosomal enzymes and acids into the space between the ruffled membrane Robson and Syndercombe Court, These enzymes dissolve the minerals and some of the bone matrix.
The minerals are released from the bone matrix into the extracellular space and the rest of the matrix is phagocytosed and metabolised in the cytoplasm of the osteoclasts Bartl and Bartl, Once the area of bone has been resorbed, the osteoclasts move on, while the osteoblasts move in to rebuild the bone matrix.
Osteoblasts synthesise collagen fibres and other organic components that make up the bone matrix. They also secrete alkaline phosphatase, which initiates calcification through the deposit of calcium and other minerals around the matrix Robson and Syndercombe Court, As the osteoblasts deposit new bone tissue around themselves, they become trapped in pockets of bone called lacunae.
Once this happens, the cells differentiate into osteocytes, which are mature bone cells that no longer secrete bone matrix. The remodelling process is achieved through the balanced activity of osteoclasts and osteoblasts. If bone is built without the appropriate balance of osteocytes, it results in abnormally thick bone or bony spurs. Conversely, too much tissue loss or calcium depletion can lead to fragile bone that is more susceptible to fracture.
Typical features on X-ray include focal patches of lysis or sclerosis, cortical thickening, disorganised trabeculae and trabecular thickening. As the body ages, bone may lose some of its strength and elasticity, making it more susceptible to fracture.
This is due to the loss of mineral in the matrix and a reduction in the flexibility of the collagen. Adequate intake of vitamins and minerals is essential for optimum bone formation and ongoing bone health. Two of the most important are calcium and vitamin D, but many others are needed to keep bones strong and healthy Box 2. Key nutritional requirements for bone health include minerals such as calcium and phosphorus, as well as smaller qualities of fluoride, manganese, and iron Robson and Syndercombe Court, Calcium, phosphorus and vitamin D are essential for effective bone mineralisation.
Vitamin D promotes calcium absorption in the intestines, and deficiency in calcium or vitamin D can predispose an individual to ineffective mineralisation and increased risk of developing conditions such as osteoporosis and osteomalacia. Other key vitamins for healthy bones include vitamin A for osteoblast function and vitamin C for collagen synthesis Waugh and Grant, Physical exercise, in particular weight-bearing exercise, is important in maintaining or increasing bone mineral density and the overall quality and strength of the bone.
This is because osteoblasts are stimulated by load-bearing exercise and so bones subjected to mechanical stresses undergo a higher rate of bone remodelling. Reduced skeletal loading is associated with an increased risk of developing osteoporosis Robson and Syndercombe Court, Bone is a dynamic structure, which is continually remodelled in response to stresses placed on the body.
Changes to this remodelling process, or inadequate intake of nutrients, can result in changes to bone structure that may predispose the body to increased risk of fracture. Part 2 of this series will review the structure and function of the skeletal system.
Tagged with: Newly qualified nurses: systems of life. Sign in or Register a new account to join the discussion. You are here: Orthopaedics. Skeletal system 1: the anatomy and physiology of bones. Abstract The skeletal system is formed of bones and cartilage, which are connected by ligaments to form a framework for the remainder of the body tissues. This article has been double-blind peer reviewed Scroll down to read the article or download a print-friendly PDF here if the PDF fails to fully download please try again using a different browser Read part 2 of this series here.
Box 1. Types of bones Long bones — typically longer than they are wide such as humerus, radius, tibia, femur , they comprise a diaphysis shaft and epiphyses at the distal and proximal ends, joining at the metaphysis. Most long bones are located in the appendicular skeleton and function as levers to produce movement Short bones — small and roughly cube-shaped, these contain mainly cancellous bone, with a thin outer layer of cortical bone such as the bones in the hands and tarsal bones in the feet Flat bones — thin and usually slightly curved, typically containing a thin layer of cancellous bone surrounded by cortical bone examples include the skull, ribs and scapula.
Most are located in the axial skeleton and offer protection to underlying structures Irregular bones — bones that do not fit in other categories because they have a range of different characteristics. They are formed of cancellous bone, with an outer layer of cortical bone for example, the vertebrae and the pelvis Sesamoid bones — round or oval bones such as the patella , which develop in tendons.
Box 2. Vitamins and minerals needed for bone health Key nutritional requirements for bone health include minerals such as calcium and phosphorus, as well as smaller qualities of fluoride, manganese, and iron Robson and Syndercombe Court, Danning CL Structure and function of the musculoskeletal system.
St Louis, MO: Elsevier. London: Elsevier. Iyer KM Anatomy of bone, fracture, and fracture healing. London: Springer. Moini J Bone tissues and the skeletal system.
In: Anatomy and Physiology for Health Professionals. Intramembranous ossification involves the replacement of sheet-like connective tissue membranes with bony tissue. Bones formed in this manner are called intramembranous bones.
They include certain flat bones of the skull and some of the irregular bones. The future bones are first formed as connective tissue membranes. Osteoblasts migrate to the membranes and deposit bony matrix around themselves. When the osteoblasts are surrounded by matrix they are called osteocytes. Endochondral ossification involves the replacement of hyaline cartilage with bony tissue.
Most of the bones of the skeleton are formed in this manner. These bones are called endochondral bones. In this process, the future bones are first formed as hyaline cartilage models. During the third month after conception, the perichondrium that surrounds the hyaline cartilage "models" becomes infiltrated with blood vessels and osteoblasts and changes into a periosteum. The osteoblasts form a collar of compact bone around the diaphysis. At the same time, the cartilage in the center of the diaphysis begins to disintegrate.
Osteoblasts penetrate the disintegrating cartilage and replace it with spongy bone.
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