Imtramembranous ossification starts during the development within the womb and where the flat bones and some bone of the skull and the collarbone are formed from connective tissue. This is also the method which helps injured bones to heal and develops when bones are broken to reconstruct the bone. This process also occurs during embryonic development which is endochondrial ossification, this is where bones are produced from cartilage. Endochodrial ossification is the process when the development of long bones occurs such as the development of arms and legs. Within intramembranous ossification there is no cartilage present as the bones develop from other connective tissues. The begin of intramembranous ossification starts from stem cells, these cells then have to develop into a particular cell type, such as osteoblasts which are cells that create bone tissue, spongy inner tissue of the marrow and the mineralised compact bone tissue which encloses the marrow. There are many different stages which take place throughout intramembranous ossification due to the stem cells change, differentiate and develop into specific types of bone cells. A related process takes place to repair broken or damaged bone as the bone tissue develops from membranous or connective tissue in a similar way that they develop within the womb. In the process of stem cells form layers of relatively primitive connective tissue and some develop into bone-producing cells. During this there is a bone matrix of calcium which eventually gathers into bone spicules. When bone matrix is expelled the bone spicules grow in size, which leads to the greater formation called a trabeculae which bulild up on top on each other to form a wovenbone where as some trabeculae stay as spongy tissues which then develop into bonemarrow. The bones which progress through the intramembranous ossification develop into the patella, the parietal, frontal, occipitial and temporal bones in the skull, the patella and the upper and low jawbones. Inramembranous ossification also adds to bone growth particularly in short bones and the thickening of long bones as they grow. The process which is responsible for the lengthening of the bones is called epiphysis and occurs at the ends of the bones, the epiphyseal place is found under the epiphyses and before the diaphysis. During puberty and early adulthood this is usually present and is the location of bone growth. When this process has fully taken place the the epiphyseal plate completely ossifies and closes after this there is a thin line still
only a thin line is still there and the bones can no longer grow in length.
Endorchondral ossification is where cartilage eventually gets replaced with bone during the development. During this process the osteoblasts occur in regions of cartilage called the ossification centres. These osteoblasts then progress into osteocytes which are mature bone cells which harden the bone matrix. The majority of bones occur from a mixture of intramembranous and endochodral ossification. During this process the mesencymal cells progress into chondroblasts which increase in number through cell division which then increase in size and expel a matrix which hardens because of the occurrence of inorganic minerals, which leads to the forming of chambers within the matrix. Osteoblasts and blood-forming cells go through these chambers. These osteoblasts expel minerals which form the bone matrix. Mature hardened bone consists of an organic element and a mineral element, mature hardened bone is also living tissue. The majority of the organic element is made up of proteins such as, collagen fibres, an extracelluar matrix, and fibroblasts, which consist of the living cells which create the collagen and the matrix. The mineral part of the bone is made up of hydroxyapatite and calcium carbonate which provides strength and rigidity. Through the life of an individual the osteoblasts constantly expel minerals and the osteoclasts constantly reabsorb the minerals.
The skeletal system starts to develop during the third week after conception with the development of the notochord with is the construction of the back of the embryo which develops into the spine, spinal cord and the brain. During the four week of conception arms and legs are visable, with the fingers and toes developing around the eighth week of conception, after this the limbs start to expand and take on specific shape. By the fifth week the embryo has doubled in size and there is a tail-like structure which eventually develops itno the coccyx. During the seventh week the embryos facial features are visible and the 206 bones are all in place. Although the process of osteogenesis has not yet taken place fully as the bones are do not harden until further development. The majority of bony nuclei of the long bones and round bones do not finish developing until after birth.
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There are many different types of the main human bones which are the long, short, flat and irregular bones. The long, short and irregular bones develop due to endochondral ossification this is the process were cartilage is replaced by bone. The development of flat bones occur during intramembranous ossificaion with the bone developong by sheets of connective tissue, around 80 percent of the mature skeleton is represented by compact cortical bone which supports the body, at the midpoint of the long bones there is extra thickness to stop the bones from bending. Cancellous bone has a porous structure which has small cavities and is similar to sponge; it is found mainly in the pelvis and within the 33 vertebrae from the neck to the tailbone. Long bones increase in length by adding to the epiphyseal plate. When the bones lengthen they go through a process called remodelling, throughout this process the outer shape of the bones change as well. The bones of the skull grow due to the layers which add to the circumference of the skull by this occuring the skll becomes thicker. During birth long bones have more than one ossification centre, which grow during childhood up until the epiphyseal plates become fused with he shaft of the diaphysis. tHis process is motivated due to the hormones in the testes and ovaries, there hormones signal the linear growth of the long bones to reach the end of the growth. Flat and round bones of the skeleton are able to continue growth throughout life.
The vertebral column is one of the primary support structures of the human skeleton, and consists of individual pseudoseparate and fused vertebrae which allow the movement and support of the skull, and the flexibility of the neck and back and provides secure locations for the ribs and protection of the spinal cord. The spine consists of seven cervial vertebrae which form the neck, tweleve toracic vertebrae forming the upper back and five lumbar vertebrae forming the lower back. Near the bottom of the spine there is the sacrum which is made up of five fused vertebrae which secures the spine to the pelvic girdle and the coccyx which is a semi-flexible sequence of around four vertebrae which assist in the support of the the lower alimentary tract. Inbetween each vertebrae there is a intervertabral disk which is made up of cartilage and cushions the vertabral column to protect the spine from trauma, the interverebral disks also act as a shock absorber. The spine is held together by a sequence of ligaments which include the intertransverse ligaments which run down the length of the spine and are attached to the transverse processes of each vertebrae. The spinal cord is a primary nerve pathway to and from the brain which proceed down a canal in the centre of the spinal column.
The spine consists of four natural curves. Two are lordotic and two are kyphotic. The cervical and lumbar curves are lordotic. The thoracic and sacral curves are kyphotic. The curves help to distribute mechanical stress as the body moves.
The cervial spine consists of seven vertebrae which are located at the top of the spine and is often referred to of the neck. One of the purposes of the cervial spine is to contain and protect the spinal cord, support the skull and to allow the head movement from side to side and to bend forward and backwards. There are many different ligaments, muscles and tendons which assist in supporting and stabilising the cervical spine. The main role of the ligaments is to stop excessive movement which could lead to damaging the cervial spine and spinal cord. The muscles in the cervical spine help the spinal balance and to provide movement, the muscles contract and relax due to the nerve impulses which are signaled from the brain. Some muscles work in pairs and some contract and the opposingmuscle relaxes, there are many different types of muscle which are lateral flexors, forward flexors, rotators and extensors.
The thoracic vertebrae form the posterior part of the bony thoracic cage. The thoracic spine consists of 12 vertebrae which eventually increase in size. The Thoracic spine is found in the chest area, and the ribs are connected to the thoracic spine and protect a number of vital organs.
The lumbar spine consists of five vertebrae which are positioned below the thoracic vertebrae and the fused vertebrae of the sacrum. These vertebraeâ€™s are a lot larger than the vertebrae of the cervical and thoracic vertebrae. The vertebral foramen is normally triangular with the spinous process pointing backwards and is rectangular or hatchet-shaped. Each vertebra are positioned on top of each other and between each one there is a intervertabral disk which is made up of cartilage and cushions the vertabral column to protect the spine from pressure, they distribute stress and prevent the vertebrae from grinding against each other.
Ligaments and Tendons
Groups of ligaments hold the vertebrae and discs together and they connect bone to bone. The main function of tendons is to hold muscle to bone. Within the spine the tendons connect muscles to the vertebrae they also help to stabilise the spine and protect them from extreme movement in any one direction.
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