a) What is meant by the terms dermal (intramembranous) and endochondral with respect to bone? Give an example of each.
b) With the aid of simple diagrams, describe the parts of a young (growing) bone, as typified by the femur.
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a) To understand the terms dermal (intramembranous) and endochondral bones, let's break them down:
- Dermal (Intramembranous) Bones: These types of bones form from mesenchymal cells directly without any cartilage precursor. They are also known as intramembranous bones because they develop within a connective tissue membrane.
An example of dermal (intramembranous) bone is the flat bones of the skull, such as the parietal bone. The mesenchymal cells in the connective tissue differentiate into osteoblasts, which then secrete bone matrix directly. This process results in the formation of flat bones with a dense outer layer and a spongy inner layer.
- Endochondral Bones: These types of bones form from a cartilage precursor. They undergo a process called endochondral ossification, where the cartilage is gradually replaced by bone.
An example of endochondral bone is the long bones of the body, such as the femur. During development, a cartilage model forms first, which is later replaced by bone tissue. The process begins with the formation of a cartilage template, followed by the invasion of blood vessels and the conversion of cartilage into bone tissue.
b) Understanding the parts of a young (growing) bone, let's focus on the femur, which is a long bone:
Below is a simple diagram outlining the parts of a young bone:
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| | growth plate or
| d | epiphyseal plate
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__________________| |__________________
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- Epiphysis: The ends of the long bone are called epiphyses. In this diagram, they are represented by the upper and lower portions of the bone. The epiphysis contains spongy bone tissue and serves as a site for joint connections.
- Diaphysis: The middle portion of the bone is called the diaphysis. It represents the shaft of the bone and contains compact bone tissue.
- Epiphyseal Plate (Growth Plate): The region between the epiphysis and the diaphysis is known as the epiphyseal plate or growth plate. It is represented by the dashed line in the diagram. The epiphyseal plate plays a crucial role in bone growth as it allows the bone to lengthen longitudinally during childhood and adolescence.
- Periosteum: The bone is covered with a tough, outer protective layer called the periosteum. It contains blood vessels, nerves, and cells involved in bone repair and growth.
- Medullary Cavity: Inside the diaphysis, there is a hollow space known as the medullary cavity. It is filled with bone marrow, which is responsible for producing blood cells.
It's important to note that this is a simplified diagram to help illustrate the main parts of a young bone. In reality, bones are more complex structures with additional components and variations.