The Biological and Biochemical Foundations of Living Systems section of the MCAT includes the skeletal system. As part of this, the syllabus identifies the following areas which you need to know for the exam:
The following guide will provide an overview of these key topic areas to support you to prepare well for the exam and achieve a good MCAT score.
The skeleton has three major functions:
The skeleton provides a rigid structure inside the body that supports the body, as well as provides an area for muscles to attach to, enabling locomotion.
Skeletal bones are made of calcium that give strength and rigidity to the skeleton, allowing them to be able to protect vulnerable internal organs that would otherwise get severely damaged from day to day activities. For example, the spine (vertebral column) protects your spinal cord which would otherwise be susceptible to damage when doing things like sitting in chairs or even lying down in bed.
As mentioned above, calcium is an important mineral that gives strength to the skeleton, but bones are also used as a storage space for minerals such as phosphorus and calcium. When there are fluctuations in the mineral balance of the bloodstream, minerals are either taken from the bones or stored in the bones to bring them back to a steady state.
The MCAT syllabus further breaks down the skeletal structure in to:
There are four main types of bone:
Long bones are cylindrical and have a length greater than their width, for example, the femur.
Short bones are equal in length, width and thickness making a cube-like shape. The only short bones in humans are the carpals and tarsals of the wrist and ankle respectively.
Flat bones are thin and usually curved like ribs and irregular bones are bones that have a structure that does not fit into any other category – often having a complex shape like the vertebrae.
A joint is an area where two bones make contact with each other. Joints can either be classified based on the connective tissue that is most dominant in that particular joint or by the level of movement that joint can make.
Fibrous joints are those where the predominant connective tissue is collagen. They are usually immovable and have no joint cavity. An example of a fibrous joint would be sutures – immovable joints in the cranium. Cartilaginous joints are characterized by the dominant presence of hyaline cartilage/fibrocartilage with these joints usually having a low range of mobility. Finally, synovial joints are characterized by the presence of a joint cavity that is filled with synovial fluid. The cavity is surrounded by hyaline cartilage and altogether allows the joint to be mobile.
The three joint types that are differentiated based on movability are: synarthrosis (immovable), amphiarthrosis (slightly moveable) and diarthrosis (freely moveable).
The endoskeleton is described as a hard, mineralised skeletal structure that is located within the soft tissue of organisms. The purpose of an endoskeleton is to protect internal organs, allow for locomotion and provide support.
An exoskeleton is a skeletal structure that lies outside the soft tissue of organisms. It provides a ‘shell’ which can be used to protect and support the organism; it is a very common feature in insects.
There are two types of bone tissue – cortical bone, which forms a rigid exterior, and cancellous bone, which forms a spongy interior.
Osteoblasts are a type of cell that is derived from stem cells and are found in cortical bone tissue. They aid in the creation of bone tissue whereas osteocytes (also found in the cortical bone) are involved in the reabsorption of bone tissue and make up 90-95% of the total bone cells. Osteoclasts are found within the osteons of the cortical bone and their function is to degrade bone tissue ready for bone remodeling or the release of calcium into the bloodstream. An osteon is a cylinder that has rings of organic matrix running through it as well as a singular nerve and blood vessel.
The matrix is composed of a protein structure with calcium phosphate crystals captured inside it. Calcium phosphate is the major constituent of mammalian bones and type I collagen is the most abundant substance in the protein part of the matrix.
Cartilage is a flexible connective tissue with an overarching function of preventing damage to your bones and joints. This group of tissue is made up of chondrocytes – the primary cell types used to create the collagen extracellular matrix. The cartilage is separated from other tissues in the body by the perichondrium – a two layered membrane that provides support, protection, and an attachment point for cartilage to join to other structures. Cartilage is also not supplied by any blood vessels and obtains nutrients via diffusion instead.
There are three different types of cartilage. The most common is hyaline cartilage which is characterized by its relatively simple structure and the high proportion of collagen. This type of cartilage is used as a temporary skeleton during embryo development. Fibrocartilage is another type, known for the large number of collagen fibers it holds – usually found in dense ligaments and tendons. Finally, elastic cartilage is known for its elastic network alongside the collagen fibers, allowing it to withstand repeated stretching.
Tendons and ligaments are described as dense connective tissue that provides connections between the muscular and skeletal systems. Both of these tissues are predominantly made up of collagen I and both have the presence of proteoglycans.
However, these two tissues differ in function. Tendons attach muscle to bone – this allows for movement of the body during muscle contraction, generating locomotion. Ligaments connect bone to bone in order to stabilize and maintain the movement of joints as well as provide a cushion between bones to stop the friction of movement in a joint from causing damage.
Endocrine control is used in the skeletal system to regulate the concentration of calcium in the blood.
When blood calcium levels drop, the parathyroid gland releases a parathyroid hormone (PTH), which causes the kidney to begin to reabsorb calcium as well as release vitamin D3. Vitamin D3 stimulates further calcium reabsorption in the gut. PTH also stimulates osteoclasts to trigger the breakdown of the bone, so that calcium is released into the bloodstream.
When blood calcium levels increase, PTH secretion is lowered, halting the actions described in the previous paragraph. Calcitonin is also released in response to increased calcium which causes osteoblasts to deposit excess serum calcium into the bones in a process called calcification. All of these actions are in aid of returning to a steady concentration of calcium – maintaining homeostasis.
Hopefully this has given you a better understanding of the skeletal system for the MCAT. For more support preparing for the exam, we have blogs covering a wide range of MCAT topics, from the skin system to the classification and structure of prokaryotic cells, and many more. And our MCAT Guide and Checklist will provide you with everything you need to know about the exam, including key dates, the exam format and how to prepare.