Bone Tissue and the Skeletal System

Chapter 1 Objectives

By the end of this chapter, you’ll be able to:

• Identify the different functions of bones and explain their roles in the body.
• Recognize the various classifications of bones and their unique characteristics.
• Understand the process of bone formation and how bones develop over time.
• Explain how bones heal and repair themselves following a fracture.
• Explore the impact of exercise, nutrition, and hormones on bone health.
• Discuss how calcium imbalances can influence bone structure and function.

Bones have the potential to become great fossils. While the soft tissues of an organism naturally break down and disappear over time, bone tissue, under the right conditions, undergoes mineralization, essentially transforming into stone. A well-preserved fossilized skeleton can reveal valuable information about an organism’s size and structure, much like how your own skeleton determines your physical shape. However, unlike fossils, your skeleton is made of living tissue that continuously grows, repairs, and regenerates. Bones are not just structural components—they are dynamic, complex organs that play a vital role in maintaining homeostasis and supporting overall body function.

Section 1 - The Functions of the Skeletal System

Section 1 Learning Objectives

By the end of this section, you will be able to:

• Define bone, cartilage, and the skeletal system.
• Identify and explain the functions of the skeletal system.

Introduction to the Skeletal System

Bone, or osseous tissue, is a dense and rigid type of connective tissue that forms the majority of the adult skeletal system, which serves as the body’s structural framework. In areas of movement, such as joints and the ribcage, cartilage—a semi-rigid form of connective tissue—provides flexibility and a smooth surface for motion (Tortora & Derrickson, 2021). The skeletal system is composed of both bones and cartilage and plays a crucial role in maintaining the body's structure, function, and homeostasis. Some of the key functions of the skeletal system include:

• Providing structural support – Bones form a rigid framework that supports the body's overall shape and structure.
• Facilitating movement – Bones act as attachment sites for muscles, and their interaction enables motion.
• Protecting vital organs – The rib cage shields the heart and lungs, while the skull safeguards the brain (Drake et al., 2020).
• Producing blood cells – The bone marrow generates red and white blood cells, essential for oxygen transport and immune defense.
• Storing and regulating minerals and fat – Bones serve as a reservoir for calcium and phosphate, which are critical for metabolic functions, and they store energy in the form of bone marrow fat.

Support, Movement, and Protection

The skeletal system’s most noticeable functions—support, movement, and protection—are easily observed in daily life. Similar to how steel beams provide stability in a building, bones and cartilage form the body's supportive scaffold (Standring, 2021). Without this framework, the body would lack structure and collapse into a mass of soft tissues.

Bones also play a key role in movement by serving as attachment sites for muscles. While some bones act purely as structural supports, others transmit forces generated during muscle contractions. From a biomechanical perspective, bones function as levers, while joints serve as fulcrums that facilitate motion (Betts et al., 2022). Movement occurs only when a muscle spanning a joint contracts, exerting force on the bone to produce motion. Understanding the interaction between the skeletal and muscular systems—known collectively as the musculoskeletal system—is essential for comprehending body mechanics and functional movement.

Bones play a crucial role in shielding vital organs from potential injury by either enclosing or surrounding them. For instance, the rib cage provides a protective barrier for the lungs and heart, while the vertebral column safeguards the spinal cord. Similarly, the skull forms a protective shell around the brain, ensuring it remains secure from external impact.

Figure 6.3 Bones Protect brain the cranium completely surrounds and protects the brain from non-traumatic injury.

Orthopedics and the Musculoskeletal System

An orthopedist is a medical specialist who focuses on diagnosing and treating conditions related to the musculoskeletal system. Treatment options for orthopedic conditions can range from medications, physical therapy, and assistive devices to surgical interventions, depending on the severity and type of injury or disorder (Betts et al., 2022). The term "orthopedics" originates from the Greek words ortho- (meaning "straight") and paed- (meaning "child"), referring to the discipline's early focus on correcting skeletal deformities in children. However, modern orthopedists treat patients of all ages, from infants to the elderly (Tortora & Derrickson, 2021). In fact, advances in medical technology now allow for prenatal surgeries to correct congenital conditions like spina bifida, a neural tube defect where the spinal canal fails to close completely during fetal development.

Common Conditions Treated by Orthopedists

Orthopedists specialize in bone and joint injuries but also address various musculoskeletal disorders, including spinal deformities. Spinal abnormalities can present in different ways, such as:

• Scoliosis – A lateral curvature of the spine that, in severe cases, can cause one shoulder blade to protrude as a visible hump (Drake et al., 2020).
• Kyphosis – An excessive dorsoventral curvature, commonly referred to as a hunchback, which can compress the thoracic cavity and impair posture.

These spinal curvatures frequently develop in pre-adolescents due to factors like poor posture, abnormal growth, or unknown causes. Fortunately, many cases can be successfully treated with physical therapy, bracing, or surgical intervention when necessary.

Orthopedics and Aging

As people age, the accumulated effects of spinal injuries and degenerative diseases like osteoporosis can result in postural changes and curvature of the spine, often leading to the characteristic stooped posture seen in elderly individuals. Osteoporosis, in particular, weakens bones, increasing the risk of vertebral fractures that contribute to spinal deformities over time (Ross & Pawlina, 2020).

Sports Medicine and Orthopedic Treatments

Some orthopedic specialists focus on sports medicine, addressing both minor injuries, such as sprained ankles, and complex injuries, such as rotator cuff tears in the shoulder. Treatment approaches vary from rehabilitation exercises to surgical procedures, depending on the severity and functional impact of the injury (Tortora & Derrickson, 2021).

Figure 6.4 Complex Brace An orthopedist will sometimes prescribe the use of a brace that reinforces the underlying bone structure it is being used to support. (credit: Becky Stern/Flickr)

Mineral Storage, Energy Storage, and Hematopoiesis

Beyond providing structural support, bone tissue plays a crucial role in metabolic functions essential for overall health. One of its primary roles is acting as a reservoir for essential minerals, particularly calcium and phosphorus, which are necessary for numerous physiological processes. These minerals are incorporated into the bone matrix and can be released into the bloodstream as needed to maintain homeostasis. Calcium ions (Ca²⁺), for example, are critical for muscle contractions and regulating the transmission of nerve impulses, ensuring proper neuromuscular function (Hall, 2021).

Energy Storage and Blood Cell Production

In addition to mineral storage, bones also function as storage sites for fat and are involved in blood cell production. The soft connective tissue inside bones, known as bone marrow, plays a vital role in these processes . There are two distinct types of bone marrow:

• Yellow marrow – Primarily composed of adipose tissue, which stores triglycerides that serve as an energy reserve.
• Red marrow – Responsible for hematopoiesis, the process of blood cell formation, which generates red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes) (Drake et al., 2020).

Red marrow plays a critical role in oxygen transport, immune defense, and blood clotting, ensuring the body's ability to function optimally. This balance between mineral storage, fat storage, and hematopoiesis highlights the dynamic nature of bone tissue, making it an essential organ system beyond its structural role.

Figure 6.5 Head of Femur Showing Red and Yellow Marrow The head of the femur contains both yellow and red marrow. Yellow marrow stores fat. Red marrow is responsible for hematopoiesis. (credit: modification of work by “stevenfruitsmaak”/Wikimedia Commons)