The Skeletal System

Overview and Functions

The skeletal system provides the structural framework for the human body and serves several essential functions necessary for survival and movement.

• Protection: The skeleton protects vital organs, such as the brain (protected by the skull) and the heart and lungs (protected by the rib cage).

• Mineral Storage: Bones act as reservoirs for minerals, especially calcium (Ca2+) and phosphate (PO43−), which are necessary for various physiological processes.

• Blood Cell Formation: Hematopoiesis occurs in the red bone marrow, producing red blood cells, white blood cells, and platelets.

• Fat Storage: Yellow bone marrow stores fat, serving as an energy reserve.

• Movement: Bones serve as levers for muscles, enabling movement.

• Support: The skeleton provides support for the body, maintaining posture and bearing weight.

Bone Development and Ossification

Introduction to Ossification

Ossification is the process by which bone tissue forms. Most bones begin ossifying during the embryonic period (first 12 weeks in utero), and the process is typically completed by age 7.

Types of Ossification

• Intramembranous Ossification:

  • Bone develops from a membrane of connective tissue.

  • Forms flat bones such as the skull and clavicles.

  • Occurs primarily during fetal development.

• Endochondral Ossification:

  • Bone develops from a hyaline cartilage model.

  • Forms most bones of the body, including long bones.

  • Begins during fetal development and is mostly completed by age 7.

Endochondral Ossification

Building a Hyaline Cartilage Model

Endochondral ossification begins with the formation of a hyaline cartilage model, which consists of chondrocytes (cartilage cells) and extracellular matrix (ECM), surrounded by connective tissue and chondroblasts.

Steps in the Ossification of a Long Bone

1. Formation of Bone Collar: Osteoblasts differentiate from the perichondrium (which becomes the periosteum) and form a bone collar around the diaphysis (shaft) of the cartilage model.

2. Primary Ossification Center: Forms at the center of the diaphysis. Chondrocytes enlarge and the surrounding matrix calcifies, then chondrocytes die, creating cavities.

3. Invasion of Periosteal Bud: Blood vessels, nerves, osteoblasts, and osteoclasts invade the cavities, forming spongy bone.

4. Formation of Medullary Cavity: Osteoclasts break down some of the newly formed spongy bone, creating the medullary cavity. The diaphysis elongates as ossification continues.

5. Secondary Ossification Centers: Develop in the epiphyses (ends of the bone) after birth. The process is similar to that in the diaphysis, except spongy bone is retained and no medullary cavity forms in the epiphyses.

Bone Growth

Growth in Length (Longitudinal Growth)

Bones lengthen by the proliferation of chondrocytes in the epiphyseal plate (growth plate), a region of hyaline cartilage located between the diaphysis and epiphysis.

• Growth occurs at the epiphyseal plate, which is composed of five zones of cells (from resting to proliferating, hypertrophic, calcification, and ossification zones).

• Osteoblasts eventually replace cartilage with bone, leading to the closure of the epiphyseal plate and cessation of growth in length.

Growth in Width (Appositional Growth)

Bones grow in width by appositional growth, which involves the addition of new bone tissue by osteoblasts beneath the periosteum and the removal of bone by osteoclasts on the endosteal surface.

• Osteoblasts build new bone on the outer surface.

• Osteoclasts resorb bone on the inner surface, enlarging the medullary cavity.

• This process can continue after longitudinal growth has ceased.

Hormonal Regulation of Bone Growth

Growth Hormone

• Secreted by the anterior pituitary gland.

• Stimulates growth of most tissues, including bone, by increasing chondrocyte division in the epiphyseal plate and osteoblast activity.

Sex Hormones (Testosterone and Estrogen)

• Stimulate cell division in the epiphyseal plate, promoting longitudinal growth.

• Promote appositional growth (testosterone more than estrogen).

• Induce closure of the epiphyseal plate (estrogen acts faster than testosterone).

• Both hormones promote osteoblast activity, increasing bone formation.

Bone Remodeling

Overview of Bone Remodeling

Bone remodeling is the continuous process of bone formation (deposition) and bone resorption, allowing bones to adapt to stress and maintain calcium homeostasis.

• Bone Deposition: Osteoblasts secrete organic matrix and assist in mineralization. Once surrounded by matrix, they become osteocytes.

• Bone Resorption: Osteoclasts secrete hydrogen ions and enzymes that dissolve the bone matrix, releasing minerals into the blood.

Factors Affecting Bone Remodeling

• Decline in growth hormone and sex hormones with age reduces bone formation.

• Mechanical stress (tension, compression, pressure) stimulates bone remodeling.

• Nutrition (adequate vitamin D, magnesium, protein, vitamins C & K) is essential for bone health.

Calcium Homeostasis and Hormonal Control

Regulation of Blood Calcium Levels

Calcium homeostasis is tightly regulated by hormones to ensure proper physiological function.

• Parathyroid Hormone (PTH): Released in response to low blood calcium; stimulates osteoclast activity, increasing bone resorption and raising blood calcium levels.

• Calcitonin: Released in response to high blood calcium; inhibits osteoclast activity, promoting bone deposition and lowering blood calcium levels.

Summary of Calcium Homeostasis

• Low blood Ca2+ triggers PTH release, increasing bone resorption and blood Ca2+.

• High blood Ca2+ triggers calcitonin release, increasing bone deposition and lowering blood Ca2+.

• Vitamin D is necessary for intestinal absorption of Ca2+.

Additional info:

• Bone remodeling is essential for repair of micro-damage, adaptation to mechanical load, and regulation of mineral homeostasis.

• Disorders of bone growth and remodeling include osteoporosis (increased bone resorption) and osteopetrosis (decreased bone resorption).