Introduction

How important are your bones and your skeletal system?

Try to imagine what you would look like without them. You would be a soft, wobbly pile of skin, muscles, and internal organs, so you might look something like a very large slug. Not that you would be able to see yourself—folds of skin would droop down over your eyes and block your vision because of your lack of skull bones. You could push the skin out of the way if you could only move your arms, but you need bones for that as well!

Humans are vertebrates, which are animals that have a vertebral column or backbone. Invertebrate animals, like the banana slug below, do not have a vertebral column, and they use a different mechanism than vertebrates to move about. The sturdy internal framework of bones and cartilage found inside vertebrates, including humans, is called an endoskeleton. The adult human skeleton consists of approximately 206 bones in adults. An endoskeleton develops within the body rather than outside like the exoskeleton of insects.

Are bones living?

It’s common to think of bones as not living. But bones are very much living. In fact, you are constantly making new bone tissue. That means that you are also constantly getting rid of bone. Bone is full of blood, nerves, and all sorts of cells and proteins, making it an extremely complex living tissue.

Many people think of bones as dry, dead, and brittle, which is what you might think if you saw a preserved skeleton in a museum. The association of bones with death is a common association because the calcium-rich bone tissue of a vertebrate is the last thing to decompose after the organism dies. However, the bones in your body are very much alive. They contain many tough protein fibers, are crisscrossed by blood vessels, and certain parts of your bones are metabolically active. Preserved laboratory skeletons are cleaned with chemicals that remove all organic matter from the bones, which leaves only the calcium-rich, mineralized (hardened) bone tissue behind.

Skeletal Structures and Functions

Bones and Their Articulations

It’s common to think of the skeletal system as being made up of only bones, but the skeletal system contains many types of structures. In addition to localizing blood cell formation and storing calcium, bones come together at locations called articulations (or joints) to allow for locomotion and work. In any complex system that moves (such as a bicycle or car), allowing functional, repetitive motion requires a lot of control and support.

Connective Tissues

In addition to bones, the skeletal system contains other important connective tissues including cartilage and ligaments. Tendons, technically a structure of the muscular system, are also important connective tissue for stabilizing joints and supporting structures.

Cartilage is a firm yet pliable substance that performs various functions: protection, shape maintenance and support, lubrication, and shock absorption. Its primary function is to coat the end of the bones where they articulate with one another, providing a smooth, cushioned surface. Furthermore, cartilage can serve as a template for bone formation during development and bone healing (this will be further discussed in the section about bone ossification).

Ligaments and tendons support articulations and control the muscle attachment to the bones. Ligaments connect bones and stabilize articulations. Tendons connect bones to muscles.

The structures of ligaments and tendons are similar in that both tissues are made of fibrous proteins aligned in the direction of the force experienced. The types of proteins and differences in stretch and recoil distinguish the mechanical behavior of ligaments and tendons. Ligaments are stiffer but deform more since they stabilize articulations. Tendons are wrapped with a continuation of the connective tissue (fascia) surrounding muscle cells to help transmit and dissipate force.

Axial Skeleton Anatomy

The adult human skeletal system is composed of 206 bones. The skeleton is divided into two functional groupings—the axial and the appendicular skeleton.

The axial skeleton comprises the skull, hyoid bone, vertebral column, and thorax (ribs and sternum). The axial skeleton protects and supports the head, neck, and trunk organs.

• The skull consists of 22 facial and cranial bones that interlock to form openings for the eyes and provide protection for the brain. The cranium, a collection of bones that protect the brain, and the mandible are the two major parts of the skull.
• The hyoid bone is not attached to any other bone, is located in the neck, and supports tongue movement.
• The vertebral column consists of individual vertebrae separated by cartilaginous disks. The vertebral column forms the middle axis of the skeleton.
• The thoracic cage protects the internal organs of the thorax and upper abdomen. The thoracic cage consists of the ribs and the sternum. The ribs articulate anteriorly with the sternum and posteriorly with the vertebrae of the thorax.

The table below lists the location and function of the major bones of the axial skeleton:

Appendicular Skeleton Anatomy

The appendicular skeleton is composed of the upper limbs, lower limbs, pectoral girdle, and pelvic girdle. The appendicular skeleton functions to anchor the limbs to the axial skeleton.

The pectoral girdle consists of a scapula and clavicle on each side of the body. The pectoral (shoulder) girdle permits movement of the upper limbs by connecting the upper limbs to the axial skeleton.

The upper limbs of the appendicular skeleton are composed of the humerus, the radius, and the ulna, which complement each other to form the forearm, and the wrist. The hand subdivides into smaller bones of the palm and fingers.

The pelvic girdle of the appendicular skeleton is composed of two coxal bones (fused ilium, ischium, and pubis bones), which attach to the vertebral column and the lower limbs.

The lower limbs each consist of the femur or thigh bone; the tibia or shinbone, the fibula, or calf bone; and the foot. The patella is the bone located at the point where the femur and tibia articulate with each other. The foot subdivides into smaller bones of the ankle, instep, and toes.

The table below lists the location and function of the major bones of the appendicular skeleton:

Bones – Classification by Microscopic Structure

The bones of your body are mineralized structures that make up a major portion of your skeleton. In the broad sense, a bone is composed of bone tissue, cartilage, ligaments, tendons, vasculature, and nervous tissue. Bone tissue is a collection of specialized cells (osteoblasts, osteoclasts, osteocytes), organic extracellular matrix proteins (collagen and proteoglycans), and inorganic salt crystals that work together to provide strength and flexibility.

Although all bones have a similar composition, their large-scale structures and functions differ. One way to classify bone tissue is based on its microscopic structure.

• Bone tissue with a tightly packed microstructure arranged into rings (osteons) is called compact bone (also called cortical or lamellar bone). Compact bone structure gives bones their stiffness. However, if bone structures were made only of compact bone, they would be very heavy and brittle.
• Bone tissue with a porous microstructure is spongy bone (also called cancellous or trabecular bone). Spongy bone consists of branching bone structures called trabeculae. Spongy bone helps to reduce the weight and brittleness of bone and is found at the ends of bone where forces are high. Spongy bone allows bones to bend a slight amount without cracking.

Classification of Bones by Shape

The bones of the human skeleton are classified by their shape: long bones, short bones, flat bones, sutural bones, sesamoid bones, and irregular bones.

Short bones, or cuboidal bones, are bones that are the same width and length, giving them a cube-like shape. For example, the bones of the wrist (carpals) and ankle (tarsals) are short bones.

Flat bones are thin and relatively broad bones that are found where extensive protection of organs is required or where broad surfaces of muscle attachment are required. Examples of flat bones are the sternum (breast bone), ribs, scapulae (shoulder blades), and the roof of the skull.

Irregular bones are bones with complex shapes. These bones may have short, flat, notched, or ridged surfaces. Examples of irregular bones are the vertebrae, hip bones, and several skull bones.

Sesamoid bones are small, flat bones and are shaped similarly to a sesame seed. The patellae are sesamoid bones. Sesamoid bones develop inside tendons and may be found near joints at the knees, hands, and feet.

Sutural bones are small, flat, irregularly shaped bones. They may be found between the flat bones of the skull. They vary in number, shape, size, and position.

Long bones are longer than they are wide and have a shaft and two ends. The diaphysis, or central shaft, contains bone marrow in a medullary cavity. The rounded ends called epiphyses are covered with articular cartilage and are filled with red bone marrow, which produces blood cells. Most of the limb bones are long bones—for example, the femur, tibia, ulna, and radius. Exceptions to this include the patella and the bones of the wrist and ankle.

All bones have surface markings and characteristics that make a specific bone unique. There are holes, depressions, smooth facets, lines, projections, and other markings. These usually represent passageways for vessels and nerves, points of articulation with other bones, or points of attachment for tendons and ligaments.

Joints and Skeletal Movement

The point at which two or more bones meet is called a joint or articulation. Joints are responsible for movement, such as the movement of limbs, and stability, such as the stability found in the bones of the skull. Joints can be classified based on their structure. The structural classification divides joints into fibrous, cartilaginous, and synovial joints depending on the material composing the joint and the presence or absence of a cavity in the joint. The bones of fibrous joints are held together by fibrous connective tissue. There is no cavity, or space, present between the bones, so most fibrous joints do not move at all or are only capable of minor movements. The joints between the bones in the skull and between the teeth and the bone of their sockets are examples of fibrous joints.

Cartilaginous joints are joints in which the bones are connected by cartilage. An example is found at the joints between vertebrae, the so-called “disks” of the backbone. Cartilaginous joints allow for very little movement. Synovial joints are the only joints that have a space between the adjoining bones. This space is referred to as the joint cavity and is filled with fluid. The fluid lubricates the joint, reducing friction between the bones and allowing for greater movement. The ends of the bones are covered with cartilage, and the entire joint is surrounded by a capsule. Synovial joints are capable of the greatest movement of the joint types. Knees, elbows, and shoulders are examples of synovial joints.

The wide range of movement allowed by synovial joints produces different types of movements. Angular movements are produced when the angle between the bones of a joint changes. Flexion, or bending, occurs when the angle between the bones decreases. Moving the forearm upward at the elbow is an example of flexion. Extension is the opposite of flexion in that the angle between the bones of a joint increase. Rotational movement is the movement of a bone as it rotates around its own longitudinal axis. Movement of the head as in saying “no” is an example of rotation.

Structure of Bones

Although bones vary greatly in size and shape, they all have certain structural similarities. Bones are organs. (Recall that organs are made up of two or more types of tissues.) The two main types of bone tissue are compact bone and spongy bone. Compact bone makes up the dense outer layer of bones. Spongy bone is lighter and less dense than compact bone and is found toward the center of the bone. Periosteum (from peri = around and osteo = bone) is the tough, shiny, white membrane that covers all surfaces of bones except the joint surfaces. Periosteum is composed of a layer of fibrous connective tissue and a layer of bone-forming cells. Bones consist of different types of tissue including compact bone, spongy bone, bone marrow, and periosteum. Compact bone makes up the dense outer layer of bones. Its functional unit is the osteon. Compact bone is very hard and strong. Spongy bone is found inside bones and is lighter and less dense than compact bone. This is because spongy bone is porous. Red bone marrow is a soft connective tissue that produces blood cells. It is found inside the pores of spongy bone. Periosteum is a tough fibrous membrane that covers and protects the outer surfaces of bones.

Compact Bone

Just below the periosteum is the hard layer of compact bone tissue. It is so-called due to its high density, and it accounts for about 80% of the total bone mass of an adult skeleton. Compact bone is extremely hard and is made up of many cylinder-shaped units called osteons. Osteons act like strong pillars within the bone to give the bone strength and allow it to bear the weight of the attached muscles and withstand the stresses of movement. Osteons are made up of rings of calcium salts and collagen fibers called bone matrix. Bone matrix is a mixture of calcium salts, such as calcium phosphate and calcium hydroxide, and collagen (a type of protein), which form hollow tubes that look similar to the rings on a tree. Each of these matrix tubes is a lamella, which means “thin plate” (plural: lamellae). The calcium salts form crystals that give bones great strength, but the crystals do not bend easily and tend to shatter if stressed. Collagen fibers are tough and flexible. All collagen fibers within a single lamella are lined up in the same direction, which gives each lamella great strength. Overall, the protein-calcium crystal combination in the matrix allows bones to bend and twist without breaking easily. The collagen fibers also act as a scaffold for the laying down of new calcium salts.

In the center of each osteon is a central canal. The canal serves as a passageway for blood vessels and nerves. Within each osteon, many mature bone cells called osteocytes are located. Osteocytes are found in little pockets called lacunae that are sandwiched between layers of bone matrix. You can see lamellae and osteocytes in their lacunae in the figure below. Osteocytes are responsible for monitoring the protein and mineral contents of the bone, and they direct both the release of calcium into the blood and the uptake up of calcium salts into the bone. Other bone cells, called osteoblasts, secrete the organic content of the matrix and are responsible for the growth of new bone. Osteoblasts are found near the surface of bones. Osteoclasts are bone cells that remove calcium salts from bone matrix. These bone cells will be discussed in further detail below.

Spongy Bone

Whereas compact bone tissue forms the outer layer of all bones, spongy bone or cancellous bone forms the inner layer of all bones. While it is less dense than compact bone, the term “spongy” refers only to the appearance of the bone, as spongy bone is quite strong. The lamellae of spongy bone form an open, porous network of bony branches, or beams, called trabeculae, that give the bone strength and make the bone lighter. This also allows room for blood vessels and bone marrow. Spongy bone does not have osteons; instead, nutrients reach the osteocytes of spongy bone by diffusion through tiny openings in the surface of the spongy bone. Spongy bone makes up the bulk of the interior of most bones including the vertebrae.

Bone Marrow

Many bones also contain a soft connective tissue called bone marrow. There are two types of bone marrow. Red marrow produces red blood cells, platelets, and most of the white blood cells in the body. Yellow marrow produces some white blood cells but is primarily made of fat cells. Both types of bone marrow contain numerous blood vessels and capillaries. In newborns, bones contain only red marrow. As the child ages, red marrow is mostly replaced by yellow marrow. In adults, red marrow is mostly found in the flat bones of the skull, the ribs, the vertebrae, and the pelvic bones. It is also found between the spongy bone at the very top of the femur and the humerus.

Periosteum and Endosteum

The outer surfaces of bones—except where they make contact with other bones at joints—are covered by periosteum. Periosteum has a tough, external fibrous layer and an internal layer that contains osteoblasts (bone-building cells). The periosteum is richly supplied with blood, lymph, and pain receptors, which make it very sensitive to manipulation. Periosteum provides nourishment to the bone through a rich blood supply. The periosteum is connected to the bone by strong collagen fibers called Sharpey’s fibers, which extend into the outer circumferential lamellae of the compact bone.

Endosteum is a thin vascular membrane that lines the medullary cavity of long bones and coats the trabeculae of spongy bone.

Cell Types in Bone

Bone, or osseous tissue, is a connective tissue that constitutes the endoskeleton. It contains specialized cells and a matrix of mineral salts and collagen fibers. The mineral salts primarily include hydroxyapatite, a mineral formed from calcium phosphate. Calcification is the process of deposition of mineral salts on the collagen fiber matrix. This crystallizes and hardens the tissue. The process of calcification only occurs in the presence of collagen fibers. Bone consists of four types of cells: osteoblasts, osteoclasts, osteocytes, and osteoprogenitor cells.

• Osteoprogenitor cells are pluripotent stem cells that divide to produce cells that differentiate into osteoblasts.
• Osteoblasts are bone-forming cells that are located on the inner and outer surfaces of bones. They make a collagen-rich protein mixture (called osteoid) that mineralizes to become solid bone matrix. Osteoblasts are immature bone cells. Osteoblasts that become trapped in the bone matrix differentiate into osteocytes.
• Osteocytes are the main cells in bony connective tissue; these cells cannot divide. These mature bone cells differentiate from osteoblasts that become trapped by mineralized bone matrix. The spaces that osteocytes occupy are called lacunae. Osteocytes are star-shaped and have many processes that reach out through narrow channels in the mineralized bone called canaliculi.
• Osteoclasts are the cells responsible for bone resorption. Located on bone surfaces, osteoclasts are large cells with as many as 50 nuclei! They secrete acids and hydrolytic enzymes that dissolve the bone matrix. The minerals are released into the blood, and so this process helps to regulate calcium concentrations in body fluids. Bone may also be resorbed if mechanical stresses on the bone change. This remodeling allows our bones to respond to changes in use.

Osteoclasts constantly remove minerals from the bone, and osteoblasts constantly produce the matrix that binds minerals into the bone, so both of these cells are important in calcium homeostasis.

Changes in the Skeletal System: Ossification, Bone Growth, Remodeling, and Repair.

Bone Formation

Bone is a replacement tissue; that is, it uses a model tissue on which to lay down its mineral matrix. For skeletal development, the most common template is cartilage. During fetal development, a framework is laid down that determines where bones will form. This framework is a flexible, semi-solid matrix produced by chondroblasts and consists of hyaluronic acid, chondroitin sulfate, collagen fibers, and water. As the matrix surrounds and isolates chondroblasts, they are called chondrocytes. Unlike most connective tissues, cartilage is avascular, meaning that it has no blood vessels supplying nutrients and removing metabolic wastes. All of these functions are carried on by diffusion through the matrix. This is why damaged cartilage does not repair itself as readily as most tissues do.

Throughout fetal development and into childhood growth and development, bone forms on the cartilaginous matrix. By the time a fetus is born, most of the cartilage has been replaced with bone. Some additional cartilage will be replaced throughout childhood, and some cartilage remains in the adult skeleton.

Ossification, or osteogenesis, is the process of bone formation by osteoblasts. Ossification is distinct from the process of calcification; whereas calcification takes place during the ossification of bones, it can also occur in other tissues. Ossification begins approximately six weeks after fertilization in an embryo. Before this time, the embryonic skeleton consists entirely of fibrous membranes and hyaline cartilage. The development of bone from fibrous membranes is called intramembranous ossification; development from hyaline cartilage is called endochondral ossification. Bone growth (often referred to as bone lengthening) continues until approximately age 25. Bones can grow in thickness throughout life, but after age 25, osteoblasts function primarily in bone remodeling and repair.

Intramembranous Ossification

Intramembranous ossification is the process of bone development from fibrous membranes. It is involved in the formation of the flat bones of the skull, the mandible, and the clavicles. Ossification begins as mesenchymal cells form a template of the future bone. They then differentiate into osteoblasts at the ossification center. Osteoblasts secrete the extracellular matrix and deposit calcium, which hardens the matrix. The non-mineralized portion of the bone or osteoid continues to form around blood vessels, forming spongy bone. Connective tissue in the matrix differentiates into red bone marrow in the fetus. The spongy bone is remodeled into a thin layer of compact bone on the surface of the spongy bone.

Endochondral Ossification

Endochondral ossification is the process of bone development from hyaline cartilage. All of the bones of the body, except for the flat bones of the skull, mandible, and clavicles, are formed through endochondral ossification. In long bones, chondrocytes form a template of hyaline cartilage that corresponds to the diaphysis. Responding to complex developmental signals, the matrix begins to calcify. This calcification prevents the diffusion of nutrients into the matrix, resulting in chondrocytes dying and the opening up of cavities in the diaphysis cartilage. Blood vessels invade the cavities, and osteoblasts and osteoclasts modify the calcified cartilage matrix into spongy bone. Osteoclasts then break down some of the spongy bone to create a medullary cavity in the center of the diaphysis. Dense, irregular connective tissue forms a sheath (periosteum) around the bones. The periosteum assists in attaching the bone to surrounding tissues, tendons, and ligaments. The bone continues to grow and elongate as the cartilage cells at the epiphyses divide. In the last stage of prenatal bone development, the centers of the epiphyses begin to calcify. Secondary ossification centers form in the epiphyses as blood vessels, and osteoblasts enter these areas and convert hyaline cartilage into spongy bone. Until adolescence, hyaline cartilage persists at the epiphyseal plate (growth plate), which is the region between the diaphysis and epiphysis that is responsible for the lengthwise growth of long bones.

Bone Growth

The skeletons of babies and children have many more bones and more cartilage than adults have. As a child grows, these “extra” bones, such as the bones of the skull (cranium) and the tailbone (sacrum), fuse together, and cartilage cells (chondrocytes) are gradually replaced by bone cells (osteocytes).

An infant is born with zones of cartilage called epiphyseal plates between segments of bone to allow further growth of the bone. When the child reaches skeletal maturity (between the ages of 18 and 25 years), all of the cartilage in the plate is replaced by bone, which stops further growth.

Bones grow in length at the epiphyseal plate by a process that is similar to endochondral ossification. The chondrocytes in the region of the epiphyseal plate reproduce by cell division and push older chondrocytes down toward the diaphysis. Eventually, these chondrocytes age and die. Osteoblasts move into this region and replace the chondrocytes with bone matrix. This process lengthens the bone and continues throughout childhood and the adolescent years until cartilage growth slows down and finally stops. When cartilage growth stops (usually in the early twenties), the epiphyseal plate completely ossifies so that only a thin epiphyseal line remains, and the bones can no longer grow in length. Bone growth is under the influence of growth hormone from the anterior pituitary gland and sex hormones from the ovaries and testes. Even though bones stop growing in length in early adulthood, they can continue to increase in thickness or diameter throughout life in response to stress from increased muscle activity or weight-bearing exercise.

Long bones continue to lengthen, potentially until adolescence, through the addition of bone tissue at the epiphyseal plate. They also increase in width through appositional growth.

Thickening of Long Bones

Appositional growth is the increase in the diameter of bones by the addition of bony tissue at the surface of bones. Osteoblasts at the bone surface secrete bone matrix, and osteoclasts on the inner surface break down bone. The osteoblasts differentiate into osteocytes. A balance between these two processes allows the bone to thicken without becoming too heavy.

Bone Remodeling

Bone renewal continues after birth into adulthood. Old bone tissue is replaced by new bone tissue. This process underlies the normal process of bone remodeling and healing of breakage. It involves the processes of bone deposition by osteoblasts and bone resorption by osteoclasts. Normal bone growth requires vitamins D, C, and A, plus minerals such as calcium, phosphorous, and magnesium. Hormones such as parathyroid hormone, growth hormone, and calcitonin are also required for proper bone growth and maintenance.

Bone turnover rates are quite high, with ten percent of the adult skeleton being replaced every year. Differences in turnover rate exist in different areas of the skeleton and in different areas of a bone. For example, the bone in the head of the femur may be fully replaced every six months, whereas the bone along the shaft is altered much more slowly.

Bone remodeling allows bones to adapt to stresses by becoming thicker and stronger when subjected to stress. Bones that are not subject to normal stress, for example when a limb is in a cast, will begin to lose mass.

Bone Repair

A fractured or broken bone undergoes repair in four stages:

1. Blood vessels in the broken bone tear and hemorrhage, resulting in the formation of clotted blood, or a hematoma, at the site of the break. The severed blood vessels at the broken ends of the bone are sealed by the clotting process, and bone cells that are deprived of nutrients begin to die.
2. Within days of the fracture, capillaries grow into the hematoma, and phagocytic cells begin to clear away the dead cells. Though fragments of the blood clot may remain, fibroblasts and osteoblasts enter the area and begin to reform bone. Fibroblasts produce collagen fibers that connect the broken bone ends, and osteoblasts start to form spongy bone. The repair tissue between the broken ends of two bones is called a fibrocartilaginous callus, as it is composed of both hyaline and fibrocartilage.
3. The fibrocartilaginous callus is converted into a bony callus of spongy bone. It takes about two months for the broken bone ends to be firmly joined together after the fracture. This is similar to the endochondral formation of bone, as cartilage becomes ossified; osteoblasts, osteoclasts, and bone matrix are present.
4. The bony callus is then remodeled by osteoclasts and osteoblasts, with excess material on the exterior of the bone and within the medullary cavity being removed. Compact bone is added to create bone tissue that is similar to the original, unbroken bone. This remodeling can take many months, and the bone may remain uneven for years.

Bone and Homeostasis

All of the organ systems of your body are interdependent, and the skeletal system is no exception. The food you take in via your digestive system and the hormones secreted by your endocrine system affect your bones. Even using your muscles to engage in exercise has an impact on your bones.

Exercise and Bone Tissue

During long space missions, astronauts can lose approximately 1 to 2 percent of their bone mass per month. This loss of bone mass is thought to be caused by the lack of mechanical stress on astronauts’ bones due to the low gravitational forces in space. Lack of mechanical stress causes bones to lose mineral salts and collagen fibers, and thus strength. Similarly, mechanical stress stimulates the deposition of mineral salts and collagen fibers. The internal and external structure of a bone will change as stress increases or decreases so that the bone is an ideal size and weight for the amount of activity it endures. That is why people who exercise regularly have thicker bones than people who are more sedentary. It is also why a broken bone in a cast atrophies while its contralateral mate maintains its concentration of mineral salts and collagen fibers. The bones undergo remodeling as a result of forces (or lack of forces) placed on them.

Numerous controlled studies have demonstrated that people who exercise regularly have greater bone density than those who are more sedentary. Any exercise will stimulate the deposition of more bone tissue, but resistance training has a greater effect than cardiovascular activities. Resistance training is especially important to slow down the eventual bone loss due to aging and for preventing osteoporosis.

Nutrition and Bone Tissue

The vitamins and minerals contained in all of the food we consume are important for all of our organ systems. However, there are certain nutrients that affect bone health.

You already know that calcium is a critical component of bone, especially in the form of calcium phosphate and calcium carbonate. Since the body cannot make calcium, it must be obtained from the diet. However, calcium cannot be absorbed from the small intestine without vitamin D which is a fat-soluble vitamin. Therefore, intake of vitamin D is also critical to bone health. In addition to vitamin D’s role in calcium absorption, it also plays a role, though not as clearly understood, in bone remodeling.

Milk and other dairy foods are not the only sources of calcium. This important nutrient is also found in green leafy vegetables, broccoli, intact salmon, and canned sardines with their soft bones. Nuts, beans, seeds, and shellfish provide calcium in smaller quantities.

Except for fatty fish like salmon and tuna, or fortified milk or cereal, vitamin D is not found naturally in many foods. The action of sunlight on the skin triggers the body to produce its own vitamin D, but many people, especially those of darker complexion and those living in northern latitudes where the sun’s rays are not as strong, are deficient in vitamin D. In cases of deficiency, a doctor can prescribe a vitamin D supplement.

Hormones and Bone Tissue

The endocrine system produces and secretes hormones, many of which interact with the skeletal system. These hormones are involved in controlling bone growth, maintaining bone once it is formed, and remodeling it.

Several hormones are necessary for controlling bone growth and maintaining the bone matrix. The pituitary gland secretes growth hormone (GH), which, as its name implies, controls bone growth in several ways. It triggers chondrocyte proliferation in epiphyseal plates, resulting in the increasing length of long bones. GH also increases calcium retention, which enhances mineralization, and stimulates osteoblastic activity, which improves bone density.

GH is not alone in stimulating bone growth and maintaining osseous tissue. Thyroxine, a hormone secreted by the thyroid gland promotes osteoblastic activity and the synthesis of bone matrix. During puberty, the sex hormones (estrogen in girls and testosterone in boys) also come into play. They too promote osteoblastic activity and production of bone matrix, and in addition, are responsible for the growth spurt that often occurs during adolescence. They also promote the conversion of the epiphyseal plate to the epiphyseal line (i.e., cartilage to its bony remnant), thus bringing an end to the longitudinal growth of bones. Additionally, the kidney converts Vitamin D into calcitriol, its active form. Calcitriol stimulates the absorption of calcium and phosphate from the digestive tract.

Bone modeling and remodeling require osteoclasts to resorb unneeded, damaged, or old bone and osteoblasts to lay down new bone. Two hormones that affect osteoclasts are parathyroid hormone (PTH) and calcitonin. PTH stimulates osteoclast proliferation and activity. As a result, calcium is released from the bones into circulation, thus increasing the calcium ion concentration in the blood. PTH also promotes the reabsorption of calcium by the kidney tubules, which can affect calcium homeostasis (see below).

Remodeling, or bone turnover, is the process of resorption of minerals followed by replacement by bone matrix, which causes little overall change in the shape of the bone. This process occurs throughout a person’s life. Osteoblasts and osteoclasts communicate with each other for this purpose. The purpose of remodeling is to regulate the level of calcium in the blood, repair micro-damaged bones (from everyday stress), and shape the skeleton during skeletal growth.

The process of bone resorption by the osteoclasts releases stored calcium into circulation and is an important process in regulating blood calcium balance. As bone deposition actively fixes circulating calcium in its mineral form by removing it from the bloodstream, resorption actively unfixes it, thereby increasing circulating calcium levels. These processes occur in tandem at site-specific locations.

When blood calcium levels decrease below normal, calcium is released from the bones so that there will be an adequate supply for blood clotting, muscle contraction, and nervous system function. When blood calcium levels are increased, the excess calcium is stored in the bone matrix. The dynamic process of releasing and storing calcium goes on almost continuously.

The small intestine is also affected by PTH, albeit indirectly. Because another function of PTH is to stimulate the synthesis of vitamin D, and because vitamin D promotes intestinal absorption of calcium, PTH indirectly increases calcium uptake by the small intestine. Calcitonin, a hormone secreted by the thyroid gland, has some effects that counteract those of PTH. Calcitonin inhibits osteoclast activity and stimulates calcium uptake by the bones, thus reducing the concentration of calcium ions in the blood. As evidenced by their opposing functions in maintaining calcium homeostasis, PTH and calcitonin are generally not secreted at the same time. The table below summarizes the hormones that influence the skeletal system.

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