What Organs Are Involved in the Skeletal System?

The skeletal system is built from organs, and the biggest surprise is that every one of your bones counts as an organ in its own right.1Innerbody. Interactive Guide to the Skeletal System Around and between the bones sit softer parts, including marrow, cartilage, ligaments, tendons and joints, that belong to the same system. The skeleton is also not a closed unit. It trades calcium and signals with glands, kidneys and muscles, and it even sends out hormones of its own.2PubMed Central. Roles for osteocalcin in brain signalling: implications in cognition- and motor-related disorders

Why Each Bone Counts as an Organ

An organ is a structure made of more than one kind of tissue that does a job together. Bone qualifies because it is made of many living cells, protein fibers and minerals.3Innerbody. Interactive Guide to the Skeletal System Bones also carry their own blood vessels and nerves, which is why a broken bone bleeds and hurts. An adult has 206 bones, while a newborn has about 270 to 300 that fuse over childhood.

Cut a bone open and the layers show. The hard outer shell is what appears bright white on an X-ray. Inside, the spongy bone is full of tiny struts that keep the bone light but strong, and the hollow middle of a long bone like the thigh holds marrow. The mineral in the spongy inner bone differs in makeup from the dense outer bone, matching their different jobs.4PubMed. Chemical and structural characterization of the mineral phase from cortical and trabecular bone

A thin living lining wraps the outside of bone, and a similar lining covers the inner surfaces. After a fracture, the outer lining, the inner lining and the marrow all supply new bone-forming cells.5PubMed Central. Skeletal Cell Fate Decisions Within Periosteum and Bone Marrow During Bone Regeneration The outer bone lining is also the main source of the temporary cartilage that bridges a break before it turns to bone.6PubMed Central. Skeletal Cell Fate Decisions Within Periosteum and Bone Marrow During Bone Regeneration Bone inside you is constantly broken down and rebuilt, which is why broken bones heal and why exercise strengthens them.

Bones also act as a storehouse. They hold most of the body’s calcium and phosphorus and release it when the blood needs more. Because lead behaves like calcium, about 90% of the lead in the body ends up stored in bone, where it can stay for decades.7PubMed Central. The Mechanisms of Lead Toxicity in Living Organisms

Bone Marrow, the Organ Inside Your Bones

Bone marrow is the soft tissue filling the hollow spaces inside bones. It is the main place where the body makes blood cells.8PubMed Central. Structural organization of the bone marrow and its role in hematopoiesis It holds stem cells that renew themselves and mature into the huge numbers of blood cells needed to stay alive.9PubMed Central. Structural organization of the bone marrow and its role in hematopoiesis There are two kinds, red and yellow marrow. In adults, blood-making red marrow is found mainly in the hips, breastbone, ribs, skull and spine.

Babies have red marrow in almost all their bones. Over the years, much of the marrow in the arms and legs turns yellow. After severe blood loss, yellow marrow can partly switch back to red.

Marrow is also part of the immune system. It helps run the body’s immune responses and clears out old, abnormal cells and debris.10PubMed. Structure and function of the bone marrow and hematopoiesis Marrow counts as a primary immune organ because it houses the stem cells that become immune cells.11PubMed Central. Crosstalk between bone and the immune system

Yellow (fatty) marrow gets its color from fat cells.12PubMed Central. Marrow fat metabolism is linked to the systemic energy metabolism It can take up and store fats circulating in the blood, much like body fat elsewhere.13PubMed Central. Marrow fat metabolism is linked to the systemic energy metabolism It also releases proteins that affect nearby marrow and may influence metabolism throughout the body.14PubMed Central. Development, regulation, metabolism and function of bone marrow adipose tissues Exactly what marrow fat does is still being worked out.15PubMed Central. Editorial: Bone Marrow Adipose Tissue: Formation, Function, and Impact on Health and Disease

Most people meet marrow through medicine. A bone marrow test usually samples the back of the hip bone. Marrow transplants replace blood-making stem cells in some blood cancers, and the donated stem cells travel through the blood and settle inside the recipient’s bones.

Cartilage and the Joints It Lines

Most of the skeleton first forms as a cartilage model that is gradually replaced by bone.16PubMed Central. Mechanisms of synovial joint and articular cartilage development Children’s growth plates are leftover cartilage near the ends of long bones that let them lengthen.

Cartilage comes in three types, each in its own places:

Fold your ear and it springs back, which is elastic cartilage. The tip of your nose is cartilage, not bone.

A joint is any place where two bones meet. Movable joints like the knee are sealed in a capsule filled with slippery fluid, with joint cartilage on the bone ends.21PubMed Central. Mechanisms of synovial joint and articular cartilage development Some joints barely move, like the seams of the skull, and some move a little, like the joints between vertebrae.

Joint cartilage has no blood supply, so it repairs itself poorly once worn or torn. Thinning cartilage is the core change in osteoarthritis, and as it goes, bone may rub on bone, causing pain and stiffness. A torn knee meniscus is a different problem: it is a fibrocartilage injury, not arthritis.

Ligaments and Tendons Hold It All Together

Ligaments are tough bands that connect bone to bone and keep joints stable. Tendons connect muscles to bones so muscle pulls can move the skeleton. Both are made mostly of tightly packed collagen fibers, which resist stretching.

The places where tendons and ligaments attach to bone take concentrated stress.22PubMed Central. Where tendons and ligaments meet bone: attachment sites (‘entheses’) in relation to exercise and/or mechanical load To cope, the attachment shifts gradually from soft tendon to hard, mineral-filled bone through a layer of fibrocartilage.23PubMed Central. Tissue Engineering Strategies for the Tendon/ligament-to-bone insertion Force then passes between soft and hard tissue without a sharp weak line where they meet.24PubMed Central. Tissue Engineering Strategies for the Tendon/ligament-to-bone insertion

The two connectors get injured in different ways. A sprain injures a ligament, like a rolled ankle. A strain injures a muscle or tendon, like a pulled hamstring. Either can range from mild stretching to a full tear.

Attachment points are also common sore spots. Tennis elbow and heel pain often start where tendon meets bone, since repeated strain at these high-stress points can inflame them. When surgeons reattach a torn tendon, it often heals weaker than the original, because that graded zone is hard to rebuild.

Bone Also Works Like a Hormone Gland

Bone cells make a hormone that acts on the kidneys.25PubMed Central. REGULATION AND FUNCTION OF THE FGF23/KLOTHO ENDOCRINE PATHWAYS It makes them let more phosphate go into the urine.26PubMed Central. Skeletal secretion of FGF-23 regulates phosphate and vitamin D metabolism It also lowers the amount of active vitamin D in the blood.27PubMed Central. REGULATION AND FUNCTION OF THE FGF23/KLOTHO ENDOCRINE PATHWAYS

Bone-forming cells also make a hormone called osteocalcin.28PubMed Central. Osteocalcin—A Versatile Bone-Derived Hormone It helps the body handle blood sugar, respond to insulin and burn energy.29PubMed Central. Roles for osteocalcin in brain signalling: implications in cognition- and motor-related disorders Mice lacking it develop insulin resistance and abnormal fat buildup.30PubMed. Osteocalcin: an osteoblast-derived polypeptide hormone that modulates whole body energy metabolism Mice without it are also more anxious, have weaker memory and a smaller memory center in the brain.31PubMed Central. Osteocalcin—A Versatile Bone-Derived Hormone Most of this comes from mice, and how strongly it works in people is still being studied.

For everyday health, no supplement or test of osteocalcin is used in routine care. The findings do help explain why bone health and metabolism seem connected. Staying active benefits both, without needing to target the hormone.

Organs Outside the Skeleton That Keep It Working

The parathyroid glands, four small glands in the neck, release parathyroid hormone (PTH).32PubMed. A central regulation of PTH secretion and function The hormone keeps blood calcium steady and is a strong controller of bone mass.33PubMed. A central regulation of PTH secretion and function When needed, it signals bone cells to break down bone and release calcium.34PubMed Central. Parathyroid hormone: anabolic and catabolic actions on the skeleton When the glands stay overactive for a long time, bone is lost overall even though bone building speeds up.35PubMed Central. Regulation of Bone Remodeling by Parathyroid Hormone

Vitamin D takes a trip through several organs. The skin makes it from sunlight, and food and supplements add more. The liver converts it into an inactive storage form.36PubMed Central. Crucial Role of Vitamin D in the Musculoskeletal System The kidneys then convert that into the active hormone.37PubMed. Vitamin D in health and disease: a literature review Active vitamin D helps regulate the minerals bones are built from,38PubMed. Vitamin D in health and disease: a literature review so long-term kidney or liver disease can weaken bones. The vitamin D blood test usually measures the liver’s storage form, listed as 25-hydroxyvitamin D.

Muscles and bones affect each other through physical pulling and through chemical signals each releases.39PubMed Central. Interactions between Muscle and Bone—Where Physics Meets Biology Muscle releases growth signals that help translate exercise into a bone response.40PubMed Central. A Role for Myokines in Muscle-Bone Interactions Bone cells sense when they are bent or squeezed and switch on signals in response.41PubMed Central. HOW DO BONE CELLS SENSE MECHANICAL LOADING?

Everyday habits support all of these partners. Regular walking, stair climbing or lifting loads the bones, and enough calcium and vitamin D from food, sun or supplements supplies the raw material. People with kidney disease often need bone checks from their doctor.

How the Skeletal System Changes With Age

Aging brings lower bone mass and more fat cells in the marrow.42PubMed. Aging and lineage allocation changes of bone marrow skeletal (stromal) stem cells The same marrow stem cells can become either bone-forming cells or fat cells.43PubMed. Aging and lineage allocation changes of bone marrow skeletal (stromal) stem cells With age they lean toward fat, and the marrow gradually loses its ability to renew bone.44PubMed. Aging and lineage allocation changes of bone marrow skeletal (stromal) stem cells

This shift, along with signals from marrow fat cells, contributes to osteoporosis, a condition of thin, fragile bones.45PubMed Central. Bone Marrow Adipose Tissue and Skeletal Health The same pattern appears in bone loss linked to diabetes, menopause and some long-term medicines.46PubMed Central. Bone Marrow Adipose Tissue and Skeletal Health Areas of marrow with more fat tend to have thinner, sparser spongy bone and fewer bone-making cells.47PubMed. Age-Related Increases in Marrow Fat Volumes have Regional Impacts on Bone Cell Numbers and Structure

Osteoporosis usually causes no symptoms until a bone breaks. The hip, wrist and spine are the common sites. A bone density scan can find it before a fracture.

Several habits slow age-related loss: weight-bearing and strength exercise at any age, not smoking, and limiting alcohol. After menopause or past age 65, it is worth asking a doctor about screening.

Are Teeth Part of the Skeletal System

Tooth enamel is the hardest substance in the body, made of the same calcium-phosphate mineral as bone.48PubMed Central. Comparison of mechanical property and role between enamel and dentin in the human teeth Most of a tooth is dentin, a mineral-filled tissue beneath the enamel.49PubMed Central. Comparison of mechanical property and role between enamel and dentin in the human teeth Unlike bone, enamel has no living cells and cannot regrow once damaged. A chipped tooth needs a dentist, while a broken bone heals on its own.

Teeth are usually grouped with digestion instead, since their main job is chewing food. They are not counted among the 206 bones, though they sit in sockets of the jawbone, which is part of the skeleton.

The two still overlap. Both rely on calcium, phosphorus and vitamin D. Jawbone loss can loosen teeth, and dental X-rays sometimes reveal signs of thinning bone.