Bones: 206 Internal Soldiers That Protect, Support, And Perform

Bone is a living connective tissue that regularly modifies throughout your lifetime.1,2 This change is called remodeling, where older or damaged bone cells are replaced by new bone cells.2  

Illustration of bone structure

Structure of Bone Tissue

The adult human bones consist of two types of bone tissues.
 

  • Compact Or Cortical Bone: It is the hard and dense outer layer that surrounds the bone marrow. It constitutes about 80% of the bone mass.4 
  • Cancellous Or Spongy Bone: It is less dense and has a honeycomb-like structure. Cancellous bone constitutes about 20% of the bone mass.4
Illustration of growing bones

Bone Growth And Structure Changes

Ever wondered how your bones grow?
 
During childhood and adolescence, the bones contain growing zones called growth plates (epiphyseal plates). These growth plates contain cartilage cells that multiply and grow in length. Eventually, the cells turn into hard, mineralized bone.4
 
The bones also undergo two important processes – bone modeling and bone remodeling.4 

Illustration of bone modeling & remodeling

Bone Modeling & Remodeling

Bone modeling refers to the process by which bones undergo a modification of an overall shape due to the activity of bone cells.4
 
During bone remodeling, the bone cells resorb old bone and form a new bone. Your bones undergo remodeling throughout your life.4

Illustrations of various types of bone cells or Illustration of osteoblast, osteocyte, osteoclast

Bone Cells

Bones contain three main types of cells

Osteoblasts: 
Osteoblasts are the bone cells that contribute to the formation of a new bone.

Osteocytes: 
Osteocytes are mature bone cells that constitute about 90%–95% of the total bone cells.5 Osteoblasts can be embedded in the bone as osteocytes.5

Osteoclasts: 
These cells conduct bone resorption or breakdown of bone. Osteoclasts help remodel the old and injured bones.4

The osteoblasts and osteoclasts are responsible for the constant remodeling of the bone.5

Functions Of Bone

Your bones are more than a structure. They are highly evolved dynamic organs, capable of functions such as protection, movement, and mineral storage.6,7
 
Below are some of its functions in brief:

3D model of the human skeleton

Structural Support

Bones offer support for internal structures. They bear the weight of your body and enable an upright posture.6,7 

3D model of human rib cage & heart

Organ Protection

Your sturdy bones along with muscles protect the delicate internal organs, which would otherwise be prone to injury.6

Graphic of a running man

Physical Activity

Your bones are highly evolved structures that perform diverse mechanical functions.8 The counterforce offered by the bones to a contracting muscle is what initiates the locomotion.9 A combined effort of bone and skeletal muscle is needed to perform any form of physical activity.6

Illustration on bone protection with minerals & vitamins

Storage Warehouse

Up to 70% of the bone weight is made of minerals; calcium is the most abundant mineral accounting for 99% of the body’s mineralized tissues.10 Bones act as a calcium warehouse, allowing nerves and muscles to function properly.6 The human body cannot produce calcium. Its only source of calcium is diet or nutritional supplements. In case your body does not absorb an adequate amount of calcium, your bones may become weak.11 

Illustration of blood cells

Blood Cell Production

Most of your bones have a soft, spongy tissue called bone marrow, which is responsible for the production of blood cells. Bone marrow has many blood vessels and has two types of cells. 

  • The red marrow cells are responsible for the production of blood cells (RBC, WBC, and platelets)
  • The yellow bone marrow is composed of fat cells.12
3D model of human pelvis

Energy Source

Did you know there are more than just minerals in your bones? Your bones have bone marrow adipose tissues, which not only act as an energy reservoir for the body but also contribute to immune function and other bodily processes.13 

Bone Types

Your body consists of four types of bones and each type of bone has a distinctive shape and function.14

 The different types of bones include:

Thus, bones are unique connective tissues that perform several vital functions, including support, mobilization, and protection of internal organs.1

Illustration of the humerus bone

Long Bones14

Long bones are thin, long cylindrical bones and are present in the arms and legs. They function as ‘levers’ that move when the muscles contract.

Illustration of palm bones

Short Bones14

Short bones have approximately equal dimensions (length, width, and thickness), and usually appear cuboid in shape. The wrist (carpals) and ankle (tarsals) contain short bones.

Illustration of the rib cage

Flat Bones14

Flat bones have a thin, flattened, and slightly curved structure. They are present in the skull (cranium), and ribs.

Illustration of the vertebrae

Irregular Bones14

Irregular bones have a complex shape. One of the examples of irregular bones is vertebrae (spine). 

References

1. Weatherholt AM, Fuchs RK, Warden SJ. Specialized connective tissue: bone, the structural framework of the upper extremity. J Hand Ther. 2012;25(2):123-31; quiz 132. 
2. American Bone Health. Bone is a living tissue, 2016. [Internet] [cited 2022 20 Mar]. Available from: americanbonehealth.org
3. NIH. National Cancer Institute. SEER Training Modules. Structure of Bone Tissue. [Internet] [Cited 28th March 2022]. Available at: training.seer.cancer.gov
4. Clarke B. Normal bone anatomy and physiology. Clin J Am Soc Nephrol. 2008; 3 Suppl 3(Suppl 3): S131-9. 
5. Florencio-Silva R, Sasso GR, Sasso-Cerri E, et al. Biology of Bone Tissue: Structure, Function, and Factors That Influence Bone Cells. Biomed Res Int. 2015; 2015:421746. 
6. DiGirolamo DJ, Kiel DP, Esser KA. Bone and skeletal muscle: neighbors with close ties. Journal of bone and mineral research. 2013;28(7):1509-18.
7. Dudek M, Meng QJ. Running on time: the role of circadian clocks in the musculoskeletal system. Biochemical journal. 2014;463(1):1-8.
8. Weiner S, Wagner HD. The material bone: structure-mechanical function relations. Annual review of materials science. 1998;28(1):271-98.
9. Sommerfeldt D, Rubin C. Biology of bone and how it orchestrates the form and function of the skeleton. European Spine Journal. 2001;10(2):S86-95.
10. Vannucci L, Fossi C, Quattrini S, et al. Calcium intake in bone health: a focus on calcium-rich mineral waters. Nutrients. 2018;10(12):1930.
11. Mayo Clinic. Nutrition and Healthy Eating. Calcium and calcium supplements: Achieving the right balance. [Internet] [Cited 2 May 2022] Available at: www.mayoclinic.org
12. National Cancer Institute. Bone marrow [Internet]. [cited 21 Mar 2022]. Available from: www.cancer.gov
13. Piotrowska K, Tarnowski M. Bone Marrow Adipocytes-Role in Physiology and Various Nutritional Conditions in Human and Animal Models. Nutrients. 2021;13(5):1412. 
14. NIH. National Cancer Institute. SEER Training Modules. Classification of Bones. [Internet] [Cited on 8th March 2022]. Available at: training.seer.cancer.gov

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