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The osteoblast    is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an osteocyte    , the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space called a lacuna and is surrounded by bone tissue. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes lack mitotic activity. They can communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi    (singular = canaliculus), channels within the bone matrix.

If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells—the osteogenic cell    . These osteogenic cells are undifferentiated with high mitotic activity and they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. They differentiate and develop into osteoblasts.

The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast    . They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone. [link] reviews the bone cells, their functions, and locations.

Bone Cells
Cell type Function Location
Osteogenic cells Develop into osteoblasts Deep layers of the periosteum and the marrow
Osteoblasts Bone formation Growing portions of bone, including periosteum and endosteum
Osteocytes Maintain mineral concentration of matrix Entrapped in matrix
Osteoclasts Bone resorption Bone surfaces and at sites of old, injured, or unneeded bone

Compact and spongy bone

The differences between compact and spongy bone are best explored via their histology. Most bones contain compact and spongy osseous tissue, but their distribution and concentration vary based on the bone’s overall function. Compact bone is dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution.

Compact bone

Compact bone is the denser, stronger of the two types of bone tissue ( [link] ). It can be found under the periosteum and in the diaphyses of long bones, where it provides support and protection.

Diagram of compact bone

A generic long bone is shown at the top of this illustration. The bone is split in half lengthwise to show its internal anatomy. The outer gray covering of the bone is labeled the periosteum. Within the periosteum is a thin layer of compact bone. The compact bone surrounds a central cavity called the medullary cavity. The medullary cavity is filled with spongy bone at the two epiphyses. A callout box shows that the main image is zooming in on the compact bone on the left side of the bone. On the main image, the periosteum is being peeled back to show its two layers. The outer layer of the periosteum is the outer fibrous layer. This layer has a periosteal artery and a periosteal vein running along its outside edge. The inner layer of the periosteum is labeled the inner osteogenic layer. The compact bone lies to the right of the periosteum and occupies the majority of the main image. Two flat layers of compact bone line the inner surface of the ostegenic periosteum. These sheets of compact bone are called the circumferential lamellae. The majority of the compact bone has lamellae running perpendicular to that of the circumferential lamellae. These concentric lamellae are arranged in a series of concentric tubes. There are small cavities between the layers of concentric lamellae called lacunae. The centermost concentric lamella surrounds a hollow central canal. A blue vein, a red artery, a yellow nerve and a green lymph vessel run vertically through the central canal. A set of concentric lamellae, its associated lacunae and the vessels and nerves of the central canal are collectively called an osteon. The front edge of the diagram shows a longitudinal cross section of one of the osteons. The vessels and nerve are visible running through the center of the osteon throughout its length. In addition, blood vessels can run from the periosteum through the sides of the osteons and connect with the vessels of the central canal. The blood vessels travel through the sides of the osteons via a perforating canal. The open areas between neighboring osteons are also filled with compact bone. This “filler” bone is referred to as the interstitial lamellae. At the far right of the compact bone, the edge of the spongy bone is visible. The spongy bone is a series of crisscrossing bony arches called trabeculae. There are many open spaces between the trabeculae, giving the spongy bone its sponge-like appearance.
(a) This cross-sectional view of compact bone shows the basic structural unit, the osteon. (b) In this micrograph of the osteon, you can clearly see the concentric lamellae and central canals. LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Questions & Answers

Between the heart and the Brain which one is more important to human being... discuss
Faith Reply
well the brain is important for motor skills, the heart is important for involuntary muscle movement supporting body functions. the body can survive without brain involvement, but the body cannot last without the heart
john
granted the heart is important, but the brain gives the body purpose
john
what is homeostasis
Rebecca Reply
what is the composition of saliva
Vijay Reply
ഫസ്റ്റ് ചാപ്റ്റർ ഇംപോർട്ടൻസ് പോസ്റ്റ്
Reshma Reply
Yes....
Loving
nhi samjh aya
Anshika
Nhi wt is this
Loving
I don't know
Anshika
Okk wre frm u r
Loving
what is mean of? reshma
Asad
I don't know but would like to
Rebecca
D bone in d ankle joint re what ?
Ifunanya Reply
can one define a cell as a basic unit of a living organism
Michael Reply
Which of the following hormones are responsible for the adolescent growth spurt? estrogen and testosterone, even in women?
Kepa Reply
estrogen
Farhana
Estrogen!
Jazil
estrogen hormone
Michael
yes estrogen hormone
Anshika
yes
Sale
What is sling give d characteristics of sling uses of sling
adamu Reply
a sling a rope used in hunting ie throwing of rocks
Michael
I went to learn anatomy of joints
Arman Reply
me 2
adamu
joint have 3 cartilaginous joint fibrous joint synovial joint U can reserch in Google can explain U well
sopheaktra
define sling
adamu
To throw with a circular or arcing motion
real
what is respiration
Osele Reply
what is respiration
Paul
Respiration is the process by which oxygen is taken in and carbon dioxide is given out.
Md
Respiration is the combination of inhalation and exhalation.
Khim
inhalation is the taking in air from environment to lung and exhalation is taking out sir from the lung to environment ..
Khim
is the process by wich gases(oxygen and carbon(IV)oxide go through the nose, trachea and the lungs to the blood stream
Michael
what is holistic
Kibrom Reply
Holistic means encompassing the whole of a thing, and not just the part. Holistic medicine looks at the whole person for answers, not just at physical symptoms. You might have heard of holistic medicine, which tries to treat someone as mind and body, instead of treating only the part of the patient
I want to learn one by one system like skeleton system... muscular system
Mary Reply
i want to learn this
Kibrom
what is the difference between regional anatomy and system anatomy
David
system anatomy is when we study the system like digestive, circulatory, reproductive, but regional anatomy is studying the anatomy by regions of body like anatomy of neck, thorax, head etc. Regional anatomy may include system anatomy...
Biplav
yes
Anshika
explain in details factors that affects bone development
Precious Reply
describe the process of bone healing
Precious
out line factors that delays bone healing
Precious
actually,for the bone formation they must be sufficient level of calcium,phosphorus and vitamin D in the body for it to happen smoothly.
lucky
the above mentioned substances are the ones able for the formation and repairing of bones.
lucky
explain in details factors that affects bone development
Precious
what are tissue
muki Reply
a lots of cells make tissue and a cell body is tissue
Rabab
cells come together to form a tissue and tissues come together to form an organ
Adoma
what is appendicular skeleton
OSE Reply
appendicular skeleton is upper limbs and lower limbs
sopheaktra

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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