Bones and Bone Tissue

Long, short, flat, irregular, and sesamoid

What are the 5 types of bones?

They draw water out of the blood vessels and cells by osmosis. This process traps water within the ECM, which helps the tissue to resist compression.

What do large compounds such as glycosaminoglycans and proteoglycans in the ECM do?

They act like molecular "glue" that binds the different components together (binding to both hydroxyapatite and bone cells).

What do most glycoproteins in the ECM do?

The bone becomes flexible and unable to resist compression

What happens if we destroy the inorganic matrix of bone and leave the organic matrix intact?

The bone becomes brittle and shatters easily

What happens if we destroy the organic matrix of bone and leave the inorganic matrix intact?

Nutrient arteries

- enter through nutrient foramen - passes through compact bone into the inner medullary cavity, where it supplies the internal structures of the long bone

growth hormone for bone

- produced by the anterior pituitary, an endocrine gland below the brain has these effects on bone: - an increase in the rate of mitosis of chondrocytes in the epiphyseal plate, promoting longitudinal growth - an increase in the activity of osteogenic cells, including their activity in the zone of ossification - direct stimulation of osteoblasts in the periosteum, triggering appositional growth

Osteocyte

A bone cell formed from an osteoblast which helps to maintain bone - osteoblasts become surrounded and eventually trapped by secreted bone matrix in a small cavity known as a lacuna; at this point, these cells are called _______________ - secrete chemicals that are required for maintaining the ECM - also appear to recruit osteoblasts to build up areas of the bone under tension

Irregular bones

A bone that has an irregular shape and that does not fit into any other category. - include the vertebrae and certain skull bones.

Short bones

A bone that is about as long as it is wide, or roughly cube-shaped - include the carpals (bones of the wrist) and tarsals (bones of the ankle)

Long bones

A bone with a shape that is longer than it is wide. - include most bones of the arms and legs (including the humerus, or arm bone), as well as bones of the hands, feet, fingers, and toes.

epiphysial line

A calcified remnant of the epiphyseal plate.

Epiphyseal plate

Also known as the growth plate; is a line of hyaline cartilage from which a long bone grows in length in children and adolescents

Hydroxyapatite crystals Ca10(PO4)6(OH)2

A calcium phosphate-based mineral that makes up the inorganic portion of the osseous tissue extracellular matrix. This mineral makes bone one of the hardest substances in the body and gives bone its strength and the ability to resist compression, which allows it to perform its functions of support and protection.

Medullary cavity

A cavity within the shaft of the long bones filled with bone marrow (marrow can be either yellow or red, depending on the bone and the person's age) - surrounded by the diaphysis - also known as the marrow cavity

Periosteum

A dense irregular collagenous connective tissue that covers the surface of bones and is richly supplied with blood vessels and nerves

calcitonin

A hormone produced by the parafollicular cells of the thyroid gland; decreases blood calcium ion concentration by stimulating osteoblasts to build bone. - has essentially the opposite effect of PTH, as it decreases both the activity of osteoclasts and the formation of new osteoclasts. This increases bone deposition, which pulls calcium ions out of the blood and deposits them into the bone. - currently thought to decrease plasma calcium levels in children and pregnant women.

Central canal

A small, cerebrospinal fluid-filled canal located in the center of the spinal cord. - also called Haversian canal - contains blood vessels and nerves that supply the cells of the osteon. This canal, like all internal surfaces of bone, is lined by endosteum.

Articular cartilage

A thin layer of hyaline cartilage covering the articular surface of a bone at a synovial joint, serving to reduce friction and ease joint movement.

Yellow bone marrow

Adipose tissue located within most bones of an adult. - stores triglycerides, consists mostly of blood vessels and adipocytes

Osteopetrosis

Also known as "marble bone disease," involves defective osteoclasts that do not properly degrade bone

Protein intake

An adequate dietary intake of protein is necessary for osteoblasts to synthesize the collagen fibers needed for the organic matrix of bone.

1/3 from periosteum and 2/3 from nutrient arteries or small blood vessels that enter and exit the epiphyses via numerous small holes in their compact bone

Blood supply to long bones comes from...

vessels in the periosteum

Blood supply to short, flat, irregular, and sesamoid bones comes largely from....

Bone deposition

Bone deposition is carried out by osteoblasts in the periosteum and endosteum. Osteoblasts make the components of the organic matrix, as well as facilitate the formation of inorganic matrix. They secrete certain proteoglycans and glycoproteins that bind to calcium ions, and it appears that they also secrete vesicles containing calcium ions, ATP, and enzymes. These vesicles bind to collagen fibers, and their calcium ions eventually crystallize, which ruptures the vesicle and begins the overall process of calcification.

Secondary bone

Bone that has fully formed lamellae with regularly arranged collagen bundles that are parallel to one another, which makes it much stronger than primary bone - contains a higher percentage of inorganic matrix, which contributes to its strength - also called lamellar bone

Osseous tissue

Bone tissue

Flat bones

Bones that are thin and broad - include most bones of the skull, the ribs, the sternum, and the bones of the pelvis

Perforating canal

Canal perpendicular to the central canal that connects different central canals - carries blood vessels from the periosteum that merge with vessels in the central canals.

Perforating fibers

Collagen fibers that secure periosteum to underlying bone. These fibers penetrate deeply into the bone matrix, securing the periosteum in place.

Lamellae, central canal, lacunae and canaliculi

Components of osteon

A section through one of these bones resembles a "sandwich" made up of two thin layers of compact bone and a middle layer of spongy bone housing bone marrow

Describe internal structure of Short, Flat, Irregular, and Sesamoid Bones

The diaphyses of long bones consist of a very thick layer of compact bone with only scant inner spongy bone surrounding the medullary cavity. As with the diaphysis, the outer parts of the epiphyses are compact bone; however, the interior of the epiphyses consists of abundant spongy bone.

Describe the arrangement of compact and spongy bone in a long bone

Epiphysis

End of a long bone

Epiphyseal lines

Found separating both proximal and distal epiphyses from diaphysis - remnants of a structure termed the epiphyseal plate (also called growth plate)

Compact bone

Hard, dense bone tissue that is beneath the outer membrane of a bone - structure enables it to resist the majority of stresses placed on it, which are linear compression and twisting forces

Red bone marrow

Hematopoietic tissue located within certain bones that produces all of the formed elements of the blood. - consists of a network of reticular fibers supporting islands of blood-forming, or hematopoietic, cells

They accomplish bone resorption by secreting hydrogen ions and enzymes from a region of the cell called the ruffled border. The hydrogen ions create an acidic environment that dissolves the inorganic matrix, and the enzymes catalyze reactions that break down the organic matrix. Notice that the liberated minerals, amino acids, and sugars then enter the osteoclast, after which they are eventually delivered to the blood for reuse in the body or excretion as waste.

How do osteoclasts accomplish bone resorption?

Excess growth hormone will continue to stimulate appositional growth even after the epiphyseal plate has closed.

How does excess growth hormone allows the bone to continue to grow after the epiphyseal plate has closed?

Osteoblasts

Immature cuboidal to columnar cells found in the inner periosteum and endosteum that build bone - derived from flattened cells called osteogenic cells, which differentiate into ______________ when stimulated by certain chemical signals - perform the process of bone deposition, during which they secrete the organic matrix and aid in formation of the inorganic matrix.

Spongy bone

Inner layer of bone tissue having many small spaces and found just inside the layer of compact bone. - forms a framework of bony struts that allows it to resist forces in many directions and provides a place for the bone marrow to reside - also called cancellous bone

Circumferential lamellae

Lamellae located deep to periosteum and superficial to endosteum - add strength and tension resistance to the bone as a whole

Osteoclasts

Large, multinucleated cells derived from the fusion of cells formed in the bone marrow - reside in shallow depressions on the internal or external surfaces of bone - responsible for the process of bone resorption, during which they break down the bone ECM

NOTE: About 65% of the total weight of bone consists of an inorganic matrix composed of minerals. The remaining 35% of total bone weight consists of an organic matrix, which contains many of the usual ECM "ingredients," including collagen fibers.

NOTE: About 65% of the total weight of bone consists of an inorganic matrix composed of minerals. The remaining 35% of total bone weight consists of an organic matrix, which contains many of the usual ECM "ingredients," including collagen fibers.

estrogen

Like testosterone, estrogen increases the rate of longitudinal bone growth and inhibits osteoclasts. When the estrogen level increases during the teenage years, a "growth spurt" similar to that in males begins in females. Like testosterone, estrogen also accelerates closure of the epiphyseal plate, although it has a much more potent effect on epiphyseal plate closure than does testosterone. This is partly the reason why women are generally shorter in stature than men.

Lamellae

Most osteons contain from 4 to as many as 20 lamellae, which are sometimes called concentric lamellae. Just as tree rings enable a tree to withstand a great deal of stress, the lamellar structure of compact bone greatly enhances its strength. The collagen fibers of adjacent lamellae run in opposite directions, a trait that allows the osteon to resist twisting and bending forces in more than one direction.

NOTE: In actively growing bones, appositional growth primarily thickens the compact bone of the diaphysis. As this occurs, osteoclasts in the medullary cavity digest the inner circumferential lamellae, so that as the bones increase in width, their medullary cavities enlarge as well. Bone growth in width may continue after bone growth in length ceases, depending on factors such as hormones, the forces to which the bone is subjected, and diet.

NOTE: In actively growing bones, appositional growth primarily thickens the compact bone of the diaphysis. As this occurs, osteoclasts in the medullary cavity digest the inner circumferential lamellae, so that as the bones increase in width, their medullary cavities enlarge as well. Bone growth in width may continue after bone growth in length ceases, depending on factors such as hormones, the forces to which the bone is subjected, and diet.

NOTE: Osteocytes have long, thin "arms," cytoplasmic extensions that extend through the canaliculi to contact the arms of other osteocytes. At these places of contact are gap junctions that allow small substances to pass from cell to cell. A lacuna and its surrounding canaliculi somewhat resemble a spider—the lacuna is the body of the spider, and the canaliculi are its legs.

NOTE: Osteocytes have long, thin "arms," cytoplasmic extensions that extend through the canaliculi to contact the arms of other osteocytes. At these places of contact are gap junctions that allow small substances to pass from cell to cell. A lacuna and its surrounding canaliculi somewhat resemble a spider—the lacuna is the body of the spider, and the canaliculi are its legs.

NOTE: Short, Flat, Irregular, and Sesamoid Bones don't have diaphyses or epiphyses and therefore also don't have a medullary cavity, epiphyseal lines, or epiphyseal plates. However they do have a periosteum and an outer layer of compact bone that surrounds the inner spongy bone.

NOTE: Short, Flat, Irregular, and Sesamoid Bones don't have diaphyses or epiphyses and therefore also don't have a medullary cavity, epiphyseal lines, or epiphyseal plates. However they do have a periosteum and an outer layer of compact bone that surrounds the inner spongy bone.

Nutrient foramen

Narrow tunnel through the diaphysis that allows blood vessels into the medullary cavity

Calcium ion intake

Obviously, if calcium ion intake is too low, bone deposition will not take place. This explains why people are encouraged to include calcium ion-rich foods in their diets.

endochondral ossification

Ossification where bones are built on a model made of hyaline cartilage - All the bones in the body below the head, except the clavicles, form by this process - begins during the fetal period for most bones, although some, such as those in the wrist and ankle, ossify much later - The hyaline cartilage model consists of chondrocytes and cartilage ECM, and is surrounded by a connective tissue membrane called the perichondrium and immature cartilage cells known as chondroblasts. - long bones also contain secondary ossification centers within their epiphyses that ossify by a very similar process

Ossification (osteogenesis)

Process of bone formation

Lacunae

Recall that osteoblasts eventually are surrounded by the ECM they secrete to become osteocytes. These cells reside in the remaining _____________, small cavities that are filled with extracellular fluid and located between lamellae.

Interstitial lamellae

Remains of old osteons that broke down as bone grew and remodeled itself

- Protection (of organs) - Mineral storage (calcium, phosphorus, and magnesium salts) and acid-base homeostasis - Blood cell formation (from red bone marrow and through process of hematopoiesis) - Fat storage (yellow bone marrow contains adipocytes with stored triglycerides) - Movement - Supports (weight of body)

Roles of skeleton system

Sesamoid bones

Small, relatively flat, and oval-shaped bones located within tendons - give the tendon a mechanical advantage, providing better leverage for muscles, and also reduce wear and tear on the tendon. - eg. patella

Trabeculae

Spicules of bone that make up spongy bone tissue. - covered with endosteum and usually do not contain osteons - contain concentric lamellae, within which we find canaliculi and lacunae housing osteocytes - however no central or perforating canals are present and the cells obtain their oxygen and nutrients from the blood vessels in the bone marrow

Bone Remodeling in Response to Tension and Stress

Stated simply, the heavier the load a bone must carry, the more bone tissue is deposited in that bone. This load doesn't necessarily refer to weight per se, but rather to the amount of compression. Compression is the act of squeezing or pressing together; this force results when the bones are, for example, pressed between the body's weight and the ground. Tension and pressure placed on the bone also affect bone remodeling. Tension is a stretching force, whereas pressure is application of a continuous downward force. Where tension is placed, osteoblasts are stimulated and bone deposition occurs. However, where continuous pressure is placed, osteoclasts are stimulated and bone resorption occurs.

*1. The chondroblasts in the perichondrium differentiate into osteoblasts. *When this happens, the perichondrium is now called the periosteum. *2. The bone begins to ossify from the outside.* a. Osteoblasts build the bone collar on the external surface of bone. Osteoblasts begin to secrete organic bone ECM deep to the periosteum, forming a ring of early compact bone called the bone collar. b. Simultaneously, the internal cartilage begins to calcify and the chondrocytes die. As the bone collar calcifies, ECM surrounding the internal chondrocytes calcifies. This cuts off the blood supply to the chondrocytes, causing them to eventually die. Their death leaves cavities surrounded by calcified cartilage. *3. In the primary ossification center, osteoblasts replace the calcified cartilage with early spongy bone; the secondary ossification centers and medullary cavity develop. *Osteoclasts etch a hole in the bone collar that allows a group of blood vessels and bone cells to enter the primary ossification center. Here, osteoblasts replace the calcified cartilage with early spongy bone while other osteoblasts continue to increase the size of the bone collar. As this happens, the cavities enlarge and combine, forming the medullary cavity. Also, secondary ossification centers develop and the epiphyses begin to ossify. *4. As the medullary cavity enlarges, the remaining cartilage is replaced by bone; the epiphyses finish ossifying.* As ossification continues, the calcified cartilage is replaced with bone. Osteoclasts degrade much of the newly formed spongy bone, which increases the size of the medullary cavity. This space becomes filled with bone marrow. Cartilage remains in only two locations: the epiphyseal plates and the articular cartilage. Cartilage in the epiphyseal plate will eventually be replaced with bone when growth in length ceases; the articular cartilage, however, remains for life.

Steps to endochondral ossification

Collagen fibers

The predominant protein fibers in bone are ______________, which form cross-links with one another and help bone to resist torsion (twisting) and tensile (pulling or stretching) forces. They also align with hydroxyapatite crystals, significantly enhancing the hardness of bone.

Calcitriol

The active form of vitamin D; increases the number of calcium ions absorbed from the small intestine. - works directly in the intestine to increase plasma calcium levels - parathyroid hormone (PTH) activates Vitamin D into calcitriol in the kidney. Calcitriol then increases absorption of calcium in the intestine.

Endosteum

The connective tissue membrane lining the internal surfaces of a bone. - thinner than periosteum and lacks the fibrous outer layer that we see with the periosteum

bone remodeling

The continual process of bone growth and bone loss that takes place within healthy bone.

maintenance of calcium ion homeostasis; bone repair; replacement of primary bone with secondary bone; replacement of older, brittle bone with newer bone; and bone adaptation to tension and stresses

The cycle of bone formation and loss is carried out for a number of reasons....

Bone resorption

The destruction of bone tissue by osteoclasts. During bone resorption, osteoclasts secrete hydrogen ions (H+) from their ruffled borders onto the bone ECM. The hydrogen ions make the pH more acidic, which breaks down the pH-sensitive hydroxyapatite crystals in the inorganic matrix. The released calcium ions and other minerals may then be reused in the body. To degrade organic matrix, osteoclasts secrete enzymes to catalyze reactions that break down proteoglycans, glycosaminoglycans, and glycoproteins. The breakdown products are taken into the osteoclasts for possible reuse.

Bone matrix

The extracellular matrix of bone tissue, including ground substance and collagen fibers. Excludes the inorganic calcium salts.

Primary bone

The first bone formed by both types of ossification - immature bone - also called woven bone - consists of irregularly arranged collagen bundles, abundant osteocytes, and little inorganic matrix - in most locations in the body, it's resorbed by osteoclasts and replaced by mature secondary bone

Osteon

The functional unit of compact bone; consists of concentric rings of bone matrix called lamellae that surround a central canal. - also called a Haversian system

Sinuses

The interior of some flat and irregular bones of the skull contains hollow, air-filled spaces called _____________________ that make the bones lighter.

Osteoid

The organic component of the extracellular matrix of osseous tissue; consists of collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins such as osteocalcin.

Inorganic Matrix

The predominant ingredient is calcium salts (in fact, bone stores about 85% of the total calcium ions in the body), with a good amount of phosphorus as well. Most of the calcium and phosphorus salts exist as part of a large mineral called hydroxyapatite crystals. Other ingredients include bicarbonate (HCO3−) potassium, magnesium, and sodium salts.

Many fractures may be stabilized through closed reduction, in which the bone ends are brought into contact by simply manipulating the body part. More severe fractures may require open reduction, in which the fracture is fixated surgically with, for example, plates, wires, and screws. After the fracture is stabilized, it is immobilized for a period of around 6 weeks while the repair process occurs.

The primary treatment for fractures is stabilization of the fracture, followed by immobilization.

appositional growth

The process by which bones grow in width during which osteoblasts between the periosteum and the bone surface lay down new bone - does not initially result in the formation of new osteons; instead, new circumferential lamellae are formed. As these new lamellae are added, the deeper circumferential lamellae are either removed or incorporated into osteons.

Intramembranous ossification

The process by which certain flat bones form from a mesenchymal model. Ossification where bones are built on starting material known as a model that is made of a membrane of embryonic connective tissue

Diaphysis

The shaft of a long bone

Diploë

The spongy bone tissue of flat bones.

Vitamin K intake

There is good evidence that vitamin K aids in the production of calcium ion-binding glycoproteins by osteoblasts. Inadequate vitamin K intake may decrease bone deposition.

Canaliculi

Tiny canals that connect lacunae - also allow oxygen and nutrients from the blood to reach every osteocyte.

Intramembranous and endochondral

Two types of ossification

Vitamin C intake

Vitamin C is required for the synthesis of collagen proteins. If vitamin C intake is inadequate, collagen synthesis in bone decreases.

Calcium Ion Homeostasis

bone stores most of the body's calcium ions. Calcium ions are required for a number of critical processes, including contraction of the heart and skeletal muscles, transmission of nerve impulses, and blood clotting. If the calcium ion concentration in the blood drops too low or rises too high, major disruptions in homeostasis, up to and including death, can occur.

Stimulus: Blood calcium ion level decreases below normal range. Receptor: Parathyroid gland cells detect a low blood calcium ion level. The cells of the parathyroid gland act as receptors that detect a lower than normal concentration of calcium ions in the blood. Control center: Parathyroid gland cells release parathyroid hormone (PTH) into the blood. The cells of the parathyroid gland also act as control centers. When they are stimulated, they release PTH into the bloodstream. Effector/response: Parathyroid hormone stimulates effects that increase the blood calcium ion level. PTH leads to increased osteoclast activity, which causes the breakdown of bone and the subsequent release of calcium ions into the blood. Note that this actually weakens the bones, but prevents much more serious imbalances in calcium ion homeostasis. In addition, PTH stimulates the intestines to absorb calcium ions and the kidneys to retain calcium ions. Each of these actions works to increase the blood calcium ion level to the normal range. Homeostasis and negative feedback: The calcium ion concentration returns to the normal homeostatic range, and negative feedback decreases parathyroid gland cell secretion of PTH. The cells of the parathyroid gland detect when the calcium ion level has returned to the normal range. In response, they secrete smaller amounts of parathyroid hormone via a negative feedback mechanism.

calcium feedback loop involving a hormone called parathyroid hormone

longitudinal growth occurs not from the division of osteocytes or other bone cells, but from the division of chondrocytes in the epiphyseal plate. (recall that the epiphyseal plate is composed of hyaline cartilage that does not ossify during endochondral ossification)

how does longitudinal growth happen

Testosterone

increases appositional bone growth, causing bones in males to become much thicker and have greater calcium salt deposits than bones in females - also increases the rate of mitosis at the epiphyseal plate, and so the large increases of this hormone that occur during the teenage years are accompanied by "growth spurts." - Although testosterone increases longitudinal bone growth, it also accelerates the closure of the epiphyseal plates.

mesenchymal membrane

membrane composed of a sheet of embryonic connective tissue called mesenchyme that is rich with blood vessels and mesenchymal cells.

note: PTH promotes the formation of calcitriol.

note: PTH promotes the formation of calcitriol.

1. A hematoma fills the gap between the bone fragments. When a bone is damaged, its blood vessels rupture and bleed into the injured site, forming a hematoma (also known as a blood clot), a mass of blood cells and proteins that resembles grape jelly. The hematoma cuts off the blood supply to the damaged area, and the bone cells surrounding the area die. 2. Fibroblasts and chondroblasts infiltrate the hematoma and a soft callus forms. Fibroblasts and small blood vessels in the periosteum invade the hematoma in the first week after the fracture. The fibroblasts secrete collagen fibers and form dense irregular collagenous connective tissue that starts to bridge the gap between the bone fragments. At the same time, some of the osteogenic cells from the endosteum become chondroblasts that secrete hyaline cartilage. This mixture of hyaline cartilage and collagenous connective tissue is called a soft callus. 3. Osteoblasts build a bone callus. Osteoblasts within the periosteum begin laying down a collar of primary bone called a bone callus (or hard callus). Initially, only part of the bone callus is actually bone tissue; the remainder is the soft callus. This process continues over the next several weeks until the entire callus is made of primary bone that forms a bridge between the bone fragments. As we discussed earlier, primary bone is not as strong as secondary bone; however, notice in Figure 6.17 that the bone callus extends beyond the border of the bone, which helps to strengthen it. 4. The bone callus is remodeled and primary bone is replaced with secondary bone. Over the next several months, the bone callus is remodeled and the primary bone is resorbed and replaced with secondary bone. Eventually, the bone callus is fully resorbed and the bone regains its previous structure and strength. However, the bone callus often remains visible for months or longer following full healing of the injury.

steps to fracture healing

*1. Osteoblasts develop in the primary ossification center from mesenchymal cells.* At the primary ossification center, mesenchymal cells differentiate first into osteogenic cells and then into osteoblasts. *2. Osteoblasts secrete organic matrix, which calcifies, and trapped osteoblasts become osteocytes.* The newly formed osteoblasts secrete the organic matrix of bone. In a few days' time, calcium salts and other components of the inorganic matrix are deposited in the primary ossification center, a process called calcification, and the early bone hardens. Trapped osteoblasts then become osteocytes. *3. Osteoblasts lay down trabeculae of early spongy bone, and some of the surrounding mesenchyme differentiates into the periosteum.* Osteoblasts continue to lay down new bone, forming the trabeculae of the early spongy bone. Over time, the trabeculae enlarge and merge, forming larger trabeculae. About this time, some of the mesenchyme surrounding the developing bone differentiates into the periosteum. Some of the vascular tissue in the forming spongy bone will become bone marrow. *4. Osteoblasts in the periosteum lay down early compact bone.* Osteoblasts within the periosteum continue to secrete organic bone matrix. This matrix becomes more heavily calcified than the deeper spongy bone trabeculae, and its structure is remodeled to become the immature compact bone.

steps to intramembranous ossification

1. Chondrocytes divide in the zone of proliferation. The stacked chondrocytes in the zone of proliferation divide, and the cells above them become part of the next zone. 2. Chondrocytes that reach the next zone enlarge and mature. 3. Chondrocytes die and their matrix calcifies. Chondrocytes then reach the zone of calcification. Here, chondrocytes are quite far from the blood supply, so they die and their ECM accumulates calcium salt deposits. 4. Calcified cartilage is replaced with bone. In the final zone, the zone of ossification, osteoblasts invade the calcified cartilage and begin to lay down bone on top of it. Eventually, the calcified cartilage/bone is resorbed by osteoclasts and completely replaced with bone. Notice that the new bone is added to the diaphysis.

steps to longitudinal growth

The zone closest to the epiphysis is the zone of reserve cartilage, and it contains cells that are not directly involved in bone growth but that can be called upon to divide if needed. The region just "above" (closer to the diaphysis) the zone of reserve cartilage is the zone of proliferation, and it has actively dividing chondrocytes in lacunae. The next region toward the diaphysis is the zone of hypertrophy and maturation, and it contains mature chondrocytes. Above that is the zone of calcification; it contains dead chondrocytes, some of which are calcified. And finally, the zone of ossification has calcified chondrocytes and osteoblasts that build bone.

the epiphyseal plate contains five different zones of cells, each having a distinct appearance

Vitamin D intake

vitamin D is a steroid synthesized by the body in response to exposure of the skin to UV light. Vitamin D acts mainly on the intestines, where it promotes calcium ion absorption, and on the kidneys, where it prevents loss of calcium ions to the urine. The net effect of vitamin D on bone is to increase bone deposition and bone mass.

primary ossification center

where ossification begins