Changes

'''Long bones''' - A secondary center of ossification develops in the epiphyses.

'''Long bones''' - A secondary center of ossification develops in the epiphyses.

==Bone Growth==

==Bone Growth==

[[Image:Growth plate.jpg|right|thumb|250px|<small><center>Growth plate (Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]

[[Image:Growth plate.jpg|right|thumb|250px|<small><center>Growth plate (Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]

===Length===

===Length===

The '''epiphyseal growth plate (EGP)''' is a transverse disc of cartilate between the two ossification centers and allows the bone to continue to grow in length after birth. It then closes. The EGP is a region of continuous chondroblast differentiation and maturation and can be divided into five zones:

The '''epiphyseal growth plate (EGP)''' is a transverse disc of cartilage between the two ossification centers and allows the bone to continue to grow in length after birth. It then closes. The EGP is a region of continuous chondroblast differentiation and maturation and can be divided into five zones:

# '''Germinal Zone''' - chondroblast division at a low rate, and self renewal.

# '''Germinal Zone''' - chondroblast division at a low rate, and self renewal.

===Matrix===

===Matrix===

'''Osteoid'''- An organic, uncalcified, homogeneous substance that stains light pink with H&E . Osteoid consists of type I collagen (90%), which resists tension and bone-specific proteins(10%), including; Osteonectin, Osteopontin and Osteocalcin.

'''Osteoid'''- An organic, uncalcified, homogeneous substance that stains light pink with H&E . Osteoid consists of type I collagen (90%), which resists tension, and bone-specific proteins (10%), including; Osteonectin, Osteopontin and Osteocalcin.

'''Mineral'''- An inorganic component, provides rigidity. Consists of a crystalline lattice of calcium phosphate and calcium carbonate. It also contains Mg, Mn, Zn, Cu, Na, F. It accounts for 65% of bone.

'''Mineral'''- An inorganic component that provides rigidity and consists of a crystalline lattice of calcium phosphate and calcium carbonate. It also contains Mg, Mn, Zn, Cu, Na, F. It accounts for 65% of bone.

===Cells===

===Cells===

'''Osteoblasts'''

'''Osteoblasts'''

A single layer of mesenchymal cells which synthesize bone extracellular matrix (osteoid). When active, thet appear plump and cuboidal, with a basophilic cytoplasm. The cell membranes are rich in alkaline phosphatase (ALP). The cells are possibly involved in pumping calcium across membranes. They are promoted by growth factors and have receptors for [[Bones - normal#Bone resorption|PTH]]. They contract in response, which provides space for osteoclasts to attach. When they are inactive, there is less cytoplasm, so they become flattened.

A single layer of mesenchymal cells which synthesize bone extracellular matrix (osteoid). When active, they appear plump and cuboidal, with a basophilic cytoplasm. The cell membranes are rich in alkaline phosphatase (ALP). The cells are possibly involved in pumping calcium across membranes. They are promoted by growth factors and have receptors for [[Calcium#Parathyroid Hormone (PTH)|PTH]]. They contract in response, which provides space for osteoclasts to attach. When they are inactive, there is less cytoplasm, so they become flattened.

'''Osteocytes'''

'''Osteocytes'''

osteoblasts are embedded in their own matrix; reside within '''lacunae''' and are interconnected via channels forming '''canaliculi'''. Canaliculi create connections to form a huge neural-like junctional organization. They contact osteoblasts and each other with cytoplasmic processes and reach through canaliculi in the mineralised bone matrix. Osteocytes regulate the composition of bone fluid.

Osteocytes are embedded in their own matrix; reside within '''lacunae''' and are interconnected via channels forming '''canaliculi'''. Canaliculi create connections to form a huge neural-like junctional organization. They contact osteoblasts and each other with cytoplasmic processes and reach through canaliculi in the mineralised bone matrix. Osteocytes regulate the composition of bone fluid.

'''Osteoclasts'''

'''Osteoclasts'''

Osteoclasts are giant (multinucleate [[Monocytes|monocytes]]) cells. Histologically, they are large, often multinucleated cells. They have an acidophilic cytoplasm. They sit in the bone surface depression known as, '''howship's lacuna'''. Osteoclasts respond to vitamin D by increasing their numbers and activity (parathyroid independent). Osteoclasts act to resorb the bone ECM. They firstly dissolve mineral followed by collagen, using the brush border. They do <u>not</u> have receptors for PTH, but do have receptors for [[Calcium#Calcitonin|calcitonin]].  

Osteoclasts are giant (multinucleate [[Monocytes|monocytes]]) cells. Histologically, they are large, often multinucleated cells. They have an acidophilic cytoplasm. They sit in the bone surface depression known as, '''howship's lacuna'''. Osteoclasts respond to [[Calcium#Calcitriol (Activated Vitamin D3)|vitamin D by increasing their numbers and activity (parathyroid independent). Osteoclasts act to resorb the bone extracellular matrix (ECM). They firstly dissolve mineral followed by collagen, using the brush border. They do <u>not</u> have receptors for PTH, but do have receptors for [[Calcium#Calcitonin|calcitonin]].  

Osteoblasts, osteocytes, chondroblasts and chondrocytes are derived from the stromal fibroblastic system ('''osteoprogenitor cells'''). Osteoclasts are derived from the haematopoietic system.

Osteoblasts, osteocytes, chondroblasts and chondrocytes are derived from the stromal fibroblastic system ('''osteoprogenitor cells'''). Osteoclasts are derived from the haematopoietic system.

1. '''Compact bone (cortical)'''  

1. '''Compact bone (cortical)'''  

A sheath covers the external surface of long bone. It is thicker in the shaft and thins over the epiphyses. It is comprised of thin lamallae in a series of concentric tubes arranged around small central canals (collectively known as an '''osteone'''). It forms 80% of the total bone mass and consists of cells and interstitial substance - 30% ossein (type of collagen) and 70% minerals, especially calcium phosphate.  

A sheath covers the external surface of long bone. It is thicker in the shaft and thins over the epiphyses. It is composed of thin lamallae in a series of concentric tubes arranged around small central canals (collectively known as an '''osteone'''). It forms 80% of the total bone mass and consists of cells and interstitial substance - 30% ossein (type of collagen) and 70% minerals, especially calcium phosphate.  

2. '''Cancellous bone (spongy or trabecular)'''  

2. '''Cancellous bone (spongy or trabecular)'''  

===Laminar bone===

===Laminar bone===

Formed on the periosteal surface of diaphysis. It accomodates rapid growth of large dogs and farm animals. Plates of woven bone from within the periosteum. As it forms, it fuses with the bone surface.

Formed on the periosteal surface of diaphysis. It accommodates rapid growth of large dogs and farm animals. Plates of woven bone from within the periosteum. As it forms, it fuses with the bone surface.

==Periosteum and blood supply==

==Periosteum and blood supply==

===Periosteum===

===Periosteum===

The periosteum is the specialised sheath of connective tissue covering bone, except at the articular surfaces. It is loosely attached, except at tendon insertions and boney prominences. Histologically, there is an outer layer which is fibrous for support, and an inner layer that is osteogenic. It consists of a rich supply of nerves and lymph vessels, including nutrient, metaphyseal and periosteal arteries. The normal flow of blood from the medulla to the periosteum is due to higher pressures in the medulla. Young animals have a greater blood supply.  

The periosteum is the specialised sheath of connective tissue covering bone, except at the articular surfaces. It is loosely attached, except at tendon insertions and bony prominences. Histologically, there is an outer layer which is fibrous for support, and an inner layer that is osteogenic. It consists of a rich supply of nerves and lymph vessels, including nutrient, metaphyseal and periosteal arteries. The normal flow of blood from the medulla to the periosteum is due to higher pressures in the medulla. Young animals have a greater blood supply.  

The '''endosteum''' lines the marrow cavity. The '''medullary cavity and cancellous interstitium''' is for bone marrow storage and production. [[Bone Marrow - Anatomy & Physiology#Red marrow|'''Red marrow''']] is a richly vascularized, gelatinous tissue with hematopoeitic properties found abundantly in young animals and [[Bone Marrow - Anatomy & Physiology#Yellow marrow|'''yellow marrow''']] has been converted from red marrow by fat infiltration, causing hematopoeitic properties to dwindle.

The '''endosteum''' lines the marrow cavity. The '''medullary cavity and cancellous interstitium''' is for bone marrow storage and production. [[Bone Marrow - Anatomy & Physiology#Red marrow|'''Red marrow''']] is a richly vascularized, gelatinous tissue with hematopoeitic properties found abundantly in young animals and [[Bone Marrow - Anatomy & Physiology#Yellow marrow|'''yellow marrow''']] has been converted from red marrow by fat infiltration, causing hematopoeitic properties to dwindle.

==Links==

==Links==

Click here for information on [[Bones - Pathology|bone and cartilage pathology]].

Click here for information on [[Bones - Pathology|bone and cartilage pathology]].

 

[[Category:Musculoskeletal System - Anatomy & Physiology]]

[[Category:Musculoskeletal System - Anatomy & Physiology]]

[[Category:To Do - AimeeHicks]][[Category:To Do - Review]]

[[Category:A&P Done]]