bone

Bone is a calcified tissue that acts as a support tissue and as a reservoir for minerals.

The extracellular matrix of bone contains extracellular MATRIX that contains Type I collagen fibers, ground substance and minerals.

1. Type I collagen fibers
2. Ground substance

Acidophilic due to the high amount of calcium

proteoglycans, GAGs, glycoproteins (osteonectin, osteocalcin, sialoprotein)

mineral salts in the form of hydroxyapatite (calcium phosphate crystals).

Bone is studied in decalcified or dried bone preparations

The matrix contains osteocytes trapped in lacunae

Bone cells are interconnected by thin channels called canaliculi

Carries nutrients and gases to nearby blood vessels

highly vascular, containing many blood vessels.

Bone matrix is constantly being remodeled

1. Spongy (cancellous, trabecular)
2. COmpact (dense, corticle)

Small pieces of bone, usually by bone marrow cavity

Large peices of bone with enclosed channels with blood vessels

Each ”gross” bone consists of a mixture of compact and spongy bone.

Bone is a tissue and an 'organ”

All exposed surfaces of bone must be covered by a lining of cells or tissues or it will be destroyed

1. Osteoprogenitor cells
2. osteoblasts
3. MAKE BONE
4. Osteocytes

Osteogenic stem cells

-They are mesenchymal stem cell hiding in the periosteum and endostum
-They can undergo mitosis
-They can differentiate into osteoblasts

-They are bone producers

Blue (they are basphilic cells with round euchromatic nucleus)

. lined up in one-cell thick rows on surfaces of bone (appositional growth)

secrete organic unmineralized matrix called osteoid (collagen &
ground substance) and matrix vesicles that initiate bone mineralization.

No, they are terminally differeniyaed

When trapped in the mineralizing matrix, they become osteocytes

They are trapped in lacunae with long, thin cell processes extendinng into canalculi

gap junctions at the tips of their processes

They maintain the bone matrix (with some synthetic and resportive ability)

No, they are terminally differentiated

It is a bone resorbing cell

Monocytes become osteoclasts and osteoclasts destroy bone

They have an acidic cytoplasm because they are abundant in mitochondria

Any of the tiny depressions, pits, or irregular grooves in bone that is being resorbed by osteoclasts

Membrane is highly folded into a ruffled border

Lysosomes, collagenase, and other lytic enzymes

Calcium and phosphate

Haversian systems or osteons, the functional unit of compact bone, consists of
concentric lamellae (layers) of bone matrix and osteocytes surrounding a canal
with blood vessels

Chains of 10-20 osteocytes in 4-10 lamellae are linked by canaliculi.

Oxygen and nutrients from the central
blood vessel, diffuses through the canaliculi to reach all the osteocytes in the osteon.

Lamellae have a slightly different angle of collagen
fibers

endosteum

-transverse canals aligned perpendicular to the long axis of bone
-bring blood vessels from the periosteum to the Haversian canals, marrow
cavity and then back out.

Endosteum

- remnants of partially resorbed osteons formed during bone
remodeling

The first few lamellae of bone near the periosteum or the
marrow cavity. T

They are supplied with blood from vessels in the periosteum or marrow

Endosteum lines internal structures including bone marrow, Haversian & Volkmann’s canals.

Periosteum lines the outside of bone. Collagen fibers (Sharpey’s fibers) in the periosteum insert
directly into the bone matrix.

Both the endosteum and periosteum are osteogenic.

Spongy bone (cancellous, trabecular) consists of small finger-like pieces of bone, usually
associated with the marrow cavity.

Blood vessels in the surrounding spaces between the
trabeculae of spongy bone supply oxygen and nutrients to nearby osteocytes.

Compact bone (dense, cortical) consists of thick pieces of bone with osteons.

Blood vessels in the Haversian canals supply oxygen and nutrients to osteocytes in the osteon/Haversian system

1. Volmann's and Harversian canals
2. Osteons
3. Circumferential and interstitial lamellae

Bone develops in mesenchyme, a loosely organized embryonic connective tissue.

Mesodermgives rise to most of the mesenchyme in the embryo except for the head and neck region
where most of the mesenchyme is derived from neural crest ectoderm.

Bone forms by two processes: intramembranous or endochondral ossification

Bones develops directly from the mesenchyme

All head related = Flat bones of face and skill (vault of cranium), manible, maxilla, and clavicle

-Temporary cartilage model is formed in mesenchyme
-Continuously growing cartilage model is progressive replaced by bone

Mesenchyme to cartilage model (chondroblasts/cytes) to bone (osteoblasts)

Extremities, vertebrae, pelvis, and bones at base of skull (NO HEAD)

The terms, endochondral and intramembranous ossification refer ONLY to the mechanism
and environment by which bone is initially formed.

1. Both give rise to same histological types of bone
2. Both processes have primary, immature bone made of collagen first that is poorly arranged
3. Bone remodeling makes all of immature WOVEN bone to strong LAMELLAR bone
4. Spongy bone is then filled in

1. Mesenchyme differeniate to osteoblasts
2. Osteoblasts secrete unmineralized bone matrix (osteoid) to form primary woven bone. Osteoblast get trapped in lacunae, making osteocytes
3. Osteoblasts differeniate and secrete osteoid at surface of developing bone (appositional growth) forming spicules
4. Bone is continuously remodeled through antagonistic actions of osteoblastsm
5. The periosteum, endosteum, and marrow develop

1. Primary bone is replaced with stronger, secondary lamellar bone
2. Compact bone forms by filling in spaces of spongy
3. Proper poertions of bone is maintained by adding new layers

1. Mesenchyme cells condense, differenitae into chondroblasts, and form HYALINE CARTILAGE MODEL
2. Blood vessels develop into a connective tissue. Increased oxygen causes conversion of PERICHONDRIUM TO PERIOSTEUM.
3. Oxgen and nutrients cannot diffuse through bone matrix of PERIOSTEAL COLLAR to cartilage within. This causes the chrondrocytes to hyperthrphy, calcify, and chondrocytes die
4. PERIOSTEAL bud breaks the bone collar in center shaft, which brings in vessels, blood cells, osteoclasts, and osteoprogenitaor cells
5. BECOMES PRIMARY OSSIFICATION CENTER.
6. OSTEOBLASTS = differentiate and deposit bone on pieces of calcifed cartilage
7. OSTEOCLASTS = reabsorb the calified cartilage and newly formed bone FORMS THE MARROW CAVITY

8. Growth in length occurs by chondrocyte division (interstital growth)
9. Growth in width occurs by appositional growth by osteoblasts that differenitae from the periosteum. Osteoclasts enlarge marrow cavity.
10. Secondary ossification center forms at end of bone restricting cartilage growth to growth/epiphyseal plate (occurs POSTNATALLY) m
11. Growth in length ends with cartilage in the growth epiphyseal plate is destroyed
12. Bones can increase in WIDTH throughout life from differentiation of osteoblasts in periosteum

1. Resting cartilage (young healthy cartilage)
2. Proliferating cartilage (condrocytes divide interstitally, proudce matrix, increase length, arrange in columns)
3. Hypertrophying cartilage (chondrocytes and lacunae enlarge - apoptosis
4. Calcifying cartilage (insoluble calcium salts in cartilage matrix
5. Resorption (osteoclasts eat clacified matrix)

Maintenance of the appropriate WIDTH of bone during development (and life) occurs by
antagonistic actions of osteoblasts and osteoclasts.

Bone matrix is added to the outside of the bone (appositional growth) by osteoblasts that
differentiate from the periosteum.

Bone matrix is resorbed from the inside by osteoclasts in the marrow cavity

1. Number or activity of osteoclasts and osteoblasts
2. Poor bone matrix construction by issues with osteoid or matrix or mineralization

Spongy bone has a high surface to volume area allowing more space for osteoblasts and
clasts.

Spongy bone

(1) Calcitonin - from tyroid gland, lowers blood calcium levels, inhibits osteoclast
(2) Parathyroid hormone - from parathyroid gland, raise blood calcium, increases osteoclast

Hyperparathyroidism causes excessive osteoclast activity and bone resorption.

Osteoporosis – is a disease in which there is a reduction in total bone mass due to an
increased rate of bone resorption. Bone appears histologically normal. Decreased bone is
especially visible in spongy or trabecular bone. Fractures are common especially in
vertebrae.

Vitamin D (and Calcium) deficiency – results in inadequate mineralization of newly
formed bone matrix resulting in an accumulation of osteoid, the unmineralized organic
components of bone matrix. Vitamin D is necessary for calcium absorption.

Osteomalacia – low vitamin D in adults leads to inadequate mineralization during
bone remodeling. Bones are soft and easily fracture.

Rickets – low vitamin D in children, leads to inadequate mineralization during bone
growth and remodeling as well as disrupts the growth plates. Cartilage does not calcify,
and bone does not mineralize adequately in the growth plate. Growth is stunted and
with weight baring the softer bones of the legs become bowed.

Growth hormone (GH), secreted from the anterior pituitary, stimulates
general tissue growth, including that of cartilage and bone.

Acromegaly - excess GH in adults increases bone formation. Causes excess growth from the periosteum that produces thicker bones and coarse facial features

Gigantism - excess GH in children results in increased and extended bone growth. Results in bones that are thicker and wider and longer due to increased cartilage proliferation at growth plate

Pituitary dawrfism - decreased GH in children reduces overall growth in all tissues, including cartilage and bone. Endochondral AND intermembranous ossification are effected. Causes decreased sature, but normal proportions.

Achondroplasia - the most common form of drawfism. Is a genetiic disease where chondrocyte division is inhibited. Bones formed by endochondral ossification are affected. Individuals will have short extremeities, bowed legs, and swayed back. Head appears large compared to short limbs.

Osteogenesis imperfecta (brittle bone disease), is a group of hereditary disorders with
defective synthesis of Type I collagen. Tissues with high Type I collagen, like bone, are
severely affected. Osteoblasts secrete the defective collagen into the matrix, with the
result that bones are weak and easily fractured.

1. large hematoma from clotted blood forms a fibrin scaffold. The damaged cells and tissue are removed and new vessels, inflammatory cells, fibroblasts, and chrondrogenic and osteogenic cells appear
2. SOFT CALLUS containing cartilage and CT connects ends of the fracture bones
3. BONY CALLUS with parts of primary woven bone replace soft cartilaginous callus
4. Bone remodeling replaces woven bone with strong lamellar bone

Growth in length ends when all chondrocytes are destroyed