Mineralized Tissues

Question 1

Background on connective tissue

Answer 1

Cells + extracellular matrix

Matrix: can be gel-like, hard, fluid; usually protein
-supports and binds cells
-exchange medium
-“rigid” connective tissue with
A. Glycosaminoglycans (usually chondroitin sulfate), GAGs
B. proteoglycans (core protein with attached GAG chain)

Question 2

Mineralization

Answer 2

Deposition of minerals (Ca and P salts) in connective tissue matrix

Question 3

Advantages of mineralization

Answer 3

Mechanical strength/support

Storage for later use

Question 4

If mineralized, cells in matrix may become isolated from blood…

Answer 4

=entombment

O2, CO2, nutrients

Question 5

Mineralized examples

Answer 5

Bone, cartilage, dentin, enamel (or enameloid)

Question 6

Common themes regarding mineralized tissue

Answer 6

A. Collagen is present
Collagen bundles form network for mineral deposition in connective tissue

B. hydroxyapatite is abundant in dentin and bone

Question 7

Mineralization processes

Answer 7

1. Calcification
2. Dentin formation
3. Ossification

Question 8

Calcification (calcified cartilage)

Answer 8

major cartilage components:

• fibroblast: collagen matrix
• chondroblast: sulfated GAGs and proteoglycans in matrix

chondroblasts entombed in lacunae = chondrocytes (cartilage cell) with reduced mitotic ability

diffusion supplies lacunar cells; poor vascular supply

repair of bone quicker than cartilage

Question 9

compared to bone, cartilage…

Answer 9

• has a lower metabolic rate; avascular
• has different cellular and extracellular components (thus different structural properties)
• cells are chondrocytes suspended in semi-rigid matrix
• matrix with collagen and/or “fibrous proteins” (collagen absent in some lampreys and hagfishes, present in others)
• matrix is rich in polysaccharides

Question 10

hyaline cartilage

Answer 10

least differentiated; translucent; often converted to bone

Ex, “growth plate” in limb bones, tracheal rings, articular surfaces of joints, larynx, nose

Question 11

fibrocartilage

Answer 11

intervertebral discs and pelvic girdle symphyses

Question 12

elastic cartilage

Answer 12

pinna, epiglottis

Question 13

calcified cartilage

Answer 13

Ca salts released into/form in matrix; usually hyaline or elastic; often appears bone-like

Question 14

calcification

Answer 14

=calcium salts entomb chondrocytes; lacunar cell death

Ex:

• jaws of chondrichthyes
• endochondral bone formation before replacement by osseous tissue
• atypical - joint damage in osteoarthritis patients (calcium pyrophosphate dihydrate and calcium phosphate)

Question 15

Dentin Formation

Answer 15

mineralization: cell vitality retained

forms only in dermis, usually near epidermal border

odontoblasts reside near dermal blood vessels

cytoplasmic extensions of ondontoblasts deposit dentin - odontoblastic process

• 30% organic (collagen and GAGs)
• 70% inorganic (hydroxyapatite)
• extensions project away from blood vessel

odontoblast “retreat” toward blood vessel; cell extenstions become embedded in dentin

odontoblastic processes become surrounded by dentin matrix

microscopic “tunnels” through mineralized tissue - canaliculi or dentin tubules

odontoblasts aggregate; dental pulp

dentin is usually covered by epidermal enameloid (enamel, ganoin)

enameloid produced by ameloblasts in epidermis

enameloid applied to surface of dentin teeth, fish scales

Question 16

Ossification

Answer 16

not restricted to dermis

bone: always with collagen fiber matrix

spaces between fibers filled with hydroxyapatite

crystals bound to collagen by cementing substance (water and mucopolysaccharides)

hydroxyapatite hardens, cells extensions surrounded

microscopic tunnels though mineralized tissue = canaliculi

clusters of 2-4 osteoblasts reside in lacunae with interstitial fluid

cells retain direct/indirect access to vascular supply via canaliculi

Question 17

cementing substance similar to dentin, but…

Answer 17

• cells (osteoblasts) with cytoplasmic extensions in several directions
• osteoblasts are perivascular regarding vascular supply
• extensions contact other osteoblasts
• usually no cell retreat as hydroxyapatite produced

Question 18

bone: similar composition as dentin

Answer 18

30% organic (collagen and GAGs)

70% inorganic (hydroxyapatite)

Question 19

enamel

Answer 19

96% inorganic hydroxyapatite; acellular

Question 20

ameloblasts

Answer 20

enamelin and amelogenin, epidermis

Question 21

types of bone cells

Answer 21

osteoprogenitor
osteoblasts
osteocytes
osteoclasts

Question 22

osteoprogenitor cells

Answer 22

unspecialized; mesodermal mesenchyme origin; mitotic

Question 23

osteoblasts

Answer 23

bone-forming cells; osteogenesis; not mitotic

Question 24

osteocytes

Answer 24

“mature” osteoblast; maintenance; not mitotic

Question 25

osteoclasts

Answer 25

derived from monocytes

Question 26

all bone ultimately derived from…

Answer 26

mesodermal mesenchyme

Question 27

Types of Bone

Answer 27

Presence/Absence of Cells:

• cellular bone
• acellular bone - common in fishes

Appearance:

• compact
• cancellous - spongy or trabecular bone (trabecular spaces and trabeculae)

Orientation of collagen:

• lamellar bone - regular arrangement of collagen and bone cells (think osteon)
• non-lamellar bone (spongy bone, dermal bone)

Formation:

• membrane bone
• endochondral bone

Question 28

Haversian bone (osteonic bone)

Answer 28

=lamellar bone

mineralized collagen bundles surrounding haversian/osteonic canal; lamellae

Each canal: arteriole, venule, lymph vessel, neurons

Canal and surrounding lamellae with lacunae = osteon

blood vessels in osteon connected to blood vessels outside bone

Question 29

Volkmann’s canal

Answer 29

perforating canal - nutrient foramen

Question 30

spongy bone (=cancellous bone)

Answer 30

porous, trabeculae

trabeculae = beams/bars, architectural trusses

trabeculae with irregular lamellae, no organized osteonic systems

Question 31

red marrow

Answer 31

red marrow in spaces between trabeculae = hemopoietic/hemapoietic tissue

compare yellow marrow (in medullary cavity)

abundant in flat bones (roofing bones of skull, ribs, scapulae)

Question 32

membrane bone

Answer 32

no cartilage precursor

mesodermal mesenchyme cells arrange in layers (sheet-like “membranes”; membranous blastema)

these cells become osteoblasts

Question 33

types of membrane bone

Answer 33

1. dermal bone
2. perichondral/periosteal bone
3. heterotopic bones

Question 34

heterotopic bones

Answer 34

usually respond to mechanical stress

• sesamoid bones
• os cordis (fibrous connective tissue between A&V, interventricular septum)
• baculum (in fibrous connective tissue above urethra; bats, rodents, dogs, some primates, otter, walrus, raccoon)
• rostral bone