HCB Objectives 6
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Note: PTH increases serum Ca2+ by stimulating osteoblasts to stimulate
osteoclasts which breakdown bone
Note: Calcitonin and estrogen decrease Ca2+ by inhibiting osteoclasts
1.
Cartilage:
a.
Hyaline cartilage: has a smooth, “glassy” appearance; lubricates joint and
provides cushioning so there is no bone-bone contact
 Found in costal cartilages, supporting rings of upper airways, and
articulating surfaces of synovial joints
b.
Elastic cartilage: hyaline cartilage with elastic fibers to provide elasticity
 Found in pinna of ear and epiglottis
c.
Fibrocartilage: hyaline cartilage with type I collagen fibers to give tensile
strength
 Found in intervertebral disks, and menisci of the knee
2
Intramembranous bone formation: flat bone formation, mesenchymal cells
give rise to osteoblasts which lay down new bone. These spicules of bone grow
by differentiation and appositional (from the outside) growth. Spicules then fuse
with adjacent spicules to form new bone
Endochondral bone formation: long bone formation, mesenchymal cells
differentiate into chondroblasts which secrete a cartilage template.
Osteoprogenitor cells differentiate into osteoblasts inside the perichondrium
which lay down a compact bony collar around the peripheral wall which will
grow appositionally to widen the bone (cells on the outside on the bony collar are
the periosteum). On the inside of the bone (marrow cavity) the hyaline cartilage
in the epiphyseal plate hypertrophy, degenerate, and are calcified. The primary
ossification center vasculature invades this area, and osteoclasts break down their
calcium remains, followed by osteoblasts filling the space with new woven bone
3.
Structural organization of lamellar bone: Alternating layers of type I collagen
laid down by osteoblasts with osteocytes between the layers (as layers are laid
down, osteoblasts mature into osteocytes). In trabecular bone, cells can interact
via gap junctions, but in compact bone there must be some passageway. In
osteons (subunits of compact bone) there are Haversian canals which carry an
arteriole, venule, and a nerve. Osteons communicate along cement lines and
spaces are occupied by interstitial lamellae (remodeled old osteons). At the
periosteal border, compact bone is arranged into circumferential lamellae that
follow the bone.
4.
Chondroblasts: present in the periphery of cartilage and dispersed in the
innermost layer of perichondrium (dense irregular CT connecting CT to adjacent
tissue and carrying nutrient supply); function is to secrete extracellular matrix.
Chondrocytes: present in cartilage extracellular matrix (lacunae); function is to
secrete extracellular matrix.
Osteoblasts: made from precursor osteoprogenitor cells, present along bone
surfaces to form sheets of osteoid (organic bone matrix), regulates osteoclast
activity; quiescent osteoblasts are called bone lining cells.
Osteoclasts: made from monocyte precursors, found along bone surface and
“crush” bone (break it down to mobilize calcium in serum). Osteoclasts form a
highly folded membrane at the tight seal with the bone matrix where acids are
pumped to breakdown bone. These components are endocytosed, dissolved
further in lysozomes and released into the serum; the resulting pit is a Howship’s
lacuna.
Osteocytes: made from precursor osteoblast, when osteoblast is surrounded by
matrix (lacunae), help maintain bone matrix, sense and respond to loading to
remodel bone. Exist in a functional syncytium with gap junctions extending
through canaliculi to other osteocytes (important because cells can only interact
with blood supply in Haversian canal).
Diagram of bone and cartilage illustrating patterns of occurrence:
5.
Compact (cortical) bone: solid bone, outer surfaces of bone and the walls of
bone cylinders
Cancellous (trabecular, spongy) bone: internal areas of bone, interconnecting
struts with open space between
Woven bone: first organization of bone; a matrix of randomly arranged collagen
fibers upon which lamellar bone is built
Lamellar bone: mature bone, collagen fibers are remodeled into a highly ordered
array
Endosteum: inner surface of bone, facing hollow (medullary) cavity, lined with
osteoblasts
Periosteum: outer layer of bones (except at articular surfaces) made of dense
irregular CT, lined with osteoblasts
Osteoid: Newly laid matrix that has not yet been inorganically mineralized
(usually by hydroxyapatite; matrix is made of inorganic hydroxyapatite and
organic type I collagen and calcium-binding glycoproteins)
Epiphyseal plate: area of cartilage in bone between marrow cavity and
epiphysis. This area is where bones grow; as cartilage grows, cartilage is also
calcifying on the diaphyseal border. On this border, osteoclasts break down the
ossified cartilage and osteoblasts lay down woven bone.
Perichondrium: the outer layer of cartilage; made of loose CT so that nutrients
and blood can enter into cartilage
6.
Stucture:
a.
Cartilage: extracellular matrix is made of type II collagen fibers (thinner
than type I, visible in EM) and aggrecan (chondroitin sulphate and keratin
sulphate GAGs bound to hyaluranon)
b.
Bone: composed of 30% organic (type I collagen, osteopontin,
osteonectin, and osteocalcin – blood osteocalcin is used to indicate
osteoblast activity) and 70% inorganic (hydroxyapatite – 99% of body
calcium)
Function:
a.
Cartilage: mostly mechanical support, often to ease bone-bone contact in
joints
b.
Bone: mostly mechanical support, marrow is site of hematopoeisis
Growth:
a.
Cartilage: Extracellular matrix is secreted by both chondroblasts and
chondrocytes
b.
Bone: Osteoid is laid down by osteoblasts and is later remodeled to
become lamellar bone
Maintenance:
a.
Cartilage: Maintains itself through diffusion in the periosteum
b.
Bone: Osteoclasts continually breakdown bone while osteoblasts build it
back up
7.
Synovial joint histology: Articular (hyaline cartilage without periosteum)
cartilage binds to bones, and synovial membrane lines the joint cavity. Synovial
membrane is made of cuboidal cells (synoviocytes which make synovial fluid) but
do not form tight junctions and interact with outlying loose CT. On the inside of
the synovial joint is loose, vascular tissue. Since the articular cartilage is
avascular, all metabolites are processed and sent through the synovial joint. A
joint capsule surrounds the joint which is made of dense irregular CT.
Why it is susceptible to infection: Because there is a lack of tight junctions, a
basal lamina. Thus, it can become easily infected by interactions with
surrounding loose CT.