BIOLOGICAL SCIENCE 102 – ANIMAL BIOLOGY – NOTES & VOCABULARY
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VERTEBRATE OSTEOLOGY: BONE STRUCTURE AND BONE GROWTH
Functions of bones = support, protection, leverage, storage of minerals and lipids, blood cell
production
Two Structurally Different Types of Bone:
1. CORTICAL OR COMPACT BONE
- white, ivory-like near the outside (cortex) of the bone, very dense
- forms the walls of long bones; made up of functional units called osteons
2. CANCELLOUS OR SPONGY BONE
- non-compact bone found at the ends of bones
- spongy bone contains trabeculae = fibrous rods for strength that bear the weight of the body in the
bone (similar to the ceiling of a cathedral); trabeculae are oriented to support the stresses put on a
particular bone according to need for each individual bone
- spongy bone is not made of osteons; spongy bone receives its nutrients by diffusion
• BONE MARROW = in the medullary or marrow cavity of the bone
yellow marrow = loose connective tissue dominanted by adipocytes (fat)
red marrow = mixture of mature and immature red and white blood cells and the cells from which all
blood cells are derived (stem cells)
MATRIX OF BONE = mostly calcium phosphate with some calcium hydroxide to form
hydroxyapatites salts that can withstand compression (but are likely to shatter when bent or twisted)
in a matrix of collagen fibers (flexible & tough; can bend, twist or stretch) = makes an protein-crystal
with intermediate properties
BONE CELLS AND TERMS
Compact bone and spongy bone have the same basic constituents, but differ in their 3 dimensional
organization
The basic functional unit of mature compact bone is the osteon or Haversian system
In an osteon, the osteocytes are arranged in concentric layers around a central canal that contains
blood vessels that supply the osteon, central canals run parallel to the surface of the bone
osteon = basic functional unit of compact bone
perforating canals (or canals of Volkmann) extend perpendicular to the surface from the central
canals; blood vessels in these canals run deeper into the bone
osteocytes = mature bone cells found in small pockets called lacunae sandwiched between layers of
calcified matrix
lacunae = pockets which contain osteocytes
lamellae = thin layers of matrix around lacunae and central canals
canaliculi = hollow channels that penetrate the lamellae and radiate through the matrix to connect
one lacunae with another to provide nutrients and oxygen to bone cells; nutrients diffuse from the
canaliculi to the osteocytes
osteoblasts = cuboidal cells that make the bone matrix that is later calcified
osteoclasts = large cells with many nuclei that dissolve the bone matrix (using acids) and release the
stored minerals, calcium and phosphate = important for regulating blood calcium and phosphate
levels in the body
Osteoblasts and osteoclasts are always working and the balance between the activites of these
2 cell types is very important; if osteoclasts remove calcium salts to fast, the bones may
become weak, if the osteoblasts dominant, the bones may become stronger and larger
cortex = the outer regions or layers of the bone
medullary (or marrow) cavity = space within the bone that contains the marrow
diaphysis (or shaft) = the shaft of a long bone; note that diaphyses is plural
epiphysis = the end of a long bone; note that epiphyses is plural
epiphyseal line = a point of fusion where the growing regions of the bone unite
BIOLOGICAL SCIENCE 102 – ANIMAL BIOLOGY – NOTES & VOCABULARY
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epiphyseal plate = a region of cartilage between the epiphysis and the diaphysis in a bone that is still
growing
nutrient foramina = small holes that allow for the passage of blood vessels into and out of the bone
periosteum = the dense, connective tissue sheath that covers the outside of a bone
endosteum = the inner lining of marrow cavity of the long bones near the marrow
ossification = the formation of bone (not just cartilage)
calcification = the deposition of calcium salts (e.g. calcium phosphate) into bone
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BONE GROWTH
There are two main processes by which bones develop:
(It is very difficult to distinguish between mature bones that developed by one process versus the
other)
1. endochondral ossification
- most bones develop using this process
- begins with a template of hyaline cartilage which is eventually calcified to form bone
- within the hyaline cartilage, primary ossification centers (in the diaphyses) are the first
areas of the bone to become mineralized
- basically the chondrocytes enlarge, the pH drops (acidity increases) and bone
minerals precipitate out
- this walls off the chondrocytes and they starve; osteoblasts then invade the cartilage
and bone mineralization is promoted
- later, secondary ossification centers (in the epiphyses) develop and expand the
mineralization of the cartilage
- for example, a fetus has a nearly transparent skeleton which is primarily composed of hyaline
cartilage
- ossification begins before birth, but large amounts of the skeleton are still cartilage at the
time of birth and the skeletal system is generally more flexible in younger individuals
- None of the growth bones are fused at the time of birth and a neonate does not possess
carpal bones (growth bones include bones such as the pelvic bones and the skull)
2. intramembranous ossification
- during this type of bone formation, mineralized bone does not replace cartilage
- instead, a fibrous membrane is laid down and ossification occurs between the fibers
- this type of ossification is typical of the complex bones of the skull and the clavicle
- the membrane is made of collagen into which osteoblasts secrete spicules (threads) of bone
- a sesamoid bone is formed in this fashion (looks like a sesame seed)
- the patella (knee bone) is an example of a sesamoid bone
- the patella begins its development at about two years of age within the tendon that
stretches from the quadriceps to the tibia
Appositional Growth = bone increases in diameter from the outside of the developing bone (like
putting layers of cement on the outside of a round building); new bone forming cells enter from the
periosteum
- Generally, remodeling of bone is accomplished from the inside by osteoclasts until bone
growth is completed (about 20 years of age)
- Bone growth is somewhat like building a snow fort, snow is packed around the outside, while
the inside is hollowed out to enlarge it
Epiphyseal Plate Growth = bones grow in length from their ends (epiphyses), as chondrocytes wall
themselves off and die, the area becomes ossified and new cartilage is formed at the ends (which will
eventually be ossified); this process occurs until bone growth ceases sometime after puberty
BIOLOGICAL SCIENCE 102 – ANIMAL BIOLOGY – NOTES & VOCABULARY
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NOTES ABOUT PUBERTY IN HUMANS:
(there is much individual variation in bone growth, the age at which bone growth ceases, etc.)
- Epiphyseal Fusion = at puberty the growth of the epiphyseal plates speeds up in response to
hormones; growth stops when the cartilage “eating” cells (chondroclasts) catch up with the cartilage
producing cells (chondroblasts) in the epiphyses.
- Bone growth is primarily controlled by hormones (growth hormone and sex steroids)
- There is a growth spurt that occurs at puberty
- Generally, developing girls experience their growth spurt before developing boys
- Girls start their growth spurt earlier (and thus are taller than boys as puberty begins, e.g. junior
high), but boys experience a longer growth spurt so that in the end, generally, boys are taller than girls
- The clavicle is usually the last bone to complete its growth; this helps determine “boy shape” from
“man shape” (that is, men have broader shoulders than boys)