Skeletal System: Bone Classification, Bone Growth and Remodeling

1. Bone Classification
a. Bones are classified based on their basic shapes:
i. Flat bones
1. Flat bones are always thin and flat.
2. They have a thin outer layer of compact bone; the inside is completely filled with
spongy bone.
a. The spaces of the spongy bone are filled with red bone marrow.
3. Examples: Cranial bones, sternum, ribs, and clavicle
ii. Irregular bones
1. Irregular bones have a variety of shapes and surfaces and do not fit into any
other category.
2. They have thin outer layers of compact bone and are completely filled with
spongy bone.
a. The spaces of the spongy bone are filled with red bone marrow.
3. Examples: Vertebrae and os coxal bones (hip bones)
iii. Short bones
1. Short bones are roughly cube-shaped bones.
2. They have thin outer layers of compact bone and are completely filled with
spongy bone.
a. The spaces of the spongy bone are filled with red bone marrow.
3. Examples: Carpal bones and Tarsal bones
iv. Sesamoid bones
1. Sesamoid bones are short bones that develop in tendons.
2. They have thin outer layers of compact bone and are completely filled with
spongy bone.
a. The spaces of the spongy bone are filled with red bone marrow.
3. The only example of a sesamoid bone that everyone will have is the patella.
Some individuals have sesamoid bones that develop in tendons of the hands or
feet but these are not present in everyone.
v. Sutural bones
1. Sutural bones are small bones that sometimes develop in joints between cranial
bones called sutures.
2. Not everyone has sutural bones so there is no example that everyone will have.
vi. Long bones
1. Long bones are roughly rectangular in shape. All the longest bones in the body
are long bones but long bones to not have to be large.
2. The detailed structure of long bones will be presented later.
3. Examples: Humerus, Femur, Metacarpals, Phalanges
2. Long Bone Structure and Growth
a. Regions
i. Epiphyses
1. Singular = Epiphysis

2. Epiphyses are the wide tips of long bones.
3. They have a thin outer layer of compact bone and are completely filled with
spongy bone.
a. The spaces of the spongy bone are filled with red bone marrow.
4. The tips of the epiphyses are covered by thin layers of hyaline cartilage called
articular cartilages.
ii. Diaphysis
1. The diaphysis is the thin, middle region of the long bone.
2. The diaphysis has a thick outer layer of compact bone, a very thin inner lining of
spongy bone, and a hollow center called the medullary cavity.
a. The medullary cavity is filled with yellow bone marrow.
iii. Metaphyses
1. Singular = Metaphysis
2. The metaphyses are the small regions of the long bone between the diaphysis
and the epiphyses.
3. What is found in the metaphysis depends on age:
a. Before young adulthood:
i. The metaphysis contains hyaline cartilage and is called the epiphyseal
plate.
1. From birth until young adulthood the cartilage in the
epiphyseal plate grows and edges are converted to bone to
increase the length of the long bone. This is how long bones
grow in length and people grow in height.
2. Starting in adolescence the growth in the epiphyseal plate
slows down and eventually the entire epiphyseal plate is
completely converted to bone (endochondral ossification) and
the growth of the bone stops at young adulthood.
b. From young adulthood until death:
i. The metaphyses contain an outer layer of compact bone and are filled
with spongy bone. The bone that replaced the epiphyseal plate is
called the epiphyseal line. There is no growth in bone length during
this time because there is no hyaline cartilage.
3. Osteogenesis
a. Osteogenesis- bone tissue formation, how bones are made initially
i. Begins at 8 weeks gestation and continues until young adulthood
b. Two types:
i. Intramembranous Ossification- production of bone from embryonic connective tissue
called mesenchyme.
1. How membrane/dermal bones are made. Examples: Most cranial bones (Flat
bones), clavicle, sternum.
2. Steps in Intramembranous Ossification- See slide 8
a. Blood vessels grow into a region of mesenchyme converting clusters of
mesenchymal cells into osteoblasts.

b. The new osteoblast clusters produce star shaped regions of solid bone
matrix called spicules.
c. Over time the spicules grow into one another and form spongy bone tissue
with trabeculae formed by fused branches of the spicules.
d. The edges of the newly formed spongy bone are remodeled into compact
bone forming a flat bone containing spongy bone in the center. The
trapped blood vessels for red bone marrow.
ii. Endochondral Ossification- Production of a bone from a hyaline cartilage model
1. Conversion of hyaline cartilage to bone.
2. How all bones from the neck down (except clavicles) are made.
3. Begins at week 8 gestation and continues in the epiphyseal plates until young
adulthood.
4. Steps in Endochondral Ossification- See slides 6 and 7
a. Production of the bone begins with the formation of a hyaline cartilage
model of the bone.
b. Blood vessels grow around the diaphysis of the cartilage model converting
perichondrium to periosteum with osteoblasts.
c. New osteoblast produce a layer of compact bone around the diaphysis
called a bone collar.
d. Inside the bone collar the cartilage matrix begins to calcify, and
chondrocytes enlarge and die leaving behind cavities in the diaphysis.
e. Blood vessels grow into the diaphysis and over time as the diaphysis grows
in length converts remaining cartilage and calcified matrix to spongy bone
and a hollow medullary cavity. This new bone tissue produced in the
diaphysis is called the primary ossification center.
f. Blood vessels grow around and then into the epiphyses causing the
conversion of cartilage in the epiphyses to spongy bone surrounded by a
thin layer of compact bone. These areas of new bone tissue produced in
the epiphyses are called secondary ossification centers.
g. The metaphysis remains as hyaline cartilage and becomes the epiphyseal
plate, the tips of the epiphyses remain as hyaline cartilage and become the
articular cartilage.
4. Bone Remodeling
a. Bone Remodeling Units = Osteoblasts + Osteoclasts
i. In locations where there are combinations of osteoblasts and osteoclasts
1. Most bone production occurs in the periosteum.
2. Most bone resorption occurs in the endosteum.
a. Bone resorption = bone tissue breakdown
ii. Used to replace bone tissue over time and regulate bone thickness
1. If osteoblasts and osteoclast function at the same rate bone tissue is replaced
without changing thickness of the bone.
2. If osteoblasts work faster than osteoclasts, there is more bone production than
bone resorption = bone becomes thicker.

3. If osteoclasts work faster than osteoblasts, there is more bone resorption than
bone production = bone becomes thinner.
iii. Regulated by hormones and physical stress on the bone
1. Hormones
a. Parathyroid hormone (PTH)
b. Calcitonin (CT)
2. Physical stress on a specific area of a bone stimulates osteoblasts = more bone
production = bone becomes thicker.