Page 231 - Fundamentals of anatomy physiology

218 Unit 2 Support and Movement Checkpoint
Intramembranous Ossification
Intramembranous (in-tra-MEM-bra-nus) ossification be-
gins when osteoblasts differentiate within a mesenchymal
or fibrous connective tissue. This type of ossification is also
called dermal ossification because it normally takes place in the
deeper layers of the dermis. The bones that result are called
dermal bones. Examples of dermal bones are the flat bones of
the skull, the mandible (lower jaw), and the clavicles (collar-
bones). The steps of intramembranous ossification are sum-
marized in Figure 6–12.
6 The Blood and Nerve Supplies to Bone
In order for bones to grow and be maintained, they require
an extensive blood supply. For this reason, osseous tissue is
highly vascular. In a typical bone such as the humerus, three
major sets of blood vessels develop (Figure 6–13):
1. The Nutrient Artery and Vein. The blood vessels that sup-
ply the diaphysis form by invading the cartilage model as
endochondral ossification begins. Most bones have only
one nutrient artery and one nutrient vein, but a few bones,
including the femur, have more than one of each. The ves-
sels enter the bone through one or more round passage-
ways called nutrient foramina in the diaphysis. Branches of
these large vessels form smaller perforating canals and ex-
tend along the length of the shaft into the osteons of the
surrounding compact bone.
2. Metaphyseal Vessels. The metaphyseal vessels supply blood to of osteocytes, osteoblasts, and osteoclasts. In adults, osteocytes
the inner (diaphyseal) surface of each epiphyseal cartilage, are continuously removing and replacing the surrounding cal-
where that cartilage is being replaced by bone.
3. Periosteal Vessels. Blood vessels from the periosteum pro-
vide blood to the superficial osteons of the shaft. During
endochondral bone formation, branches of periosteal ves-
sels also enter the epiphyses, providing blood to the sec-
ondary ossification centers.
Following the closure of the epiphyses, all three sets of
vessels become extensively interconnected.
The periosteum also contains a network of lymphatic ves-
sels (lymphatics) and sensory nerves. The lymphatics collect
lymph from branches that enter the bone and reach individ-
ual osteons by the perforating canals. The sensory nerves pen-
etrate the compact bone with the nutrient artery to innervate
the endosteum, medullary cavity, and epiphyses. Because of
the rich sensory innervation, injuries to bones are usually very
painful.
In the next section, we examine the maintenance and re-
placement of mineral reserves in the adult skeleton.
9. In endochondral ossification, what is the original
source of osteoblasts?
1 0. During intramembranous ossification, which type
of tissue is replaced by bone?
1 1. How could x-rays of the femur be used to determine
whether a person has reached full height?
See the blue Answers tab at the back of the book.
6-6 Bone growth and development
depend on a balance between bone
formation and bone resorption
Learning Outcome Describe the remodeling and homeostatic
mechanisms of the skeletal system.
The process of remodeling continuously recycles and renews
the organic and mineral components of the bone matrix. Bone
remodeling goes on throughout life, as part of normal bone
maintenance. Remodeling can replace the matrix but leave the
bone as a whole unchanged, or it may change the shape, inter-
nal architecture, or mineral content of the bone. Through re-
modeling, older mineral deposits are removed from bone and
released into the circulation at the same time that circulating
minerals are being absorbed and deposited.
Bone remodeling involves interplay among the activities
cium salts. Osteoclasts and osteoblasts also remain active, even
after the epiphyseal cartilages have closed. Normally, their
activities are balanced: As quickly as osteoblasts form one os-
teon, osteoclasts remove another by osteolysis.
The turnover rate of bone is quite high. In young adults,
almost one-fifth of the skeleton is recycled and replaced each
year. Not every part of every bone is affected equally. The rate
of turnover differs regionally and even locally. For example,
the spongy bone in the head of the femur may be replaced two
or three times each year, but the compact bone along the shaft
remains largely unchanged.
Because of their biochemical similarity to calcium, heavy-
metal ions such as lead, strontium, cobalt, or radioactive ura-
nium or plutonium can be incorporated into the matrix of
bone. Osteoblasts do not differentiate between these heavy-
metal ions and calcium. This means that any heavy-metal ions
present in the bloodstream will be deposited into the bone
matrix. Some of these ions are potentially dangerous, and the
turnover of bone matrix can have detrimental health effects as
ions that are absorbed and accumulated are released into the
circulation over a period of years. This was one of the major