Architecture of bone

There are two types of bone tissue: woven or primary
bone and lamellar or mature bone. All of the bone in
the newborn baby is woven and this bone type occurs
also in localized regions of the growing skeleton and
in repairing fractures. Woven bone is characterized
by a random orientation of its collagen fi bres and a
low mineral content relative to that of lamellar bone.
Except in a very few locations, such as tooth sockets,
woven bone is replaced in infants over one year old
by lamellar bone, so-called because the collagen fi bres
are orientated in parallel arrays. The bone tissue described
in the following is lamellar bone.
There are two types of lamellar bone: cortical bone
and cancellous or trabecular bone. Cortical bone is a
dense solid mass which forms the external layer of all
bones of the body and predominates in the shafts of
long bones. Cancellous bone takes the form of an irregular
latticework of rods, plates and arches individually
known as trabeculae. Cancellous bone makes up most
of the bone tissue of short, fl at and irregularly shaped
bones such as the vertebrae, pelvis and shoulder blade;
it is also found in the bulbous ends of long bones.
In early life, the spaces between the trabeculae of all
cancellous bones are fi lled with haemopoietic (bloodproducing)
red bone marrow, but later the red marrow
is confi ned to the cancellous bones of the trunk. The
spaces in other cancellous bones are fi lled with yellow
bone marrow, which consists mostly of fat. In a typical long bone (Fig. 3.34) the shaft (diaphysis)
is composed almost entirely of cortical bone with
a small component of cancellous bone surrounding
the medullary cavity. In the adult, the medullary cavity
contains yellow bone marrow. The bulbous ends
(epiphyses) and the metaphyses consist of cancellous
bone covered by a shell of cortical bone. In the
growing animal, the epiphysis is separated from the
metaphysis by a thick plate of hyaline cartilage known
as the epiphyseal plate; it is in this region that bone
elongation occurs. In the adult, the cartilaginous plate
has been replaced by cancellous bone.
The outer surfaces of most bones, but not the articular
surfaces of joints, are covered by a bilayered
sheath called the periosteum. The outer layer of the periosteum consists of dense fi brous connective tissue
and contains a network of blood vessels. The inner
layer is composed of more loosely arranged connective
tissue and numerous osteoprogenitor cells. Some
component collagenous fi bres of the periosteum penetrate
the bone matrix as Sharpey’s fi bres. Inside the
bone, the medullary cavity and the cavities of cancellous
bone are lined by the endosteum. This membrane
is composed of a single layer of osteoprogenitor cells
and a very small amount of connective tissue. The
principal functions of the periosteum and endosteum
are nutrition of the osseous tissue and the provision
of new osteoblasts for bone repair.