Chapter 6
Bones and Skeletal Tissues
179
6
has distinguishing features and functions. In most cases, bone
projections indicate the stresses created by muscles attached to
and pulling on them or are modified surfaces where bones meet
and form joints.
Bone markings that are depressions and openings include
fossae (singular: fossa), sinuses, foramina (singular: foramen),
and grooves. Tey usually allow nerves and blood vessels to
pass.
Table 6.1
describes the most important types of bone
markings. Familiarize yourself with these terms because you
will meet them again as identifying marks of the individual
bones studied in the lab.
Microscopic Anatomy of Bone
Cells of Bone Tissue
Five major cell types populate bone tissue: osteogenic cells, os-
teoblasts, osteocytes, bone lining cells, and osteoclasts. All of
these except for the osteoclasts originate from mesenchymal
cells. Each cell type is essentially a specialized form of the same
basic cell type that transforms to a mature or functional form
that serves bone growth in some specific way
(Figure 6.5)
.
Bone cells, like other connective tissue cells, are surrounded by
an extracellular matrix of their making.
Osteogenic Cells
Osteogenic cells
, also called
osteoprogeni-
tor cells
, are mitotically active stem cells found in the membra-
nous periosteum and endosteum. In growing bones they are
flattened or squamous cells. When stimulated, these cells dif-
ferentiate into osteoblasts or bone lining cells (see below), while
others persist as osteogenic cells.
Osteoblasts
Osteoblasts
are bone-forming cells that secrete
the bone matrix. Like their close relatives, the fibroblasts and
chondroblasts, they are actively mitotic. Te unmineralized
bone matrix they secrete includes collagen (90% of bone pro-
tein) and calcium-binding proteins that make up the initial un-
mineralized bone, or
osteoid
. As described later, osteoblasts also
play a role in matrix calcification.
(a) Osteogenic cell
(b) Osteoblast
(c) Osteocyte
Stem cell
Mature bone cell that monitors
and maintains the mineralized
bone matrix
Matrix-synthesizing cell
responsible for bone growth
(d) Osteoclast
Bone-resorbing cell
Figure 6.5
Comparison of different types of bone cells.
The bone lining cell, similar in
appearance to the osteogenic cell and similar to the osteocyte in function, is not illustrated.
the underlying bone (Figure 6.4). Te periosteum also provides
anchoring points for tendons and ligaments. At these points the
perforating fibers are exceptionally dense.
A delicate connective tissue membrane called the
endos-
teum
(en-dos
9
te-um; “within the bone”) covers internal bone
surfaces (Figure 6.4). Te endosteum covers the trabeculae of
spongy bone and lines the canals that pass through the compact
bone. Like the periosteum, the endosteum contains osteogenic
cells that can differentiate into other bone cells.
Location of Hematopoietic Tissue in Bones
Hematopoietic tissue,
red
marrow
, is typically found within the
trabecular cavities of spongy bone of long bones and in the diploë
of flat bones. For this reason, both these cavities are o±en called
red marrow cavities
. In newborn infants, the medullary cavity
of the diaphysis and all areas of spongy bone contain red bone
marrow. In most adult long bones, the fat-containing medullary
cavity extends well into the epiphysis, and little red marrow is
present in the spongy bone cavities. For this reason, blood cell
production in adult long bones routinely occurs only in the heads
of the femur and humerus (the long bone of the arm).
Te red marrow found in the diploë of flat bones (such as the
sternum) and in some irregular bones (such as the hip bone) is
much more active in hematopoiesis. When clinicians suspect
problems with the blood-forming tissue, they obtain red mar-
row samples from these sites. However, yellow marrow in the
medullary cavity can revert to red marrow if a person becomes
very anemic and needs more red blood cells.
Bone Markings
Te external surfaces of bones are rarely smooth and featureless.
Instead, they display projections, depressions, and openings.
Tese
bone markings
serve as sites of muscle, ligament, and
tendon attachment, as joint surfaces, or as conduits for blood
vessels and nerves.
Projections—bone markings that bulge outward from the
surface—include heads, trochanters, spines, and others. Each
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