Covering, Support, and Movement of the Body
Flat bones consist of two thin plates of compact bone enclosing a
diploë (spongy bone layer). Short and irregular bones resemble
ﬂat bones structurally.
A long bone is composed of a diaphysis (sha±) and epiphyses (ends).
Te medullary cavity of the diaphysis contains yellow marrow; the
epiphyses contain spongy bone. Te epiphyseal line is the remnant of
the epiphyseal plate. Periosteum covers the diaphysis; endosteum lines
inner bone cavities. Hyaline cartilage covers joint surfaces.
In adults, hematopoietic tissue (red marrow) is found within the
diploë of ﬂat bones and occasionally within the epiphyses of long
bones. In infants, red marrow is also found in the medullary cavity.
Bone markings are important anatomical landmarks that reveal
sites of muscle attachment, points of articulation, and sites of
blood vessel and nerve passage.
Microscopic Anatomy of Bone
Tere are ﬁve types of bone cells—osteogenic cells (bone stem
cells), osteoblasts (matrix-synthesizing cells), osteocytes (bone
matrix maintenance cells), bone lining cells (line surfaces where no
bone activity is ongoing), and osteoclasts (bone destruction cells).
Te structural unit of compact bone, the osteon, consists of a
central canal surrounded by concentric lamellae of bone matrix.
Osteocytes, embedded in lacunae, are connected to each other
and the central canal by canaliculi.
Spongy bone has slender trabeculae containing irregular lamellae,
which enclose red marrow–ﬁlled cavities.
Chemical Composition of Bone
Bone is composed of living cells and matrix. Te extracellular
matrix includes osteoid, organic substances that are secreted by
osteoblasts and give the bone tensile strength. Its inorganic (mineral)
components, the hydroxyapatites (calcium salts), make bone hard.
Formation of the Bony Skeleton
Intramembranous ossiﬁcation forms the clavicles and most skull
bones. Te ground substance of the bone matrix is deposited
between collagen ﬁbers within the ﬁbrous membrane to form
bone. Eventually, compact bone plates enclose the diploë.
Most bones are formed by endochondral ossiﬁcation of a hyaline
cartilage model. Osteoblasts beneath the periosteum secrete bone
matrix on the cartilage model, forming the bone collar. As the
cartilage model deteriorates, internal cavities open up, allowing
periosteal bud entry. Bone matrix is deposited around the
cartilage remnants but is later broken down.
Postnatal Bone Growth
Long bones increase in length by interstitial growth of the
epiphyseal plate cartilage and its replacement by bone.
Appositional growth increases bone diameter/thickness.
Bone Homeostasis: Remodeling and Repair
Bone is continually deposited and resorbed in response to
hormonal and mechanical stimuli. ²ogether these processes
constitute bone remodeling.
An unmineralized osteoid seam appears at areas of new bone
deposit; calcium salts are deposited a few days later.
Osteoclasts release lysosomal enzymes and acids on bone surfaces
to be resorbed. Te dissolved products are transcytosed to the
opposite face of the osteoclast for release to the extracellular ﬂuid.
Te hormonal controls of bone remodeling serve blood calcium
homeostasis. When blood calcium levels decline, P²H is released
and stimulates osteoclasts to digest bone matrix, releasing ionic
calcium. As blood calcium levels rise, P²H secretion declines.
Mechanical stress and gravity acting on the skeleton help maintain
skeletal strength. Bones thicken, develop heavier prominences, or
rearrange their trabeculae in sites where stressed.
Fractures are treated by open or closed reduction. Te healing
process involves formation of a hematoma, a ﬁbrocartilaginous
callus, a bony callus, and bone remodeling, in succession.
Homeostatic Imbalances of Bone
Imbalances between bone formation and resorption underlie all
Osteomalacia and rickets occur when bones are inadequately
mineralized. Te bones become so± and deformed. Te most
frequent cause is inadequate vitamin D.
Osteoporosis is any condition in which bone breakdown outpaces
bone formation, causing bones to become weak and porous.
Postmenopausal women are particularly susceptible.
Paget’s disease is characterized by excessive and abnormal bone
Developmental Aspects of Bones: Timing of Events
Osteogenesis is predictable and precisely timed.
Longitudinal long bone growth continues until the end of
adolescence. Skeletal mass increases dramatically during puberty
and adolescence, when formation exceeds resorption.
Bone mass is fairly constant in young adulthood, but beginning
in the 40s, bone resorption exceeds formation.
(Some questions have more than one correct answer. Select the best
answer or answers from the choices given.)
Which is a function of the skeletal system?
providing levers for
all of these.
A bone with approximately the same width, length, and height is
a long bone,
a short bone,
a ﬂat bone,
an irregular bone.
Te sha± of a long bone is properly called the
Sites of hematopoiesis include all but
red marrow cavities of
the diploë of ﬂat bones,
medullary cavities in
bones of infants,
medullary cavities in bones of a healthy adult.