Bones and skeletal tissues 2014

Bones and
Skeletal
Tissues
BIO 351
Human Anatomy &
Physiology I
Dr. Barbara T. Wizer

Skeletal Cartilages
 Skeletal cartilages are made from cartilage,
surrounded by a layer of dense irregular connective
tissue called the perichondrium.
 Hyaline cartilage is the most abundant skeletal cartilage,
and includes the articular, costal, respiratory, and nasal
cartilages.
 Elastic cartilage is more flexible than hyaline, and is
located only in the external ear, the epiglottis of the
larynx, and the eustachian (auditory) tube.
 Fibrocartilage is located in areas that must withstand a
great deal of pressure or stretch, such as the cartilages of
the knee (menisci), and the intervertebral discs

Fibrocartilage
in the menisci
of the knee

Hyaline cartilage covers the articular surfaces of
bones.

The costal cartilages are made up of hyaline cartilage.

Classification of Bones, 1
 There are two main divisions of the bones of
the skeleton: the axial skeleton and the
appendicular skeleton. The axial skeleton
consists of the skull, vertebral column, and rib
cage
 The appendicular skeleton consists of the
bones of the upper and lower limbs, and the
girdles that attach them to the axial skeleton

Structural Classification of Bones
5 Types:
 Long bones are longer than they are wide, have a definite
shaft and two ends, and consist of all limb bones except the
patellas, carpals, and tarsals.
 Short bones are somewhat cube-shaped and include the
carpals and tarsals.
 Flat bones are thin, flattened, often curved bones that include
most skull bones, the sternum, scapulae, and ribs.
 Irregular bones have complicated shapes that do not fit in any
other class, such as the vertebrae and coxae.
 Sesamoid bones – “extra” bones that develop in tendons or
near joints, esp. in feet + hands. Include the patellas, which
are the only sesamoid bones consistently present in skeleton

Functions of Bones
 Bones support the body and cradle the soft
organs,
 protect vital organs,
 allow movement,
 store minerals such as calcium and phosphate,
 and house hematopoietic tissue (which forms
blood cells) in specific marrow cavities.

Gross Anatomy
 Bone markings are projections, depressions,
and openings found on the surface of bones
that function as sites of muscle, ligament, and
tendon attachment, as joint surfaces, and as
openings for the passage of blood vessels and
nerves.

Compact and Spongy Bone
 All bone has a dense outer layer consisting of
compact bone that appears smooth and solid.
 Internal to compact bone is spongy bone,
which consists of honeycomb, needle-like, or
flat pieces, called trabeculae.

Structure of a Typical Long Bone
 Long bones have a tubular bone shaft, consisting of a bony
collar surrounding a hollow medullary cavity, which is filled
with yellow bone marrow in adults.
 Epiphyses are at the ends of the bone, and consist of internal
spongy bone covered by an outer layer of compact bone.
 The epiphyseal line (or scar) is located between the epiphyses
and diaphysis, and is a remnant of the epiphyseal plate (=
growth plate).
 The external surface of the bone is covered by a connective
tissue membrane called the periosteum.
 The internal surface of the bone is lined by a connective
tissue membrane called the endosteum.

Bones and skeletal tissues 2014

Location of Hematopoietic Tissue
 Hematopoietic tissue of bones, red bone
marrow, is located within the trabecular
cavities of the spongy bone in flat bones, and
in the epiphyses of long bones.
 Red bone marrow is found in all flat bones,
epiphyses, and medullary cavities of infants,
but in adults, distribution is restricted to flat
bones and the proximal epiphyses of the
humerus and femur.

Microscopic Anatomy of Bone,1
 The structural unit of compact bone is the osteon, or
Haversian system, which consists of concentric tubes of bone
matrix (the lamellae) surrounding a central Haversian canal
that serves as a passageway for blood vessels and nerves.
 Perforating, or Volkmann’s, canals lie at right angles to the
long axis of the bone, and connect the blood and nerve supply
of the periosteum to that of the central canals and medullary
cavity.
 Osteocytes occupy lacunae at the junctions of the lamellae,
and are connected to each other and the central canal via a
series of hair-like channels, canaliculi.

Microscopic Anatomy of Bone, 2
 Circumferential lamellae are located just
beneath the periosteum, extending around the
entire circumference of the bone, while
interstitial lamellae lie between intact osteons,
filling the spaces in between.
 Spongy bone lacks osteons but has trabeculae
that align along lines of stress, which contain
irregular lamellae.

The microscopic
image of bone
tissue show
circumferential
lamellae.

Note the
trabeculae
in spongy
bone.

Chemical Composition of Bone
 Organic components of bone include cells
(osteoblasts, osteocytes, and osteoclasts) and osteoid
(ground substance and collagen fibers), which
contribute to the flexibility and tensile strength of
bone.
 Inorganic components make up 65% of bone by
mass, and consist of hydroxyapatite, a mineral salt
that is largely calcium phosphate, which accounts for
the hardness and compression resistance of bone.

Hydroxyapatite, the inorganic component of bone

Formation of the Bony Skeleton
Bone develops in one of 2 ways:
 Intramembranous ossification - bones form
within fibrous connective tissue membranes.
The cranial bones and clavicles develop in
this way.
 In endochondral ossification hyaline cartilage
models are laid down first, then replaced by
calcified bony matrix, forming all bones
below the skull except for the clavicles.

Fetal skull
showing the
fontanel
between the
frontal and
parietal bones
—what is the
function of the
fontanels?

The epiphyseal
plates are made
up of hyaline
cartilage and
are the places
where long
bones grow in
length.

The epiphyseal
plates are
replaced with
epiphyseal
lines when
growth in
height is
completed.

Postnatal Bone Growth
 Growth in length of long bones occurs at the
osteogenic zone through the rapid division of the
upper cells in the columns of chondrocytes,
calcification and deterioration of cartilage at the
bottom of the columns, and subsequent replacement
by bone tissue — endochondral growth.
 Growth in width, or thickness, occurs through
appositional growth due to deposition of bone
matrix by osteoblasts beneath the periosteum.

Hormonal Regulation
 During infancy and childhood, the most
important stimulus of epiphyseal plate activity
is growth hormone from the anterior
pituitary, whose effects are modulated by
thyroid hormone.
 At puberty, testosterone and estrogen
promote a growth spurt, but ultimately induce
the closure of the epiphyseal plate.

Bone Remodeling
 In adult skeletons, bone remodeling is a balance of
bone deposit and removal; bone deposit occurs at a
greater rate when bone is injured, and bone
resorption allows minerals of degraded bone matrix
to move into the blood.
 Osteoblasts are the bone forming cells and
osteoclasts are the cells that break down bone.
 In response to mechanical stress and gravity, bone
grows or remodels in ways that allow it to withstand
the stresses it experiences.

Osteoporosis
 A group of disorders in which the rate of bone resorption
exceeds the rate of formation.
 Bones have normal bone matrix, but bone mass is reduced
and the bones become more porous and lighter-- increasing
the likelihood of fractures.
 Older women are especially vulnerable to osteoporosis, due
to the decline in estrogen after menopause.
 Other factors that contribute to osteoporosis include a petite
body form, insufficient exercise or immobility, a diet poor in
calcium and vitamin D, abnormal vitamin D receptors,
smoking, and certain hormone-related conditions.

Bone Diseases
 Osteoporosis
Bone loss - results in increased risk of
fractures – esp. vertebrae, hip (proximal
femur), wrist
 Rickets (children)/ Osteomalacia (adults)
demineralization of bone due to Vit. D
deficiency
 Osteomyelitis – infection of bone
 Osteosarcoma – malignant tumor

Regulation of Blood Calcium Level
An example of Homeostasis
 Hormones
1 – parathyroid hormone (PTH)
produced by the parathyroid glands in the neck,
secreted when blood Ca level is low, causes bone
breakdown and release of Ca from the bone into
the blood
2 – calcitonin
produced by the thyroid gland in the neck,
secreted when blood Ca level is high, causes
decreased breakdown of bone matrix so blood Ca
level drops

Causes of Hypercalcemia
 Hyperparathyroidism
 Malignancy – cancer with metastasis to bone,
certain solid tumors, certain hematologic
malignancies
 Renal failure
 Certain meds
 Hypervitaminosis D

Bones and skeletal tissues 2014

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