Bone Histology

What is the calcified matrix composed of?

• minerals (65%): mostly calcium hydroxyapatite crystals,

• organic matter :

  • type I collagen,

  • sulphated glycoproteins, and

  • proteoglycans (including bound water)

What are the functions of bones

• Support and protection for the body and its organs.

• Reservoir for calcium and phosphate ions.

• Plays an important role in hematopoiesis

HOw are bones classified?

• according shape;

  • location of spongy and compact bone varies with shape

Describe the structure of the long bone

In shaft:

• entire thickness of bone tissue = compact, (only small portion = spongy), which faces the marrow cavity.

At ends:

• most of the bone is spongy, and only the outer shell of bone is compact.

Describe the structure of short, flat, and irregular bones

• Short

  • equal in length and diameter.

  • carpal bones in hand.
    

• Flat

  • thin and plate-like,

  • consist of two layers of thick compact bone; intervening layer of spongy bone.

  • bones of the skull.

    
    

• Irregular bones,

  • contain air sinuses

  • vertebrae, ethmoid bone

Describe what is on the outer surface of bone? What are the two layers of this? Describe each layer? How is collagen arranged? Exeption?

• covered by periosteum (except in areas covered by articular cartilage)

  • sheath of dense connective tissue consisting of:

    • outer fibrous layer

    • inner cellular layer

      • contains osteoprogenitor cells.

      • more prominent in growing bone.

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• Collagen fibers = parallel to bone surface

  • except at tendon/ligament’s attachment

    • here, collagen fibers extend directly into bone tissue

    • This is called Sharpey’s fibers

What is inside the bone cavity (aka marrow cavity) of both compact/spongy bones? How is this different in adults/ kids

• lined by endosteum,

  • a layer of connective tissue containing
    osteoprogenitor cells (can → osteoblasts and bone–lining cells.)
    

Younger people/kids:

• marrow cavity contains red bone marrow → later replaced by yellow bone marrow (fat)

  • red b/c blood cells in different stages and reticular fiber network

Adults:

• red bone marrow is restricted to spaces of spongy
  bones at a few places in the body,

  • ex: sternum and the iliac crest

    • site for taking diagnostic bone marrow samples

What are the five types of cells associate with bone tissues; how are osteocytes different than the rest of these cell types

1. Osteoprogenitor cells,

2. Osteoblasts,

3. Osteocytes,

4. Bone lining cells

5. Osteoclasts.

Similarities (except for OSTEOCLASTS):

• differentiates from mesenchymal stem cell

• all transform from less mature → more mature form (bone growth)

ONLY OSTEOCLASTS:

• originates from granulocyte / monocyte progenitor

• responsible for bone resorption , an activity associated with bone remodeling.

What is osteogenesis? Where are Osteoprogenitor found? Describe their cell type/structure

• Process of new bone formation

  • requires osteoprogenitor cells

    • responsive to molecular stimuli → transformation into bone-forming cells

• Osteoprogenitor cells location:

  • periosteal and endosteal

  • microvasculature of bone. ( Harversian &
    Volkmann's canals)

• flattened or squamous-shaped cells with a very lightly staining oval nucleus and a pale cytoplasm. EM reveals rER and free ribosomes

What are osteoblasts and their functions. Describe their cell type. how do they communicate? What receptors do they possess

• Differentated from bone-forming cells

• Function:

  • secrete bone matrix (type I collagen) and bone matrix proteins (BMPs),

    • onstitute the initial unmineralized bone or osteoid.

  • responsible for the calcification of the bone matrix

• communicate with other osteoblasts and with osteocytes by gap junctions.

• possess Parathyroid hormone receptors, estrogen, and progesterone on cell membrane

• Histology:

  • Under the light microscope, osteoblasts appear as cuboidal or polygonal in shape and in a single layer lying in apposition to the forming bone.

  • cytoplasm is intensely basophilic because of an extensive rER.

What are osteocytes and their function; what happens when they die; what space do they occupy? What do they possess

• Osteocyte = osteoblasts surrounded by osteoid or bone matrix

• Function:

  • Maintain bone matrix via synthesizing or degrading

• Death (like fracture):

  • resorption of bone by osteoclasts → repair or remodeling by osteoblast activity

• occupies a space or lacuna that conforms to the shape of the cell.

• has cytoplasmic processes, → extend through canaliculi → contact processes of neighboring osteocytes and bone lining cells by means of gap junctions.

what are bone-lining cells derived from? Function? What are they called when on the external/internal side

• from osteoblasts

• cover bone that is not remodeling.

• external = periosteal cells,

• internal = endosteal cells

What are osteoclasts responsible for? Where are they found? what happens to the location that osteoclasts occupy? How are they created? What are their three zones? Whats on their cell membrane

• responsible for for bone resorption.

• large multinucleated cells, found at sites where bone is being removed.

  • rest directly on the bone tissue where
    resorption is taking place.

  • result of activity = shallow bay called a
    resorption bay or Howship’s lacuna forms under osteoclasts

  • exhibit 3 specialized zones: ruffled border, clear zone, basolateral region

• created by fusion of mononuclear hemopoietic progenitor cells under influence of cytokines.

  • Newly formed osteoclasts undergo an activation
    process to become bone–resorbing cells.

• has calcitonin receptors
  

histology: marked acidophilia and exhibits a strong histochemical reaction with acid phosphatase because of the numerous

What are the three functions of osteoclasts

1. Bone remodeling by the process of bone turnover.

2. Proper shaping of the bone.

3. Enlargement of the medullary spaces to enable hematopoiesis

What is the fundamental units of mature compact bone? Describe the parts of this unit

composed of cylindrical structural units called osteons (Haversian systems).

osteon consists of concentric lamellae of
bone matrix surrounding a central canal, called
the osteonal (Haversian ) canal, which contains
the blood vessels and nerves of the osteon.

Between the osteons are remnants of previous
concentric lamellae called interstitial lamellae.
Because of the lamellar arrangement, mature
bone is also called lamellar bone.

What and where are canaliculi; function?

Canaliculi containing the processes of
osteocytes are generally arranged in a radial
pattern with respect to the Haversian canal. This system of
canaliculi serves for the passage of substances
between osteocytes and blood vessels.

Describe the orientation of osteons? Orientation of collagen fibers on osteon? What benefit does this arrangement give?

The long axis of an osteon is parallel
to the long axis of he bone.

The collagen fibers in an osteon are
laid down parallel to one another but
in different directions from adjacent
lamellae. This arrangement gives the
cut surface of lamellar bone the
appearance of plywood and imparts
great strength to the osteon

where could lamellar bone also could be found? WHat is the orientation of this?

Lamellar bone is also found at sites
other than the osteon.
Circumferential lamellae follow the
entire inner and outer circumference
of the shaft of a long bone, appearing
much like the growth rings of a tree.

What are vokmann’s channels? What do they connect? What is a key histological feature of these channels

Perforating canals (Volkmann's
canals) are channels in the lamellar
bone through which blood vessels
and nerves travel from the periosteal
and endosteal surfaces to reach the
osteon canals.
• They also connect osteonal canals
and run at a right angle to the long
axis of the osteon and of the bone.
• Volkmann’s canals are not
surrounded by concentric lamellae, a
key feature in their histological

Describe mature spongy bone

Also called cancellous bone, e.g.,
present inside the epiphyses
(heads) of long bones, is always
surrounded by compact bone.
• It is structurally like the mature
compact bone except that the
tissue is arranged as trabeculae or
spicules, and numerous
interconnecting marrow spaces of
various sizes are present between
the bone tissue. The matrix of the
bone is lamellated.

What are immature bones? how does it differ from a mature bone?

Bone tissue initially formed in the skeleton of a
developing fetus is called immature bone.
• It differs from mature bone in several respects :
• Immature bone does not display an organized
lamellated appearance and is hence called non-
lamellar bone. It is also called bundle bone or
woven bone because of the interlacing
arrangement of the collagen fibers.
• Immature bone contains relatively more cells per
unit area than does mature bone.
• The cells in immature bone are randomly
organized, whereas cells in mature bone are
usually arranged with their long axis in the same
direction as the lamellae.

The matrix of immature bone has more
ground substance than the mature
bone.
• The matrix in immature bone stains
more intensely with hematoxylin
• Immature bone forms more readily
than mature bone

The matrix of compact bone stains
more intensely with eosin.
• Mature bone is the main type of
bone in an adult whereas immature
bone in adults is seen where bone is
being remodeled , at the insertion
of tendons and in alveolar sockets
of the teeth . (orthodontic
corrections)

What are the two types of bone development

endochondral
(cartilage precursor ) or intramembranous ossification ( without
intervention of cartilage model).
• The bones of the extremities and those parts of the axial skeleton
that bear weight develop by endochondral ossification.
• The flat bones of the skull & face, mandible, and clavicle develop by
intramembranous ossification.

Describe the mechanism of intramembranous ossification

Bone is formed by the differentiation of
mesenchymal cells into osteoblasts.
• Intramembranous ossification begins
around the 8th week of gestation.
• Mesenchymal cells migrate and aggregate in
specific areas destined to form bones. This
condensation of mesenchymal cells
(ossification centers) starts the process of
intramembranous ossification.

As the process continues, the site becomes
vascularized, and the aggregated mesenchymal
cells become larger and rounded. The cytoplasm
of osteoprogenitor cells becomes more
basophilic, Golgi area becomes more evident.
These cytoplasmic changes result in a
differentiated osteoblast, which then secretes
type I collagen and other components of bone
matrix.
• Newly formed bone matrix appears in histologic
sections as small, irregularly shaped spicules and
trabeculae.

how does intramembranous ossificaiton lead to appositional growth

With time, the matrix is calcified, and osteoblasts
become osteocytes with their cytoplasmic
processes connecting with neighboring osteocytes.
This is the process of appositional growth, and
bony spicules increase in size and form, woven
bone or intramembranous bone.
• Mesenchymal regions that do not undergo
ossification give rise to endosteum and
periosteum.
• Most bones of the skull and jaws, scapula, and
clavicle.

describe the mechanism of enchondral ossification

Responsible for the formation of long and short
bones, which rely on the presence of a hyaline
cartilage model, which is used as a template and
within which the bone is made.
• Cartilage does not become bone; instead, a bony
subperiosteal collar is formed around the midriff of
the cartilaginous template. The collar increases in
length and width.
• The chondrocytes in the center of the template
hypertrophy and resorb some of their matrix, thus
enlarging their lacunae so much that the lacunae
become confluent.
• The hypertrophied chondrocytes assist in the
calcification of the cartilage and later die.
• The newly formed spaces are invaded by the
periosteal bud ( composed of blood vessels,
mesenchymal cells, and osteoprogenitor cells)

Endochondral ossification :
• Osteoprogenitor cells differentiate into
osteoblasts, which then lay a bony
matrix on the surface of calcified
cartilage.
• As the subperiosteal bone collar
increases in thickness and length,
osteoclasts resorb the calcified
cartilage–calcified bone complex,
leaving an enlarged space, the future
marrow cavity

describe the growth of endochondral bone:

Endochondral bone growth occurs in the 2nd
trimester of pregnancy and continues into
early adulthood.
• The entire process of ossification will spread
away from the primary ossification center,
and eventually, most of the cartilage
template will be replaced by bone, forming
the diaphysis of the long bone.
• As the diaphyseal marrow cavity enlarges, a
distinct zonation can be recognized in the
cartilage at both ends of the cavity ...
Epiphyseal cartilage

describe the different zones of Epiphyseal cartilage

Zone of reserve cartilage ( R)
• Zone of proliferation (P)
• Zone of maturation (M)
• Zone of hypertrophy and calcification ( H)
• Zone of cartilage degeneration (CD)
• Osteogenic zone (O)
• Shortly after birth, a secondary ossification center develops in
the proximal epiphysis. As a result, the cartilage remains only
as the articular cartilage at the end of long bones and as a
transverse disc of cartilage known as the epiphyseal growth
plate, which is responsible for maintaining the growth process

describe bone remodeling

dult bone is continuously being
remodeled.
• Harversian canal systems must be
modified by osteoclastic resorption
followed by osteoblastic bone
formation.
• When an individual achieves maximal
growth, proliferation of new cartilage
within the epiphyseal plate
terminates ( epiphyseal closure ).
• Vestigial evidence of the site of the
epiphyseal plate is reflected by an
epiphyseal line, consisting of bone
tissue.