The document summarizes the gross anatomy and microstructure of bone. It describes the parts of long bones including the diaphysis, epiphyses, and growth plates. It explains the differences between compact and spongy bone and their histological features. It also outlines the four main types of bone cells - osteoblasts, osteocytes, osteoclasts, and osteogenic cells - and their functions in bone formation and resorption.

3. Gross Anatomy of Bone:
• The structure of a long bone allows for the best
visualization of all of the parts of a bone.
• A long bone has two parts: the diaphysis and the
epiphysis.
• The diaphysis: is the tubular shaft that runs
between the proximal and distal ends of the
bone.
• medullary cavity: which is filled with yellow
marrow.
• The walls of the diaphysis are composed of dense
and hard compact bone.

5. • Epiphysis (plural = epiphyses): is the wider section at
each end of the bone which is filled with spongy
bone.
• Red marrow fills the spaces in the spongy bone.
• Metaphysis: is the area were each epiphysis meets
the diaphysis.
• Epiphyseal plate (growth plate): which is a layer of
hyaline (transparent) cartilage in a growing bone.
• When the bone stops growing in early adulthood
(approximately 18–21 years), the cartilage is replaced
by osseous tissue and the epiphyseal plate becomes
an epiphyseal line.

9. • Endosteum (end- = “inside”; oste- = “bone”): is a
delicate membranous lining the medullary cavity.
• This endoseum is where the bone growth,
repair, and remodeling occur.
• Periosteum (peri- = “around” or “surrounding”):
is a fibrous membrane that covers the outer
surface of the bone.
• The periosteum contains blood vessels, nerves,
and lymphatic vessels that nourish compact
bone.

11. • Tendons and ligaments also attach to bones at
the periosteum.
• The periosteum covers the entire outer surface
except where the epiphyses meet other bones to
form joints.
• In this region, the epiphyses are covered with
articular cartilage: a thin layer of cartilage that
reduces friction and acts as a shock absorber.

13. • Flat bones, like those of the cranium, consist of a
layer of diploe (spongy bone), lined on either
side by a layer of compact bone.
• The two layers of compact bone and the interior
spongy bone work together to protect the
internal organs.
• If the outer layer of a cranial bone fractures, the
brain is still protected by the intact inner layer.
• Overall, about 80% of the skeleton is compact
bone and 20% is spongy bone.

16. Compact and Spongy Bone:
• The differences between compact and spongy
bone are best explored via their histology.
• Most bones contain compact and spongy osseous
tissue, but their distribution and concentration
vary based on the bone’s overall function.
• Compact bone is dense so that it can withstand
compressive forces.
• while spongy (cancellous) bone has open spaces
and supports shifts in weight distribution.

20. • Compact bone is the denser, stronger of the two
types of bone tissue .
• It can be found under the periosteum and in the
diaphyses of long bones, where it provides support
and protection.
• Osteon, or Haversian system: is the microscopic
structural unit of compact bone.
• Lamellae (singular = lamella): is a concentric rings of
calcified matrix that forms the osteon.
• Central Canal, or Haversian Canal: is a central
opening of the osteon which contains blood vessels,
nerves, and lymphatic vessels.

23. • Volkmann’s canals: is a perforating canal in which
the vessels and nerves branch off at right angles and
extends to the periosteum and endosteum.
• lacunae (singular = lacuna): is the spaces b/w
borders of adjacent lamellae, contains an osteocytes
are located inside the lacunae.
• Canaliculi connect with the canaliculi of other
lacunae and eventually with the central canal.
• This system allows nutrients to be transported to
the osteocytes and wastes to be removed from
them.

27. Spongy (Cancellous) Bone:
• Spongy bone, also known as cancellous bone, contains
osteocytes housed in lacunae, but they are not arranged in
concentric circles.
• Instead, the lacunae and osteocytes are found in a lattice-like
network of matrix spikes called trabeculae (singular =
trabecula).
• The trabeculae may appear to be a random network, but each
trabecula forms along lines of stress to provide strength to the
bone.
• The spaces of the trabeculated network provide balance to the
dense and heavy compact bone by making bones lighter so that
muscles can move them more easily.
• In addition, the spaces in some spongy bones contain red
marrow, protected by the trabeculae, where hematopoiesis
occurs.

29. Blood and Nerve Supply:
• The spongy bone and medullary cavity receive nourishment
from arteries that pass through the compact bone.
• Nutrient foramen (plural = foramina): small openings in
the diaphysis, In which the arteries enter through it.
• The osteocytes in spongy bone are nourished by:
1. blood vessels of the periosteum that penetrate spongy
bone and
2. blood that circulates in the marrow cavities.
• As the blood passes through the marrow cavities, it is
collected by veins, which then pass out of the bone through
the foramina.

30. • In addition to the blood vessels, nerves follow
the same paths into the bone where they tend
to concentrate in the more metabolically active
regions of the bone.
• The nerves sense pain, and it appears the
nerves also play roles in regulating blood
supplies and in bone growth, hence their
concentrations in metabolically active sites of
the bone.

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33. Bone Cells and Tissue:
• Bone contains a relatively small number of cells
entrenched in a matrix of collagen fibers that provide a
surface for inorganic salt crystals to adhere.
• These salt crystals form when calcium phosphate and
calcium carbonate combine to create hydroxyapatite,
which incorporates other inorganic salts like magnesium
hydroxide, fluoride, and sulfate as it crystallizes, or calcifies,
on the collagen fibers.
• The hydroxyapatite crystals give bones their hardness and
strength, while the collagen fibers give them flexibility so
that they are not brittle.
• Although bone cells compose a small amount of the bone
volume, they are crucial to the function of bones.

34. Four types of cells are found within bone tissue:
1. Osteoblasts.
2. Osteocytes.
3. Osteogenic cells.
4. Osteoclasts.

35. 1. Osteogenic cells:
• Are unspecialized stem cells derived from
mesenchyme, the tissue from which almost all
connective tissues are formed.
• They are the only bone cells to undergo cell
division; the resulting cells develop into osteoblasts.
• Osteogenic cells are found along the:
1. inner portion of the periosteum.
2. in the endosteum.
3. in the canals within bone that contain blood
vessels.

36. 2. Osteoblasts:
• Are bone-building cells.
• They synthesize and secrete collagen fibers
and other organic components needed to
build the extracellular matrix of bone tissue,
and they initiate calcification.
• As osteoblasts surround themselves with
extracellular matrix, they become trapped in
their secretions and become osteocytes.

37. 3. Osteocytes:
• mature bone cells, are the main cells in bone
tissue and maintain its daily metabolism, such
as the exchange of nutrients and wastes with
the blood.
• Like osteoblasts, osteocytes do not undergo
cell division.
• (Note: The ending -cyte in the name of a bone
cell or any other tissue cell means that the cell
maintains the tissue.)

38. 4. Osteoclasts:
• are huge cells derived from the fusion of as many as 50
monocytes (a type of white blood cell) and are
concentrated in the endosteum.
• On the side of the cell that faces the bone surface, the
osteoclast’s plasma membrane is deeply folded into a
ruffled border.
• Here the cell releases powerful lysosomal enzymes and
acids that digest the protein and mineral components of
the underlying bone matrix.
• This breakdown of bone extracellular matrix, termed
resorption (re¯-SORP-shun), is part of the
, and
• (Note: The ending -clast in a bone cell means that the cell
breaks down extracellular matrix.)

39. • As you will see later, in response to certain hormones,
osteoclasts help regulate blood calcium level.
• They are also target cells for drug therapy used to treat
osteoporosis.
• Bone is not completely solid but has many small spaces
between its cells and extracellular matrix components.
• Some spaces serve as channels for blood vessels that supply
bone cells with nutrients.
• Other spaces act as storage areas for red bone marrow.
• Depending on the size and distribution of the spaces, the
regions of a bone may be categorized as compact or
spongy.

40. 5. Bone - lining cells:
• Are found on the surface of most bones in the adult
skeleton.
• These cells are believed to be derived from osteoblasts
that cease their physiological activity and flatten out on
the bone surface.
• Like a oseogenic cells that can divide and differentiate into
osteoblasts.
• Probably they serve as an ion barrier around bone tissue.
• regulate movements of calcium and phosphate into and
out of the bone matrix, which in turn helps control the
deposition of hydroxyapatite in the bone tissue.

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