Histology if bone

1. Bone
By dr Abdiasis Omar Mohamed
MBBS

2. Introduction
• As the main constituent of the adult skeleton, bone
tissue solid support for the body, protects vital organs
such as those in the cranial and thoracic cavities, and
encloses internal (medularly) cavities containing bone
marrow where blood cells are formed.
• Bone (or osseous) tissue also serves as a reservoir of
calcium, phosphate, and other ions that can be
released or stored in a controlled fashion to maintain
constant concentrations in body fluids.

3. • In addition, bones form a system of levers that
multiply the forces generated during skeletal muscle
contraction and transform them into bodily
movements.
• This mineralized tissue therefore confers mechanical
and metabolic functions to the skeleton.
• Bone is a specialized connective tissue composed of
calcified extracellular material, the bone matrix.

4. Bone cells
1. Osteocytes (Gr. osteon, bone + kytos, cell), which
are found in cavities (lacunae) between bone matrix
layers (lamellae), with cytoplasmic processes in
small canaliculi (L. canalis, canal) that extend into
the matrix.
2. Osteoblasts (osteon + Gr. blastos, germ), growing
cells which synthesize and secrete the organic
components of the matrix

5. 3. Osteoclasts (osteon + Gr. klastos, broken), which are
giant, multinucleated cells involved in removing
calcified bone matrix and remodeling bone tissue.

7. Bone matrix
• About 50% of the dry weight of bone matrix is
inorganic materials.
• Calcium hydroxyapatite is most abundant, but
bicarbonate, citrate, magnesium, potassium, and
sodium ions are also found.
• Significant quantities of noncrystalline calcium
phosphate are also present.

8. • The organic matter embedded in the calcified matrix
is 90% type I collagen, but also includes mostly small
proteoglycans and multiadhesive glycoproteins such
as osteonectin.

9. Periosteum & endosteum
• External and internal surfaces of all bones are
covered by connective tissue of the periosteum and
endosteum, respectively.

10. • The periosteum is organized much like the
perichondrium of cartilage, with an outer fibrous
layer of dense connective tissue, containing mostly
bundled type I collagen, but also fibroblasts and
blood vessels.
• Periosteal blood vessels branch and penetrate the
bone, carrying metabolites to and from bone cells.

11. • The periosteum’s inner layer is more cellular and
includes osteoblasts, bone lining cells, and
mesenchymal stem cells referred to as
osteoprogenitor cells.
• With the potential to proliferate extensively and
produce many new osteoblasts, osteoprogenitor cells
play a prominent role in bone growth and repair.

12. • Internally the very thin endosteum covers small
trabeculae of bony matrix that project into the
marrow cavities.
• The endosteum also contains osteoprogenitor cells,
osteoblasts, and bone lining cells.

13. Types of bone
• Macroscopically bone is divided;
1. compact (cortical) bone, which represents 80% of
the total bone mass.(lamellar bone and woven bone)
2. cancellous (trabecular) bone, constituting about 20%
of total bone mass.(lamellar bone and woven bone)

15. Osteogenesis
• Bone development or osteogenesis occurs by one of
two processes:
1. Intramembranous ossification, in which osteoblasts
differentiate directly from mesenchyme and begin
secreting osteoid.
2. Endochondral ossification, in which a preexisting
matrix of hyaline cartilage is eroded and invaded by
osteoblasts, which then begin osteoid production

18. Bone remodeling and repair
• The rate of bone turnover is very active in young
children, where it can be 200 times faster than that of
adults.
• In adults the skeleton is also renewed continuously in
a process of bone remodeling that involves the
coordinated, localized cellular activities for bone
resorption and bone formation

19. • Bone repair after a fracture or other damage uses
cells, signaling molecules, and processes already
active in bone remodeling.
• Surgically created gaps in bone can be filled with new
bone, especially when periosteum is left in place.
• The major phases that occur typically during bone
fracture repair include initial formation of
fibrocartilage and its replacement with a temporary
callus of woven bone.

21. Metabolic role of the bone
• The skeleton serves as the calcium reservoir,
containing 99% of the body’s total calcium in
hydroxyapatite crystals.
• The concentration of calcium in the blood (9-10
mg/dL) and tissues is generally quite stable because
of a continuous interchange between blood calcium
and bone calcium.

22. • The principal mechanism for raising blood calcium
levels is the mobilization of ions from hydroxyapatite
to interstitial fluid, primarily in cancellous bone.
• Ca2+ mobilization is regulated mainly by paracrine
interactions among bone cells:
- parathyroid hormone
- calcitonin

23. Joints
• Joints are regions where adjacent bones are capped
and held together firmly by other connective tissues.
• Synarthroses( permits no movement)
- Synostoses involve bones linked to other bones (
sutures)
- Syndesmoses join bones by dense connective tissue
only (tibiofibular joint and sacroiliac joint)
- Symphyses have a thick pad of fibrocartilage(
intervertebral discs and pubic symphysis,)

24. • Diarthroses: permit free bone movement.
• elbow and knee
• In a diarthrosis ligaments and a capsule of dense
connective tissue maintain proper alignment of the
bones.
• The capsule encloses a sealed joint cavity containing
a clear, viscous liquid called synovial fluid.

26. THE END