HISTOLOGY OF MUSCULOSKELATAL SYSTEM

1. HISTOLOGICAL FEATURES OF MUSCULOSKELETAL TISSUES
Presenter – Dr Suman Subedi
First year Resident
Moderator- Dr Bidur Gyawali

2. Musculoskeletal Tissues
• Bone
• Cartilage
• Skeletal muscle
• Tendon, ligaments,Joint capsule
• Peripheral nerves

4. Histological features:
1. Bone:
• The strength and stiffness of bone combined with its light weight gives
vertebrates their mobility, dexterity, and strength.
• Like the other musculoskeletal tissues, bone consists of mesenchymal cells and an
extracellular matrix, but unlike the other tissues bone matrix mineralizes.

6. Matrix

7. Osteoblasts
• Cuboid cells aligned in layers along immature osteoid
• Derived from undifferentiated mesenchymal stem cells
• Osteoblastic differentiation controlled by- Bone
morphogenetic proteins (BMP), PDGF,IGF
• Concerned with- bone formation
- regulate osteoclastic activity
• Stimulated by pulsatile PTH
• Mature osteoblasts form rows of small mononuclear cells
along free surfaces of trabeculae and haversian systems where
osteoid is laid down prior to calcification

8. Osteocytes
• Former osteoblasts surrounded by newly formed matrix
• Constitute 90% of the cells in the mature skeleton
• Lying in their bony lacunae, with long interconnecting cytoplasmic processes
projecting through canaliculi
• Important for control of extracellular calcium and
phosphorus concentration
• Directly stimulated by calcitonin, inhibited by PTH
• Sclerostin secreted by osteocytes helps negative
feedback on osteoblasts’ bone deposition

9. Osteoclasts
• Large multinucleated giant cells
• Develop from mononuclear precursors in the haemopoietic
marrow under influence of RANKL secreted by osteoblasts
which binds RANK receptor on osteoclast precursors
• Principal mediators of bone resorption

10. Osteoclast
• Foamy appearance due to
presence of numerous
vesicles in cytoplasm
• Possess a ruffled (“brush”)
border of plasma membrane
enfoldings that increase
surface area for resorption
• Bone resorption occurs in
depressions: Howship
lacunae

11. Osteoprogenitor cells
• Originate from mesenchymal stem cells
• Line haversian canals, endosteum, and periosteum. Awaiting the
stimulus to differentiate
• Become osteoblasts under conditions of low strain and increased
oxygen tension
• Become cartilage under conditions of intermediate strain and low
oxygen tension
• Become fibrous tissue under conditions of high strain

12. Bone

13. cortical and cancellous bone.
• Examination of the cut surface of a
bone shows that the tissue assumes
two forms: the outer cortical or
compact bone and the inner cancellous
or trabecular bone.

14. Cortical bone
• Strong, dense bone, makes up 80% of the skeleton
• Composed of multiple osteons (haversian systems) with intervening interstitial
lamellae
• Osteons are made up of concentric bone lamellae with a central canal (haversian
canal) containing osteoblasts (new bone formation) and an arteriole supplying
the osteon.

15. • Lamellae are connected by canaliculi.
• Cement lines mark outer limit of osteon (bone resorption ended).
• Volkmann's canals: radially oriented, have arteriole, and connect adjacent
osteons
• Thick cortical bone is found in the diaphysis of long bones

18. Cancellous Bone
• Crossed lattice structure, makes up 20% of the skeleton
• High bone turnover rate.
• Bone is resorbed by osteoclasts in Howship's lacunae and formed on the opposite side
of the trabeculae by osteoblasts
• Osteoporosis is common in cancellous bone, making it susceptible to fractures (e.g.,
vertebral bodies, femoral neck, distal radius, tibial plateau).
• Commonly found in the metaphysis and epiphysis of long bones

21. Woven Bone
• Random organization
• Not stress-oriented
• Weaker
• Immature or pathologic bone
• Eg- embryonic skeleton
fracture callus
osteosarcoma
fibrous dysplasia

22. Physis
• Two growth plates exist in immature long bones:
(1) Horizontal (the physis) and
(2) Spherical (growth of the epiphysis)

25. Histologic zone of failure varies with the type of
loading applied to a specimen

26. Joints
1. Fibrous joints-
Join bone or cartilage by fibrous tissue, allow very little
movement. Eg- sutures of skull.
2. Cartilaginous joints-
• Primary- between bone and hyaline cartilage
• Secondary- two bone ends covered in a thin layer of hyaline
cartilage are joined by interposed fibrous cartilage. Eg-
symphysis pubis
3. Synovial joints

27. Synovial joint
• Adjoining bone ends covered
by very smooth hyaline
cartilage
• Enclosed within joint capsule
• Synovial fluid bathes and
lubricates cartilage
• Lined by synovial membrane
with cells called
synoviocytes-produce
lubricant and hyaluronic acid
responsible for viscosity of
the fluid.

28. PERIOSTEUM :
• Two layers: an outer fibrous and an inner cellular and vascular layer.
• outer layer consists of a dense fibrous tissue matrix and fibroblast-like cells
• inner osteogenic, or cambium, layer contains cells capable of forming cartilage
and bone.

30. • The portion of the femoral neck within the hip joint capsule has no cambial
layer in its fibrous covering to participate in callus formation during fracture
healing.
• With increasing age periosteum becomes thinner and less vascular and its ability
to form new bone decline.

31. Cartilage
• articular bearing surface
• Slick bearing surface: decreases friction and distributes loads
• Coefficient of friction less than ice on ice
• Shock-absorbing cushion resists shear/compression.
• Withstands impact loads up to 25 N/mm2
• Avascular, aneural, and alymphatic
• Receives nutrients and oxygen from synovial fluid via diffusion

32. Cartilage
• Cartilage is hypoxic; ATP from glycolysis
• Heals poorly
• Anisotropic—properties of material vary with direction of force
• Biphasic—property of liquid and solid
• Viscoelastic—strain (change in L/L) varies by rate of loading

33. Cartilage homeostasis disrupted by:
• Direct trauma/excess or inadequate forces
• Loss of underlying bone structure
• Genetic defects in normal structure/function
• Chemical/enzymatic threats

34. types of cartilage:
• Elastic cartilage forms the auricle of the external ear, a portion of the epiglottis,
and some of the laryngeal or bronchiolar cartilages.
• Fibrous cartilage forms part of the intervertebral discs, pubic symphysis, and
tendon, ligament, and joint capsule insertions.
• Hyaline cartilage forms most of the skeleton and the physeal cartilages; in
adults it persists as the nasal, laryngeal, bronchiolar, articular, and costal
cartilages.

35. Hyaline cartilage
• Has a gel-like matrix consisting of a proteoglycan ground substance in which are
embedded an architecturally structured collagen network and a relatively sparse
scattering of chondrocytes, which are responsible for producing all the structural
components of the tissue.

36. Composition of hyaline cartilage
• Water (approx 75% of wet weight)-
More at the surface. Responsible for nutrition and lubrication
• Collagen (approx 15% of wet weight)-type II
• Proteoglycan (approx 10% of wet weight)–
Glycosaminoglycan, chondroitin sulfate, keratin sulfate
• Chondrocytes (1-5% of wet weight)-
Produces collagen, proteoglycan and maintains matrix

39. Aging cartilage
• • Decreased number of chondrocytes (but larger in size)
• • Increased lysosomal enzymes
• • Senescence markers of chondrocytes
• • Telomere erosion and β-galactosidase
• • Decreased response to growth factors (TGF-β)

40. Aging Cartilage
• Decreased water content—“dried up old cartilage”
• Increased advanced glycosylation end products
• Collagen—increased cross-links and diameter—stiffens
• cartilage
• Increased protein
• Decreased tensile strength
• Increased modulus of elasticity (more stiff)

41. Skeletal muscle
• Fascial coverings-
Epimysium surrounds each
skeletal muscle belly or the
bundles of fascicles
Perimysium surrounds
individual muscle fascicles.
Endomysium surrounds
individual fibers (myofibers).

42. • Sarcolemma is the plasma membrane surrounding a skeletal muscle cell. It
surrounds the contractile elements, forming the transverse tubules
• Sarcoplasmic reticulum is a smooth endoplasmic reticulum that surrounds the
individual myofibrils, storing calcium in intracellular membrane–bound channels.

43. Sarcomere
• Each muscle fibre consists of many tiny Myofibrils (1-3 μm in
diameter and 1-2 cm long) that are highly organized, with
individual contractile units called sarcomeres.
• Sarcomere organization causes the banding pattern (striations)
seen in skeletal muscle
• Sarcomeres (Z line to Z line) are arranged into bands and lines.

44. • Dark band( A band) consists of thick myosin filaments
• Light band ( I band) consists of thin actin filaments
• In the middle of A band is a lighter H zone
• In the middle of I band there is a dark thin Z line.

45. Peripheral nerves
• main components of the peripheral
nervous system (PNS) are the nerves,
ganglia, and nerve endings.
• Nerves are bundles of nerve fibers
(axons) surrounded by Schwann cells
and layers of connective tissue.

46. Myelinated nerve fibres-
• all motor axons and large sensory axons carrying touch, pain and proprioception
are coated with myelin sheath ,derived from the accompanying Schwann cells.
• Every few millimetres the myelin sheath is interrupted, leaving short segments of
bare axon called the nodes of Ranvier. Nerve impulses leap from node to node at
the speed of electricity, much faster than unmyelinated fibres

47. Non-Myelinated nerve fibres-
• Most axons, particularly small-diameter fibres carrying crude sensation and the
efferent sympathetic fibres – are unmyelinated but wrapped in Schwann cell
cytoplasm.

48. Nerve organization
• Nerve fibers are grouped into bundles to form nerves
• Outside the Schwann cell membrane the axon is covered by a
connective tissue stocking,the endoneurium.
• The axons that make up a nerve are separated into bundles
(fascicles) by fairly dense membranous tissue, the
perineurium. In a transected nerve, these fascicles are seen
pouting from the cut surface, their perineurial sheaths well
defined and strong enough to be grasped by fine instruments
during operations for nerve repair.
• The groups of fascicles that make up a nerve trunk are
enclosed in an even thicker connective tissue coat, the
epineurium

49. • The nerve is richly supplied by blood vessels that run longitudinally in the
epineurium before penetrating various layers to become the endoneurial
capillaries.
• These fine vessels may be damaged by stretching or rough handling of the nerve
• However, they can withstand extensive mobilization of the nerve, making it
feasible to repair or replace damaged segments by operative transposition or
neurotization.

51. • Afferent fibres- carry afferent(sensory) impulses from peripheral receptors via
cells in the dorsal root ganglia to the spinal cord
• Efferent fibers- carry efferent (motor) impulses from cells in the anterior horn of
the spinal cord to the muscles
• The larger trunks are mixed, with motor and sensory axons running in separate
bundles.

52. • The peripheral ends of all the neurons are branched.
• A single motor neuron may supply 10 to several thousand muscle fibres(the
smaller the ratio, the finer the movement)
• Peripheral branches of each sensory neuron may serve from a single muscle
spindle to a comparatively large patch of skin(the fewer the end receptors served,
the greater the degree of discrimination)

53. Tendons
• Dense, regularly arranged groups of collagen bundles that attach muscle to bone
at the enthesis and covered with synovium or paratenon
• Collagen bundles are arranged in parallel rows.

54. Tendon composition
• Collagen (75% dry weight)
95% type I collagen, 5% type III collagen
• Proteoglycans (5% dry weight)
• Decorin—most predominant proteoglycan. Regulates tendon
diameter and provides cross-links between collagen fibers.
• Aggrecan—present at points of tendon compression

55. Tendon Composition
• Tenocytes—
o Fibroblast-like differentiated cells from the mesoderm
o Synthesize extracellular matrix(ECM) and assemble early collagen
fibrils
o Tenocyte production of collagen increases tendon healing and
reduces repair ruptures.
o produce type III collagen in response to rupture.

56. Structure
• Fibers and bundles are surrounded by endotenon and
epitenon.
• In tendons that endure less friction and are without synovial
fluid and tendon sheath, paratenon functions as a sleeve.
• Endotenon and epitenon are composed of type III collagen and
carry the nerves, arteries, veins and lymphatics of tendons.

58. • Paratenon-covered tendons (Achilles tendon): high vascularity, with
better healing results
• Synovium-covered tendons (flexor tendons): allow for gliding
motion, with vincula that carry the blood supply to tendon
segment

59. Ligaments
• Consist of dense connective tissue, similar to tendons.
• Characteristics:
o Originate and insert on bone
o Stabilize joints and prevent displacement of bones
o Contain mechanoreceptors and nerve endings that facilitate joint proprioception
o Surrounding epiligamentous coat—analogous to the epitenon of tendons and
carries neurovascular structures

60. Composition
• Primarily type I collagen (80% of dry weight)
• Water
• Elastin (1% dry weight)
• Lipids
• Proteoglycans (1% dry weight)—function in water retention
Compositional differences-compared to tendons, ligaments have
• Less collagen
• More proteoglycans and therefore more water
• Higher elastin content
• Less organized collagen fibers that are more highly crosslinked
• “Uniform microvascularity”—receive supply at insertion sites
by the epiligamentous plexus

61. Enthesis
• The connective tissue between tendon/ligament and bone
• Two types of entheses:
1. Fibrous enthesis-
The collagenous tendon or ligament directly attaches to the bone.
2. Fibrocartilaginous enthesis-
Tendon/Ligament-to-bone transition in four phases (zones):
Tendon/ligament, fibrocartilage, mineralized fibrocartilage and
bone

62. MENISCI
• mechanical functions of load bearing, shock absorption, joint lubrication
• In knee, superficially it consists of a mesh of fine fibrils.
• deep to this, small diameter collagen fibrils with a radial orientation relative to
the body of the meniscus.

63. • deeper central or middle region, consist of circumferential large fibril bundles
with radial fiber weaved among the circumferential fibril .
- The circumferential collagen bundles gives tensile strength for loads
- The radial fibers may resist the development and propagation of
longitudinal tears between the larger circumferential collagen fiber.

64. SYNOVIUM
• Loose connective tissue rich in capillaries
• Lacks a basement membrane
• No tight junctions
• No epithelial cells
• • Type A synovial cells—macrophage-like
• Phagocytosis
• From bone marrow
• • Type B synovial cells—fibroblast-like
• • Make golden synovial fluid with lubricin
• • Mesenchymal derived

65. References
• Millers Review of Orthopaedics, Seventh Edition
• Apley and Solomon’s System of Orthopaedics and Trauma, Tenth
Edition
• diFiore’s Atlas of Histology with functional correlations ,Twelveth
edition
• Netters Concise Orthopaedic Anatomy, 2nd Edition by Jon C.
Thompson MD
• Turek’s Orthopaedics: Principles and Their Application, 6th Edition