Endochondral ossification

• A cartilage model exists
• Through intramembraneous ossification in the perichondrium a collar of bone forms around the middle part of the cartilage model
• The perichondrium change to a periostium
• The bone collar cuts off the nutrient and oxygen supply to the chondrocytes in the cartilage model
• The chondrocytes then increase in size and resorb the surrounding cartilage matrix until only thin vertical septae of matrix are left over
• These thin plates then calcify after which the chondrocytes die
• The osteoclasts make holes in the bone collar through which blood vessels can now enter the cavities left behind by the chondrocytes
• With the blood vessels osteoprogenitor cells enter the tissue
• They position themselves on the calcified cartilage septae, change into osteoblasts and start to deposit bone to form trabeculae
• In the mean time the periosteum is depositing bone on the outside of the bone collar making it thicker and thicker
• The trabeculae,consisting of a core of calcified cartilage with bone deposited on top of it, are eventually resorbed by osteoclasts to form the marrow cavity
• The area where this happens is the primary ossification centre and lies in what is called the diaphysis (shaft)
• This process spreads in two directions towards the two ends of the bone the epiphysis
• In the two ends (heads) of the bone a similar process takes place
• A secondary ossification centre develops from where ossification spreads radially
• Here no bone collar forms
• The outer layer of the original cartilage remains behind to form the articulating cartilage
• Between the primary and the secondary ossification centers two epiphyseal cartilage plates remain
• This is where the bone grows in length

• From the epiphyseal cartilage plate towards the diaphysis a number of zones can be identified:

 Resting zone of cartilage

 Hyaline cartilage

 Proliferation zone

 Chondrocytes divide to form columns of cells that mature.

Hypertrophic cartilage zone

 Chondrocytes become larger, accumulate glycogen, resorb the surrounding matrix so that only thin septae of cartilage remain 

Calcification and degeneration zone

The thin septae of cartilage become calcified.

The calsified septae cut off the nutrient supply to the chondrocytes so subsequently they die.

Ossification zone.

Osteoclasts make openings in the bone collar through which blood vessels then invade the spaces left vacant by the chondrocytes that died.

Osteoprogenitor cells come in with the blood and position themselves on the calcified cartilage

septae, change into osteoblasts and start to deposit bone.

 When osteoblasts become trapped in bone they change to osteocytes.

Growth and remodeling of bone

Long bones become longer because of growth at the epiphyseal plates

They become wider because of bone formed by the periosteum

The marrow cavity becomes bigger because of resorbtion by the osteoclasts

Fracture repair

When bone is fractured a blood clot forms

 Macrophages then remove the clot, remaining osteocytes and damaged bone matrix

The periosteum and endosteum produce osteoprogenitor cells that form a cellular tissue in the fracture area

 Intramembranous and endochondral ossification then take place in this area forming trabeculae.

Trabeculae connect the two ends of the broken bone to form a callus

Remodelling then takes place to restore the bone as it was

Joints

The capsule of a joint seals off the articular cavity,  

The capsule has two layers

 fibrous (outer)

synovial (inner)

The synovial layer is lined by squamous or cuboidal epithelial cells,  Under this layer is a layer of loose or dense CT, The lining cells consists of two types:

- A cells

- B cells

They secrete the synovial fluid

They are different stages of the same cell, They are also phagocytic., The articular cartilage has fibres that run perpendicular to the bone and then turn to run parallel to the surface

Initially, four clefts exist; however, only one gives rise to a definite structure in adults.

The Superior Roof of the Orbit

• The superior wall or roof of the orbit is formed almost completely by the orbital plate of the frontal bone.
• Posteriorly, the superior wall is formed by the lesser wing of the sphenoid bone.

• The roof of the orbit is thin, translucent, and gently arched. This plate of bone separates the orbital cavity and the anterior cranial fossa.

• The optic canal is located in the posterior part of the roof.

Muscles Around the Eyelids

• The function of the eyelid (L. palpebrae) is to protect the eye from injury and excessive light. It also keeps the cornea moist.

The Orbicularis Oculi Muscle

• This is the sphincter muscle of the eye.
• Its fibres sweep in concentric circles around the orbital margin and eyelids.
• It narrows the eye and helps the flow of tears from the lacrimal sac.

• This muscle has 3 parts: (1) a thick orbital part for closing the eyes to protect then from light and dust; (2) a thin palpebral part for closing the eyelids lightly to keep the cornea from drying; and (3) a lacrimal part for drawing the eyelids and lacrimal punta medially.

• When all three parts of the orbicularis oculi contract, the eyes are firmly closed and the adjacent skin becomes wrinkled.
• The zygomatic branch of the facial nerve (CN VII) supplies it.

The Levator Palpebrae Superioris Muscle

• This muscle raises the upper eyelid to open the palpebral fissure.
• It is supplied by the oculomotor nerve (CN III).

3 basic functions
o    protection of respiratory tract during swallowing food/air pathways cross.
    epiglottis provides protection
o    control intra-thoracic pressure (in coughing) -    close off airway to build pressure then rapidly open to release stuff
o    production of sound (in speaking, singing, laughing)

Important structures

o    hyoid bone
o    thyroid cartilage
o    arytenoids cartilage: vocal and muscle process
    sits on slope on posterior side of cricoid - spin and slide
o    cricoid cartilage: signet ring
o    thyroepiglottic ligament

Membranes and ligaments

o    membrane: general; ligament: thickening of membrane
o    folds: free edges of membranes or ligaments
o    names: tell you where located

Important membranes:
    quandrangular/vestibular membrane—from epiglottis to arytenoids
•    inferior edge: false vocal fold
    thyrohyoid membrane
    conus elasticus = cricothyroid = cricovocal
•    superior/medial edge = vocal fold
•    vocal ligaments: true folds, top of cricothyroid membrane