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

Muscles of the Pharynx

• This consists of three constrictor muscles and three muscles that descend from the styloid process, the cartilaginous part of the auditory tube and the soft palate.

External Muscles of the Pharynx 

• The paired superior, middle, and inferior constrictor muscles form the external circular part of the muscular layer of the wall.
• These muscles overlap each other and are arranged so that the superior one is innermost and the inferior one is outermost.

• These muscles contract involuntarily in a way that results in contraction taking place sequentially from the superior to inferior end of the pharynx.
• This action propels food into the oesophagus.

• All three constrictors of the pharynx are supplied by the pharyngeal plexus of nerves, which lies on the lateral wall of the pharynx, mainly on the middle constrictor of the pharynx.
• This plexus is formed by pharyngeal branches of the glossopharyngeal (CN IX) and vagus (CN X) nerves.

The Superior Constrictor Muscle

• Origin: pterygoid hamulus, pterygomandibular raphe, posterior end of the mylohyoid line of the mandible, and side of tongue.
• Insertion: median raphe of pharynx and pharyngeal tubercle.
• Innervation: though the pharyngeal plexus of nerves.

• The pterygomandibular raphe is the fibrous line of junction between the buccinator and superior constrictor muscles.

The Middle Constrictor Muscle

• Origin: stylohyoid ligament and greater and lesser horns of hyoid bone.
• Insertion: median raphe of pharynx.
• Innervation: through the pharyngeal plexus of nerves.

The Inferior Constrictor Muscle

• Origin: oblique line of thyroid cartilage and side of cricoid cartilage.
• Insertion: median raphe of pharynx.
• Innervation: through the pharyngeal plexus of nerves.

• The fibres arising from the cricoid cartilage are believed to act as a sphincter, preventing air from entering the oesophagus. 

Gaps in the Pharyngeal Musculature

• The overlapping arrangement of the three constrictor muscles leaves 4 deficiencies or gaps in the pharyngeal musculature.
• Various structures enter and leave the pharynx through these gaps.

• Superior to the superior constrictor muscle, the levator veli palatini muscle, the auditory tube, and the ascending palatine artery pass through a gap between the superior constrictor muscle and the skull.
• Superior to the superior border of the superior constrictor, the pharyngobasilar fascia blends with the buccopharyngeal fascia to form, with the mucous membrane, the thin wall of the pharyngeal recess.

• Between the superior and middle constrictor muscles, the gateway to the mouth, though which pass the stylopharyngeus muscle, the glossopharyngeal nerve (CN IX), and the stylohyoid ligament.

• Between the middle and inferior constrictor muscles, the internal laryngeal nerve and the superior laryngeal artery and vein pass to the larynx.

• Inferior to the inferior constrictor muscles, the recurrent laryngeal nerve and inferior laryngeal artery pass superiorly into the larynx.

•  Provides a rigid support system
• Protects delicate structures (e. g., the protection provided by the bones of the vertebral column to the spinal cord)
• Bones supply calcium to the blood; are involved In the formation of blood cells (hemopoiesis)
• Bones serve as the basis of attachment of muscles; form levers in the joint areas, aIlowing movement

Superior Constrictor Muscle

• Origin: Hamulus, pterygo-mandibular raphe, and mylohyoid line of the mandible.
• Insertion: Median raphe of the pharynx.
• Nerve Supply: Vagus nerve via the pharyngeal plexus.
• Arterial Supply: Ascending pharyngeal artery, ascending palatine artery, tonsillar branch of the facial artery, and dorsal branch of the lingual artery.
• Action: Constricts the wall of the pharynx during swallowing.

• BONES OF THE CRANIUM   Occipital (1) Frontal    (1) Sphenoid (1) Ethmoid  (1) Parietal    (2) Temporal  (2)

  BONES OF THE FACE   Mandible (1) Vomer     (1) Maxillae  (2) Zygomae  (2) Lacrimal   (2) Nasal        (2) Inferior nasal conchae (2) Palatine     (2)

   

Internal Ear

• Osseous labyrinth: a complex system of cavities in the substance of the petrous bone.
• Membranous labyrinth: filled with endolymph, bathed in perilymph.