NEET MDS Lessons
Anatomy
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
- U-shaped bone
- Body
- Greater horn
- Lesser horn
- Suspended by ligaments from the styloid process
Structure of the Nasal Septum
- This part bony, part cartilaginous septum divides the chamber of the nose into two narrow nasal cavities.
- The bony part of the septum is usually located in the median plane until age 7; thereafter, it often deviates to one side, usually the right.
- The nasal septum has three main components: (1) the perpendicular plate of the ethmoid bone; (2) the vomer, and (3) the septal cartilage.
- The perpendicular plate, which forms the superior part of the septum, is very thin and descends from the cribiform plate of the ethmoid bone.
- The vomer, which forms the posteroinferior part of the septum, is a thin, flat bone. It articulates with the sphenoid, maxilla and palatine bones.
The Nasal Mucosa
- Mucosa lines the entire nasal cavities except for the vestibule of the nose.
- The nasal mucosa is firmly bound to the periosteum and perichondrium of the supporting structures of the nose.
- It is continuous with the adjoining cavities to which the nasal cavity communicates (e.g., the nasopharynx and paranasal sinuses).
- The inferior 2/3 of the nasal mucosa is called the respiratory area and air passing over this is warmed and moistened before it passes into the lungs.
- The superior 1/3 is called the olfactory area.
The Olfactory Area of Nasal Mucosa
- This area contains the peripheral organ of smell.
- Sniffing draws air into this area
- Olfactory receptor cells (from the olfactory nerve, CN I, are located in the mucosa of this area in the nose.
Nerves to the Respiratory Area of Nasal Mucosa
- The inferior 2/3 of the nasal mucosa are supplied chiefly by the trigeminal nerve (CN V).
- The mucous membrane of the nasal septum is supplied chiefly by the nasopalatine nerve, a branch of the maxillary nerve (CN V2).
- Its anterior portion is supplied by the anterior ethmoidal nerve (a branch of the nasociliary nerve) which is derived from the ophthalmic nerve (CN V1).
- The lateral walls of the nasal cavity are supplied by branches of the maxillary nerve (CN V2); the greater palatine nerve, and the anterior ethmoidal nerve.
Arteries of the Nasal Mucosa
- The blood supply of the mucosa of the nasal septum is derived mainly from the maxillary artery.
- The sphenopalatine artery, a branch of the maxillary, supplies most of the blood of the nasal mucosa.
- It enters by the sphenopalatine foramen and sends branches to the posterior regions of the lateral wall and to the nasal septum.
- The greater palatine artery, also a branch of the maxillary, passes through the incisive foramen to supply the nasal septum.
- The anterior and posterior ethmoidal arteries, branches of the ophthalmic artery, supply the anterosuperior part of the mucosa of the lateral wall of the nasal cavity and nasal septum.
- Three branches of the facial artery (superior labial, ascending palatine, and lateral nasal) also supply the anterior parts of the nasal mucosa.
Veins of the Nasal Mucosa
- The veins of the nasal mucosa form a venous network of plexus in the connective tissue of the nasal mucosa.
- Some of the veins open into the sphenopalatine vein and drain to the pterygoid plexus.
- Others join the facial and infraorbital veins.
- Some empty into the ophthalmic veins and drain into the cavernous sinus.
Intramembranous ossification
- Flat bones develop in this way (bones of the skull)
- This type of bone development takes place in mesenchymal tissue
- Mesenchymal cells condense to form a primary ossification centre (blastema)
- Some of the condensed mesenchymal cells change to osteoprogenitor cells
- Osteoprogenitor cells change into osteoblasts which start to deposit bone
- As the osteoblasts deposit bone some of them become trapped in lacunae in the bone and then change into osteocytes
- Osteoblasts lie on the surface of the newly formed bone
- As more and more bone is deposited more and more osteocytes are formed from mesenchymal cells
- The bone that is formed is called a spicule
- This process takes place in many places simultaneously
- The spicules fuse to form trabeculae
- Blood vessels grow into the spaces between the trabeculae
- Mesenchymal cells in the spaces give rise to hemopoetic tissue
- This type of bone development forms the first phase in endochondral development
- It is also responsible for the growth of short bones and the thickening of long bones
The Muscles of Facial Expression
- These lie in the subcutaneous tissue and are attached to the skin of the face.
- They enable us to move our skin and change our facial expression. They produce their effects by pulling on the skin but do not move the facial skeleton.
- These muscles surround the facial orifices and act as sphincters and dilators.
- All facial muscles receive their innervation from the branches of the facial nerve (CN VII)-temporal, zygomatic, buccal, marginal mandibular, cervical.
Skull bones
- 26 bones: 22 bones + hyoid (small bone in neck for swallowing) + 3 auditory ossicles (middle ear: incus, malleus, stapes)
- 21 bones: tightly connected; mandible is freely mobile at temperomandibular joint (synovial)
- connective-tissue interface b/w bones = suture
- bones – mandible = cranium
- cranium
- neurocranium: covers brain anteriorly, laterally and posteriorly
- brain supported by bones of basicranium
- also contributes to interorbital region; b/w eyes and superior to nasal passages
- viscerocranium/splanchnocranium: bones of face
- sutures
- coronal: separates frontal from parietals
- sagittal: separates two parietal bones
- lambdoidal: separates parietal form occipital
- squamosal: b/w temporal and parietal; overlapping sutures
- At birth: 2 frontal bones which eventually fuse; metopic suture disappears
Cranial Cavities: 5 major cavities
Endocranial, left and right orbits, nasal cavities, oral cavity, middle ear cavities
Endocranial cavity
- contains brain, meninges, cerebrospinal fluid, brain’s vascular supply and most proximal portion of cranial nerves
- enclosed by neurocranium and basicranium
- basicranium: foramina for neurovascular bundles
- foramen magnum: spinal cord exit
- floor of endocranial cavity divide into fossae
- anterior: frontal lobes of brain
- middle: pair temporal lobes
- posterior: cerebellum and brainstem