Ossification : easy and brief description

Ossification: it is the process of bone formation

Classification of ossification centre:

1. A primary ossification center is the first area of a bone to start ossifying.

 It usually appears during intra-uterine life in the central part of each developing bone.

Exception: carpal bones of hand

 In long bones the primary ossification centers appear in the diaphysis/shaft and in irregular bones the primary centers appear usually in the body of the bone.

All long bones have only one primary center but some irregular bones such as the hip bone and vertebrae have multiple primary centers.

2. Secondary ossification centre: A secondary ossification center is the area of ossification that appears after the primary ossification center has already appeared - most of which appear during the postnatal and adolescent years. Most bones have more than one secondary ossification center. In long bones, the secondary centers appear in the epiphyses (upper and lower end).

Classification of mechanism of ossification

Membranous ossification:

Ex: head and neck: parietal bone, frontal bone, maxilla, zygomatic,

Sup extre: clavicle

Cartilaginous ossification: all vertebrae, all ribs, sternum, scapula, all bones of upper limb except clavicle, all bones of lower limb

Membrano-cartilaginous ossification:

Occipital: membranous ossification area above the highest nuchal line, rest of the parts arises from cartilaginous ossification  

Sphenoid: membranous ossification above the lateral part of greater wing and pterygoid process except the hamulus, rest of the part arises from cartilage,

Temporal: membranous ossification – squamous part and tympanic part, rest of the parts arises from cartilage,

 Mandible: cartilaginous ossification anterior part of body and part of ramus above mandibular foramen, rest of the body arises from membranous ossification

Name of bone	Primary ossification center	Secondary ossification center
Clavicle	Two	One for sternal end
Humerus	One for shaft	Three for upper end – head , greater & lesser tubercle Four for lower end – lateral & medial epicondyle, capitulum, trochlear
Radius	One primary ossification center for shaft	One for upper end One for lower end
Ulna	One primary ossification center for shaft	One for upper end One for lower end
Carpal bone	One primary ossification center which appear after birth	No secondary ossification centre
Metacarpal bone	One primary ossification center for shaft	One 2nd ossification centre :  base of  1st metacarpal bones is ossified from 2nd ossification center Heads of  2-5 metacarpal bone are ossified from 2nd ossification center
Phalanges	One primary ossification center for shaft and head	One 2nd ossification center for base  of each phalanx

Lower limb

Name of bone	Primary ossification center	Secondary ossification center
Hip bone	Three – ilium, ischium, pubis	5 secondary ossification center
Femur	One for shaft	Three for upper end – head , greater & lesser trochanter One for lower end
Tibia	One primary ossification center for shaft	One for upper end One for lower end
Fibula	One primary ossification center for shaft	One for upper end One for lower end
Tarsal bone (except calcaneus)	One primary ossification center	No secondary ossification centre
Calcaneus	One primary ossification center	One secondary ossification centre
Metatarsal bone	One primary ossification center for shaft	One 2nd ossification centre :  base of  1st metatarsal bones is ossified from 2nd ossification center Heads of  2-5 metatarsal bone are ossified from 2nd ossification center
Phalanges	One primary ossification center for shaft and head	One 2nd ossification center for base  of each phalanx

Handout on occipital bone : helpful for viva examination

Occipital

1.       Anatomical  points:

·         Occipital condyles are directed  downwards

·         Basilar part is directed forwards and upwards

·         External surface of squamous part is convex and directed backward

2.       Morphological type : Flat bone

3.       Structure of bone : inner and outer compact and middle diploe

4.       Ossification :  area of occipital bone above the highest nuchal line  is arisen from membranous ossification and rest of the parts of occipital bone is arisen from cartilaginous ossification

5.       Development : paraxial mesoderm

6.       Parts : Squamous part , basilar part and condylar part

Squamous part :

1.       Two surfaces : external and internal

External surface of squamous part

·         External occipital protuberance : gives attachment to the highest point of the ligamentum nuchae

·         Highest nuchal line : gives attachment to epicranial aponeurosis :

·         Superior nuchal line:  its

·          medial part attached with origin of trapezius muscle and

·          lateral part from before backwards attached with origin of occipital belly of occipito frontalis muscle and insertion of sternocleidomastoid and splenius capitis

·         Inferior nuchal line: medially:  rectus capitis posterior minor and

                                     Laterally rectus capitis posterior major  

·         External occipital crest : gives attachment of ligamentum nuchae

·         Area between superior and inferior nuchal line :

·          medially insertion of semispinalis capitis and

·          laterally obliquus capitis superior  

·         Area between inferior nuchal line and foramen magnum

·         Medially attachment of rectus capitis posterior minor and

·         laterally attachment of rectus capitis posterior major

Internal surface of squamous part of occipital bone

Internal occipital protuberance : an irregular elevation opposite the external occipital protuberance

Sagittal sulcus : lodges superior sagittal sinus and its margin of sulcus attached with falx cerebri

Transverse sulcus  (right and left)  : lodges  transverse sinuses  and its margin of sulcus attached with tentorium cerebella

Right transverse sinus is the continuous of superior sagittal sinus and

left transverse sinus is the continuous of straight sinus

Internal occipital crest lodges occipital sinus and its margin attached with falx cerebelli.

Fossae :

Two upper triangular fossae : lodge occipital lobes of the cerebrum with meninges

Two lower quadrilateral fossae : lodge cerebellar hemispheres

Vermian fossa: a small depression which lodge vermis of the cerebellum

Angles:

Superior angle :  is meeting points of occipital bone and two parietal bone

Lateral angle : is the meeting points of occipital bone, parietal bone and mastoid part of temporal bone

Two lambdoid borders: they are articulates with posterior border of occipital bone

Two mastoid borders: they are articulates with mastoid part of temporal bone

Basilar part of occipital bones

Parts

2.        Three surfaces and three borders

3.       Anterior surface: articulate with body of sphenoid , forming primary cartilaginous joint , ossifies by the end of the 25^th years

4.       Smooth Superior surface (also known as clivus) it is related with meninges , ligament , lower part of pons and medulla oblongata , near the foramen magnum it is related with following structures above downward

·         Membrana tectoria

1.       Upper band of crucial ligament

2.       Apical ligament

3.       Anterior atlanto –occipital membrane

Inferior surface of basilar part of occipital bone :

Pharyngeal tubercle: a small middle bony elevation attached with fibrous raphe of pharynx and superior constrictor muscle

Lateral to pharyngeal tubercle : insertion of longus capitis muscles

Behind the attachement of longus capitis or in front of occipital condyles : insertion of rectus capitis anterior

Borders of basilar part of occipital bones

Posterior border is formed the anterior margin of foramen magnum

Two Lateral borders : articulate with petrous part of temporal bone

Condylar part of occipital bone

Parts

Superior surface : contain jugular tubercle an oval eminence overlying the hypoglossal canal

Inferior surface:

Two Occipital condyles : articulate with superior articular processes of atlas forming atlanto –occipital joint

Hypoglossal canals : they are  situated anterior part of each condyle

 transmits :

1.       Hypoglossal nerve

2.       Meningeal branch of ascending pharyngeal artery 

3.       Emissary vein connecting basilar venous plexus with pterygoid venous plexus

Condylar fossa : a depression behind the condyle

Condylar canal occationally a foramen present within the condylar fossa which transmit emissary vein connect sigmoid sinus with veins of suboccipital triangle

Jugular process : it is situated just lateral to occipital condyles

Anterior surface of jugular process bears jugular notch which form jugular foramen by articulating with petrous part of temporal bone

Jugular foramen : transmitted

1.       Anterior compartment  :  inferior petrosal sinus

2.       Middle compartment : glossopharyngeal

Vagus nerve , accessory nerve 

Meningeal branch of ascending pharyngeal artery

Emissary vein

3.       Posterior compartment :

Sigmoid sinus continued as internal jugular vein

Foramen magnum:

Smaller anterior compartment transmits

1.       Apical ligament

2.       Upper band of cruciate ligament

3.       Membrane tectoria

Larger posterior compartment transmits

1.       Lower end of medulla oblongata

2.       Right and left vertebral artery

3.       Anterior and posterior spinal artery

4.       Spinal root of accessory nerve

5.       Sympathelic plexus of nerves

6.       Veins joining the venous plexus of medulla oblongata

7.       Lower part of tonsil of cerebellum

Jugular foramen:

1. glossopharyngeal nerve : mixed nerve : sensory supply motor supply
2. vagus nerve : mixed nerve : sensory supply  motor supply
3. Cranial root of accessory nerves: motor nerve. supply
4. Internal jugular vein

Summary of cranial nerves

Cranial nerves

Pure Sensory nerve: 

 olfactory nerve, optic nerve and vestibulocochlear nerve (1, 2, 8)

Function of sensory nerve in short:

1. Olfactory nerve : carry smell

1. Optic nerve : responsible for vision

1. Vestibulocochlear nerve : vestibular part responsible for balance and cochlear nerve responsible for vision

No.	Sensory nerve	Component	Name of the opening  through which nerve  enter  into the  cranium	Arises from
I	Olfactory nerve	SVA	Cribriform plate of  ethmoid	Axon of olfactory nerve   arises from the Olfactory  receptors cells which are  situated in the nasal mucosa
II	Optic nerve	SSA	Optic canal	Optic nerve fibres are arises  from  the ganglionic cells  of the retina
VIII	Vestibulocochlear  nerve	SSA	Internal acoustic meatus	Nerves are arises from inner ear

Pure motor nerve:  Oculomotor,  trochlear and abducent , accessory nerve and hypoglossal 

 nerve (3, 4, 6,   11, 12)

Oculomotor, trochlear and abducent nerves are pure motor nerve which innervate extraocular muscles

	Nerves	Functions	Nucleus	Name of the opening through which nerve exit the cranium
III	Oculomotor nerve	All extra-ocular muscles of  eye are supplied by oculomotor nerve except SO and LR	Oculomotor nucleus Edinger –Westphal  nucleus	Superior orbital fissure
IV	Trochlear nerve	SO 4: superior oblique muscle is supplied by trochlear nerve	Trochlear nucleus	Superior orbital fissure
VI	Abducent nerve	LR 6: lateral rectus muscle is supplied by abducent nerve	Abducent nucleus	Superior orbital fissure

XI	Accessory nerve	Functions	Nucleus	Name of the opening  through which nerve exit  the cranium
	Cranial root	It supplies all muscles of soft palate  ( except tensor veli  palatine)  and all muscles of  the pharynx  except stylopharyngeus	Spinal accessory nucleus	Jugular foramen
	Spinal root	This part supplies the sternocleidomastiod and trapezius muscles		Foramen magnum
XII	Hypoglossal nerve	It supplies all muscles of the tongue  except  palatoglossus	Hypoglossal nucleus	Hypoglossal canal

Mixed nerves: (V, VII, IX, X)

 Trigeminal nerve,

Facial,

Glossopharyngeal,

 Vagus 

Trigeminal nerve (V)

It has three parts

		Foramen	Nucleus of trigeminal nerve
Ophthalmic nerve	sensory nerve	Superior orbital  fissure	Motor nucleus of trigeminal  nerve Spinal nucleus of trigeminal  nerve Main sensory nucleus of  trigeminal nerve and  mesencephalic nucleus of  trigeminal nerve
Maxillary nerve	sensory nerve	Foramen  Rotundum
mandibular nerve	Mixed : both  sensory and  motor	Foramen ovale

Functions of mixed nerves in short:

Trigeminal nerve functions (in short):

Sensory function: carry sensation from face

Motor function: ---Muscles of mastication, tensor tympani, tensor veli palatine

Facial nerve (VII)Functions (in short):

Sensory function: carry taste sensation from ant 2/3 of tongue

Motor function: motor innervation of muscles of facial expression

Secretomotor function: innervate lacrimal gland through pterygopalatine ganglion 

Glossopharyngeal nerve functions(IX) (in short):

Sensory function: carry taste sensation and general sensation from pos 1/3 of tongue 

Motor function: innervate stylopharyngeus muscle

Secretomotor function: innervations of parotid gland through otic ganglion

Vagus nerve(x) function in short :

Sensory function: carry taste sensation from post most part of tongue, carry sensation from larynx, 

external ear

Motor innervations: muscles of larynx