Development of the Head and Neck 1 Flashcards
What does the fetal skull have, that the adult skull doesn’t?
Fontanelles
What are fontanelles?
Membranous areas of unfused skull
When will the fontanelles be closed?
Within the first 2 years of life
What is the function of fontanelles?
- Flexibility allows passage through the birth canal
- Allow for brain growth
What changes in the fontanelles can indicate pathology?
- Sunken
- Bulging
- Enlarged
What do sunken fontanelles indicate?
Dehydration
What do bulging fontanelles indicate?
Increased intracranial pressure
When do enlarged fontanelles often occur?
In prematurity
What is the problem with enlarged fontanelles?
They can squash the brain, causing brain damage
When does the neural tube form?
Week 3
How is the neural tube formed?
- The notocord is formed at gastrulation
- The notocord sends signals causing the overlying ectoderm to thicken
- Edges of the ectoderm elevate out of slipper-shaped neural plate, out of the plane of the disc
- Edges curl towards one another, creating neural tube
What happens to the anterior end of the neural tube?
It begins to form the brain
Describe the anterior end of the neural tube when it’s beginning to form brain?
Initially in 3 vesicle stage, and vesicles quickly specialise
What do the vesicles of the neural tube specialise to form?
- Prosencephalon - forebrain
- Mesencephalon - midbrain
- Rhombencephalon - hindbrain
What has happened to the embryo by the end of week 4?
It has folded
What helps to fold the embryo?
The growth of the neural tube
What does folding of the embryo create?
Primitive gut tube
Does the primitive gut tube include the primitive pharynx?
Yes
What is the primitive gut tube lined with?
Endoderm
Describe the head and neck region of the embryo early in week 4?
- Face has no distinguishing external features
- Head and neck represent half the length of the embryo
- There is an open pore in the neural tube cranially and caudally, which is fusing from the middle upwards
Where does the pharynx extend in the adult?
From the base of the skull to the inferior border of the cricoid cartilage of the larynx
What are the divisions of the pharynx?
- Nasal
- Oral
- Laryngeal
Where is the nasopharynx?
Superior to the soft palate
Where is the oropharynx?
Between the soft palate and the larynx
Where is the laryngopharynx?
Posterior to the larynx
What are the pharyngeal (or brachial) arches?
A system of mesenchymal proliferations in the neck region of the embryo
Where do the pharyngeal arches form?
In the lateral walls of the embryonic pharynx, towards the cranial end of the neural tube
What gives the bumps of the pharyngeal arches?
Lots of cell division and plastic mesenchyme production
What are the pharyngeal arches involved in?
Formation of the head and neck
What type of tissues do the pharyngeal arches give rise to?
- Muscles
- Cartilages
- Nerves
- Arteries
What systems of the body do the pharyngeal arches notably involve?
- Brain
- CVS
- Special sensory organs
What is the pharyngeal apparatus very closely associated to?
Primitive heart and neural tube
When are the pharyngeal arches apparent from?
About 4 weeks in the developing embryo
What does each arch have?
Regarding its core
A large mesenchyme core
What migrates in to the mesenchyme core of the phayngeal arches?
Some neural crest cells
What are the pharyngeal arches covered in on either external surface?
Ectoderm
What is found between each arch on its external surface?
Pharyngeal groove, or cleft
What happens to the phayngeal groove?
They eventually disappear, except the first (between the first and second arches)
What does the first pharyngeal cleft become?
The external auditory meatus of the ear
What forms around the entrance to the external auditory meatus?
Swellings
What forms from the swellings of the entrance to the meatus?
The external ear (auricle)
How do the clefts disappear?
The second cleft grows down to cover the others, obliterating all the other clefts
What happens if the cervical sinus is not obliterated?
Cysts or fistulae can occur
Where can cysts or fistulae resulting from failure of obliteration of cervical sinus occur?
Anywhere along the anterior border of the sternocleidomastoid
What is the internal surface of the pharyngeal arch?
The part facing into the primitive gut tube
What is the internal surface of each pharyngeal arch covered by?
Endoderm
What is formed by the internal surface of the pharyngeal arch?
A similar pattern of grooves between each arch, known as pharyngeal pouches
What arises from the pharyngeal pouches?
- Parathyroid
- Thymus
- Tonsils
- Middle ear
What are the pharyngeal arches, grooves, and pouches collectively known as?
Pharyngeal apparatus
How many pharyngeal arches are there?
5, numbered 1-6
The 5th arch doesn’t form in humans
Which pharyngeal arch is biggest?
1st
What happens to the size of the phayngeal arches?
They get progessively smaller from 1 to 6, so the 4th and 6th arch are not as readily visible as the others
What is associated with each pharyngeal arch?
- Cartilage bar
- Nerve
- Artery
What is the cartilage bar of each pharyngeal arch dervied from?
Neural crest cells
What will the cartilage bars subsequently become?
Skeletal elements of head and neck
Which cartilaginous bar is largest?
Cartilage bar of 1st pharyngeal arch
What does the cartilage bar of the first pharyngeal arch do?
Divides into maxillary and mandibular prominences
What is the mandibular prominence known as?
Meckel’s cartilage
What does Meckels cartilage give rise to?
- Malleus
- Incus
How to the malleus and incus arise from the Meckel’s cartilage?
Remodelling
What does the mandibular prominence provide?
Template for mandible
How does the mandible form from the mandibular prominence?
Membranous ossification
What is the cartilage bar of the 2nd arch known as?
Reichert’s
What does the cartilage bar of the 4th and 6th arch give rise to?
- Thyroid
- Arytenoids
- Cricoids
What are the ossicles of the middle ear derived from?
Cartilage bar
How many pairs of cranial nerves are there?
12
What happens to the cranial nerves during development?
They loose their segemental arrangement
How are the cranial nerves classified?
Based on their function and their embryological origin
What are the classifications of cranial nerves?
- Somatic efferent
- Special sensory
- Nerves of pharyngeal arches
What parts of the neural tube are the cranial nerves derived from?
The mid- or hind-brain, except CN I and II
What cranial nerves are associated with the pharyngeal arches?
V, VII, IX, and X
Where do the cranial nerves associated with the pharyngeal arches arise from?
The lowest part of the brainstem
What do the cranial nerves associated with the pharyngeal arches innervate?
They innervate the pharyngeal arch, and therefore the structural derivates of each pharyngeal arch is innervated by the particular cranial nerve associated with that arch
What cranial nerve is associated with the first pharyngeal arch?
Trigeminal (V)
What cranial nerve is associated with the second pharyngeal arch?
Facial (VII)
What cranial nerve is associated with the 3rd pharyngeal arch?
Glossopharyngeal (IX)
What cranial nerve is associated with the 4th pharyngeal arch?
Vagus (X)
What cranial nerve is associated with the 6th pharyngeal arch?
Vagus (X)
What cranial nerves have a relationship wtih the pharyngeal arch system?
- CN XI (cranial accessory)
- CN XII
Where does the aortic sac lie?
On the floor of the pharynx
What does each pharyngeal arch become associated with?
Regarding blood vessels
An aortic arch of blood vessels
What does each aortic arch blood vessel run through?
The mesenchyme of its respective pharyngeal arch
How does the arrangement of the aortic arch blood vessels initially appear?
Very organised
What happens to the arrangement of the aortic arch blood vessels?
As the embryo develops, and structures start to derive from the pharyngeal arches, much of the order is lost
What is the rearrangement of developing structures the reason for?
The recurrent laryngeal nerve of the vagus becomes looped under the arch of the aorta on the left side, and the subclavian artery on the right side
What artery is derived from the first arch?
Common, external, and internal carotid arteries and branches
What artery is derived from the second arch?
Common, external, and internal carotid arteries and branches
What artery is derived from the third arch?
Common, external, and internal carotid arteries and branches
What artery is derived from the forth arch?
Aortic arch (on left) and subclavian artery (on right)
What artery is derived from the sixth arch?
Pulmonary arteries
What are the muscular derivatives of the first arch?
- Muscles of mastication
- Mylohyoid
- Anterior belly of digastric
- Tensor tympani
- Tensor veli palatine
What are the muscular derivates of the second arch?
- Muscles of facial expression
- Stapedius
- Stylohyoid
- Posterior belly of digastric
What are the muscular derivates of the third arch?
Stylopharyngeus (muscle of the pharynx)
What are the muscular derivates of the fourth arch?
- Cricothyroid
- Levator palatine
- Constrictors of pharynx
What are the muscular derivates of the sixth arch?
Intrinsic muscles of the larynx
What are the sensory derivatives of the first arch?
Main sensory supply to the face and nasopharynx
What are the sensory derivatives of the 2nd arch?
Special sensory (taste) to anterior 2/3 of tongue
What are the sensory derivates of the 3rd arch?
Sensory to posterior 1/3 of the tongue, and upper part of the pharynx
What are the sensory derivatives of the 4th and 6th arches?
Sensory to linings of the lower pharynx and all larynx
What are the skeletal derivative of the first arch?
- Maxillae and zygomatic bones
- Meckel’s cartilage
- Malleus
- Incus
- Mandible
What are the skeletal derivates of the second arch?
Reichert’s cartilage;
- Hyoid
- Lesser cornu
- Upper body
- Stapes
- Part of middle ear
- Styloid process
- Stylohyoid ligament
What are the skeletal derivatives of the 3rd and 4th arch?
Epiglottis
What are the skeletal derivatives of the 4th and 6th arches?
Cartilages of larynx
What does the facial skeleton arise from?
The frontonasal prominence and the first pharyngeal arch
Why do the muscles of facial expression and the muscles of mastication have different cranial nerve innervation, despite being so topographically related?
Due to them being derived from different pharyngeal arches
What are the muscles of mastication derived from?
The first pharyngeal arch
What is the result of the muscles of mastication being derived from the first pharyngeal arch?
They are innervated by the facial nerve (CN VII)
What is CN VII the nerve of?
The second arch
What does CN VII pass through?
The stylomastoid foramen and parotid gland
What happens in the second pharyngeal pounch?
There is epithelial proliferation, followed by migration in and colonisation by lymphoid precursors
What happens to the 3rd and 4th pharyngeal pouches?
They divide into dorsal and ventral components
What are the derivatives of the first pharyngeal pounch?
- Pharyngotympanic (Eustachian) tube
- Middle ear cavity
What are the derivatives of the second phayngeal pouch?
Palatine tonsil
What are the derivatives of the third pharyngeal pouch?
- Inferior parathyroid (dorsal)
- Thymus (dorsal and ventral)
What are the derivatives of the 4th pharyngeal pouch?
- Superior parathyroid
- C cells of thyroid
