Embyology Flashcards
Define Mesenchyme
Embryonic connective tissue
It develops into bone, cartilage, vessels etc
Explain the naming of the pharyngeal arches?
They develop around the future mouth and; pharynx, supporting the primitive pharynx hence their name.
(Also known as branchial arches)
How many pharyngeal arches are there?
Technically 6 but the 5th regresses so early that we refer to 5 (1-4 & 6)
Describe the structure of the pharyngeal arches?
Ectoderm - Lines the outside forming clefts in between the arches
Endoderm - Lines the inside forming pouches between the arches
The mesoderm is between the two, mixed up with neural crest cells. They become cartilage and muscles in the head/neck.
Describe the nervous innervation of the pharyngeal arches.
1) Trigeminal Nerve - Mandibular Branch (Cr N V3)
2) Facial Nerve (VII)
3) Glossopharyngeal (Cr N IX)
4) Vagus - Superior Laryngeal branch (Cr N X)
6) Vagus - Recurrent Laryngeal Branch (Cr N X)
Why is it so important to know the nervous innervation of the pharyngeal arches?
Any structure formed from an arch will be innervated by the same nerve.
So if we know what innervates a muscle we know where that muscle formed form.
Examples of structure innervation relating to that of its pharyngeal arch
All muscles of facial expression are innervated by the facial nerve so we know they developed from the 2nd branchial arch
The Digastric is split:
Ant Belly - Cr N V3 - So developed from 1st branchial arch
Post Belly - Cr N VII - So developed from 2nd branchial arch
Describe the bones/cartilage formed from each branchial arch
The region formed formed from each starts at the front and extends postior/superiorly:
1st - Mandible up to malleus/incus
2nd - Upper half of hyoid + lesser horn, follows stylohyoid ligament up to styloid process and stapes
3rd - Lower half of hyoid and greater horn
4th Thyroid cartilage
6th Cricoid Cartilage
What does the face develop from?
5 processes:
Mesenchyme superior to the future mouth -> One Frontonasal process
Mesoderm of the 1st arch -> 2 mandibular processes that meet in the middle
2 maxillary processes grow up from the maxillary section of the 1st arch mesoderm
Describe how the mandible forms:
The two mandibular processes fuse to form the mandible.
Describe the formation of the nose?
1) Ectoderm in the Frontonasal process thickens to form 2 nasal placodes
2) The placodes then invaginates to form nasal pits which become nostrils
- 1) As the placodes form they split the nose into 4
- Two lateral nasal processes
- Two medial nasal processes which fuse into 1
3) Then the medial nasal process grows down and laterally, separating the nasal opening from the mouth and forming a process called the philtrum (groove above upper lip)
How does the palate form?
Starts as the medial nasal process grows inf/lateral to form the philtrum.
The philtrum becomes the primary palate.
2 palatine shelves grow inwards from the maxillary process and fuse in the midline to form the secondary palate
The oral and nasal cavity are now fully seperate
How does the cranial vault form?
Made up of flat bones. There are membranous bones that are formed by membranous ossification calvaria
Not fully formed at birth
Sutures between them are soft and allow deformation during through the birth canal. 6 membrane covered fontanelle of clinical significance
How does the base of the skull form?
Made up of irregular bones. Bones are formed by endochondral ossification
How does the viserocranium form?
Made up of skeleton of face
Partly from brachial arches and partly from sensory capsules
