Embyology

Question 1

Define Mesenchyme

Answer 1

Embryonic connective tissue

It develops into bone, cartilage, vessels etc

Question 2

Quick recap of the embryo from zygote -> Neural tube formation

Answer 2

Zygote -> Mass of cells (Morula) -> Blastocyst

• Mass of cells in blastocyst form a bilaminar germ disc (epi/hypoblast)
• develops into a trilaminar disc (Ecto-Meso-Endoderm)
• Germ disc splits blastocystic cavity in two. The dorsal side (against ectoderm) becomes the amniotic cavity. The ventral side (against endoderm) becomes the yolk sac
• Ectoderm cells drop into mesoderm forming the notochord. The embryo now has an axis (head vs tail end and left vs right)
• At each end is section lacking mesoderm, this is where the mouth and anus will form
• Notochord stimulates ectoderm abode to form neural plate
• Neural plate invaginates to form neural tube
• As it invaginates some cells form neural crest cells which migrate and give rise to structures elsewhere e.g. dorsal root ganglia, autonomic plexus and sympathetic chain
• Finally paraxial mesoderm forms 33 pairs of somites which become the axial skeleton, muscles, cranial skeleton etc.

Question 3

Explain the naming of the pharyngeal arches?

Answer 3

They develop around the future mouth & pharynx, supporting the primitive pharynx hence their name.

(Also known as branchial arches)

Question 4

How many pharyngeal arches are there?

Answer 4

Technically 6 but the 5th regresses so early that we refer to 5 (1-4 & 6)

Question 5

Describe the structure of the pharyngeal arches?

Answer 5

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.

Question 6

Describe the nervous innervation of the pharyngeal arches.

Answer 6

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)

Question 7

Why is it so important to know the nervous innervation of the pharyngeal arches?

Answer 7

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.

Question 8

Examples of structure innervation relating to that of its pharyngeal arch

Answer 8

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

Question 9

Describe the bones/cartilage formed from each branchial arch

Answer 9

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

Question 10

What does the face develop from?

Answer 10

5 processes:
Mesenchyme sup. to the future mouth -> 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

Question 11

Describe how the mandible forms:

Answer 11

The two mandibular processes fuse to form the mandible.

Question 12

Describe the formation of the nose?

Answer 12

1) Ectoderm in the Frontonasal process thickens to form 2 nasal placodes
2) The placodes then invaginates to form nasal pits which become nostrils

2. 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)

Question 13

How does the palate form?

Answer 13

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 & nasal cavity are now fully seperate

Question 14

How does the cranial vault form?

Answer 14

Mesoderm above the developing brain -> Mesenchyme -> Membranous ossification to form cranial fault.

Question 15

When does the cranial vault finish forming?

Answer 15

The sutures are still soft at birth to allow deformation during birth.
The anterior fontanelle is the last to close, at around it 1.5yrs it closes to form the coronal suture.

Question 16

How does the base of the skull form?

Answer 16

Starts with Mesenchyme around the notochord (mainly derived from neural crest cells)
Then it preforms the skull base in cartilage (endochondral ossification)
It Ossifies later.

Part of the base of the skull is made up of sensory capsules.

Question 17

How do the sensory capsules form?

Answer 17

Starts with the head and neck somites (So para-axial mesoderm)
They preform in cartilage then ossify to form the bones around the special sense organs

Question 18

Define the cranial vault, base of skull & Viscerocranium

Answer 18

The cranial vault are flat bones formed by membranous ossification

The base of the skull is made of irregular bones formed by endochondral ossification

The viscerocranium is the skeleton of the face formed from the sensory capsules and branchial arches