Embryology

Question 1

List the weeks of embryogenesis

Answer 1

Week 1: Fertilisation 
End of week 1: Blastocyst
Week 2: Implantation and bilamina
Week 3: Gastrulation
Week 4: Folding morphogenesis
End of week 4: 6 pharyngeal arches

Question 2

Describe the blastocyst stage of embryogenesis

Answer 2

• Embryo is at the blastocyst stage

• Differentiated into:
• Inner cell mass (will give rise to entire body system)
* Outer cell layer or trophoblast (forms placental organ)

Question 3

Describe the implantation and bilamina stage of embryogenesis

Answer 3

• In week 2, zona hatching occurs; this is when the blastocyst hatches out of the zone pellucida covering. This allows it to attach to the uterus wall, and obtain nutrients from it
• The inner cell mass develops two layers. This is known as Bilamina formation:
• Epiblast (ectoderm)
• Hypoblast (endoderm)

Question 4

Describe the gastrulation stage of embryogenesis

Answer 4

• Formation of guts
• It is a cell differentiation of the blastocyst into three layers:
• Ectoderm
• Mesoderm
* Endoderm

Question 5

State the fate of the three layers of the blastocyst

Answer 5

Ectoderm: neural tissue and skin. Dentine, enamel and pulp

Mesoderm: Muscle (heart). Masticatory and facial muscles, connective tissue and bone

Endoderm: Digestive system, respiratory tract, endocrine. Submandibular and sublingual glands

Question 6

What are the fate of the pharyngeal arches?

Answer 6

First arch:

• Maxillary and mandibular processes
• Muscles: masticatory muscles, mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini
• Nerve: Trigeminal (V2, V3)
• Maxilla, Meckel’s cartilage
• Maxillary artery and external carotid artery
• Majority of the tongue

Second arch:

• Muscle of facial expression, posterior belly of digastric
• Nerve: Facial nerve (VII)
• Hyoid bone, part of the temporal bone

Question 7

Describe the folding morphogenesis of embryos

Answer 7

Ventral foldings:
* This type of folding ensures that the skin covers everything

Neural folding:
• Is the folding of the ectoderm, which has differentiated into the neural ectoderm
* The neural plate and plate border which eventually forms the neural tube
• This neural tube becomes the brain and spinal cord later

Neural crest cells:

• Following the closure of the trunk neural folds, the neural crest cells migrate from the dorsal aspect of the neural tube and become mesenchymal cell type
• They later give rise to teeth and supporting structures

Caudal folding:
• Enables the embryo to curl into the fetal position
* `This establishes the two ends of the embryo; head end and tail end