Introduction to Embryology

Question 1

How long is the embryonic period and what does it accomplish?

Answer 1

0 - 8 weeks

Establishes organ systems

Question 2

How long is the foetal period and what does it accomplish?

Answer 2

From 9 weeks to term

Growth and maturation of organ systems

Question 3

What are the 6 stages of implantation / impregnation?

Answer 3

1. Ovulation
2. Fertilisation
3. Cleavage
4. Morula
5. Blastocyst
6. Implantation

Question 4

What occurs during week 1?

Answer 4

Fertilisation takes place in uterine tube
Calculated from day 0 OR 2 weeks before last menstrual period
Union of genetic material in sperm and ovum
Formation stages = fertilisation to blastocyst

Question 5

What is a blastocyst?

Answer 5

Three days after fertilisation, a normally developing embryo will contain about 6 - 10 cells. By the fifth or sixth day, the fertilised egg is known as a blastocyst - a rapidly dividing ball of cells

In vitro fertilisation, emryos will be cultured until this stage before being implanted into the mother

Question 6

What are the different layers of a blastocyst and their purpose?

Answer 6

Inner group of cells becomes the embryo

Outer group of cells becomes trophoblast (placenta) - nourish and protect the embryo

Question 7

What three key steps occur during week 2?

Answer 7

Zona hatching = shedding the zona pellucida (surrounding glycoprotein sphere fof blastocyst) which allows implantation on the uterine wall

Implantation = affixation to uterine wall

Bilaminar formation = formation of two layers of the inner cell mass, seperated by an extracellular basement membrane

Question 8

What are the two layers called in the bilaminar?

Answer 8

Epiblast = top layer
Hypoblast = bottom layer

Question 9

Gastrulation occurs in week 3 of the embryonic period, what does this mean?

Answer 9

Epiblast reorganises into a multilayered and multidimmensial structure called the gastrula or trilaminar embryonic disc

Question 10

What are the layers and components of the trilaminar embryo post-gastrulation and what will they develop into?

Answer 10

Ectoderm = outer layer (skin, brain, and spinal cord)

Mesoderm = middle layer (muscle, connective tissue, cardiovascular system, and kidney)

Endoderm = bottom layer (Lines visera and the organs of gastrointestinal tract)

Supported by newly formed cartilaginous rod called the notochord which will become the neural tube

Question 11

In terms of the orofacial region, what is the fate of the 3 germ layers?

Answer 11

Endoderm = submandibular and sublingual glands

Mesoderm = Heart and vascular tissues, masticatory / expression muscles, bone, and other connective tissues

Ectoderm = Parotid gland, enamel, dentine, pulp, cranial nerves, and taste buds

Question 12

How is the neural tube formed?

Answer 12

Day 19 = neural folds start pinching towards the midline of the embryo

Day 22 = Convergence of neural folds

Day 23 = Neural folds fused together to form neural tube

Day 24 = Anterior neuropore closes

Day 26 = Posterior neuropore closes

Question 13

What occurs in week 4 of the embryonic period?

Answer 13

Caudal and ventral body folding / embryonic folding

This forms the basic physiological structure (head, butt etc) as well as internalises the gut endoderm (future respiratory and digestive tracts)

Question 14

What are pharyngeal arches and what is their function?

Answer 14

The result of mesenchymal tissue (connective tissue) in the cranial region of the embryo

Precursors for many crucial structures eg msucles, cartilage / bone, cranial nerves, blood, vessels

Question 15

What is the structure of the pharyngeal arches?

Answer 15

Arches are seperated by clefts (pharyngeal grooves) on the ectodermal surface and pockets (pharyngeal pouches) on the endodermal surface

Question 16

What are examples of muscle structures formed from pharyngeal arch 1?

Answer 16

Masticatory muscles
Mylohyoid
Anterior belly of digastric
Tensor tympani
TVP

Question 17

What are examples of cartilage / bone structures formed from pharyngeal arch 1 by invading neural crest cells?

Answer 17

Meckel’s cartilage (mandible)
Palatopterygoquadrate bar (part of sphenoid)
Malleus
Incus

Question 18

What is an example of a cranial nerve structure formed from pharyngeal arch 1 by invading neural crest cells?

Answer 18

Trigeminal nerve

Question 19

What are examples of blood vessels formed from pharyngeal arch 1?

Answer 19

Maxillary artery

External carotid artery

Question 20

What are examples of muscle structures formed from pharyngeal arch 2?

Answer 20

Facial expression
Posterior belly of digastric
Stylohyoid
Stapedius

Question 21

What are examples of cartilage / bone structures formed from pharyngeal arch 2?

Answer 21

Stapes
Part of hyoid
Temporal bones

Question 22

What is an example of a cranial nerve structure formed from pharyngeal arch 2?

Answer 22

Facial nerve

Question 23

What is an example of a blood vessel formed from pharyngeal arch 2?

Answer 23

Internal carotid

Question 24

What is an example of a mucle structure formed from pharyngeal arch 3?

Answer 24

Stylopharyngeus

Question 25

What is an example of a cartilage / bone structure formed from pharyngeal arch 3?

Answer 25

Part of hyoid

Question 26

What is an example of a cranial nerve structure formed from pharyngeal arch 3?

Answer 26

Glosspharyngeal

Question 27

What is an example of a blood vessel formed from pharyngeal arch 3?

Answer 27

Common carotid

Question 28

What is an example of a muscle structure formed from pharyngeal arch 4 and 6?

Answer 28

Intrinsic laryngeal and pharyngeal

Question 29

What is an example of a cartilage / bone structure formed from pharyngeal arch 4 and 6?

Answer 29

Larnyx cartilage

Question 30

What is an example of a cranial nerve structure formed from pharyngeal arch 4 and 6?

Answer 30

Vagus

Question 31

What is an example of a blood vessel formed from pharyngeal arch 4 and 6?

Answer 31

Arch of the aorta

Question 32

What are the 5 key processes in shaping the nose and mouth?

Answer 32

Frontonasal Process (FNP)
Medial Nasal Process (MNP)
Lateral Nasal Process (LNP
Maxillary Process
Mandibular Process

Question 33

How does the face develop?

Answer 33

Day 24 = Presence of stomodeum (what will become the mouth) which is between the maxillary processes. FNP is above and mandibular process is below

Day 28 = Nasal placodes form and mouth starts forming where stomodeum was

Day 31 = Nasal placodes become the MNP, the LNP, and form nasal pits

Day 35 = Various fusions of processes to form the base of the face

Face is fully developed by week 10

Question 34

What processes form the nose?

Answer 34

Nasal septum = fused MNP
Alae = fused LNP
Bridge = FNP

Question 35

What processes for the lips?

Answer 35

Philtrum = fused MNP
Upper lip = fused maxillary processes and MNP
Lower lip = fused mandibular processes

Question 36

What is macrostomia / microstomia mouth and how is it formed?

Answer 36

A deformity making the mouth appear larger / smaller due to lateral clefts

Due to failure of fusion between maxillary and mandibular processes

Question 37

What are the different kinds of cleft lips and how do they form?

Answer 37

Unilateral / bilateral cleft lips = failure of fusion of MNP and maxillary process

Oblique facial cleft = failure of fusion of LNP and maxillary process

Median cleft lip = failure of fusion of MNP and MNP

Cleft lower lip = Failure of fusion of mandibular process and mandiular process

Question 38

How does the hard palate develop?

Answer 38

From week 7 - 10

Fusion of the palatal shelves with each other and with the nasal septum separates the nasal cavities from the oral cavity

The epithelial cells either undergo EMT or apoptosis to allow the fusion of the processes

Ossification of the palate starts at week 8, mid-palatal suture ossifies by around 12 - 14 years old