Embryological and feta development

Question 1

What does embryology do?

Answer 1

Embryology provides an understanding by which the human body develops and is essential for understanding:

• gross anatomy
• congenital anomalies

Question 2

What does gross anatomy mean

Answer 2

Macroscopic structures

Question 3

What does congenital anomalies mean

Answer 3

They are not usually inherited but are born with it. They are not always found straight away.

Question 4

When does embryology begin?

Answer 4

When the sperm fertilises an oocyte.

Question 5

What is an embryo made up of?

Answer 5

An embryo comprises of the tissues formed once mitosis of an ovum begins, therefore, even at the 2 cell stage, it is still an embryo.

Question 6

What are the 3 periods?

Answer 6

• Pre-embryonic period
• Embryonic period
• The fetal period

Question 7

Describe the pre-embryonic period?

Answer 7

This is the first 2 weeks after fertilisation. The embryo moves through the uterine tube to the uterus where it will implant into the uterus wall.

Question 8

Describe the embryonic period?

Answer 8

This is around 3 - 8 weeks. During this period, differentiation occurs in order to establish the different organ systems. At this stage there is the potential for malformation as the embryo is vulnerable to environmental agents such as viruses and tetratogens.

Question 9

Describe the fetal period?

Answer 9

This is from 8 weeks up to full term and is the phase of growth and enlargement.

Question 10

What are the carnegie stages?

Answer 10

They are named after carnegie institute of Washington. They found that age and size is a poor way to classify because the embryo can shrink up to 50% in the fluid. They are therefore based on morphological characteristics.

Question 11

What happens in week 1 of the carnegie stages?

Answer 11

This is stage 1, 2 and 3.
There is the fertilisation of the egg and sperm to form a zygote. The zygote then moves along the Fallopian tube, where there is a cell division, forming the morula and blastocyst.
A blastocyst is a fluid filled ball with rudimentary embryo cells called the inner cell mass.
Around day 5 or 6, the inner cell mass moves into the uterus in order to implant in the uterine wall.
At this stage, it is about 0.1 - 0.15 mm.

Question 12

What is a morula?

Answer 12

This is formed of 10-30 cells and is the final stage before the formation of the fluid filled cavity, the blastocyst.

Question 13

What does rudimentary mean?

Answer 13

immature, undeveloped or basic form.

Question 14

What happens during week 2 of the carnegie stages?

Answer 14

This is stages 4-7.
Cavities within the centre of the blastocyst develop into blastocoele.
The outer layer of the blastocyst thins to single-cell thickness and becomes a trophoblast.
The trophoblast differentiates further to form a synaytiotrophoblast which is the main fetal component of the placenta.
The central group of cells move to one pole to form the inner cell mass.
The inner cell mass develops into 2 layers:
- epiblast
- hypoblast
These 2 layers are in contact to form bilaminar embryonic disc.
The blastocyst becomes completely embedded.

Question 15

What happens during week 3 of the carnegie stages?

Answer 15

This is stages 7-9
The bilaminar embryonic disc forms 3 layers now in a process called gastrulation.
- Epiblast forms the ectoderm
- Hypoblast forms the endoderm
These give rise to the 3rd layer in between them known as the mesoderm.
Endoderm — lining of the gastrointestinal and respiratory systems
Mesoderm — skeleton, muscle, connective tissue
Ectoderm — epidermis of skin nervous system.
At this stage the size is 1-1.5mm.

Question 16

What does a yolk sac do?

Answer 16

Produces blood cells.

Question 17

What happens in the latter stages of week 3?

Answer 17

There is the development of a groove like midline depression in the caudal end of the embryonic disc.
This marks the appearance of the primitive streak.
Cells then migrate cranially and fold to form the solid cylinder of the notochord in readiness for the neural tube.

Question 18

What is the caudal end?

Answer 18

Tail end

Question 19

What is the primitive streak?

Answer 19

Linear band of thickened epiblast that first appears at the caudal end.

Question 20

What is the cranially end?

Answer 20

The head end.

Question 21

What is the notochord?

Answer 21

Skeletal rod supporting body.

Question 22

What is the process of neurulation?

Answer 22

Process of the formation of the brain and spinal cord.

Question 23

What happens during week 4 of the carnegie stages?

Answer 23

This is stages 10-13.
The mesoderm forms somites.
Somites give rise to muscles such as the heart, blood vessels, skeletal structures and dermis.
The mesoderm also contributes to urogenital system and the serous membranes of the body.
The endoderm gives rise to linings of gastrointestinal and respiratory systems and the urinary bladder, as well as the parenchyma of the liver and pancreas.
The trilaminar disc undergoes longitudinal and lateral folding to form a folded embryo with organs inside.
The neural fold depends and fuses, the neural plate folds ventrally in the brain region.

Question 24

Summarise what happens in week 4 of the carnegie stages:

Answer 24

• Beginning of the brain and spinal cord
• Beginning of the formation of the heart
• Beginning of the gastrointestinal tract and liver
• Beginning of limb buds.

Question 25

What do teratogens in the week 4 period cause?

Answer 25

Absence of limbs and heart outside the chest.

Question 26

What are the mesoderm?

Answer 26

The middle layer of cells of embryo

Question 27

What are somites?

Answer 27

They are blocks of mesoderm locatied either side of the neural tube.

Question 28

What is the dermis?

Answer 28

The inner layer of the 2 skin layers.

Question 29

What is the urogenital system?

Answer 29

The organs of the reproductive system and urinary system.

Question 30

What is the gastrointestinal system?

Answer 30

The digestive tract.

Question 31

What happens during week 5 of the carnegie stages?

Answer 31

This is stages 14-16. 
The embryo is the size of a raisin. 
What start to develop? 
- eye gets retinal pigment 
- nasolacrimal groove 
- heart begins to beat
- neural tube forms into 3 parts 
- the 4th ventricle of the brain starts developing 
- Arm and hand plates 
- Foot plate 
- Placenta starts to function

Question 32

What is the nasolacrimal groove?

Answer 32

Produces tears

Question 33

What is the nasal pit?

Answer 33

Gives rise to nostrils

Question 34

What can teratogens effect at this stage?

Answer 34

• Transoesophageal fistulas
• nuclear cataract
• abnormally small eyes
• facial clefts
• absence of hands and feet

Question 35

What is the transoeophageal fistulas?

Answer 35

Connecting tissues between the oesophagus and the trachea.

Question 36

What happens during week 6 of the carnegie stages?

Answer 36

This is stages 17-19 
- pigmented eyes 
- nasal pit 
- nasola crimal groove 
- external acoustic meatus 
- Auricular hillock 
- Heart 
- liver prominence 
- thigh 
- ankle 
- foot plate 
The size is 13-17 mm. 
Head forms its 1st, 2nd and 3rd pharyngeal arch, there is further development of the faebrain, eyes, auricular hillocks, and the upper and lower jaws recognisable. The upper lip and palate are also forming. 
COMPLETE

Question 37

What is the acoustic meatus?

Answer 37

Eardrum membrane

Question 38

What is the auricular hillock?

Answer 38

Collects sound