1. Human Embryo Development
Extended Study

2. Learning Objectives List the sequence of development of an embryo
Explain the term fertilized egg
Explain the term blastocyst
Explain the term amnion
Explain how the placenta is formed
Explain how the embryo develops up to the third month of gestation

3. Sequence of development from fertilised egg
Early stages

4. Sequence of development from fertilised egg
The zygote contains 46 chromosomes, twenty three from the egg and 23 from the sperm

5. It divides rapidly by mitosis to produce 2 cells, then 4, then 8, 16 etc. and continues to divide

6. At this point the developing individual is referred to as the morula

7. Around 5 days after fertilisation the morula forms a hollow ball of cells called the blastocyst

8. The outer layer of the blastocyst forms the trophoblast
The outer layer of the blastocyst forms the trophoblast. This will later develop into the layer of membranes that surround the embryo (placenta and amnion)
Trophoblast

9. The inner cells (called the inner cell mass) of the blastocyst will eventually form the embryo. These cells are not yet specialised. They have a phenomenal ability to differentiate – divide to give rise to many different types of tissue
Inner cell mass

10. Stem cells
Huge research potential to renew or repair damaged body parts.

11. Learning check How many chromosomes in a fertilised egg?
Is a fertilised egg haploid or diploid?
What is the developing individual referred to when it is made up of 8 cells?
What is it referred to after a number of days?
What is unusual about the cells of the inner cell mass?

12. The morula/blastocyst is pushed along the fallopian tube until it enters the uterus

13. Here it will implant into the uterus wall
Here it will implant into the uterus wall. The endometrium now provides nourishment for the developing blastocyst
Connections with the mother will begin to form (placenta and umbilical cord)

14. This point marks the beginning of pregnancy

15. Sequence of development from fertilised egg
Development of the embryo

16. About 10 days after fertilisation the inner cell mass forms the embryonic disc
This usually consists of three layers called germ layers
Ectoderm (outside)
Mesoderm (middle)
Endoderm (inside)

17. Each of these layers gives rise to specific structures in the developing embryo
In humans the mesoderm is split by a layer called the Coelom
This allows space for more complex organs such as heart, lungs and kidneys to develop

18. Ectoderm – skin, nervous system
Coelom – heart, lungs
Mesoderm – muscles, skeleton
Endoderm – inner lining of digestive system

19. The Amnion

20. The Amnion
When first formed the amnion is in contact with the embryo, but at about the fourth or fifth week fluid begins to accumulate within it (amniotic fluid)
The primary function of the amnion and amniotic fluid is the protection of the embryo for its future development

21. Four to five weeks after fertilisation
The heart forms and starts to beat
The brain also develops
The limbs have started to form

23. By the 6th week Eyes are visible The mouth, nose and ears are forming
The skeleton is at the early stages of development

24. By the 8th week the major body organs are formed
Sex glands have developed into testes or ovaries
Bone is beginning to replace cartilage

25. By the 8th week
At this stage the embryo has taken on a recognisably human from.
From this point it is referred to as a foetus
The foetus continues to grow. No new organs are formed from this point

26. By the 12th week (3 months)
The nerves and muscle become co-ordinated allowing the arms and legs to move
The feotus sucks its thumb, urinates and even releases feaces into the amniotic fluid

27. By the 12th week (3 months)
The gender of the foetus can be seen in scans

28. The time from fertilisation to birth (the gestation period) lasts about 38 weeks (9 months)

29. Learning Check What are germ layers? Name them
What features have already appeared by the fifth week?
At what stage is the developing individual referred to as a feotus?

30. Placenta formation

31. Placenta formation

32. Placenta formation
The placenta forms from a combination of the tissues of the uterus and the embryo
Soon after implantation a membrane called the chorion completely surrounds the amnion and embryo

34. The chorionic villi emerge from the chorion and invade the endometrium allowing the transfer of nutrients from maternal blood to fetal blood

35. This combination of the chorionic villi and the endometrium will eventually form the placenta which becomes fully operational about three months into the pregnancy

36. The Placenta Chorion Embryo Placenta Mother’s blood Amnion
Nutrients, Oxygen, antibodies
Wastes, Carbon Dioxide, Water
Amnion
Amniotic fluid
Umbilical cord
Embryo’s blood
Embryo

37. Placenta allows gases, nutrients, waste, antibodies, some drugs, hormones and micro-organisms to be exchanged between mother and baby
Placenta also produces hormones

38. Blood supplies of mother and embryo do not mix
Blood types may not be compatible
Mother’s blood pressure might damage embryo

39. Umbilical cord connects the embryo with the placenta
it takes blood from the embryo to the placenta and back again
It must be cut at birth to separate mother and baby

40. 45 seconds after birth!

41. Syllabus

42. Learning check
Name the structures that move into the endometrium and eventually become part of the placenta
At what point does the placenta become fully operational?
Why is it important that the blood of the mother and baby do not mix?

43. Depth of treatment
Sequence of development from fertilised egg, morula, blastocyst, existence of amnion, placenta formation from embroyonic and uterine tissue.
Development of embryo up to third month.