Early human development

Question 1

What is anterior in embryological terms?

Answer 1

-Anterior in embryological terms is the head

Question 2

What is posterior in embryological terms?

Answer 2

-Posterior in embryological terms is the chordal end

Question 3

What do polar bodies contain?

Answer 3

Contain female genetic material

Question 4

What does 1 germ cell give rise to in females?

Answer 4

1 germ cell gives rise to 1 oocyte in females

Question 5

What does 1 germ cell give rise to in males?

Answer 5

1 germ cell gives rise to 4 sperms in males

Question 6

Where do polar bodies come from?

Answer 6

• Nuclear DNA duplicated
• Cells briefly enter meiosis then arrest after the first prophase of meiosis
  
  • Remains in arrested state till adult life
  • Only when single oocyte is selected, then meiosis resumes
• When an adult, first meiotic division occurs and 1st polar body is extruded within the perivitelline space
• Selected oocyte arrest again in meiosis 2 until fertilisation occurs
• After fertilisation meiosis resumes and 2nd polar body is extruded

Question 7

Where does oocyte selection occur?

Answer 7

-Oocyte selection occurs in ovary

Question 8

What is the oocyte collected by?

Answer 8

Oocyte collected by fimbriae

Question 9

What region does fertilisation occur?

Answer 9

Fertilisation occurs in ampulla region

Question 10

What happens in the cleavage stage?

Answer 10

• The two haploid pronuclei fuse and the zygote enters mitosis
• We then get the formation of a 2 cell zygote and then a 4 cell zygote

Question 11

What is the morula?

Answer 11

Morula is the embryo when it has 12-16 cells after mitotic division from the 2 cell zygote.

Question 12

What happens from a morula to a blastocyst?

Answer 12

• Embryo undergoes a process called compaction and cells start to become flatter and from an epithelial barrier
• This allows fluid to accumulate in the central region of the embryo and this region is known as the blastocele and the embryo is now referred to as the blastocyst

Question 13

What are the outer and inner cells of the blastocyst?

Answer 13

Outer cells of the blastocyst are trophoblasts and the inner cells are known as the inner cell mass

Question 14

What do the trophoblasts of the blastocyst take up the behaviour of?

Answer 14

Trophoblasts take up the behaviour of the epithelium

Question 15

Where do blastocysts implant and what process must occur in order for this to happen?

Answer 15

• Blastocyst implants on posterior aspect of uterine wall

- In order for this implantation, the zona pellucida needs to be removed by a process called embryo hatching

Question 16

What day does full implantation usually occur on?

Answer 16

Day 9

Question 17

What cells carry on to proliferate and what does it form in full implantation?

Answer 17

Cells of inner cell mass continue to proliferate and form the bilaminar germ disk(2 layered embryo)

Question 18

What happens to the layers in the process of gastrulation?

Answer 18

In this process, embryo goes from 2 layers to 3 layers

Question 19

Where does the process of gastrulation start and progress to where?

Answer 19

This process starts in the future posterior end of the embryo and progresses to the anterior end

Question 20

What can the progress of gastrulation be seen by?

Answer 20

Progress of gastrulation can be seen by the appearance of the primitive streak

Question 21

What node does the tip of the primitive streak have?

Answer 21

Tip of primitive streak has Hensen’s node

Question 22

What happens at the hensens node in humans?

Answer 22

• This sight in humans is the organiser of gastrulation and acts as an important signalling centre
  - Secretes growth factors

Question 23

What is the epiblast?

Answer 23

Epiblast is a single layer of epithelial cells

Question 24

What is a hypoblast?

Answer 24

Hypoblast is a single epithelial layer of primitive endoderm

Question 25

Steps involved in gastrulation

Answer 25

• Epithelial cells of hypoblast are migrating the outer parts of the embryo in towards the midline
  
  • Cells in the midline undergo epithelial to mesenchymal transition(EMT)
    
    • Epithelial cell junctions are broken down allowing mesenchymal cells to migrate away from neighbouring cells
  • Mesenchymal cells then move down the midline and undergo this process called ingression and eventually replace endoderm cells
• After a couple of days, there’s an endoderm layer and mesoderm cells, which are places in between endoderm and epiblast cells

Question 26

Steps involved in neural tube closure

Answer 26

• Neural folds rise out of the plane of disk
• Neural folds meet and fuse to create neural tube
• Surface ectoderm covers the tube