1.2 Beginnings of life

Question 1

What is the zygotic stage?

Answer 1

Entry of sperm into the oocyte leads to formation of zygote - diploid (2N); 1n from sperm 1n from oocyte
Sperm entry causes surface of oocyte (zona pellucida) to become impenetrable, prevents more than 1 sperm fertilising the egg (ovum)

Question 2

What is the cleavage stage?

Answer 2

Division initiated, each cell called a blastomere (solid mass of cells each in contact with zona pellucida)
Cell division asynchronous in mammals

Question 3

What is the morula stage?

Answer 3

16 cell stage = organisational change - cells divide but don’t grow no change in size

Question 4

What is the blastocyte stage?

Answer 4

Continued cell division makes some cells become located on the inner cell mass (icm)
1st differentiation event
Cells on outside pump fluid into embryo to form blastocyst cavity - trophoblasts
Blastocyst “hatches” - breaks out of the zona pellucida

Question 5

What is an ectoptic pregnancy?

Answer 5

Implantation in extra uterine >90% in oviduct because narrowing due to infection impeads movement.
As pregnancy develops, ruptures oviduct, embryo usually dies and a massive haemorrhage for mother
Surgery required

Question 6

How is totipotency lost through cell development?

Answer 6

Decease in totipotency
Morula, blastocyst, ICM (embryonic stem cells)
epi/hypoblast