Fertilisation

Question 1

Describe the events involved in fertilisation and the results of fertilisation in terms of chromosome number and genetic makeup of the individual

Answer 1

in stages 3,4 of fertilisation = fusion of plasma membranes of oocyte and sperm = completion of meiosis 2= both head and tail of sperm are inside the egg
In stage 5 = formation of the female and male pronuclei = 23 chromosomes each , membranes of the pronuclei break down= 46 chromosomes now
so RESULTS
1) secondary oocyte = completes meiosis 2
2)diploid number is restored
3)variation
4)determination of sex
5)first cleavage division

Question 2

Briefly explain the importance of sperm capacitation

2

Answer 2

Sperm = can’t fertilise the oocyte when they are newly ejaculated because they have masking membrane proteins .
In uterus oviduct the plasma protein covering is removed exposing binding portions of the sperm head

Question 3

Describe the cellular events in the embryo and the uterine wall up to and including implantation and the development of the embryoblast and trophoblast layers

Answer 3

Cleavage occurs (mitotic divisions) = blastomeres become smaller with each division

1) 16 cells = morula
2) Blastocyst = embryo after 5-7 days
- Cavity = filled with blastocoel , hatches from zona pellucida before implantation
- Inner cell mass = Embryoblast
- Outer cell mass = trophoblast

Question 4

Describe differentiation of embryoblast into epiblast and hypoblast layers

Answer 4

Implantation occurs:

• outer cell mass = trophoblast divided and differentiates into :
  1) Outer syncytiotrophoblasts
  2) Inner cytotrophoblasts

Inner cell mass:
Embryoblast divides and differentiates into epiblast (top) and hypoblast (bottom)

Question 5

Describe the cellular events in the embryo leading to the development of the bilaminar disk

Answer 5

day 8= cavity in the epiblast layer
Week 2 = epiblast, hypoblast form bilaminar disc
WEEK 3 = Trilaminar disc of ectoderm , mesoderm and endoderm

Question 6

what are the three layers of the trilaminar disc , and what are the parts of the primitive streak
(6)

Answer 6

week 3= 3 layers to form tilaminar disc 
1)Ectoderm 
2)Mesoderm
3)Endoderm
Primiive streak forms and it has 3 parts ; the groove , node and pit (Hensen's node) , it defines the anterior , posterior and lateral demotions of the foetus.

Question 7

List the tissues/organs derived from the three germ layers

Answer 7

Ectoderm:means outer& external forms hair, nails, brain ,spinal chord, PNS
Mesoderm: muscle, bone, CT, notochord, kidney, gonads circulatory system
Endoderm: means within so it forms the epithelial lining of the digestive tract up + lungs

Question 8

summary of fertilisation
the week of 2’s
(4)

Answer 8

1) Outer cell mass = trophoblast differentiates into 2 = synctiotrophoblast and cytotrophoblast
2)Inner cell was splits into 2 =Epiblast and hypoblast
3) 2 cavities form = amniotic cavity in the epiblast
yolk sac = outside the embryo
4) Outside the embryo = 2 layers too extra embryonic somatopleure , extra embryonic splanchnopleure