04 - Early Embryonic Development 1

Question 0

Discuss the changes taking place as the fertilised human ovum (zygote) travels down the Fallopian tube to the uterus.

Answer 0

Cleavage occurs after 30 hours into two blastomers of equal size inside the zona pelluida.

After further division a morula forms.

Compaction occurs: embryoblast (inner cell mass) and trophoblast (outer cell mass) appear and cells become pluripotent due to secreted substances pushing cells away.

Hatching: blastocyst hatches from the zona pelluida and is free to enlarge and implant.

Question 1

Define the pre embryonic, embryonic and fetal periods of human development.

Answer 1

Pre embryonic period: 1-2 weeks, cleavage, compaction and beginning of implantation

Embryonic: 3-8 weeks

Foetal: 9-38 weeks

*NB weeks calculated from date of LMP so conception is +2 weeks so term is 40

Question 2

Describe the meaning of the following terms: zygote, cleavage, zona pellucida, morula, ovary, Fallopian tube, uterus

Answer 2

Zygote: the initial cell formed when two gametes combine

Cleavage: the first mitosis division of the zygote

Zona pellucida: the glycoprotein membrane surrounding the plasma membrane of an oocyte which only allows one sperm to penetrate.

Morula: 8-16 totipotent cells

Ovary: produces ovum at the end of the Fallopian tube

Fallopian tube: connects the ovary to the uterus

Uterus: the site of implantation, where the embryo develops.

Question 3

Discuss the formation of the blastocyst and the initial stages of implantation.

Answer 3

Compaction occurs: embryoblast (inner cell mass) and trophoblast (outer cell mass) appear and cells become pluripotent due to secreted substances pushing cells away. - this forms the blastocyst and blastocyst cavity

Hatching: blastocyst hatches from the zona pelluida and is free to enlarge and implant.

Day 9: inner cell mass becomes bilaminar disk consisting of the epi blast and the hypoblast. This forms the amniotic cavity.
The outer cell mass becomes the synciotrophoblast and the Cytotrophoblast. At embryonic pole there is rapid development of the synciotrophoblast
At the abembryonic pole the primitive yolk sac forms within the blastocyst cavity and is in contact with the cytotrophoblast

Question 4

Describe the meaning of the following terms: blastocyst, trophoblast, inner cell mass, implantation, cytotrophobkast, synciotrophoblast

Answer 4

Blastocyst: stage 5 days after fertilisation, contains inner cell mass (embryoblast) and outer cell mass (trophoblast)

Trophoblast: largely develops into placenta and provides nutrients for the embryo

Inner cell mass: the cells of a blastocyst that will give rise to the foetus.

Implantation: day 9, uterine epithelium is breached and the conceptus implants within the uterine stroma

Cytotrophoblast: inner layer of trophoblast which plays an important role in implantation

Syncytiotrophoblast:outer layer of trophoblast which allows transfer of nutrients for the embryo

Question 5

Describe the formation of the embryonic disc in the inner cell mass and initial cell differentiation within it.

Answer 5

Day 9: inner cell mass becomes bilaminar disk consisting of the epi blast and the hypoblast. This forms the amniotic cavity.