Early Embryonic Develoment 1

Question 0

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

Answer 0

Oocyte released from ovary –> travels along uterine (Fallopian) tube –> fertilisation by sperm in ampulla –> zygote –> implantation in posterior uterine wall

Question 1

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

Answer 1

Pre-embryonic - weeks 1-2
Embryonic - weeks 3-8
Fetal - weeks 9-38

Question 2

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

Answer 2

Zygote - fertilised oocyte
Cleavage - the dividing of cells into two masses (blastomeres) that happens 30 hours after fertilisation
Zona pellucida - glycoprotein shell to prevent polyspermy
Morula - result of cleavage of fertilised oocyte, each cell is totipotent
Ovary - female reproductive organ that produces eggs for fertilisation
Fallopian tube - tube the oocyte travels along to reach uterus, egg is fertilised at ampulla of Fallopian tube
Uterus - female reproductive organ where implantation of the zygote and growth of embryo occurs

Question 3

Discuss the formation of the blastocyst and initial stages of implantation

Answer 3

Compaction (day 4) - forms blastocyst made up of embryoblast (inner cell mass), trophoblast (outer cell mass) and blastocyst cavity
Hatching (day 5) - blastocyst hatches from zona pellucida, no longer constrained and free to enlarge, can impant
Implantation (day 6-7) - conceptus now has 107 cells (8 for embryo, 99 for foetal membrane)

Question 4

Describe the meaning of the following terms: blastocyst, trophoblast, inner cell mass, implantation, cytotrophoblast, syncytiotrophoblast

Answer 4

Blastocyst - formed from the compaction of cells that make up morula
Trophoblast - outer cell mass that will later form structures to support embryo (placenta)
Embryoblast - inner cell mass that will later become bilaminar disc
Implantation - the attachment of the blastocyst to the uterine wall at day 6-7
Cytotrophoblast - placental membrane around yolk sac
Syncytiotrophoblast - cells that invade maternal sinusoids resulting in uteroplacental circulation

Question 5

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

Answer 5

Bilaminar disc and amniotic cavity are formed
Implantation is interstitial - uterine epithelium is breached and conceptus implants in stroma
Implantation defects - placentia praevia, ectopic pregnancy
Day 9 - embryonic pole - rapid development of syncytiotrophoblast
Abembryonic pole - primitive yolk sac formed
Day 11 - primitive yolk sac pushed away from cytotrophoblast by extraembryonic reticulum (–> extraembryonic mesoderm)
Day 12 - maternal sinusoids invaded by syncytiotrophoblast
Day 13 - formation of secondary yolk sac aka definitive yolk sac
Day 14 - spaces in extraembryonic mesoderm merge to form chorionic cavity with embryo suspended by connecting stalk (future umbilical cord)