Early Embryonic Development

Question 0

What changes take place as a fertilised ovum moves down the Fallopian tubes?

Answer 0

CLEAVAGE = formation of morula
COMPACTION = formation of blastocyst
IMPLANTATION begins

Question 1

Define the periods of embryonic development and the timeframe.

Answer 1

Pre-embryonic period (1-2wks)
Embryonic period (3-8wks)
Foetal period (9-32wks +-2wks)

Question 2

Define zygote.

Answer 2

ZYGOTE = fusion of haploid nuclei of gametes (fertilisation) to form a diploid cell (before cleavage).

Question 3

Define cleavage.

Answer 3

CLEAVAGE = first cell division (30hrs+) forming two equal blastomeres surrounded by zona pellucida (glycoprotein shell).

Question 4

Define morula.

Answer 4

MORULA = ball of totipotent cells formed on day 3 (capacity to become any cell type) surrounded by zona pellucida.

Question 5

Define ovary.

Answer 5

OVARY = female reproductive organ that produces ova (egg cell).

Question 6

Define Fallopian tubes.

Answer 6

FALLOPIAN TUBES = (uterine tubes) allow passage of ova from ovaries to uterus (ciliated epithelia).

Question 7

Define uterus.

Answer 7

UTERUS (womb) = muscular organ in which ova implant (posterior uterine wall) and foetuses gestate, opens to the vagina.

Question 8

Describe the formation of the blastocyst.

Answer 8

COMPACTION (day 4) = first differentiation step. Formation of fluid-filled first cavity (blastocyst), embryoblast (inner cell mass), and trophoblast (outer cell mass).
Each cell is pluripotent (capacity to become many cell types).

Question 9

Define blastocyst.

Answer 9

BLASTOCYST = fluid filled cavity formed during compaction (day 4).

Question 10

Define embryoblast and trophoblast.

Answer 10

EMBRYOBLAST = inner cell mass -> destined to become embryonic tissue.
TROPHOBLAST = outer cell mass -> destined to become supporting tissue
Formed during compaction (day 4).

Question 11

Define implantation.

Answer 11

IMPLANTATION = adhesion of embryo to wall of uterus. Early implantation (day 6-7) = blastocyst.
Stage determines whether pregnancy continues or not.

Question 12

Define syncytiotrophoblast and cytotrophoblast.

Answer 12

SYNCYTIOTROPHOBLAST = outer multinucleated layer of trophoblast (invades endometrium of uterus).
CYTOTROPHOBLAST = proliferative inner layer of trophoblast.

Question 13

Describe the formation of the embryonic disk and the initial differentiation within it.

Answer 13

BILAMINAR DISK = inner cell layer forms during differentiation (day 8) creating amniotic cavity.

• epiblast = all embryonic tissues develop from this.
• hypoblast = yolk sac & embryonic mesoderm develop from this.