Embryology - The First Two Weeks

Front 1

Key events in First Two Weeks

Back 1

- Fertilisation
- Formation of Zygote
- Implanting of Blastocyst
- Formation of bilaminar germ disc
- Formation of amniotic cavity, yolk sac cavity and yolk sac

Front 2

How long does development last?

Back 2

~40 weeks.

Front 3

Differences in embryological and G.P/everyday development timings.

Back 3

- Embryology: from fertilisation
- G.Ps: From last period

Front 4

Gametes: Origin, types, development

Back 4

- Primordial Germ Cells in Gonads
- Undergo gametogenesis:
- Meiosis (n=23)
- Morphological Changes
- Differs in men and women.
Produces:
- Spermatazoa
- Oocytes

Front 5

Fertilisation

Back 5

- Several oocytes mature
- only one reaches full maturity
- Fimbrae wrap around ovum.
- Burst from follicle (=ovulation)
- Fimbrae waft oocyte into uterine tube
- Sperm deposited
- Sperm + oocyte meet at Ampulla

- Sperm shed glycoprotein coat, release acrosomal enzymes in vesicles.
- Only one penetrates fully
- Looses its membrane
- Forms pro-nuclei
- Pro-nuclei of oocyte + sperm fuse
- Chromosome number restored: 46
- Zygote forms.

Front 6

Cleavage

Back 6

- Zygote divides into two cells (=blastomere) within 24 hours.
- Divides into 4 cells within 48 hours (=4 cell stage)
- Day 3, rapid increase in division. 6 - 12 cell morula forms.
- Fluid enters around day 4-5 forming blastocyst.
- Two cell sets become distinct
- Trophoblast (outer-cell mass)
- Embryoblast (inner-cell mass)

Front 7

Implantation

Back 7

- Around day 6
- Blastocyst hatches from the zona pellucida + implants INTO uterine will
- Immune supressing agents released to stop mother mounting immune response
- Also begin HCG production to maintain Uterine wall.
-

Front 8

Second Week: Important features

Back 8

- The week of twos
- Division of trophoblast/embryoblast to form bilaminar germ disc

Front 9

Trophoblast divisions

Back 9

- Cytotrophoblast (innermost)
- Syncytiotrophoblast (outermost)

Front 10

Epiblast Divisions

Back 10

- Epiblast
- Hypoblast

Front 11

Blastocyst cavity becomes....

Back 11

- Yolk-Sac Cavity
- Cells of yolk-sac cavity form extraembryonic mesoderm which replaces the yolk-sac cavity
- Remnant of this cavity = Cyst

Front 12

Amniotic Cavity

Back 12

- Forms 'within' the epiblast and is lined with epiblast cells
- Will increase in size
- Will provide nutrition

Front 13

Yolk Sac

Back 13

- Forms from the hypoblast, lined with hypoblast cells
- Will become the gut tube

Front 14

Chorionic Cavity

Back 14

- Behind the somatic mesoderm
- Will form the placenta