8.1 The Carnegie stages

Synoptic table of the Carnegie stages 1 - 6

Stage 1

Approx. 1rst day

0.1 - 0.15 mm

`1 2 3 4`	`Male pronucleus Female pronucleus Doubled paternal centrosome "Inner bodies"`

Characteristic signs:Fertilized oocyte Polar bodies Male and female pronucleus Formation of the zygote

Stage 2

Approx. 2nd - 3rd day

0.1 - 0.2 mm

`1 2 3`	`Zona pellucida Blastomere Polar body`

The axis (dashed) lies through the two future (embryonic and abembryonic) poles. The embryo is divided into right and left halves (A and B).

Characteristic signs:Separation of the blastomeres Formation of the blastomeres (totipotent to ca. the 12-cell stage) Morula stage Compaction (8 to 16 cells) The polarization of the blastomeres becomes slowly visible.

Stage 3

Approx. 4th - 5th day

0.1 - 0.2 mm

`1 2`	`Embryoblast Zona pellucida`

`3 4`	`Trophoblast Blastocyst cavity`

Characteristic signs:After the blastocyst cavity has formed, one speaks of a blastocyst (16 to 32 cells) Segmentation (inner and outer cell mass) Embryonic and abembryonic poles

`1 2 3`	`Decomposed zona pellucida with hatching blastocyst Trophoblast cells Hypoblast`

`4 5`	`Blastocyst cavity Epiblast`

Characteristic signs:Free blastocyst Hatching of the blastocyst through disintegration of the zona pellucida Epiblast and hypoblast are formed and thereby the didermic embryonic disk.

Stage 4

Approx. 6th day

0.1 - 0.2 mm

`1 2 3`	`Epithelium of the uterine mucosa Hypoblast Syncytiotrophoblast`

`4 5`	`Cytotrophoblast Epiblast`

Characteristic signs:Syncytiotrophoblast Cytotrophoblast Anchoring of the blastocyst on the endometrium and interaction between the embryo and maternal tissue. Implantation of the blastocyst in the mucosa of the uterus

Stage 5a

Approx. 7th - 8th day

0.1 - 0.2 mm

`1 2 3 4`	`Syncytiotrophoblast Cytotrophoblast Hypoblast Epiblast`

`5 6 7`	`Blastocyst cavity Amniotic cavity Maternal blood vessels`

Characteristic signs:Development of the bilaminar germ disk Genesis of the amniotic cavity and the primary yolk sac (= primary umbilical vesicle) Invasion of the trophoblasts into the uterine mucosa Solid trophoblast

Stage 5b

Approx. 9th day

0.1 - 0.2 mm

`1 2`	`Lacunar trophoblast Definitive amniotic cavity`

`3 4`	`Primary yolk sac Maternal blood vessels`

Characteristic signs:Formation of the trophoblast lacunae and complete penetration of the embryo into the endometrium Lacunar trophoblast Definitive amniotic cavity Primary umbilical vesicle (= primary yolk sac)

Stage 5c

Approx. 11th - 13th day

0.15 - 0.2 mm

`1 2 3`	`Extraembryonic mesoblast Amniotic cavity Primary yolk sac (= primary umbilical vesicle)`

`1 2 3`	`Extraembryonic mesoblast Chorion cavitySecundary yolk sac (= secundary umbilical vesicle)`

Characteristic signs:Erosion of the maternal vessels: maternal blood in the lacunae of the throphoblast The prechordal plate is formed (see stage 6) Extraembryonic mesoblast Transformation of the primary into the secundary yolk sac (= secundary umbilical vesicle)

Stage 6

Approx. 17th day

0.2 mm

1 2 3 4 5 6 7 8 9 NB Primitive groove Primitive pit Primitive node Oropharyngeal membrane Cardial plate Cut edge of amniotic membrane Mesoderm Endoderm Future cloacal membrane 1+2+3 = primitive streak

Characteristic signs:Primitive streak, Primitive node Primitive groove Secundary umbilical vesicle Allantoic diverticulum Cloacal membrane Primordial vessels in the embryonic body stalk Primordial germ cells Body stalk

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