Embryonic Development

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Introduction

Author Comments
Mark Hill.jpg Start here by looking at the external appearance of embryos in sequence from 1 to 23. It is not so important to memorise the dates, as they are only approximate, but more important to understand growth (size changes) and the development (overall sequence of events) during this period.

Clicking the Carnegie stage numbers opens a page dedicated to describing that single stage and the associated developmental events.

This page shows some key events of human development during the embryonic period of the first eight weeks (weeks 1 - 8) following fertilization. This period is also considered the organogenic period, when most organs within the embryo have begun to form.


There are links to more detailed descriptions which can be viewed in a week by week format, by the Carnegie stages or integrated into a Timeline of human development.


Online resources include: individual images of all Carnegie stages, scanning electron micrographs of the earlier stages, cross-sections showing internal structures at mid- and late-embryonic, 3D reconstructions of internal structures, animations of processes, ultrasound scans and information about abnormalites of development.


Note that there is variability in the actual timing of specific events and at the end of this period fetal development begins.

Human Embryo - Biological definition  
The following biological definition comes from the Australian National Health and Medical Research Council (NHMRC) discussion paper (2006).
"human embryo means a discrete entity that has arisen from either:
(a) the first mitotic division when fertilisation of a human oocyte by a human sperm is complete; or
(b) any other process that initiates organised development of a biological entity with a human nuclear genome or altered human nuclear genome that has the potential to develop up to, or beyond, the stage at which the primitive streak appears;
and has not yet reached 8 weeks of development since the first mitotic division."

This definition was also published by the same group in 2007.

J K Findlay, M L Gear, P J Illingworth, S M Junk, G Kay, A H Mackerras, A Pope, H S Rothenfluh, L Wilton Human embryo: a biological definition. Hum. Reprod.: 2007, 22(4);905-11 PubMed 17178746


Historically: "The distinction between the embryonic and the fetal periods at 8 postovulatory weeks has proved valuable. It is based primarily on the probability that more than 90 percent of the more than 4,500 named structures of the adult body have appeared by that time."

O'Rahilly R. 1979. Early human development and the chief sources of information on staged human embryos. Europ. J. Obstet. Gynec. Reprod. Biol., 9, 273-280. PMID 400868

O'Rahilly R. and Müller F. Developmental Stages in Human Embryos. Carnegie Institution of Washington Publication 637 (1987).


Links: NHMRC paper PDF | PMID 17178746 | Embryonic Development | Fertilization | Cell Division - Mitosis)
Embryo stages 002 icon.jpg
 ‎‎Human Embryo
Page | Play
Kyoto Collection Carnegie Collection
Human Carnegie stage 1-23.jpg Human Carnegie stage 10-23.jpg
Human Embryo, Carnegie stages 1-23 Human Embryo, Carnegie stages 10-23
Use the stage number links to images and information about each specific stage of human development over the first 8 weeks. The links below give a broad overview of developmental events during each week.
Week: 1 2 3 4 5 6 7 8
Carnegie stage: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Stage14compare23.jpg

Embryo sizes - stage 14 compared to 23

Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9

Links: Embryo Virtual Slides | Human Embryo Collections | Carnegie stage 13 - serial sections | Carnegie stage 22 - serial sections

Week 1

Week 1 - Early zygote


Week 1 Links: stage 1 | stage 2 | stage 3 | Menstrual Cycle | Fertilization | Zygote | Morula | Blastocyst | Lecture - Fertilization | Meiosis | Mitosis | Lecture - Week 1 and 2 | Menstrual Cycle | Oocyte | Spermatozoa | Twinning | Genetic risk maternal age | Trisomy 21 | Trisomy 18 | Trisomy 13 | Hydatidiform Mole | GA week 3
Carnegie stages
Early zygote labelled.jpg Stage2.jpg CSt3.jpg Human embryo day 5.jpg
stage 1 stage 2 stage 3 stage 4

Week 2

Week 2 Implantation


Week 2 Links: stage 4 | stage 5 | stage 6 | Lecture - Week 1 and 2 | Implantation | Trophoblast | Human Chorionic Gonadotropin | Pregnancy Test | Twinning | Category:Week 2 | GA week 4
Carnegie stages
Stage5 bf11L.jpg Stage6 bf03.jpg
stage 5 stage 6

Week 3

Week 3 - Embryonic disc


Week 3 Links: stage 7 | stage 8 | stage 9 | Gastrulation | Endoderm | Mesoderm | Ectoderm | Placenta | Coelomic Cavity | Lecture | Practical 1-3 | Practical 3-4 | Category:Week 3 | GA week 5
Carnegie stages
Stage7-sem4.jpg Stage8 SEM1.jpg Stage9 sem1c.jpg
stage 7 stage 8 stage 9

Week 4

Week 4 - The heart begins


Week 4 Links: stage 10 | stage 11 | stage 12 | 13 Placodes | Pharyngeal arches | Lecture - Mesoderm | Lecture - Ectoderm | Lecture - Early Vascular | Category:Week 4 | GA week 6
Carnegie stages
Stage10 sem2.jpg Stage11 sem13c.jpg Stage12 sem1c.jpg Stage13 sem1c.jpg
stage 10 stage 11 stage 12 stage 13

Week 5

Week 5 embryo
Carnegie stages
Stage14 sem1c.jpg Stage15 bf1c.jpg
stage 14 stage 15

Week 6

Week 6 - Early face deevelopment
Carnegie stages
Stage16 bf1c.jpg Stage17 bf1c.jpg
stage 16 stage 17

Week 7

Week 7 - Head and limb development
Carnegie stages
Stage18 bf1c.jpg Stage19 bf1c.jpg
stage 18 stage 19

Week 8

Week 8 - Last embryonic stage 23
  • Week 8 - Carnegie stage - 20 | 21 | 22 | 23
  • Last week of embryonic development.
Carnegie stages
Stage20 bf1c.jpg Stage21 bf1c.jpg Stage22 bf1c.jpg Stage23 bf1c.jpg
stage 20 stage 21 stage 22 stage 23
Carnegie Stages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | About Stages | Timeline

Carnegie Stage Table

Weeks shown in the table below are embryonic post ovulation age, for clinical Gestational Age (GA) measured from last menstrual period, add 2 weeks.

Stage
Days (approx)
Size
(mm)
Images
(not to scale)
Events
1
1
(week 1)
0.1 - 0.15
Human zygote two pronuclei 02.jpg
fertilized oocyte, zygote, pronuclei
2
2 - 3
0.1 - 0.2
Human embryo day 3.jpg
morula cell division with reduction in cytoplasmic volume, blastocyst formation of inner and outer cell mass
3
4 - 5
0.1 - 0.2
Human embryo day 5.jpg
loss of zona pellucida, free blastocyst
4
5 - 6
0.1 - 0.2
Week2 001 icon.jpg
attaching blastocyst
5
7 - 12
(week 2)
0.1 - 0.2
Stage5 bf11L.jpg
implantation
6
13 - 15
0.2
Stage6 bf03.jpg
extraembryonic mesoderm, primitive streak, gastrulation
7
15 - 17
(week 3)
0.4
Stage7 features.jpg
gastrulation, notochordal process
8
17 - 19
1.0 - 1.5
Stage8 bf4.jpg
primitive pit, notochordal canal
9
19 - 21
1.5 - 2.5
Stage9 dorsal.jpg
Somitogenesis Somite Number 1 - 3 neural folds, cardiac primordium, head fold
10
22 - 23
(week 4)
2 - 3.5
Stage10 bf4b.jpg
Somite Number 4 - 12 neural fold fuses
11
23 - 26
2.5 - 4.5
Stage11 bf7b.jpg
Somite Number 13 - 20 rostral neuropore closes
12
26 - 30
3 - 5
Stage12 bf5b.jpg
Somite Number 21 - 29 caudal neuropore closes
13
28 - 32
(week 5)
4 - 6
Stage13 bf2c.jpg
Somite Number 30 leg buds, lens placode, pharyngeal arches
Stage 13/14 shown in serial embryo sections series of Embryology Program
14
31 - 35
5 - 7
Stage14 bf2c.jpg
lens pit, optic cup
15
35 - 38
7 - 9
Stage15 bf1c.jpg
lens vesicle, nasal pit, hand plate
16
37 - 42
(week 6)
8 - 11
Link=Carnegie_stage_16
nasal pits moved ventrally, auricular hillocks, foot plate
17
42 - 44
11 - 14
Stage17 bf1c.jpg
finger rays
18
44 - 48
(week 7)
13 - 17
Stage18 bf1c.jpg
ossification commences
19
48 - 51
16 - 18
Stage19 bf1c.jpg
straightening of trunk
20
51 - 53
(week 8)
18 - 22
Stage20 bf1c.jpg
upper limbs longer and bent at elbow
21
53 - 54
22 - 24
Stage21 bf1c.jpg
hands and feet turned inward
Stage 22 shown in serial embryo sections series of Embryology Program
22
54 - 56
23 - 28
Stage22 bf1c.jpg
eyelids, external ears
23
56 - 60
27 - 31
Stage23 bf1c.jpg
rounded head, body and limbs
Following this stage Fetal Development occurs until birth (approx 37 weeks)

The embryos shown in the table are from the Kyoto and Carnegie collection and other sources.


Glossary Links

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Cite this page: Hill, M.A. (2016) Embryology Embryonic Development. Retrieved July 27, 2016, from https://embryology.med.unsw.edu.au/embryology/index.php/Embryonic_Development

What Links Here?
© Dr Mark Hill 2016, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G