Key events of human development during the third week (week 3) following fertilization or clinical gestational age GA week 5, based on last menstrual period.. Note that during this time the conceptus cells not contributing to the embryo are contributing to placental membranes and the early placenta.
During the third week of development conceptus implantion in the uterus wall is complete and trophoblast cells continue to invade uterine wall in the process of early placentation (villi formation). Within the conceptus, gastrulation converts the bilaminar embryo into the trilaminar embryo (ectoderm, mesoderm, endoderm). Morphological changes include an epithelial to mesenchymal cell transition and folding of the embryonic disc. Carnegie stages 7 to 9 covers this period of human embryo development.
Outside of the embryo, the extraembryonic spaces (chorionic, amniotic, yolk) and intraembryonic spaces (coeloms) continue to develop. Trophoblast cells continue to invade uterine wall and the emdometrium is being converted into the decidua, the process of villi formation and early placentation has begun.
|Week 3 - Human Embryo Stages and Events (GA week 5)
|Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9
|| Gastrulation primitive node (Hensen's node, primitive knot) The small circular region located at the cranial end of the primitive streak, where gastrulation occurs, and is a controller of this process. The process establishes the 3 germ cell layers: Endoderm, Mesoderm and Ectoderm. The second role is to act as an initial generator of the left-right (L-R) body axis.
|| Neural - neurogenesis, neural groove and folds are first seen
|| Musculoskeletal - somitogenesis, first somites form and continue to be added in sequence caudally (1 - 3 somite pairs).
Neural - the three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open
Neural Crest - mesencephalic neural crest is visible
||Heart - cardiogenesis, week 3 begins as paired heart tubes.
|Note - the day timing of stages is only approximate, system names link to first page of that specific system, and events are based upon the literature cited below.
- ↑ <pubmed>17848161</pubmed>
Week 3 Movies
- ↑ O'Rahilly R & Müller F. (2007). The development of the neural crest in the human. J. Anat. , 211, 335-51. PMID: 17848161 DOI.
Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9
- 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
- Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link
Cite this page: Hill, M.A. (2019, September 23) Embryology Week 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Week_3
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- © Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G