Week 3

Embryology - 5 Dec 2023 Expand to Translate
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Introduction

Week 3 embryonic disc

Gastrulation

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.

Carnegie Stages

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

Historic Carnegie Stages 7 to 9.

Historic Papers: 1923 head-process | 1933 tubal | 1940 | 1949

Historic Papers: 1920 Mateer Embryo | 1931 Head-Process | 1931 Prochordal Plate | 1931 neurenteric canal

Historic Papers: 1920 | 1926 | 1945

Timeline

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
Day	Stage	Event
15
16	Stage 7	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.
17
18	Stage 8	Neural - neurogenesis, neural groove and folds are first seen
19
20	Stage 9	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^[1]
21		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.
References ↑ <pubmed>17848161</pubmed>

Day	Stage	Event
15
16	Stage 7	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.
17
18	Stage 8	Neural - neurogenesis, neural groove and folds are first seen
19
20	Stage 9	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^[1]
21		heart - cardiogenesis, week 3 begins as paired heart tubes.