2011 Lab 2 - Week 3

From Embryology
2011 Lab 2: Introduction | Week 1 | Week 2 | Week 3 | Online Assessment | Group Project


Human embryo day 18.jpg

Approximate cross-section of an 18 day human conceptus.

Identify the 3 layers of the trilaminar embryo: ectoderm (columnar cells), intraembryonic mesoderm (mesenchymal cells, endodermal cells (cuboidal single layer). Identify primitive groove with dense cluster of primitive streak cells below it. (Image: Nishimura etal., 1977)

Key events of human development during the third week (week 3) following fertilization or Clinical week 5 (LMP). 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 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.

Gastrulation means "gut forming" and converts the inner cell mass which then formed the bilaminar embryo (epiblast, hypoblast) into the trilaminar embryo (ectoderm, mesoderm, endoderm).

The process involves the migration of cells from the epiblast layer through the primitive streak to form first the endoderm layer and then a second intermediate layer the mesoderm layer. Once all cells have left the epiblast layer it now becomes the ectoderm layer.

These three germ cell layers (ectoderm, mesoderm, endoderm) will form in a layer specific manner all the future tissues of the developing embryo.

Week 3 Links: Lecture - Week 3 | Gastrulation | Placenta | Coelomic Cavity
Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9

Conceptus Cavities Week 2 and Week 3

File:Chorion 001 icon.jpg</wikiflv> Animation shows the events following implantation and focuses on changes in the the spaces surrounding the embryonic disc, the extraembryonic coelom. The blastoceol cavity is converted into two separate spaces: the yolk sac and the chorionic cavity. The third space lies above the epiblast, the amniotic cavity.

blue - epiblast layer

yellow - hypoblast layer

red cells - extraembryonic mesoderm layer

green - trophoblast layer

red spaces - blood-filled spaces, maternal lacunae

white cells - (left) endometrial gland (right) endometrial epithelium

Quicktime version

Germ Layers and Dynamic Processes

From 1 layer of cells to 3 layers that define all tissues of the entire embryo.

The conceptus is now fully implanted in the uterine wall. This time is now when a small cluster of cells (the inner cell mass) will differentiate into two simple cell layers (epiblast and hypoblast). From the single epiblast layer cells will migrate through a narrow region (primitive streak) to form firstly two layers epiblast and endoderm, then later the three layers ectoderm, mesoderm and endoderm. These final three layers (germ layers) are the trilaminar embryo and will form all the tissues of the body.

During implantation cells on the outside (trophoblast layer) of the conceptus are forming part of the early placenta. The cells on the inside (inner cell mass) now begin to form the three germ layers that will form the entire embryo. Note that as the conceptus also forms the fetal component of the placenta, not all cells from our blastocyst will form part of the developing embryo.

The images below show the embryo (week 3 between 15 to 17 days) as a disc 0.4 mm diameter in size.

Stage7 features.jpg Stage7-sem1.jpg Stage7-sem4.jpg

Stage 7 Human Embryo (15 to 17 days)

Stage 10 Human Embryo (22 to 23 days)

Stage 7

Stage7-sem2.jpg Stage7 features.jpg
Features: embryonic disc, primitive node, primative streak, primitive groove, yolk sac

Facts: Week 3, 15 - 17 days, 0.4 mm

View 1: embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side.

Events: Gastrulation is continuing as cells migrate from the epiblast, continuing to form mesoderm.

Mesoderm lies between the ectoderm and endoderm as a continuous sheet except at the buccopharyngeal and cloacal membranes. These membranes have ectoderm and endoderm only and will lie at the rostral (head) and caudal (tail) of the gastrointestinal tract.

Gastrulation: Through the primitive streak cells migrate continuously through week 3 into week 4. Initial cells replace hypoblast as an epithelial layer the endoderm. Later migrating cells spread between the two epithelial layers to form mesoderm.
Stage7 primitive streak labelled.jpg Stage7 axes.jpg
Axes: embryonic disc is shown rostral (head) to top and caudal (tail) to bottom. Left and right are the lateral margins of the disc as shown.


Carnegie Stage 7 and 8, gastrulation, migration of cells through the primitive streak to form endoderm and mesodermal layers of embryo.


Trilaminar embryo.jpg Scanning electron micrograph showing the early forming 3 layers: ectoderm, mesoderm and endoderm.

Mesoderm 001 icon.jpg

Timeline - Week 3

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 second role is to act as an initial generator of the left-right (L-R) body axis.
Stage 8
Stage8 human.jpg
Neural - neurogenesis, neural groove and folds are first seen
Stage8 SEM1.jpg
Stage 9 Stage9 bf1c.jpg Musculoskeletal - somitogenesis, first somites form and continue to be added in sequence caudally

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]

  Heart - cardiogenesis, week 3 begins as paired heart tubes.
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

Week 2 and 3 Movies

Week2 001 icon.jpg Mesoderm 001 icon.jpg Chorion 001 icon.jpg Amnion 001 icon.jpg Week3 folding icon.jpg
Implantation Mesoderm Chorionic Cavity Amniotic Cavity Week 3


  1. <pubmed>17848161</pubmed>
2011 Lab 2: Introduction | Week 1 | Week 2 | Week 3 | Online Assessment | Group Project

Glossary Links

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Cite this page: Hill, M.A. (2024, April 13) Embryology 2011 Lab 2 - Week 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2011_Lab_2_-_Week_3

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G