2010 BGD Practical 3 - Week 3 Summary

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Practical 3: Oogenesis and Ovulation | Gametogenesis | Fertilization | Early Cell Division | Week 1 | Implantation | Week 2 | Extraembryonic Spaces | Gastrulation | Notochord | Week 3 | Quiz


This page is a overview of events that occur in human development up to week 3 post-fertilization. From this Practical understand concepts of: fertilization, blastocyst development, implantation, bilaminar and trilaminar embryo formation, development of embryonic cavities and brief understanding of early placenta development.

Trilaminar Embryo

By the end of week 3, segmentation of the 3 germ layers has begun:

  1. Ectoderm - central neural plate and lateral parts form epidermis
  2. Mesoderm - midline notochord, adjacent somites, formation of the internal embryonic space (intraembryonic ceolom)
  3. Endoderm - epidermal lining of gastrointestinal tract

Carnegie Stage 8

Stage8 SEM1.jpg


Human embryonic stage 8 occurs during week 3 between 17 to 19 days.

The embryo is now 1.0 - 1.5 mm in size.


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.

From the primitive node a tube extends under the ectoderm in the opposite direction to the primitive streak. This tube forms first the axial process then notochordal process, then finally the notochord.

The notochord is a key to embryonic folding and regulation of ectoderm and mesoderm differentiation. It lies in the rostrocordal axis and the embryonic disc will fold either side ventrally, pinching off a portion of the yolk sac to form the lining of the gastrointestinal tract.


  • embryonic disc
  • primitive node, primative streak, primative groove
  • connecting stalk
  • cut amniotic membrane

Carnegie stage 8

Carnegie Stage 9

Stage9 sem1c.jpg


Human embryonic stage 9 occurs during week 3 between 19 to 21 days.

The embryo is now 1.5 to 2.5 mm in size and somites have begun to form and number between 1 to 3 somite pairs during this stage.

The initial images are displayed unlabeled to allow you to explore the embryo for yourself, linked labeled versions are also available for some images.


Ectoderm - Neural plate brain region continues to expand, neural plate begins folding over the notochord. Gastrulation continues through the primitive streak region.

Mesoderm - Paraxial mesoderm segmentation into somites begins (1 - 3 somite pairs). Lateral plate mesoderm begins to vacuolate, dividing it into somatic and splanchnic mesoderm and to later form the intra-embryonic coelom. Prechordal splanchnic mesoderm begins to form the cardiogenic region, from which the primordial heart will develop.

Endoderm - Notochordal plate still visible which will form the notochord. Endoderm is still widely open to the yolk sac and germ cells form part of this layer. Extra-embryonic mesoderm on the yolk sac surface begins to form "blood islands".


  • Neural groove and neural folds, the mesoderm, which segments beside the neural groove to form somites but extends laterally to margin of embryonic disc lateral plate mesoderm, where it merges with the covering extraembryonic mesoderm.
  • The intra-embryonic coelom develops in the middle of the lateral plate mesoderm. Note amniotic ectoderm covered by extra-emebryonic mesoderm (empty spaces above and below the mesoderm are artefacts, as are the lateral folds in the ectoderm).

The first two images using bright field microscopy approximate the orientation of the scanning electron micrographs below. There are additional scanning electron micrographs showing selected features in detail.

Carnegie stage 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

Human Development Timeline

The table below shows human development features and approximate timing during the menstrual cycle to fertilization and the first 3 weeks of development.

The timing assumes fertilization the day after ovulation and the "weeks" refer to embryonic development and differ from clinical weeks (shown in brackets, from last menstrual period) and "stages" refer to Carnegie stages of development.

Week -2

(Clinical Week 1)

Menstrual cycle
Menstrual Phase
Menstrual cycle.png

Menstrual Cycle changes: Uterine endometrium (loss), Ovary (Follicle Development)

  Human- menstrual uterine endometrium.jpg
Proliferative Phase Smear- early proliferative.jpgOva41he.jpg Menstrual Cycle changes: Uterine endometrium (proliferation), Ovary (Follicle Development)

Week -1

(Clinical Week 2)

Menstrual cycle Event
Proliferative Phase
  Smear- mid-proliferative.jpg Human- mid-proliferative uterine endometrium.jpg Ovary10x.jpg Ova20he.jpg Menstrual Cycle - Mid proliferative
  Smear- late-proliferative.jpg Human- late proliferative uterine endometrium.jpg Menstrual cycle.png Menstrual Cycle - Late Proliferative


Human oocyte.jpg Follicle 001 icon.jpg

Week 1

Week 1 (Clinical Week 3)

Secretory PhaseStage 1 Early zygote.jpg Smear- secretory.jpg Human- secretory uterine endometrium.jpg Fertilization, Secretory Phase
Stage 2 Stage2.jpg Week1 001 icon.jpg Morula, Blastula
Stage 3 CSt3.jpg Blastocyst Hatching (zona pellucida lost)
  Smear- late secretory.jpg Human- late secretory uterine endometrium.jpg Late Secretory, Blastocyst (free floating)
Stage 4 Adplantation
Stage 5 Week2 001 icon.jpg

Week 2

Week 2 (Clinical Week 4)

Week2 001 icon.jpg Implantation
Stage 6 Chorion 001 icon.jpg Chorionic Cavity

Week 3

Week 3 (Clinical Week 5)

Stage 7 Stage7-bf1.jpg Stage7-sem2.jpg Stage7.jpg
Stage 8 Stage8 human.jpg Neuralplate 001 icon.jpg Neural neurogenesis, neural groove and folds are first seen
Stage8 SEM1.jpg
Stage 9 Stage9 bf2c.jpg Stage9 sem1b.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 PMID: 17848161

  Heart cardiogenesis, week 3 begins as paired heart tubes.


Finished Lab 3 !

If you have finished and would like to apply your knowledge, I have also included some [BGDlabfertilization12.htm Clinical Questions] based around this period of development.

If you have finished and need some more help understanding this period of development, I have included some links to [BGDlabfertilization14.htm Online References].

If you have finished and are interested in looking at tissues involved in this period of development, I have included some links to [BGDlabfertilization13.htm Histology Images].

Note that this Practical has discussed mainly development of the embryo as placental development will be covered in detail in another practical (Practical 8 - Placenta and Fetal Membranes).

The next Practical will continue on through embryonic development (Practical 6 - Implantation to 8 Weeks).


  • bilaminar- having 2 layers
  • blastocyst- the developmental stage following morula, as this stage matures, the zona pellucia is lost allowing the conceptus to adplant and then implant into the uterine wall.
  • blastomeres-the cells resulting from the initial rounds of mitotic division of the zygote. These cells become smaller (in cytoplasmic volume) with each division.
  • corona radiata- Layer of follicle cells of cumulus oophorus remaining attached to zona pellucida of oocyte after ovulation.
  • inner cell mass- the clump of cells found inside the blastocyst. These cells will go in to form the embryo, these are the "stem cells" (we here about in the media) that are totipotential, they can form any tissue in the embryo. Mature oocyte-the female germ cell released at ovulation from the ovary.
  • morula &endash;(L. morus = mulberry) early stage of development (12-15 cells) Followed by formation of a cavity in the mass (blastocyst stage).
  • parental genomes- the male (sperm) and female (oocyte) DNA which contributes to the embryo's cells.
  • polar bodies- 3 exclusion bodies which contain the DNA not used by the embryo. Contributed to initially by the meiotic division of the oocyte.
  • pronuclei- the male (sperm) and female (oocyte) nuclei within the fertilized oocyte, prior to their combination to form the new embryo's nuclei.
  • trilaminar embryonic disc- the 3 layered embryo stage.
  • Trophoblasts- (Gr. trophe = nutrition) outer layer of cells on blastocyst that will generate the embryonic part of the placenta.
  • zona pellucida- glycoprotein shell that surrounds the oocyte through to blastula stage of development.
  • Zygote- The first cell stage following fertilization of the oocyte by the sperm. This is the first cell of the conceptus which will divide into blastomeres.

Additional Images

Glossary Links

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
2010 BGD: Lecture 1 | Lecture 2 | Practical 3 | Practical 6 | Practical 12

Cite this page: Hill, M.A. (2021, May 16) Embryology 2010 BGD Practical 3 - Week 3 Summary. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2010_BGD_Practical_3_-_Week_3_Summary

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© Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G