BGDA Practical 3 - Extraembryonic Spaces

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



The conceptus is now fully implanted witin the endometrial wall and the uterine epithelium has reformed over the site of implanation.

Note that all future development will now occur within the wall of the uterus, not within the cavity of the uterine body.

As the conceptus continues to implant and grow a series of fluid-filled cavities form both outside and inside the implanting conceptus.

  • Outside the Conceptus - maternal blood-filled lacunae
  • Inside the Conceptus - 3 separate spaces - amniotic sac, yolk sac, chorionic sac (but outside the embryo, therefore "extra-embryonic coeloms")

Conceptus Cavities Week 2 and Week 3

<html5media height="580" width="500">File:Chorion 001.mp4</html5media>

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Chorion 001 icon.jpg
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 layer of the embryonic disc, the amniotic cavity.

  • white cells - (left) endometrial gland (right) endometrial epithelium
  • blue - epiblast layer (forming ectoderm and amniotic cavity lining)
  • yellow - hypoblast layer (replaced with endoderm during gastrulation)
  • red cells - extraembryonic mesoderm layer
  • green - trophoblast layers (cytotrophoblast and syncitiotrophoblast)
  • red spaces - blood-filled spaces, maternal lacunae

Links: MP4 version | Implantation Movie | Week 2 | Movies

Stage 7 - Implanted Conceptus

Stage7 bf5.jpg

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Stage 7 | Embryo Slides


Continued expansion of syncitiotrophoblasts within the endometrial wall opens both uterine glands and uterine blood vessels. These spaces outside the conceptus fill with uterine gland secretions and maternal blood forming maternal blood-filled lacunae. These lacunae provide the initial nutrition to the growing conceptus, which will later be provided through the placenta.


A cavity forms now between the inner cell mass and the trophoblast (cytotrophoblasts) wall. This cavity is the amniotic cavity.

A new cell layer forms from cells proliferating and lining the inside of the blastoceol cytotrophoblastic layer (extraembryoic mesoderm). This extraembryoic mesoderm layer continues to proliferate and then vacuolates, splitting into two separate cavities.

Overview of blastocyst implantation in uterine wall during the second week of development. (Image: Moore & Persaud, 1998)

Beginning of Carnegie Stage 6, blastocyst fully implanted in endometrial wall.

Continued expansion of the cavities in the extraembryonic mesoderm, allow the primary yolk sac to collapse. The collapsed space later becomes lined with endodermal cells and forms the yolk sac.

The space formed outside is lined with extraembryonic mesoderm and forms the third cavity the chorionic cavity.

A section of extraembryonic mesoderm links the now bilaminar embryonic disc to surrounding shell and is called the connecting stalk.

Human Conceptus Implantation 2nd and 3rd Week of Development

Identify chorionic sac, yolk sac and amniotic sac, all containing different fluids.

Identify secondary chorionic villi, maternal blood "spaces" (empty) and uterine tissue (maternal decidua).

  • The embryo lies between the yolk sac fluid and the amniotic sac fluid.
  • Note that the ends ("bases") of some long secondary chorionic villi are attached to the maternal (decidual) tissue by dense clusters of trophoblast cells (the cytotrophoblastic column) - these are anchoring secondary villi.
  • Other shorter, secondary villi are "free-floating" in the maternal intervillous spaces, from which the blood has drained into the large endometrial veins.

Nishimura H. Semba R. Tanimura T. and Tanaka O. Prenatal development of the human with special reference to craniofacial structures : An atlas. (1977) U.S. Dept. of Health, Education, and Welfare, Public Health Service, National Institutes of Health ; Washington. Bethesda, Md.

(Image: Nishimura etal., 1977)

Human embryo 16-18 days 01.jpg

Human conceptus approximately Carnegie stage 6-7 (16-18 days).

This is about the time of the first missed menstrual period.

Human Conceptus (high power view)

Identify the three fluid-filled sacs, the embryo, and the body (connecting) stalk indicating the caudal end of the embryo.
  • Note the thin amnion and the thicker wall of the yolk sac and conversely, the thick ectoderm and thin endoderm.
  • Vascular channels in connecting stalk.
  • The broad elevation in the ectoderm is the primitive single brain bulge and the cluster of cells at the caudal end of the embryonic disc is the primitive node region.
  • Note the start of the formation of the intra-embryonic coelom between ectoderm and entoderm, at the cranial (rostral) end of the expansion dome.
Human embryo 16-18 days 02.jpg

Human Conceptus (high power view)

Human embryo day 18.jpg Approximate cross-section of an 18 day human conceptus.

Identify the 3 cavities: yolk sac, amniotic sac and chorionic sac.

Identify the extraembryonic mesoderm which surrounds the yolk and amniotic sacs, and continues to form the connecting stalk, where it then continues to line the chorionic sac. Extra- and intra-embryonic mesoderm merge along the perimeter of the embryonic disc.

At the top right of the micrograph is a secondary chorionic villus, lying in an empty maternal intervillus space. The villous has an outer layer of syncytiotrophoblast (smaller, darker nuclei), an intermediate layer of cytotrophoblast and a core of mesoblast (extra-embryonic mesoderm).

(Image: Nishimura etal., 1977)

Stage 7 - Embryonic Disc

Stage7 bf51.jpg

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Stage 7 | Embryo Slides

Extraembryonic Spaces Interactive Component

Attempt the Quiz - Extraembryonic Space 

Here are a few simple Quiz questions that relate to Extraembryonic Spaces from the lecture and practical.

See your Quiz Result - Answer all the questions, then click "submit" to complete. The page will reload and you can then reopen this table to see your result and feedback.


1 The extra-embryonic coelomic spaces are the:

blastocoel, previtelline space and villous chorion
decidua parietalis, decidua basalis and decidua capsularis
pericardial, pleural and peritoneal
smooth chorion, villous chorion and amnion
amnion, chorion and yolk sac

2 The trophoblast differentiate to form part of the:

outer cell mass
none of the above

3 The yolk sac proper:

is limited by the exocoelomic membrane
is formed by separation of the cytotrophoblastic layers
disappears after the third week
is lined by endodermal cells
none of the above

4 Which of the following spaces can be sampled as a prenatal diagnostic test:

chorionic space
amniotic space
allantoic space
chorionic villi space
peritoneal space
none of the above

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


  • 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.
  • 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.
  • parental genomes- the male (sperm) and female (oocyte) DNA which contributes to the embryo's cells.
  • 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.

Additional Information

Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page.
Placental membranes.jpg Stage 13 image 073.jpg
Week 7 fetal membranes Week 5 pericardial and peritoneal cavities
Extra-embryonic Coeloms Intra-embryonic Coeloms

Stage 7 - Implanted Conceptus

Stage7 bf5.jpg

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Stage 7 | Embryo Slides
Stage 7 - Embryonic Disc

Stage7 bf51.jpg

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Stage 7 | Embryo Slides



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Cite this page: Hill, M.A. (2024, May 24) Embryology BGDA Practical 3 - Extraembryonic Spaces. Retrieved from

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