Talk:2010 BGD Practical 3 - Extraembryonic Spaces

From Embryology

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 - blood-filled lacunae

Inside the Conceptus - 3 separate spaces

  1. amniotic sac
  2. yolk sac
  3. chorionic sac
Human Conceptus
(Carnegie Stage 9, Day 20)


Conceptus Cavities Week 2 and Week 3

[../Movies/week2.htm Implantation Blastoceol]
[../Movies/larsen/ Conceptus Cavities Early ]
[../Movies/larsen/ Conceptus Cavities Late ]

File:Day10and12.jpgOverview of blastocyst implantation in uterine wall during the second week of development. (Image: Moore & Persaud, 1998)


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. (Image: Nishimura etal., 1977)

File:Day16lp.jpgCarnegie Stage 6-7, human conceptus approximately 16-18 days. (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 expansion dome (primitive single brain bulge) and the cluster of cells at the caudal end of the embryonic disc is the primitive node region. Note the precipitated protein in the extra-embryonic coelom and in the yolk sac, and the start of the formation of the intra-embryonic coelom between ectoderm and entoderm, at the cranial (rostral) end of the expansion dome. File:Day16hp.jpg


Human Conceptus (high power view)

Approximate cross-section of a different, 18 day human conceptus.

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

Identify the 3 layers of the trilaminar embryo: ectoderm (columnar cells), intraembryonic mesoderm (mesenchymal cells, endodermal cells (cuboidal single layer).

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.

Identify primitive groove with dense cluster of primitive streak cells below it.

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)





Outside the Conceptus - blood-filled lacunae

Inside the Conceptus - 3 separate spaces

  1. amniotic sac
  2. yolk sac
  3. chorionic sac


  • 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). (More? Week 1 Notes)
  • 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.