Week Two
Overview
During week two, the blastocyst continues the process of implantation. The second week of development is sometimes called the "Week of Twos" because of the appearance of several pairs of related structures.
Trophoblast
The outer cell mass or trophoblast forms 2 layers:
- Cytotrophoblast - a layer of cuboidal, epithelial cells that fuse into the syncytiotrophoblast.
- Syncytiotrophoblast - amorphous mass of protoplasm derived from fusion of cells from cytotrophoblast that possesses enzymes to facilitate implantation.
Extraembryonic Mesoderm
Extraembryonic mesoderm forms 2 layers during the second week of development:
- Somatic layer of extraembryonic mesoderm develops adjacent to the cytotrophoblast.
- Splanchnic layer of extraembryonic mesoderm develops adjacent to the hypoblast.
Embryoblast
Embryoblast forms a 2 layered disc (bilaminar germ disc)
- Epiblast - dorsal layer that gives rise to all three germ layers of the trilaminar embryonic disc in the third week
- Hypoblast - layer of cells ventral to the epiblast and which will contribute to the formation of the primary yolk sac.
Cavities
Two cavities form during the second week of development:
- Amniotic cavity develops dorsal to the epiblast.
- Primary yolk sac develops ventral to the hypoblast.
Week 2 - An Animation
- The blastocyst enters the uterine cavity and hatches from zona pellucida.
- It attaches to the uterine mucosa. The trophoblast differentiates a syncytium, syncytiotrophoblast that expands by the addition of cells from the inner, cytotrophoblast.
- The syncytiotrophoblast secretes enzymes that allow it to burrow into the stratum functionals. It gains sustenance from glycogen and fatty acids contained within the decidual cells.
- Lacunae, hollow spaces, form within the syncytium. These will eventually erode into the uterine arteries to become filled with maternal blood and establishing the hemochorial placenta.
- Meanwhile the inner cell mass forms a bilaminar (two layered) embryonic disc. The dorsal epiblast is covered by an amniotic cavity and the ventral hypoblast is located above the primary yolk sac.
- A layer of connective tissue, the extraembryonic mesoderm forms the lining of the chorionic cavity and forms a connecting stalk to hold the embryonic disc to the newly forming placenta. It will form the mucous connective tissue of the umbilical cord.
Bubble on a Bubble in a Balloon
A fanciful way of describing the embryo at the end of the second week of development is "A bubble on top of a bubble inside a balloon."
The amnion and epiblast make the top bubble, the hypoblast and yolk sac make the bottom bubble. They are attached by the connecting stalk to the inside of the balloon formed by the chorionic cavity.
Fig 2. In simple terms, the implanted conceptus can be described as a bubble on top of a bubble in a balloon. Trophoblastic lacunae mark the location where the placenta is developing.
This relationship is demonstrated in the modern day ultrasound image made sometime during week two of pregnancy and is supported by the image of a 2 week conceptus drawn by Henry van Dyke Carter for the 1848 edition of Gray's Anatomy.
Fig 3. The ultrasound appearance of a 2 week conceptus compared to an anatomical drawing of a 2 week conceptus from the late 1800's. The similarity is remarkable.
Bilaminar Embryonic Disc
Fig 4. Enlarged view of a bilaminar embryonic disc at the end of the second week of development. Two distinctive layers of cells define the dorsal and ventral surfaces of the embryonic disc and their relationships to the amniotic cavity and primary yolk sac.
At the end of week 2, the embryo is a bilaminar (two layered) disk consisting of epiblast and hypoblast. These layers determine the dorsal and ventral surfaces, respectively of the embryo. Cranial and caudal axes and right/left axis will develop a bit later. These axes are critical to the development of the body plan during weeks 3 and 4.
The syncytiotrophoblast and cytotrophoblast continue to establish an intimate connection to the uterine wall. At the end of week 2, blood from the maternal sinusoids will begin to flow through lacunar spaces in the trophoblast, Together, the endometrial stroma and the trophoblast (chorion) will form the placenta, the organ necessary to support the life of the fetus through development.
Clinical Correlates
Abnormal Implantations
Ectopic sites of implantation include any location other than the endometrium of the uterus. Most commonly they are within the uterine tubes, but also can lodge in the peritoneal cavity or the ovary itself.
Placenta previa is implantation over the cervical os and creates its own issues such as vaginal bleeding, spontaneous abortion and necessity for c-section if the fetus come to term.
Fig 5. Diagram depicting some common abnormal sites for implantation.