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 - Day by Day
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Start Slide Show
Day 09
Fig 1a. Day 09 implantation. A mucus plucker's seals the defect in the endometrium where the blastocyst began implantation. The syncytiotrophoblast ingests decimal cells as lacunae form within it. The bilaminar embryonic disc is largely unchanged. The amniotic and yolk sac cavities enlarge slightly.
Trophoblast
Syncytiotrophoblast invades into the endometrium, ingesting decidual cells, as a source of nutrition.
Trophoblastic lacunae form in the syncytiotrophoblast.
These will eventually fill with maternal blood as a precursor to the development of the placenta.Cavities
The primitive amniotic cavity expands slightly and the primary yolk sac, lined by hypoblast cells remains relatively unchanged. -
Day 12
Fig 1b. Day 12 implantation. This gif animation shows maternal blood percolating into the trophoblastic lacunae, establishing the hemochorial placenta and the splitting of the extra embryonic mesoderm to form the lining of the chorionic cavity, the extraembryonic coelom.
Trophoblast
Syncytiotrophoblast eventually erodes into the maternal sinusoids allowing maternal blood to percolate through the trophoblastic lacunae.
This is the beginning of the uteroplacental circulation that is necessary to support development of the embryo and later the fetus.
Humans are said to have a hemochorial placenta because maternal blood flows through spaces in the chorionic plate. Here, gas and nutrient exchange between fetal and maternal blood occurs. Capillaries and the chorionic villi form a physical barrier between fetal and maternal blood.Extraembryonic Mesoderm
A loose collection of cells develops between the cytotrophoblast and the primitive yolk sac forming the extraembryonic mesoderm.
This mesoderm will eventually break down into two layers:- somatic layer of extraembryonic mesoderm - in contact with cytotrophoblast
- splanchnic layer of extraembryonic mesoderm - in contact with primary yolk sac
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Day 13
Fig 1c. Day 13 implantation. A column of extraembryonic mesoderm maintains connection between the embryo blast and the inner wall of the chorionic cavity. It will later form the core of the umbilical cord.
Extraembryonic Mesoderm
A column of mesodermal tissue, the connecting stalk, attaches the bilaminar embryonic disk to the wall of the chorionic sac and will eventually form the core of the umbilical cord.
The side of the chorionic cavity where the connecting stalk attaches is the embryonic pole, the side opposite is the abembryonic pole.
At the embryonic pole of the blastocyst, the cytotrophoblast begins to develop finger-like projections that will eventually from the core of the primary villi of the placenta.
At the abembryonic pole, the lacunae are small and cytotrophoblast remains as a single layer of cells.Embryoblast
Beginning in week 3 there will be major morphological changes in the bilaminar embryonic disk as it is converted into a trilaminar (three-layered) embryonic disk consisting of ectoderm, mesoderm and endoderm that will develop into all of the organs and tissues of the body.
Some cells in extra embryonic splanchnic mesoderm, covering the yolk sac, differentiate into angiogenic cell clusters that will eventually contribute to the cardiovascular system.
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.