Trilaminar blastocyst and fetal membranes

The Inner Cell Mass (ICM) ultimately gives rise to the three major embryonic cell layers- Ectoderm, Mesoderm, and Endoderm. But the first major division occurs when the ICM separates into the hypoblast, which will give rise to the yolk sac, and the epiblast, which will give rise to the three germ layers.  The epiblast cells eventually migrate through the region known as the primitive streak during gastrulation. The primitive streak is a transient but critical component of the embryo that not only establishes the axes of the embryo but its formation marks the beginning of gastrulation. The major axes of the developing organism are the anterior-posterior (head-anus), dorsal-ventral (back-belly) and left to right. All development going forward will be arranged based on these axes for proper orientating purposes. The endoderm is the inner-most layer and will ultimately become the lining of the gut, and other internal organs (such as the thyroid and lung), the mesoderm or middle layer will make up muscle, the skeletal system, the circulatory system, and most of the reproductive system; and the outer layer- the ectoderm will become the skin, eyes, brain, and nervous system. Ultimately the three germ layers develop into all distinguished tissue layers of the developing fetus. Gastrulation takes place in week four of the pregnancy.

Source: http://www.cosmic-mindreach.com/Gene_Expression.html

There are several fetal membranes to be aware of- the innermost amnion, allantois, and the yolk sac, which together make up the gestational sac that holds the developing embryo. The amnion acts as a shock absorber for the fetus, it also regulates fetal temperature, and aids in movement and thus development. The allantois makes early blood vessels which will eventually become the umbilical arteries and vessels. The term umbilical vesicle is the more appropriate way to refer to the yolk sac in humans since there isn’t actual yolk in it like there is in mammalian predecessors’ (reptiles and birds). Instead the human version is filled with vitelline which does provide nutrient for the early embryo. The yolk sac also functions as the primitive circulatory system for the embryo before the real one is constructed and up and running. The yolk sac is used to gauge proper development in the embryo at the first ultrasound as it is present by week 5 and its size is a good indicator for how things are going. In the simplest terms the amniotic cavity, lined by the amnion, and the yolk sac, lined by the endoderm, start out looking like the number 8 with the amniotic cavity stacked on top of the yolk sac.

Another key membrane, the chorion- the embryonic portion of the placenta, plays several key roles- it induces the uterus to make its portion of the placenta, known as the decidua. The chorion also establishes the connection between the fetus and mother for oxygen and nourishment to be delivered to the fetus, via the chorionic villi. It helps convince the uterus to keep the fetus by releasing hormones and puts out immune response regulators so the mother’s immune system won’t reject the fetus. The placenta is a multifunctional organ providing a nutritional role, as well as an endocrine and immunologic structure. By the sixth week the embryo is wrapped up in the chorion and the amnion.

Now is a good time to also mention the umbilical cord, which is probably the most well-known embryonic structure. It comes about by week three and houses two umbilical arteries and one umbilical vein which allow the growing organism to tap into mommy’s blood supply via the placenta. By the fifth week it takes over for the yolk sac, which was providing early nutrient to the embryo. In addition to bringing oxygenated blood to the embryo, it brings the deoxygenated blood back to the placenta as well as metabolic waste.

Source: http://www.studyblue.com/notes/note/n/winterfellllll/deck/1261762