Module 4 Lesson 2 - 2

Embryonic Development – 2 to 8 weeks

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By the end of embryonic development, almost all the organ systems are formed and the embryo appears human-like. All this in just two short months! No wonder pregnant women are very tired during the first few weeks of their pregnancies. Remember that all the requirements for prenatal growth come from the mother's body.

Extra Embryonic Membranes – second week

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In addition to the development of the embryo, supporting structures must be formed at the same time. At about the second week of development, the embryo implants in the uterus and the cells of the blastocyst organize into layers into the extra-embryonic membranes. These supporting structures provide the embryo, and later the fetus, with means of nutrition, respiration, excretion, and protection.

Simply put, the extra-embryonic membranes are structures that are external to (extra-) the embryo; they include the amnion, the yolk sac, the allantois, and the chorion.

A space called amniotic cavity opens between the trophoblast (outer layer) and the inner cell mass. The layer of cells between the amniotic cavity and trophoblast forms into the amnion. The amnion is a membranous sac that will fill with fluid and protect the fetus from trauma and temperature changes.

The layer of cells on the other side of the amniotic cavity is called the embryonic disk. The disk is located on the opposite side of the amnion.

The layer of cells at the bottom of the embryonic disk form into a yolk sac. The yolk sac provides nutrients and blood circulation for the embryo during the first two weeks after implantation. The yolk sac develops finger-like projections called allantois. The allantois and yolk sac form the foundation for the umbilical cord. The remaining part of the allantois  becomes part of the urinary bladder of the fetus.

The chorion is the outermost membrane and surrounds all other membrane. The chorion develops into the fetal portion of the placenta.

Gastrulation – third week

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Read pages 511 – 512

Initially, the embryonic disk consists of two layers; later, it forms a third layer. The formation of the third layer is called gastrulation. The three layers called germ layers consist of endoderm, mesoderm, and ectoderm. The cells in each of these germ layers differentiate into specific structures in the embryo. All the cells, tissues, and organs in the body come from these germ layers.

• The endoderm layer becomes the lining of the digestive and respiratory systems, and endocrine glands.
  


• The mesoderm layer becomes the circulatory system, reproductive structures, skeletal system, and muscular system.


• The ectoderm layer becomes hairs, nails, skin (epidermis), and the nervous system.

Neurulation and Organ Formation – fourth week to eighth week

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Read pages 512 – 515

After gastrulation, the central nervous system begins to form in the ectoderm through a process called neurulation.

Specialized cells in the ectoderm thicken about the fourth week of gestation and form a neural plate. The cells surrounding the neural plate fold and converge to form the neural tube. The neural tube forms the brain and the spinal cord of the fetus.

Folic acid (vitamin B9) is essential in the development of the neural tube. Without adequate amounts of folic acid in the mother’s diet, a neural tube defect can occur in the embryo. A neural tube defect can be a malformation of the neural tube or an incomplete closure of the neural tube. It can result conditions such as spina bifida, where the spinal cord is not closed properly, or the brain tissue can be partially missing. The risks of neural tube defects can be reduced greatly by ensuring the prospective mothers are taking folic acid supplements.

For the next four weeks, primitive structures of all the organs and tissues are formed. The digestive system forms from a portion of the yolk sac. The heart forms and starts pumping after the liver (and later the bone marrow) starts to produce red blood cells. The respiratory system forms next, and limb buds develop.

The digestive system continues to develop and the limb buds form fingers and toes about the sixth week. By the seventh week, basic facial structures form and the reproductive structures become apparent although the sex of the embryo cannot be determined visually.

By the end of eighth week, the bones begin to replace cartilages. At this point, the embryo is about 3 cm and weighs approximately 8 g. After this time, the organs enlarge and mature until birth.

The Placenta and Umbilical Cord

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Read pages 515 – 516

After the implantation, the endometrium cells provide the embryo with the necessary nutrients. After the fourth week, the placenta begins to develop and provide support to the growing embryo. The placenta connects to the embryo through the umbilical cord. The umbilical cord is attached to the embryo at its navel.

The umbilical cord consists of two umbilical arteries and one umbilical vein. The two umbilical arteries carry deoxygenated blood and waste from the fetus and the umbilical vein carries oxygenated blood and nutrients to the fetus.

The placenta forms from the chorion tissue of the embryo and the endometrium tissue of the mother. A fully developed placenta provides the fetus with nutrients, vitamins, minerals, and oxygen through diffusion. By the same method, carbon dioxide and fetal wastes are filtered from the fetal blood. Protein (amino acids) and iron are transferred to the fetal blood by means of active transport.

Note that the placenta does not allow fetal blood to mix with maternal blood. The fetal blood cells come incredibly close to the maternal blood to allow for diffusion and active transport of molecules. This prevents the maternal immune system from being triggered by the fetus. Although some maternal antibodies can pass through the placenta to the fetus, other antibodies cannot. This is why some infections during pregnancy can be dangerous to the fetus.

Although the fetal and maternal bloods are not mixed, lipid-soluble molecules are transferred to the fetal blood. Some of the substances can be dangerous to the fetal development, such as alcohol, nicotine, antibiotics, pathogens, etc. Because the fetus is much smaller than an adult human, small amounts of these substances can have devastating effects on the developing fetus. For example, alcohol consumption by pregnant women can result in fetal alcohol spectrum disorder (FASD). 

The placenta also takes the role of corpus luteum and produces hormones such as hCG, estrogen, and progesterone to maintain the pregnancy.

Watch and Listen

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View the following video clip on Carnegie stages to gain a better understanding of the events of early embryonic development: