The embryonic stage of development begins at the start of the third week of development and is completed at the end of the eighth week. During this time, the embryo undergoes rapid development, forming the rudiments of all body organs, extraembryonic membranes, and the placenta. By the end of the eighth week, it has a distinct human appearance.
Germ Layers
After implantation, the embryoblast grows to become the embryonic disc, which is supported by a short stalk extending from the wall of the blastocyst. The embryonic disc consists of three embryonic tissues: ectoderm, mesoderm, and endoderm. The ectoderm forms the posterior surface of the developing embryo, while the endoderm forms the anterior surface. The mesoderm is the middle tissue layer. These embryonic tissues are called germ layers because all body tissues and organs are formed from them. Figure 18.4 illustrates the formation of the germ layers.
Briefly, ectoderm forms all of the nervous system and the epidermis of the skin. Mesoderm forms muscles, bones, blood, and other forms of connective tissues. Endoderm forms the epithelial lining of the digestive, respiratory, and urinary tracts.
Structures Formed By The Primary Germ Layers
| Ectoderm | Mesoderm | Endoderm |
| All nervous tissue Sensory epithelium of sense organs Pituitary gland, pineal gland, and adrenal medulla Epidermis of the skin including nails, hair follicles, sebaceous glands, sweat glands, and mammary glands | All connective tissues including bone and cartilage Skeletal, cardiac, and most smooth muscle tissue Red bone marrow and lymphoid tissue Blood and lymphatic vessels | Epithelial lining of alimentary canal (except oral cavity and anal canal) Liver and pancreas Epithelial lining of respiratory system (except nasal cavity and paranasal sinuses) Epithelial lining of urethra and bladder |
| Cornea, lens, and internal muscles of eye | Dermis of the skin | Thymus, thyroid and parathyroid glands |
| Epithelial lining of oral and nasal cavities, paranasal sinuses, and anal canal | Kidneys, ureters, gonads, and reproductive ducts | Epithelial lining of accessory reproductive glands |
| Salivary glands Tooth enamel | Adrenal cortex Synovial and serous membranes | Epithelial lining of tonsils, auditory tubes, and tympanic cavity |
| Cranial and spinal meninges | Spleen |
Extraembryonic Membranes
While the embryonic disc is forming, slender extensions from the trophoblast grow into the surrounding endometrium, firmly anchoring the blastocyst. The trophoblast of the blastocyst is now called the chorion (ko’-re-on), the most superficial extraembryonic membrane, and the slender extensions are known as chorionic (ko-re-on’-ik) villi.
At about the same time, two other extraembryonic membranes separate from the embryonic disc. The amnion (am’-ne-on) is formed posterior to the embryo and the yolk sac is formed anterior to the embryo (figure 18.4). Amniotic fluid fills the amniotic cavity, the space between the embryonic disc and the amnion. As the embryo develops, the amnion margins move toward the anterior surface of the embryo. In a short time, the embryo is enveloped by the amnion (figure 18.5).
Amniotic fluid serves as a shock absorber for the developing embryo. It also prevents adhesions from developing between various parts of the embryo. Later in development, the fetus swallows and inhales amniotic fluid and discharges dilute urine into it.
The yolk sac forms an outpocketing that becomes the allantois (al-lan’-to-is), the last of the extraembryonic membranes. Both the allantois and the yolk sac subsequently become part of the umbilical (um-bil’-i-kal) cord, which attaches the embryo to the placenta (figure 18.5). The yolk sac forms the early formed elements and germ cells for the embryo. It also serves as a shock absorber for the embryo, in addition to forming the primitive gut. The allantois also produces formed elements and brings umbilical blood vessels to the placenta.
Placenta
As the embryo continues to grow and the chorion enlarges, the layer of the endometrium covering the blastocyst becomes increasingly thinner. The chorionic villi in this region disintegrate, and only those in contact with the thick, spongy endometrium persist. This leads to the formation of the placenta (plah-sen’-tah), a disc-shaped structure formed of both embryonic and maternal tissues. The embryonic portion is formed of the chorion and chorionic villi, and the maternal portion is formed of the associated endometrium (figure 18.5).
The developing embryo is attached to the placenta by the umbilical cord. Two umbilical arteries bring embryonic blood to the placenta, and a single umbilical vein returns the blood to the embryo. There are no nerves in the umbilical cord.
The placenta provides an interface between the embryonic and maternal bloods for the exchange of water, respiratory gases, nutrients, wastes, hormones, and antibodies. The embryonic blood must receive all required substances from the mother’s blood and pass metabolic wastes into the mother’s blood. The placenta is usually fully functional by the end of the eighth week.
The embryonic and maternal bloods do not mix in the placenta. The maternal blood vessels open into blood- filled spaces called lacunae into which the embryonic blood vessels extend and branch repeatedly. This arrangement provides a large surface area for the exchange of substances between embryonic and maternal bloods. Observe this relationship in figure 18.6.
External Appearance
By the fourth week of development, the head and limb buds of the embryo are recognizable. By the seventh week, the rudiments of all organs are present, and the eyes and ears are visible. Figure 18.7 illustrates the visible changes in the embryo from the fourth to the seventh week.
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