Human Embryonic Development: The Third Week
The Third Week of Pregnancy
Amenorrhea and Pregnancy Confirmation
Amenorrhea is not a definitive pregnancy test and can be caused by various factors. Implantation bleeding can occur. Pregnancy is typically diagnosed around the third or fourth week after a missed period, sometimes even later. By the end of the third week, some clinical changes may appear, such as breast tension and swelling, constipation, frequent urination, and nausea/vomiting. These signs are not always consistent but hold significant diagnostic value. HCG can be detected in maternal serum or urine through various techniques, confirming pregnancy.
Biological and Immunological Pregnancy Reactions
In most cases, pregnancy diagnosis is established clinically. Biological and immunological tests are less frequently used.
Start of the 3rd Week (Days 15-17)
Mesoderm Development
The primitive streak, an axial thickening of the primary ectoderm, plays a crucial role in mesoderm formation. Cells from the primitive streak invaginate and infiltrate between the ectoderm and endoderm, creating the mesoderm, the third germ layer. Embryonic areas initially consist of ectoderm and endoderm, giving rise to the buccopharyngeal and cloacal membranes. The ectoderm overlying the newly formed mesoderm is termed secondary ectoderm.
Allantoic Diverticulum
A small, endodermal outgrowth, the allantoic diverticulum, appears in the secondary yolk sac region.
Mid-3rd Week (Days 17-19)
Notochord Formation
Hensen’s node, a swelling at the cranial end of the primitive streak, contributes to notochord development. Cells from Hensen’s node invaginate along the midline, moving towards the pharyngeal membrane between the ectoderm and endoderm, forming the notochordal process. This solid cord hollows out to become the notochord. The ventral wall of the notochordal canal fuses with the endoderm, while the dorsal wall thickens into the notochordal plate.
Early Vasculogenesis
Within the extraembryonic mesenchyme, certain cells differentiate into blood islands, marking the beginning of capillary and blood cell formation. This occurs in the chorionic plate, splanchnopleure, and the mesenchyme of the secondary trophoblastic villi, which then become tertiary villi.
End of the 3rd Week (Days 19-21)
Neural Plate and Groove Differentiation
Under the influence of the notochord and paraxial mesoderm, the axial midline ectoderm transforms into the neural plate. The edges of the neural plate fold inwards and grow, forming the neural groove. The remaining secondary ectoderm becomes the epiblast, which later gives rise to the epidermis.
Mesoderm Development (Continued)
Anterior to the pharyngeal membrane, mesenchymal condensations appear, forming the septum transversum and the outline of the primitive heart. The first intraembryonic capillaries also develop. The mesoderm on either side of the notochord divides into three zones: paraxial mesoderm (segmenting into somites), intermediate mesoderm (future nephrogenic cord), and lateral mesoderm. A cavity, the intraembryonic coelom, forms within the lateral mesoderm and communicates with the extraembryonic coelom.
Primordial Germ Cells
The first primordial germ cells appear in the mesenchyme adjacent to the endoderm of the allantoic diverticulum.