Migration
(11th days)
Extraembryonic mesoderm
Formation of chorion and connecting stalk
The result of the early gastrulation is the formed bilaminar embryo (embryonic disk) situating between two bubbles (amnion and yolk sac) and keeping with connecting stalk to the outermost bubble - chorion.
So, at the 11–14 days of the embryogenesis, the embryo proper consists of the bilaminar disc of cells – the upper thick plate of the ectoderm and the lower thinner layer of the endoderm, but the surrounding extraembryonic organs have got a great development. It is a particularity of the early gastrulation of the human embryo.
Late gastrulation – formation of the third germ layer (mesoderm) and axial organs (neural tube, notochord, somites)
Late gastrulation begins with migration of the embryonic disk cells in direction from anterior margin to posterior.
Let us look at the embryonic disc from above. The ectodermal and endodermal cells proliferate and migrate and at the future caudal end of the embryonic disc form the assemblage of the cells between the ectoderm and endoderm, which is called the primitive streak and is the source of the embryonic mesoderm.
On the transverse section through the caudal end of the embryo the upper layer – the ectoderm, the lower – the endoderm and situating between them the primitive streak can be seen. The ectoderm over the primitive streak forms deflection (future neural plate).
Further, in the way of migration and proliferation, in front of the primitive streak a small assemblage of cells produce a node – the primitive knot. A cord of cells from this primitive knot sinks inside and grows forward in the axis of the embryonic disc between the ectoderm and the endoderm. This is the notochordal process. The notochord forms the nuclei pulposus in each intervertebral disc.
At that time the embryonic mesoderm, which is produced from the primitive streak and notochord at the caudal end of the embryonic plate, migrates out, not only laterally, but also forward in between the ectoderm and endoderm – it comes to lie everywhere in the embryonic plate laterally to the notochord.
Further the cell division, cell migration and conduction lead to the subsequent formation of the neural tube and segmented intraembryonic mesoderm. Just the notochondral process growth inducts the cells of the mesoderm and ectoderm to divide, migrate and differentiate.
The ectodermal deflection overlying the notochord (neural plate) thickens, becomes deeper. Meanwhile the lateral margins of neural plate rise up to form folds, the neural crests, in between which the neural groove is formed. The folds eventually meet and fuse to form a tube – the neural tube, over the top of which the ectoderm re-establishes its continuity. With the separation of the neural tube from the ectoderm, the cells of the neural crests appear as groups of cells, lying along the dorsolateral sides of the neural tube.
The formation of the neural tube by the way of invagination begins at the future neck region of embryo and continues toward the posterior direction. At the head place neural tube enlarges and forms three brain bubbles, while the neural crests will be the source of the spinal ganglia.
Growing mesoderm becomes segmented and forms 43 – 44 paired round masses situated along the neural tube and notochord and between ectoderm and endoderm. These round structures are called somites and are formed by the 35-th day.
So, at the end of gastrulation we can see the formation of 3 germ layers and axial organs – neural tube, notochord and somites. They are called axial organs, because of their situation along the forming body of the embryo. They are temporary structures, forming the embryo body axis and participating in the formation of the different tissues and organs of the human body.
At this time the intraembryonic mesoderm consists of the dense formations – somites (paraxial mesoderm), continuing into the intermediate parts (nephrotome). Nephrotomes are the sources of the kidneys and gonads epithelium. Each nephrotome connects the somate with the lateral plates of the mesoderm (or splanchnotomes). The lateral plates of mesoderm are as follows: somatopleuric mesoderm (parietal), applied to the ectoderm and splanchnopleuric (visceral) mesoderm, applied to the endoderm. The cavity that is formed in between two layers is the intraembryonic coelom (the secondary body cavity). From the parietal and visceral plates the coelomic epithelium will be formed, while the cavity between plates will become secondary body cavity, represented in the formed organism as abdominal, pleural and pericardial cavities. Notes that lateral plates continue to the extraembryonic mesoderm of the amnion and yolk sac.
Late stage of the human embryo gastrulation