Embryology: 3rd week of development

Steps and processes

Gastrulation begins when a linear region of cells of the epiblast layer become thicker at the caudal aspect of the embryo. This primitive (Spemann’s) streak develops as epiblast cells replicate and migrate to the midline of the bilaminar disc under the influence of nodal. Nodal is a transformation growth factor β (TGF β) protein that not only initiates, but also maintains the primitive streak. The streak is comprised of totipotent stem cells from the epiblast that grow in a caudocranial manner. As cells are added to the caudal end of the primitive streak, the cranial end begins to enlarge and forms a primitive (Hensen’s) node.

The primitive node (and streak) is maintained by the hepatocyte nuclear factor 3β (HNF-3 β; a product of the FOXA2 gene). The presence of this protein is also crucial for the formation of forebrain and midbrain structures as well. Simultaneously, a slender depression develops within the streak that is continuous with the sunken area at the primitive node (i.e. the primitive groove and primitive pit, respectively). The establishment of these structures allow identification of the cranial (near the primitive node) and caudal (towards the tail of the primitive streak) poles of the embryo. It also facilitates the identification of the left and right sides, as well as dorsal and ventral surfaces of the embryo.

Germ layers

Formation

The cells of the primitive streak synthesize and secrete fibroblast growth factor 8 (FGF8). FGF8 downregulates the expression of E-cadherin, which is intended to promote cellular adhesion. As a result, epiblast cells that have lost their adhesion molecules will subsequently undergo invagination. Not only does this give rise to the previously described depressions (primitive groove and pit), but it also results in migration of epiblast cells between the epiblast and the hypoblast layers. These cells lose their tall columnar appearance and become loosely arranged spindle-shaped cells suspended in collagenous reticular fibers known as mesenchyme. The mesenchyme is made up of pluripotent cells that will provide structural support for the embryo. They also have the ability to differentiate into osteoblasts, chondroblasts, and fibroblasts, in addition to participating in vasculogenesis and angiogenesis.

FGF8 stimulates the expression of another protein, called Bachyury T, which regulates the transformation of other mesenchyme cells to the middle embryonic layer known as the mesoderm. As other epiblast and primitive streak cells migrate deeper, they eventually displace cells of the hypoblast to form the embryonic endoderm. The cells remaining in the epiblast are subsequently referred to as the ectoderm. Therefore, all three germ layers of the gastrula are epiblast derivatives. The mesoderm will eventually separate the ectoderm from the endoderm, except at the points where the two layers are fused (i.e. caudally at the cloacal membrane, and cranially at the prechordal plate). The cloacal membrane is a circular structure that marks the future location of the anus. On the other hand, the prechordal plate gives rise to the oropharyngeal membrane (also a bilaminar region), which will form the future mouth and pharynx.

Function

The three germ layers are responsible for forming all tissues within the body. The fate of each layer is as follows:

• The embryonic ectoderm, which is located on the dorsal surface of the embryo, is subdivided into neural ectoderm and surface ectoderm. The neural ectoderm will give rise to the sensory organs (eyes, internal ears). It also subdivides into the neural tube (which will form the brain and spinal cord) and the neural crest (which differentiates into the head mesenchyme and the peripheral nervous system). The surface ectoderm develops into the epidermis.
• Embryonic mesoderm is a bit more diverse and gives rise to paraxial, intermediate, and lateral plate mesoderm. The lateral plate mesoderm has cardiac, haematological, vascular, and smooth muscle fates. It also gives rise to the spleen, lymphatics, and adipose tissue. Intermediate mesoderm is responsible for the formation of the lower urinary tract, kidneys, and the reproductive system.  The paraxial mesoderm first form somites. The somites then differentiate into the rigid structural components of the body (i.e. bone, ligaments and tendons, cartilage and skeletal muscle). They also give rise to the dermis.
• The embryonic endoderm gives rise to the aero-digestive epithelium, as well as the glandular cells of the gastrointestinal tract and its associated organs. The lungs, thymus, thyroid, and prostate glands are also derived from endoderm.

Primitive streak cells continue to migrate and differentiate into mesoderm up until the beginning of the 4th gestational week. As mesoderm production declines, the primitive streak also begins to regress, becoming a small structure in the sacrococcygeal part of the embryo. Eventually, it completely disappears by the end of week 4.

Steps and processes

As the primitive node and streak are formed, invaginating mesodermal cells migrate cranially through the structure. They become prenotochordal cells that travel cranially towards the prechordal plate, in the midline. The primitive pit then projects into the notochordal process, giving rise to the notochordal canal. The notochordal process is now a tubular structure occupying the space between the primitive node to the prechordal plate. The prenotochordal cells of the notochordal process interdigitate with the cells of the hypoblast layer, prior to the invasion of the endoderm. The fused layer subsequently becomes perforated, allowing communication between the notochordal canal and the umbilical vesicle. As these perforations coalesce, the floor of the notochordal canal is lost. This is followed by flattening of the remaining notochordal process to form the notochordal plate.

Proliferation of the notochordal cells at the cranial end of the notochordal plate results in infolding of the tubular structure. Subsequently, a solid cord of cells that is definitively the notochord arises. At the area where the primitive pit descends into the epiblast, the notochordal canal persists; giving rise to a neurenteric canal that provides temporary communication between the amniotic cavity above and the umbilical vesicle below. This communication is obliterated as the notochord detaches from the endoderm.

The notochord projects from the primitive node to the oropharyngeal membrane. As the primitive streak extends caudally, the notochord also follows. Therefore, a craniocaudal growth pattern of the notochord is observed. Also, note that at this stage of development, prechordal mesoderm arises from neural crest cells, just rostral to the notochord. Therefore, the cloacal and oropharyngeal membranes are the only bilaminar regions of the embryo that remain. Notochordal migration is also associated with movement of pluripotent mesoderm that also move cranially, bilaterally with respect to the notochord process and prechordal plate. Once they have established a cranial position in the cardiogenic area of the embryo, the heart primordium is formed from the cardiogenic mesoderm by the end of week 3.

Functions of the notochord

The functions of the notochord in the development of the embryo are as follows:

• It defines the primitive longitudinal axis of the embryo.
• It provides some rigidity to the fragile developing human.
• It contributes to the formation of the intervertebral discs. Fragments of the notochord persist into adulthood as the nucleus pulposus of the intervertebral disc.
• It plays an important role in the formation of the axial musculoskeletal system.
• It is important in the development of the central nervous system.

Steps and processes

Not only does the notochord influence the epiblast to migrate and form the two deeper layers, but it also stimulates regions of the ectoderm to form the neural plate. This thick, elongated area of epithelial cells is a midline structure is adjacent to the midline and is superficially related – and equal in length – to the notochord. Growth of the notochord corresponds with an increase in width of the neural plate. The plate also grows cranially and meets the oropharyngeal membrane until it eventually outgrows the notochord. There is a prominent invagination along the long axis of the neural plate called the neural groove.

There are raised neural folds on either side of the groove that appear more pronounced at the cranial pole; indicating early brain development has commenced. Inward migration and subsequent fusion of the neural folds occur as the third week comes to a close. The neural plate has now become a neural tube, which serves as the primitive spinal cord and brain vesicles. Some neural crest cells that were not incorporated in the neural tube transform from epithelium to mesenchyme, after which they move away from the fusing neural folds. The underlying neural tube separates from the ectoderm and the non-neural edges of the ectoderm fuse to close the dorsum of the embryo; forming the epidermis.

During the closure of the neural tube, there is dissociation of some underlying neuroectodermal cells at the inner border of the folds. The neural crest cells completely separate from both the surface ectoderm and neural tube during this migration process. They aggregate as flat, irregular cells known as the neural crest that lies between the neural tube and surface ectoderm. The neural crest divides into left and right halves and migrates laterally, to the dorsal region of the embryo (relative to the neural tube).

Functions

They then develop into sensory ganglia of cranial (CN V, VII, IX, and X) and spinal (dorsal root) nerves. Other structures that are derived from the neural crest include, but are not limited to:

• Neurilemma sheaths of peripheral nerves
• Leptomeninges of the arachnoid mater and pia mater
• Pigment cells
• The medulla of the suprarenal glands

Importantly, the neural tube contributes significantly to lateralization. The floor of the neural tube expresses the LEFTY protein on the left side of the embryo. Studies suggest that it restricts crossing over of left-sided signals to the right.

Blood and blood vessel formation

Fibroblast growth factor 2 (FGF2) is the primary instigator for vasculogenesis. By binding to fibroblast growth factor receptors (FGFR) on mesoderm, it induces their differentiation into hemangioblasts. These pluripotent cells then aggregate in the yolk sac and give rise to blood islands. With the action of vascular endothelial growth factor (VEGF) acting on vascular endothelial growth factor receptors (VEGF-R2), hemangioblasts eventually differentiate into endothelial cells. VEGF then acts on VEGF-R1 in order to stimulate the characteristic tubular arrangement of the endothelial cells in the blood vessels.

The hemangioblasts that did not contribute to vasculogenesis will have one of two options:

• The peripheral cells can give rise to angioblasts that will participate in angiogenesis.
• Centrally located cells can become hematopoietic stem cells that will differentiate into all blood cells.

Vasculogenesis commences in the extraembryonic mesoderm of the connecting stalk, umbilical vesicle and chorion. It is followed briskly by embryonic vasculogenesis.

Primitive heart formation

As the blood and blood vessels begin to form, there is a concomitant establishment of the cardiogenic area. A pair of longitudinal tubes with endothelium called the endocardial heart tubes are formed during this time. They eventually fuse to form the primordial heart tube. It merges with embryonic and extraembryonic blood vessels to establish the primordial cardiovascular system. In most cases, heartbeats commence at the end of week three, making the heart the first functional organ of the embryo. However, this heartbeat is not readily appreciated until the 5th week of gestation.