5.1 Gastrulation and Formation of the Neural Tube (Neurulation)

Gastrulation

Development of the nervous system begins with the process of gastrulation, one of the earliest developmental events in the embryo. The goal of gastrulation is to start the subdivision and specialization of cells in the developing embryo. When complete, gastrulation results in the production of 3 primary germ layers. The creation of germ layers can be thought of as the earliest step towards successfully partitioning an organism. Each of the 3 germ layers are made of cells that will give rise to specific tissue types depending on their location.

Gastrulation begins several days after the fertilization of an egg in species like mice and humans. In those intervening days after fertilization (but before gastrulation), the egg undergoes several rounds of cleavage (cell division) events to produce a sphere of cells called the blastocyst. The sphere of cells surrounds a fluid-filled cavity called the blastocoel (step 1 in Figure 5.3). Reorganization of the blastula will produce two early layers of cells, the inner cell mass and the outer trophoblast layer (step 2 in Figure 5.3). The trophoblast layer will become the mammalian placenta. Through cellular movements, migration, and invagination, the cells of the inner cell mass will produce the 3 germ layers (step 2 in Figure 5.3).

Gastrulation begins after the formation of the primitive streak, which is a long groove that forms across the developing embryo. At the anterior region of the primitive streak is the node (De Robertis, 2006). This node represents the organizing center of gastrulation and is the site where invagination of cells begins. Figure 5.3 step 3 demonstrates an example of early invagination. After the cellular movements are complete, the gastrula contains 3 germ layers: the endoderm, mesoderm, and ectoderm (Figure 5.3). They are named based on where they are located within the developing embryo: the endoderm is the innermost layer, the mesoderm is in the middle layer, and the ectoderm is the outermost layer. The CNS is produced from the ectoderm, although all layers communicate with one another to coordinate development.

Figure 5.3 Gastrulation

Neuroscience Across Species

Compare and contrast: gastrulation in other species

Gastrulation is highly conserved across species. However, some anatomical names and regions differ. Many of the cellular movements and signaling events that regulate gastrulation were originally identified in other embryos such as amphibians. In some species such as frogs and zebrafish, the egg is externally fertilized, and the events of gastrulation occur external to the mother. Despite differences in species and location, similar structures arise and organize gastrulation in amphibians and other species. For example, the mammalian blastocyst is akin to a structure called the blastula, which contains a primitive streak-like structure called the blastopore. The dorsal end of the blastopore is an organizing center called the blastopore lip, equivalent to the node. Observable comparisons can be viewed in Figure 5.4 of mouse (mammalian) and zebrafish development.

Figure 5.4 Prenatal development across species

The early embryo in both species undergoes cleavage events, rapidly increasing the number of cells. In zebrafish, the cells are located above a large extra embryonic structure called the yolk sac. The human yolk sac is essential for the early production of blood and germ cells. For example, it is the site of production of microglia, the immune cells in the brain (see Chapter 17 Neuroimmunology). In humans, gastrulation occurs in the 3rd week of development and begins with the formation of the primitive streak. In mice, the blastocyst is formed by embryonic (E) day 3 and gastrulation begins at embryonic day 6.5. This is equivalent to approximately 6 hours post fertilization in zebrafish. In both species, the end of gastrulation produces the gastrula, which contains three complete germ layers (Figure 5.3). Gastrulation in chick embryos is very similar to the process in humans. The epiblast layer will produce the 3 germ layers and cells will migrate through the primitive streak as in higher vertebrates. The movements are organized at Hensen’s node which is the equivalent of the node in mammals.

Neurulation

Completion of gastrulation lays the foundation for a process called neurulation, which will generate the early brain and spinal cord tissues via the formation of a structure called the neural tube (summarized in Figure 5.5). Of the 3 germ layers, the ectoderm, or outer layer, will generate cells that become neural tissue or epidermis: two vastly different tissues. To make neural tissue (instead of epidermis), a portion of the ectoderm called the neural plate must undergo neural induction.

The neural plate is a single layer of cells located in a small, central region of the ectoderm (Figure 5.5, step 1). It is induced to become neural tissue by the expression of molecules we call neural inducers. These are secreted factors that direct the surrounding tissues through neurulation (Harland and Gerhart, 1997). One example of a neural inducer is noggin (Valenzuela et al., 1995). Localized expression of noggin induces the neural plate to invaginate (step 2 of Figure 5.5). Invagination can be described as the process of folding cells inward, to produce a small cavity. In step 2 of Figure 5.5, the cells of the neural plate that are pushing down into the tissue beneath them are migrating and growing towards cells that are secreting noggin (and other factors). This migration of neural plate cells into the underlying tissue results in the formation of the neural groove (Figure 5.5, step 2), a center region that is surrounded by indentations that form the neural folds. Ultimately by cellular movements, the two neural folds connect producing a neural tube that sits ventral to the outer epidermis (Figure 5.5 steps 3 and 4). The inner region of the newly closed neural tube develops into the central canal of the spinal cord and what will eventually form the ventricles of the brain.

Figure 5.5 Neurulation

Environment and Bidirectional communication: Neural tube defects and folic acid

Neurulation is a critical step in early brain development and if this process does not occur properly a class of disorders called neural tube defects arise. The most common neural tube defects are the result of abnormal neural tube closure, which can be affected by environmental and genetic influences. These defects fall into different classes based on type and severity. In the severest forms such as anencephaly, the neural tube does not close at the anterior end and causes missing regions of the skull and brain. Other examples include encephalocele, when the brain protrudes through the skull, and hydrocephalus, which is characterized by an accumulation of cerebrospinal fluid in the brain. Spina bifida is one of the most well-known examples of a neural tube defect and occurs when the posterior end of the neural tube does not close fully. Spina bifida can range in severity, causing either a complete opening along the vertebrae of the back or a minor separation in the bones of the vertebrae (Figure 5.6).

Figure 5.6 Defects of neurulation

Interestingly, the different types of neural tube defects observed occur at the extreme ends of the neural tube (not in the middle) as a result of how the neural tube closes. The neural tube begins closure in the middle and slowly closes on either end. Spina bifida is localized to the posterior regions of the neural tube, while anencephaly, encephalocele, and hydrocephalus occur in the anterior regions of the neural tube.

It is predicted that approximately 70% of neural tube defects occur due to genetic factors. There have been over 240 genes associated with neural tube defects (Harris and Juriloff, 2010), which strongly supports the link between genetic predisposition and neural tube abnormalities. However, genetics do not solely determine the presence of neural tube defects, as a huge proportion of neural tube defects can be prevented with the nutritional supplementation of folic acid in the mother’s diet. Folic acid is a water-soluble B-vitamin and deficiencies in folic acid have been associated with an increased risk of neural tube defects (Wallingford et al., 2013). Many foods including orange juice and some cereal grains are fortified with folic acid. The addition of folic acid to specific food products of general consumption has had an immediate impact on the prevalence of neural tube defects. Despite the efficacy of folic acid supplementation, it is still unclear how folic acid directly prevents neural tube defects.