5.2 Growth and Development of the Early Brain

Environment and Bidirectional communication: Genetic diseases of development with environmental influence

Disorders that lead to structural damage or malformations which impair lifelong function are the most common forms of congenital brain disorders. While there are a variety of environmental teratogens that can alter brain development, a significant number of congenital brain malformations are associated with a genetic mutation. For many of these disorders, a gene of interest has been assigned or associated with the disease, but how a specific mutation disrupts overall brain development is still a puzzle. These mysteries persist most likely because, in many cases, genes interact with environmental influences to determine their final effect on the brain. For example, holoprosencephaly is a congenital brain disorder characterized by abnormal separation of the forebrain (Weiss et al., 2018). This occurs prenatally and mutations in various genes have been shown to cause holoprosencephaly (Geng and Oliver, 2009). However, the degree of illness is often variable amongst individuals (Figure 5.9). Severe forms of the disease exhibit cyclopia (a single eye) and the brain does not divide into two hemispheres whereas some individuals diagnosed with holoprosencephaly can present with semilobar holoprosencephaly, in which the brain has a small separation between each hemisphere (Figure 5.9). It is generally accepted that environmental exposures in combination with genetic mutations ultimately dictate the presence and severity of holoprosencephaly. For example, some environmental teratogens such as ethanol are known to exacerbate the phenotypes of holoprosencephaly (Hong and Krauss, 2012). Additional environmental interactions include maternal diabetes and prenatal exposure to specific pharmaceutical drugs (Cohen and Shiota, 2002).

Figure 5.9 Holoprosencephaly

Neural stem cells and proliferation

As the neural tube forms, we see the emergence of a special class of cells that will proliferate (divide) to create numerous daughter cells that go on to become neurons and glia of the CNS, except for microglia which are formed in the peripheral yolk sac. These cells are committed to become nervous system cells and are referred to as neural epithelial cells or neural stem cells. In the very early stages of neural tube formation (Step 1 in Figure 5.10), these cells divide rapidly to create identical daughter cells, thereby expanding the neural tissue and growing the neural tube. Some of these daughter cells go on to become a type of cell called a radial glial cell (RGC).

RGCs are a class of neural stem cells located in a proliferative region called the ventricular zone, the thin layer of tissue surrounding the fluid-filled central cavity of the neural tube (i.e., the future ventricular system). RGCs have a unique, bipolar shape, shown in Figure 5.10, where they extend one process medially, to contact the ventricular space, and another laterally, to contract the edge of the developing neural tube (known as the pial surface). Within the ventricular zone, RGCs divide (proliferate). The goal of this expansion is to create enough RGCs to produce the neurons and glia required for a fully formed brain.

RGCs are unique in that they have the ability to self-renew, which means they produce more of themselves through symmetric cell divisions where one cell produces two independent identical cells. RGCs also have the ability to divide asymmetrically, which means they can produce one cell that is identical to themselves, but at the same time produce a different, more committed cell that we call an intermediate progenitor (IP) (Step 2). IPs migrate along the radial processes of the RGCs into the subventricular zone (Step 3 and detailed location on the y-axis) where they will undergo symmetric cell divisions to produce neurons (Step 4 and 5). We call this process of creating new neurons neurogenesis. Later, RGCs will switch towards the development of daughter cells that go on to become astrocytes and oligodendrocytes, processes referred to as gliogenesis (Step 6). Gliogenesis occurs well after neurogenesis and closer to birth, with production of astrocytes occurring first followed by production of oligodendrocytes.

Figure 5.10 Embryonic corgical neurogenesis

As neurogenesis and gliogenesis progress, the brain, of course, grows. For humans and several other highly intelligent mammals, our brain growth presents an exceptional challenge. In short, we need to fit a lot of brain, particularly a lot of cortex, into a small space, otherwise our heads would be too big for our necks to support. All mammalian species have a cortex, but it differs in size and texture (see Chapter 4 Comparative Neuroscience). In smaller mammals such as mice, the cortex is smooth and smaller relative to skull size. However, in higher mammals, the cortex is much larger. It has a surface area the size of a pillowcase, approximately 40 by 62.5 centimeters. This size difference can be attributed to increased proliferation and production of neurons. To accommodate the increase in cell number, the human brain produces cortical folding, which is the formation of gyri and sulci. In rare instances, the human brain lacks the formation of gyri and sulci, this condition causes lissencephaly, or smooth brains. Lissencephaly is associated with intellectual disability.

As the human brain grows during development, we see it go from a relatively smooth structure to one with many sulci and gyri. Figure 5.11 shows how during development the large cortical lobes curl over on top of the lower brain regions, expanding to cover and surround mesencephalic regions. This curling over during development helps explain why many of our subcortical structures (hippocampus, thalamus, basal ganglia, and lateral ventricles) have a curved shape. To get an idea of how brain folding occurs during brain development, take out four sheets of paper. First take one sheet and crumple it tightly. The paper will slightly expand then settle into a crumpled state. Now get the remaining three sheets of paper and crumple all three together. Notice how thicker paper leads to fewer folds. The crumpled pieces will be much larger and harder to fold. Now apply that logic to the growing brain. Thicker cortices fold less and end up with less cortical material hiding below the surface.

Figure 5.11 Brain growth and folding As the human brain grows, it develops sulci and gyri and curves around within the expanding skull. These adaptations help humans fit a large surface area in a small space. Image credit: Brain images by ScienceSourceImages.

Zika virus infection of neural stem cells

Zika virus is an arbovirus that is transmitted through the bite of the mosquito, Aedes aegypti. The most recent outbreak of Zika virus was in 2016. While the disease is relatively mild or asymptomatic in most individuals, it has been associated with microcephaly or smaller head size in newborns exposed to viral infection during prenatal development (Mlakar et al., 2016). The probability of a fetus to be born with microcephaly after Zika infection of the mother is between 1-13% (Antoniou et al., 2020). Such an association prompted scientists to study the effects of Zika viral infection on neural stem cell function, survival, and differentiation. In 2016, at the height of the Zika virus outbreak, Garcez and colleagues used organoid culture to study the effects of Zika virus on neural stem cell function. Organoids are stem cells that are grown in a culture dish but take on the 3-dimensional structure of an organ (see Chapter 4 Comparative Neuroscience). Using this system, it was demonstrated that infection of neural stem cells with Zika virus causes increased cell death and reduced the growth of the organoid (Garcez et al., 2016). These studies confirmed that Zika virus directly infects neural stem cells and hinders their development. They also demonstrate how proper neural stem cell function is critical for building a healthy, functional brain.

Developing layers of the cerebral cortex

The mammalian adult cortex is composed of 6 layers with a unique composition of cell types. The image on the left side of Figure 5.14 is a drawing by Ramon y Cajal, showing how these layers look in a cross-section of an adult brain where neurons have been stained a dark color. Over the course of time, scientists have asked, how are the layers of the cortex formed? Which layers (1-6) are formed first? In other words, can experiments be designed to determine which neurons from each layer are born first? The right side of Figure 5.14 shows one classic experiment that was used this way.

Figure 5.14 Cortical layer formation Image credit: Santiago Ramón y Cajal, "Comparative study of the sensory areas of the human cortex"; published 1899, ISBN 9781458821898, Public Domain, https://commons.wikimedia.org/w/index.php?curid=8513016

To determine which neurons are born first and in what location, a tracer molecule was injected into pregnant female mice at different embryonic stages of pregnancy (E11, E13, or E15) (step 1 of Figure 5.14). The tracer molecule labeled the neurons produced (via cell division) right around the time of its injection in developing pups. The pups were then euthanized after birth and researchers looked in their brains to see where the marker-labeled neurons could be found (step 2 of Figure 5.14). The early injection of the tracer molecule at E11 was associated with tracer-labeled neurons in the innermost layer of the cortex (Layer 6) while injection at E13 and E15 were associated with tracer-labeled neurons in the middle layers and more superficial layers, respectively (step 3 of Figure 5.14). Hence, the earliest born neurons are present in the innermost layers and later born neurons are in the superficial layers, producing an inside-out development of the cortex.