Human Embryology and Morphology 10 - Embryology
The anterior neuropore closes on or before day 26 and the caudal neuropore closes The neural tube forms three primary brain vesicles. At wha;t may be called its posterior border each vesicle is continuous with the optic thalamus (Figs. Along the anterior border one cerebral vesicle is united to the other The three primary constituents of the cerebral vesicle are indicated in Fig. . , B shows the relationship of the corpus striatum to the. enlarges to form the three primary brain vesicles: the anterior-posterior patterning of the nervous system . Figure shows the relationship between.
The view of brain development that has emerged from the developmental neurobiology literature presents both challenges and opportunities to psychologists seeking to understand the fundamental processes that underlie social and cognitive development, and the neural systems that mediate them.
This chapter is intended to provide an overview of some very basic principles of brain development, drawn from contemporary developmental neurobiology, that may be of use to investigators from a wide range of disciplines. Brain development; maturation, Magnetic resonance imaging, Diffusion weighted imaging, Genetic patterning of brain, Neurogenesis, Myelination, Effects of experience on connectivity Human brain development is a protracted process that begins in the third gestational week GW with the differentiation of the neural progenitor cells and extends at least through late adolescence, arguably throughout the lifespan.
The processes that contribute to brain development range from the molecular events of gene expression to environmental input. Critically, these very different levels and kinds of processes interact to support the ongoing series of events that define brain development.
Both gene expression and environmental input are essential for normal brain development, and disruption of either can fundamentally alter neural outcomes.
But neither genes nor input is prescriptive or determinative of outcome. Rather brain development is aptly characterized as a complex series of dynamic and adaptive processes that operate throughout the course of development to promote the emergence and differentiation of new neural structures and functions. These processes operate within highly constrained and genetically organized, but constantly changing contexts that, over time, support the emergence of the complex and dynamic structure of the human brain Waddington ; Morange ; Stiles This paper will review some of the major events that contribute to the development of the human brain from its early embryonic state through adolescence.
It begins by examining the foundational changes that occur during the embryonic period, which in humans extends through the eighth week post conception gestational week eight, or GW8. The anterior end of the neural tube will develop into the brain, and the posterior portion will become the spinal cord.
The neural crest develops into peripheral structures.
At this point, the early nervous system is a simple, hollow tube. It runs from the anterior end of the embryo to the posterior end. Beginning at 25 days, the anterior end develops into the brain, and the posterior portion becomes the spinal cord.
This is the most basic arrangement of tissue in the nervous system, and it gives rise to the more complex structures by the fourth week of development. Primary Vesicles As the anterior end of the neural tube starts to develop into the brain, it undergoes a couple of enlargements; the result is the production of sac-like vesicles. Three vesicles form at the first stage, which are called primary vesicles. The prefix to each generally corresponds to its position along the length of the developing nervous system.
The third vesicle at this stage is the rhombencephalon. One way of thinking about how the brain is arranged is to use these three regions—forebrain, midbrain, and hindbrain—which are based on the primary vesicle stage of development Figure Secondary Vesicles The brain continues to develop, and the vesicles differentiate further see Figure The three primary vesicles become five secondary vesicles.
The prosencephalon enlarges into two new vesicles called the telencephalon and the diencephalon. The telecephalon will become the cerebrum. The diencephalon gives rise to several adult structures; two that will be important are the thalamus and the hypothalamus.
In the embryonic diencephalon, a structure known as the eye cup develops, which will eventually become the retina, the nervous tissue of the eye called the retina. This is a rare example of nervous tissue developing as part of the CNS structures in the embryo, but becoming a peripheral structure in the fully formed nervous system. The mesencephalon does not differentiate into any finer divisions. The midbrain is an established region of the brain at the primary vesicle stage of development and remains that way.
The rest of the brain develops around it and constitutes a large percentage of the mass of the brain. Dividing the brain into forebrain, midbrain, and hindbrain is useful in considering its developmental pattern, but the midbrain is a small proportion of the entire brain, relatively speaking.
The rhombencephalon develops into the metencephalon and myelencephalon. The metencephalon corresponds to the adult structure known as the pons and also gives rise to the cerebellum.
The most significant connection between the cerebellum and the rest of the brain is at the pons, because the pons and cerebellum develop out of the same vesicle. The myelencephalon corresponds to the adult structure known as the medulla oblongata. The structures that come from the mesencephalon and rhombencephalon, except for the cerebellum, are collectively considered the brain stem, which specifically includes the midbrain, pons, and medulla.
The embryonic brain develops complexity through enlargements of the neural tube called vesicles; a The primary vesicle stage has three regions, and b the secondary vesicle stage has five regions.
External Website Watch this animation to examine the development of the brain, starting with the neural tube.Vesicles of Brain Development
As the anterior end of the neural tube develops, it enlarges into the primary vesicles that establish the forebrain, midbrain, and hindbrain. Those structures continue to develop throughout the rest of embryonic development and into adolescence.
They are the basis of the structure of the fully developed adult brain. How would you describe the difference in the relative sizes of the three regions of the brain when comparing the early 25th embryonic day brain and the adult brain?
The Neural Tube
Spinal Cord Development While the brain is developing from the anterior neural tube, the spinal cord is developing from the posterior neural tube. However, its structure does not differ from the basic layout of the neural tube. It is a long, straight cord with a small, hollow space down the center. The neural tube is defined in terms of its anterior versus posterior portions, but it also has a dorsal—ventral dimension. As the neural tube separates from the rest of the ectoderm, the side closest to the surface is dorsal, and the deeper side is ventral.
As the spinal cord develops, the cells making up the wall of the neural tube proliferate and differentiate into the neurons and glia of the spinal cord.
The Basics of Brain Development
The dorsal tissues will be associated with sensory functions, and the ventral tissues will be associated with motor functions. Relating Embryonic Development to the Adult Brain Embryonic development can help in understanding the structure of the adult brain because it establishes a framework on which more complex structures can be built. First, the neural tube establishes the anterior—posterior dimension of the nervous system, which is called the neuraxis.
The embryonic nervous system in mammals can be said to have a standard arrangement. Humans and other primates, to some degree make this complicated by standing up and walking on two legs.
The Basics of Brain Development
The anterior—posterior dimension of the neuraxis overlays the superior—inferior dimension of the body. However, there is a major curve between the brain stem and forebrain, which is called the cephalic flexure. Because of this, the neuraxis starts in an inferior position—the end of the spinal cord—and ends in an anterior position, the front of the cerebrum. If this is confusing, just imagine a four-legged animal standing up on two legs.