Week 8- The change from embryo to fetus occurs, now all major organs have at least begun to development
I’m throwing the nervous system in here, it along with the other major systems are continuing to develop in week eight and I’ve yet to talk about it in detail so here we go. We’ve already discussed the very beginning of the nervous system, the neural tube gives rise to the Central Nervous System- the brain and spinal cord while the neural crest cells gives rise to the Peripheral Nervous System- the sensory neurons, nerves, and ganglia. To be honest this system is the system of the body I need to most review for because I get confused by all the terminology and classification. The CNS and PNS are the two major subdivisions of the nervous system, with the PNS acting as the network throughout the body that allows for information to be relayed to the CNS. The PNS includes the Autonomic Nervous System, which in turn includes both the Parasympathetic and Sympathetic components. As well there is the Enteric Nervous System, which is either its own separate entity or part of the ANS depending where you fall on that debate. Regardless, it functions to innervate the gastrointestinal system.
People commonly call the major functional unit of the nervous system nerves. Nerves technically speaking are neurons where each individual neuron contains a dendrite, which receives information and sends it to the soma or cell body, the main component of the neuron; and an axon which relays information from the soma. The major two types of neurons are afferent/sensory neurons which transmit information from the body to the CNS via the spinal cord, and efferent/motor neurons which transmit information from the CNS via the spinal cord to the body. Ganglion (singular, ganglia is plural) is a mass of nervous tissue located in the PNS. Glia is the collective term for supporting cells in the nervous system. Now see why this system gets me all confused?
Now to go back to the start we’ve briefly discussed the origin of the neural tube and the neural crest cells giving rise to the Central and Peripheral Nervous Systems. The neural tube will give rise to the brain and eventually all of its regions which are broken down to five areas. The front end of the neural tube will first make three major areas- the forebrain, midbrain, and hindbrain known at this stage in development as the prosencephalon, mesencephalon, and rhombencephalon. At around seven weeks, the forebrain and hindbrain area divided again giving rise to the telencephalon and diencephalon in the forebrain region and the metencephalon and myelencephalon in the hindbrain area. The mesencephalon does not further divide and thus continues to be known by this name, it gives house the tectum and the cerebral aqueduct. The telencephalon area becomes the largest part of the brain, the cerebrum which contains the cerebral cortex, hippocampus, basal ganglia, and the olfactory bulb. The diencephalon makes up the optic vessel and the hypothalamus. The hindbrain houses the pons and the cerebellum, and the myelencephalon eventually becomes the medulla oblongata at 20 weeks. As well the neural tube will give rise to the spinal cord at the tail end of the tube. When the spinal cord is complete, again I’ll use the term adult as in the adult spinal cord, it will run from the occipital bone down to the lumbar vertebra. In order to fully understand the development of the spinal cord, a cross section of the adult spinal cord is a good place to start.
Ok so there is a lot going on in the spinal cord and I’m really not going to get into it in a lot of detail because like I said earlier this system is one I am not really familiar with and I’d rather be vague then wrong! Going back to the neural tube, the end of it will become the spinal cord so a cross-section of the neural tube and its different regions will begin to tell the tale of how the adult spinal cord gets the way it is in the above picture. The cell type that gets this party started is neuroepithelial cell, located in the wall of the neural tube, and it gives rise to a lot of the various nervous system cells, including the neuroblasts which are precursors to all neurons, as well as macroglia and accessory cells like astrocytes and oligodendrocytes.
A distinction is made in the tube that gives a dorsal portion known as the alar plate from the ventral portion, the basal plate. The alar plate will give rise to the afferent, or sensory, neurons while the basal plate will give rise to the efferent, or motor, neurons. Collectively these two areas will become the interior of the spinal cord, known as the grey matter. The interior of the spinal cord is known as the grey matter due to its appearance directly related to the relatively few myelinated axons it has, this is in contrast to the surrounding area in the spinal cord, known as the white matter which has a lot of myelinated axons. As you can probably guess it is the myelin, a protective sheath that provides insulation for axons, that gives this white color. The grey matter contains a large amount of cell bodies, along with dendrites, some unmyelinated axons and glial cells. Conversely the white matter contains few cell bodies. The grey matter is made up of the dorsal and ventral horn which are the top and bottom of the grey matter, respectively with the lateral horn in the middle of the two. The dorsal horn consists of sensory neurons, it leads into the dorsal root and ultimately the dorsal root ganglion. The dorsal root ganglion, found on either side of neural tube, is formed from the neural crest cells. This group of nervous tissue is also referred to as the Spinal Ganglion and it carries signals from the sensory organs to the integration center. The ventral horn, made up of motor neurons, which will makes its way into the ventral root. The lateral horn deals with the sympathetic nervous system as well, its neurons travel along the ventral horn root along with the motor neurons. The last piece to this puzzle that I’ll mention is the spinal nerves, where both the ventral and dorsal roots go on to. Spinal nerves carry sensory, motor, and autonomic information from the intertwined fibers that came from both the ventral and dorsal root. Theses nerves continue on their path depending on where in the body they are located. There are 31 pairs of spinal nerves (that for the most part correspond with the vertebra) cervical (8), thoracic (12), lumbar (5), sacral (5) and 1 pair of spinal nerves in the coccygeal area.
All the previous discussion was about the neural cord giving rise to the brain and spinal cord. The other major contributor to the nervous system development is the neural crest cells. The neural plate border is a distinct region at the end of the neural plate (go figure), this border will give rise to the neural crest cells. During neurulation these cells will undergo a transition in type (epithelial to mesenchyme) that will allow them to migrate and differentiation into a lot of different cell types. These cells position themselves along the anterior-posterior axis and develop into four functional regions- cranial, trunk, vagal & sacral, and cardiac region. Each give rise to different nervous system anatomy but also cartilage, bone, and connective tissue. Of note the vagal & sacral region of neural crest cells gives rise to the Enteric Nervous System and the Parasympathetic ganglia, while the trunk region gives rise to the other major component of the ANS- the sympathetic ganglia.
Fast fact- The adult brain has over 100 billion neurons.