Day-3: Vertebrates did not evolve in a day!
The Inexplicable Wonders of the Vertebrate Nervous System!
As I dive deeper into the realm of the evolution of the Animalian Nervous System, I slowly reach into the world of vertebrates. The wonders of these organisms can be truly understood during the process of embryogenesis. The early stages of embryonic development in all vertebrates are pretty much the same, and hence the development of the nervous system has a great deal of resemblance among these species. The major axis that runs along the length of a vertebrate’s body is termed as the rostrocaudal axis. Now the interesting point to be noted over here is that this axis undergoes bending during the embryonic development and this bending is unique to each type of species. Apart from this, the other two axes are orthogonal dorso-ventral axis which runs from the back to the belly and the mediolateral axis which runs from the midline to the lateral margin of the animal.
During embryogenesis, the CNS develops a hollow cylinder, called as the neural tube from a flat sheet of cells called neural plate, by the process of Neurulation. The wide end of the neural plate develops further into the brain (rostral end) whereas the narrower end (caudal end) develops into the spinal cord. The neural groove divides the neural plate into right and left halves so the plate displays the three cardinal morphogenetic attributes: polarity, regionalization and bilateral symmetry.
Neural plate further regionalizes into rostral and caudal ends and the brain plate regionalizes first into optic vesicles, that evaginates into the possible hypothalamus. The junction between the neural plate and the left over ectoderm that later on becomes the skin, possesses the neural crest. The neural crest is what develops into the Peripheral Nervous System- PNS.
Further regionalization results in the folding of the neural plate into a U- shaped structure. The dorsal ends of the tube fuse together and form an open tube, eventually forming a complete hollow tube with a thick wall. The folds are called neural folds, the tube ends are termed as neuropores and the wall is termed as neuroepithelium. Marcello Malpighi was able to observe the capillary network which was present between arteries and veins in an early developing chick neural tube. He was able to distinguish the rostrocaudally divided segments- now known as the primary brain vesicles.
Marcello Malpighi was able to observe the capillary network which was present between arteries and veins in an early developing chick neural tube. He was able to distinguish the rostrocaudally divided segments- now known as the primary brain vesicles.
The key components of the primary brain vesicles are as follows:-
· The three parts of the brain are- forebrain (prosencephalic), midbrain (mesencephalic) and the hindbrain (rhombencephalic).
· As embryogenesis progresses, the forebrain divides into endbrain (telencephalic) and interbrain (diencephalic), and the hindbrain develops into rostral pontine (metencephalic) and caudal medullary (myencephalic).
· The neural tube lumen differentiates into the CNS ventricular system
· The endbrain consists of the lateral ventricles communicating through the intraventricular foramen with the third ventricle in the interbrain.
· Third ventricle continues in the midbrain’s cerebral aqueduct and becomes the fourth ventricle of the hindbrain and spinal cord’s central canal.
· The ventricular system of the fully developed brain comprises of the cerebrospinal fluid.
Now, when we take the neuron cell development into consideration, we focus on the secondary vesicle neural tube. It undergoes repetitive division and forms a layer of stem cells termed as the pseudostraitified epithelium. These cells undergo a terminal differentiation and form a fresh layer of neurons, also known as the mantle layer, which further regionalizes into the Central Nervous System.
The Ventral Mantle Layer results in the formation of a groove, also known by the term limiting sulcus.
The development of the nervous system is highly complex and I would not like to bore with all the small theoretical detail of it, but by the end of studying this portion, I realized that every little anatomical detail matters. Any error in the development of even the most minuscule part can have dire consequences.
In our lives we tend to skip those small little instances which can either act as a sign for a prominent event in our lives. The ability to comprehend and take into account those little nuances can enlighten us and make us feel more aware about others as well as ourselves. Now, in the end it is definitely upto our prioritization capability to filter out the not so important details and interpret those intricacies in a holistic manner. Being mindful and hyper aware might sound extremely exhausting but the consumption of a little ATP in absorbing those details can benefit us a great deal.
In the end, all the above interpretation that I derive out of those little details is a result of the functional capacity of my neurons, and I feel quite grateful for a fully functional and well developed cerebral matter.
The brain development is a journey, and full comprehension of it is a time consuming process. I will make sure to enjoy every little step.
Until then, keep absorbing the little details!
