This post is not about the origin of neurons and the nervous system in the animals!
Nor is it about the origin and evolution of the vertebrates and chordates (or the history of research into that subject, which can take up a worthy post all on its own!), but a short phylogenetic note is definitely needed. The bilaterians are split into two main groups: the protostomes and the deuterostomes. They differ in the order in which the mouth and anus appear in the embryo: the protostomes first develop a mouth and then an anus independently, while the deuterostomes develop an anterior opening which wanders to the back to become the anus, and then develop the mouth (very basically). The other major difference is the cleavage pattern: most protostomes exhibit spiral cleavage, while all deuterostomes have a radial cleavage pattern.
- A notochord: a stiff skeletal rod that helps keep the animal stable while swimming.
- A neural cord dorsal to the notochord, which basically makes up the central nervous system (CNS). In the center of this neural cord lies Reissner’s fiber, a fluid-filled canal that is posteriorly connected to the archenteron by the neurenteric canal.
- Neurulation, described later and in the next post, is a chordate-only process (see the picture below for a short look at it).
- A pharyngeal-branchial complex, i.e. gill slits in the pharynx.
- A pituitary gland (sometimes called the hypophysis).
- An endostyle
- The inverted dorsoventral (DV) axis
- A post-anal tail
Neurulation is basically when the mesoderm differentiates into the notochord and the somites. What must be noted is that in chordates, the midline of the embryo (originally dorsal) becomes the ventral neural tube, so everything in chordates is ‘flipped’. The picture below shows that this is true down to a genetic level.
The diagram below shows the major steps in development in amphioxus (lancelet, cephalochordate).
In A, you see the result of the radial cleavage (never mind the colours). C shows the beginning of blastulation, which ends up with D: a 128-cell sphere with a specified outer layer (not yet anatomically differentiated, but with different gene expression patterns) surrounding a fluid-filled blastocoel (b). Next comes gastrulation, where the vegetal pole (marked by a dot in E, F, G) invaginates due to mechanical pressure from the expanding ectoderm (arrows in F and I), ending up with the siuation in J, where the ectoderm surrounds the meso- and endoderms, and a small hole (bp; blastopore) remaining – this is the anus (remember the difference between proto- and deuterostomes). Finally, in K to Q, neurulation begins. The dorsal part of the embryo flattens, making the neural plate (np). In N, several things happen. On the dorsal side, the epidermis is mingling with the neuroectoderm; on the posterior side, the neuroectoderm stays separated from the epidermis (the blastopore is not visible – at this point, it is blocked by epidermal tissue). O shows how this looks like in a transversal section: the chordamesodermal plate is what will later give rise to the notochord (chorda dorsalis, for the Latin-inclined), neural tube and the somites, as seen in P. The neural tube (nt) is closed, with various somites (s) visible and hiding the notochord behind. The tailbud (tb) is what the notochord and posterior segments will derive from as the embryo grows, as seen in Q. There you can also see the beginnings of the digestive tract’s formation, where the archenteron (yellow) is bending in on itself.
We will especially be interested in the neural plate, so I will explain what it is here. It’s a single layer of columnar epithelial cells that extend along the whole axis of the body. The plate eventually curls in on itself and forms the neural tube, which is what the entire nervous system derives from. Although initially a single layer, it gathers several layers and regions after multiple cell divisions; one of the results of these divisions is neuroblasts – neural stem cells – and it is they that form the bulk of the central nervous system. This will be discussed in more detail in the next post.
Vertebrates are the largest group of chordates and are characterised by a dorsal nerve cord protected a bony (or at least cartilaginous) covering (a spine), a skull with a centralised brain (hence why they’re sometimes called Craniata), a migrating neural crest, placodes and teeth. The other chordate subphyla are the Urochordata (tunicates), the Cephalochordata/Acraniata (lancelets). Of those, it is now generally agreed (although not conclusively!) that the tunicates are sister to the vertebrates, and the lancelets are sister to that group (i.e. they are the most basal chordates; they have a notochord, a segmented mesoderm and a tail). The picture below summarises all of this.
Just as reference, I will list the main apomorphies for each of the three taxa that prove their monophyly.
Vertebrates (besides the obvious ones described above):
- The epidermis with many differently-functioning layers is unique to the vertebrates.
- Clearly demarcated blood vessels are also unique – in all other taxa, including all invertebrates (except nemerteans), blood vessels are just holes and tunnels. In vertebrates, they have an epithelial lining.
- The heart divided into several compartments is also characteristic.
- The anterior sense organs and their associated brain areas.
- The brain itself!
- The spinal arrangement of nerves, with spinal ganglia connecting the CNS to the peripheral nervous system (PNS).
Cephalochordates (these are small and not so obvious characters!):
- Hesse ocelli, cup eyes composed of two cells found serially along the neural cord.
- Corpuscules of de Quatrefage are unique sensory cells found in the anterior of the organism.
- Cyrtopodocytes, a cell type not found in any other animal group that is specialised for excretion.
- A mouth surrounded by a ring of moveable cirri.
- A notochord that can be muscularly contracted.
Tunicates (arguably the most diverse animals, surpassing the arthropods and molluscs, so finding apomorphies here is slightly difficult):
- Their name-giving epidermis (the tunic) is unique to them; incidentally, it’s the only structure in the animal kingdom capable of producing cellulose, an ability most likely inherited by horizontal gene transfer.
- They can reverse their heartbeat, i.e. pump blood up into the pharynx for a while, then switch and pump it down to the gut.
- A pyloric gland in the gut, most likely used for excretion (analogous to a kidney) (not to be confused with these!)
We won’t look at lancelet brains in detail, other than saying that there is a general concensus on how their embryonic nervous system is homologous to the vertebrate nervous system, as shown in the picture below.
We will also not discuss the tunicates in any detail; suffice it to say that there are enough similarities in the development of tunicates and vertebrates to join them together. See the picture below for an example. A shows a standard chordate tailbud embryo. B shows the ascidian fate map. C is the interesting part: Xenopus is a model frog that’s used in developmental biology, ascidian is a type of tunicate. As you can see, the fate maps are quite similar.
Let’s expand on the most important verteberate characteristics a bit. Their major advantage is the head, that’s clear. We’ll look at that and the neural crest in the next post. Placodes are thickenings of the ectoderm, which later on become the organs (remember, the ectoderm is exclusive for the nervous system, so the organs referred to here are ‘nervous organs’): one type of placode, the sensory placodes, become the eye lens and the olfactory capsules; another type, the neurogenic placodes, give rise to the neurons in the cerebral ganglia.
See the next post in the series for the developmental biology behind the vertebrate head!
References and Further Reading