In this section, we will discuss what is called a body plan, which begins with germ layers. Then we will discuss the neural tube, which develops from one of these germ layers and eventually develops into a brain. To clarify just how complexly connected our brains are,this section will be ended discussing neuronal circuits.
Embryology, an introduction:
Neil Shubin, in Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (2009), provides a wonderful introduction to something called germ layers and also introduces embryology. Shubin is quoted in the following four paragraphs. The image of an eight-cell embryo below links to its source.
In the 1800s, some natural philosophers looked to embryos to try to find the common plan for life on earth. Paramount among these observers was Karl Ernst von Baer. Born to a noble family, he initially trained to be a physician. His academic mentor suggested that he study chicken development and try to understand how chicken organs developed.
Unfortunately, von Baer could not afford incubators to work on chickens, nor could he afford many eggs. This was not very promising. Lucky for him, he had an affluent friend, Christian Pander, who could afford to do the experiments. As they looked at embryos, they found something fundamental: all organs in the chicken can be traced to one of three layers of tissue in the developing embryo. These three layers became known as the germ layers. They achieved almost legendary status, which they retain even to this day.
Pander's three layers gave von Baer the means to ask important questions. Do all animals share this pattern? Are the hearts, lungs, and muscles of all animals derived from these layers? And, importantly, do the same layers develop into the same organs in different species?
Von Baer compared the three layers of Pander's chicken embryos with everything else he could get his hands on: fish, reptiles, and mammals. Yes, every animal organ originated in one of these three layers. Significantly, the three layers formed the same structures in every species. Every heart of every species formed from the same layer. Another layer gave rise to every brain of every animal. And so on. No matter how different the species look as adults, as tiny embryos they all go through the same stages of development.
The photograph of an embryo on the left, below, is that of a mouse and links to an article titled "How Genes Orchestrate Facial Expressions," from the University of Utah. The Gray's Anatomy illustration on the right is of a human embryo at 18-21 days of development (image links to source).
Neural tube brain organization:
The brain begins as a neural tube. Andrew Lautin, author of The Limbic Brain (2001), tells me that the beginnings of the entire central nervous system can be envisioned as an inflated, elongated balloon of the kind that clowns sometimes use to make animal figures.
As Lautin explains, like a clown flexing a balloon, imagine the neural tube flexing at two points—the cervical and cephalic flexures—both bending in similar concavity. When you put in a middle flexure, the pontine flexure, a four-sided (rhomboidal) shape emerges, that is to say the rhombencephalon, also called the hind brain. In the illustration above (links to source), a yellow box is drawn around the rhombencephalon. Thomas A. Marino at Temple University prepared the course work that includes this illustration. Click on the illustration to link to a more advanced overview of the development of the brain, spinal cord, and nerves.
The neural tube illustration at right is from course material in developmental biology from the University of Guelph in Ontario and links to source. In early development, there are only three bulges, or vesicles, in the neural tube. As an embryo develops, these vesicles begin to differentiate into subdivisions which are commonly called the forebrain, midbrain, and hindbrain. We humans share these developmental differentiations in the brain with all other vertebrates including bony fish, amphibians, reptiles, birds, and of course, other mammals.
The illustration below (links to source), also from the University of Guelph, shows the beginning of evaginations. Lautin points out that because everything is initially positioned in the mid-line, the lateral ventricles, an internal cavity in both the left and right cerebral hemispheres, have to be lateral evaginations wherein the neural tube turns outward. In the illustration below, you can see how these prosencephalon evaginations begin to take shape to produce the telencephalon (which develops into the left and right cerebral hemispheres and ventricles) and diencephalon (which develops primarily into the thalamus and hypothalamus).
Lautin explains that the telencephalon is the "most forward projected ventricle," sort of an "end-brain." The prefix tele, from the ancient Greek, means "at a distance" or "end." Given the use of the prefix di, Lautin explains that the diencephalon is a "between brain" or "inter-brain."While the mesencephalon remains relatively undifferentiated, the rhombencephalon will also develop into two vesicles—the metencephalon (further develops into the pons and cerebellum) and the myelencephalon (develops into the medulla oblongata). The image below is from a Stuart Clare's thesis submitted to the University of Nottingham for the degree of Doctor of Philosophy in 1997 (links to source). You can see the lateral ventricles, to which Lautin refers, and how they develop forward from the foramen of Monro, a single ventricle at the midline.
the following illustrations are found in a primer called Brain Basics: Know Your Brain. The National Institute of Neurological Disorders and Stroke produced the primer. The diagrams (image links to source) illustrate the locations of the fully developed forebrain, midbrain, and hindbrain in the human brain.
The diagram above is useful as an illustration of anatomical differentiation. Such a depiction can, however, create a misperception by suggesting simplicity where, in fact, none exists. As the brain develops, neuronal circuits are created to integrate the brain into a functional whole. Antonio R. Damasio makes this clear in Descartes' Error: Emotion, Reason, and the Human Brain (1994): "There are several billion neurons in the circuits of one human brain. The number of synapses formed among those neurons is a least 10 trillion, and the length of the axon cable forming neuron circuits totals something on the order of several hundred thousand miles. [Special thanks to Charles Stevens, a neurobiologist at the Salk Institute, for the informal estimate.] … The time scale for the firing is extremely small, on the order of tens of milliseconds—which means that within one second in the life of our minds, the brain produces millions of firing patterns over a large variety of circuits distributed over various brain regions." Later, Damasio adds, "…the elementary secrets of mind reside with the interaction of firing patterns generated by many neuron circuits, locally and globally, moment by moment, within the brain of a living organism."