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THE BRAIN

The human brain can answer calculus problems, compose music, coordinate the muscles for a dance routine, remember the junior prom, and generate anger towards the driver who abruptly entered your lane. The human brain is composed of six major regions, each of which contains important subregions. The brain includes spaces through which cerebrospinal fluid passes, regions which secrete hormones, and 12 cranial nerves. How could the most complex structure in the known universe evolve? Brains, including those of the human lineage, have evolved over hundreds of millions of years through many transitional steps.

Cnidarians (jellyfish, coral, Hydra ), only achieve the tissue level of organization and also have the primitive condition of radial symmetry. Rather than having right and left halves, there is more than one plane of symmetry around a central axis. In these radially symmetrical animals, the nervous system consists of a diffuse nerve net. Since sensory structures are simple and equally distributed on all sides of the body, there is no need for a concentration of nervous tissue (that is, a brain) in any one part of the body.

A group of jellyfish called the ctenophores (comb jellies) are elongated and possess a gravity-detecting statocyst at one end of the body. There is some evidence that ctenophores are the sister group of bilaterans. There are a few examples of a concentration of neural tissue in cnidarians. Marginal ganglia of Scyphomedusae show a slight centralization and in ctenophores the ganglion is associated with a sense organ (aboral) at one pole of the body.(Beklemishev, vol. 2 p. 79-80). In cnidarian larvae, nervous tissue may concentrate in anterior end of the body (Hyman). Hydra and bilaterans both express homeogenes of the Prd group, nuclear orphan receptors COP-TF, msh (or the Antennapedia group) and thrombospondin in the developing head or neural tissue (Miljkovic-Licina, 2004).

All animals with organs have bilateral symmetry: the only way to divide them into two equal halves is to divide them into right and left halves. Sense organs tend to be located at one end, the end that is usually propelled forward. With sense organs concentrated at the front end of early bilaterans, nervous tissue began to accumulate there to process this sensory input, thus forming a brain.

The most primitive flatworms (the group called Acoela) are the most primitive bilateral animals. Their nervous system consists only of a diffuse nerve net, like that of cnidarians.

Higher flatworms possess the first central nervous systems. In addition to a primitive nerve net, they possess longitudinal nerve cords and a cerebral ganglion (“brain”), although it is not well developed. The brain develops around a statocyst, homologous to the statocyst of ctenophores. These first “brains” developed at the end of the animal which was becoming specialized to deal with sensory information. This brain has little connection with or control over the diffuse nerve net throughout the rest of the body (Beklemishev, vol. 2, p. 50-1; Raikova, 2000). There are cranial nerves which consist of short pairs of nerves which stretch from the brain to the pharynx and regions of the head (Rieger, from Harrison, 1991).

In the brain of urochordates, there exists a neural gland which may be homologous to the pituitary gland (Hickman 714). As larvae they possess a dorsal nerve cord and a cerebral eye (Hickman, 716). In larvae, there are a group of cranial nerves (2 to 5) proceeding from the brain (Beklemishev, vol. 2, p. 137-8). A vesicle in the tunicate brain may be homologous to the saccus vasculosus present in the brains of many fish (Svane, 1982). There is no trace of a telencephalon or an olfactory region (Nieuwenhuys, 2002). Of the 335 cells in the tuncate brain, 70% are a group of glial cells called ependymal cells (Nieuwenhuys, 2002).

AMPHIOXUS BRAIN

In Amphioxus, there are paired nerves leaving the CNS from the anterior end (the brain) but towards the posterior end they begin to alternate. Although lancelets possess a definite brain, the CNS is actually wider in the middle of the body than at the anterior end (Willey, p. 82). The first two cranial nerves carry sensory information only (as in vertebrates) and innervate the tip of the head. The brain includes both a cerebral vesicle and a medulla-like posterior region. There are no ventricles (Willey, p. 90-1). In lancelets as in vertebrates, a diffuse nerve net exists along the digestive tract which may be a derivative of the ancestral nervous system (Beklemishev, vol. 2, p. 137)

AMPHIOXUS BRAIN

Although hagfish are craniates, they are the most primitive craniates. Their brains lack a cerebellum and a pineal complex. Lamprey brains more similar to those of gnathostomes. The majority of fiber tracts in lampreys have homologs in gnathostomes (Nieuwenhuys, 2002). The brain of a larval lamprey is depicted in the adjacent photo.

SEGMENTATION

Many animals possess segmented regions of their body, including the nervous system. Flatworms are the most primitive animals which possess brains and there is some evidence of compartmentalization of the flatworm (planarian) brain determined by differing patterns of gene expression (Marshal, 2003). The vertebrate nervous system is segmented, although different segmentation patterns are observed in different parts of the nervous system. In the diencephalon, distinct prosomeres are more obvious through the analysis of molecular differences between cells rather than by an obvious macroscopic segmentation. In contrast, the hindbrain forms a series of macroscopic rhombomeres and the spinal cord is obviously segmented with its metameric repetition of nerves. The nervous system of invertebrates is often segmented as well, although it is not obvious to what extent the segmentation of vertebrate and invertebrate brains is related (Mazet, 2002).

Note the segments (rhombomeres) in the hindbrain of the chick.

The vertebrate brain begins its development as a tube whose separate regions differentiate into the brain regions found in the adult.

Are the divisions of vertebrates and advanced invertebrates homologous? In other words, were some of the divisions of the brain of advanced vertebrates inherited from invertebrate ancestors? Molecular and developmental evidence supports the conclusion that the brains of bilateran animals are homologous organs which are derived from a common ancestral structure (Reichert, 2001). Both vertebrates and higher invertebrates (such as Drosophila) have the common feature of a brain which is divided into a forebrain, midbrain, and hindbrain with an important midbrain/hindbrain boundary characterized by the expression patterns of Otx, Hox, and Pax2/5/8 genes (Hirth, 2003). The brain of urochordates seems to be divided into three regions: a forebrain/midbrain (as indicated by the expression of Hroth, the homolog of vertebrate Otx), an anterior hindbrain (as indicated by the tunicate homolog of vertebrate Pax genes), and a posterior hindbrain (as indicated by the expression of a tunicate Hox gene) (Wada, 1998).

Gene expression patterns (such as those of AmphiFoxB and AmphiSim) indicate that the cerebral vesicle of Amphioxus is organized into regions homologous to the vertebrate diencephalon and midbrain. Posterior to this, there is a region of the neural tube which expresses Hox genes, as does the vertebrate hindbrain, although it does not form obvious rhomobmeres. Nevertheless, there are segmented blocks of tissue revealed by the expression pattern of AmphiFoxB which may represent an early stage in rhombomere evolution in which the signals for segmentation originated from surrounding tissues rather than retinoic acid gradients in the nervous tissue as in vertebrates (Mazet, 2002). The lancelet hindbrain also reveals segmentation in the expression pattern of islet1 (a gene important in developing motor neurons). Islet1 expression also reveals that Amphioxus may have homologs of the pineal gland and adenohypophysis (Jackman, 2000).

In coelomate animals, the gene Evx has also been recruited into the development of the nervous system. In vertebrates (but not Amphioxus), the role of Evx in the nervous system is augmented to include expression in the midbrain-hindbrain boundary (MHB) which is an important organizing region for the vertebrate brain. (Ferrier, 2001). Pax6 expression patterns help to mark the diencephalon-mesencephalon border in lampreys and gnathostomes (Derobert, 2002). The midbrain-hindbrain boundary is marked by the expression patterns of a number of additional genes such as Otx-2, Wnt-1, FGF8, En (2,5,8), and Pax(2,5,8) (Hall, 1999). Hox genes are involved in the segmentation of the hindbrain. For example, Hoxb-1 is produced only in rhombomere 4 while Hoxa3 is expressed at the boundary between rhombomeres 4 and 5 (Hall, 1999).

In vertebrates, the boundary of the diencephalon and mesencephalon includes a structure known as the subcommissural organ (SCO) which secretes products important in the development of the nervous system, including the spinal cord. A protein secreted here, SCO-spondin, is homologous with F-spondin secreted from the floor plate and thrombospondin. The SCO-spondin gene is present in all chordates and there is some evidence that it exists in more basal deuterostomes as well. The infundibular organ in Amphioxus may be homologous to the subcommissural organ of vertebrates (Gobron, 1999).

Hagfish possess a cerebrum, diencephalon, midbrain, and medulla, as do all vertebrates. Lampreys and all vertebrates possess a cerebellum. Mammals possess a pons. The brains of vertebrates do not seem to be unrelated structures, but rather homologous organs which are modified versions of a common ancestral plan. The ostracoderms possessed brains intermediate between those of modern jawless fish and jawed fish (Janvier, 2008).

Note that the brains of the following diverse organisms are composed of the same regions. Also note the gradual increase in the overall size of the brain through the taxonomic groups.

HAGFISH

LAMPREY

SHARK

PERCH

LUNGFISH

FROG

TURTLE

OPOSSUM

PIG

MODEL OF HUMAN BRAIN MODEL

Placental mammals studied to date share the production of new neurons in adults in the dentate gyrus of the hippocampus and the subventricular zone lining the lateral ventricles (Bernier, 2002).