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DIENCEPHALON

The human diencephalon is composed of several regions. The thalamus which relays sensory information to the cerebrum, processes visual and auditory stimuli, and generates some emotions, among other functions. The hypothalamus contains the primary pleasure center, regulates the autonomic nervous system, regulates the release of many hormones, regulates circadian rhythms, and determines many sexual dimorphic behaviors, among other functions. The pineal gland secretes melatonin and helps to generate circadian rhythms. Many of these regions and functions were established early in the evolution of vertebrates.

The diencephalon forms as four separate neuromeres expressing distinct combinations of genes such as Wnt3, Wnt3A, Dlx1, Dlx2, Emx, Pax6, and PaxZf. Segments of the Drosophila brain are not only defined by homologs of some of these proteins (such as wingless, the homolog of Wnt, and Dll, the homolog of Dlx), but expression patterns relative to the borders of certain segments are homologous (Fidor, 1993).

Amphioxus possesses a diencephalon although it is not well differentiated from the cerebrum. (Ariens, p. 868). Lancelets possess a region similar to the hypothalamus (Murakami, 2005). Hagfish possess a habenular complex and separate nuclei in the dorsal thalamus. (Butler, 1996, p. 303). Hagfish lack a pineal complex (Nieuwenhuys, 2002).

HAGFISH

In all vertebrates, the diencephalon can be divided into an epithalamus, dorsal thalamus, ventral thalamus, and hypothalamus. The lamprey diencephalon possesses a lateral geniculate nucleus, dorsal and ventral thalamus, hypothalamus, neural hypophysis, pars intermedius, lateral hypothalamus (Ariens, p. 1192), pineal body and a habenular nucleus (Hardisty, p. 310). While the geniculate primordium of the thalamus does receive visual input in lampreys, the tectum is the major visual processing center (Hardisty, p. 319). The pineal gland projects to the pretectum (Butler, 1996, p. 302) and the retina projects to the pretectum and the tegmentum (Butler, 1996, p. 290).

In lampreys and apparently in some fossil fish, the parapineal body and the pineal body (epiphysis) are a bilateral pair of structures rather than being located in series, one in front of the other. A pair of visual structures may have been the ancestral condition of this pineal complex (Weichert, 1970, p.622). The parapineal organs of lampreys, tadpoles, and lizards possess cells similar to rods and cones. Frogs possess a brow spot or frontal organ where ancestral pineal eye might have been (Weichert, 1970, p.622).

LAMPREY

In all vertebrates, the hypothalamus integrates visceral systems. The hypothalamus in lampreys contains a preoptic nucleus, a mammillary body, and neurons capable of the secretion of hormones (Hardisty, p. 320-1). All vertebrates possess at least 4 circumventricular organs (including the hypothalamus) which can monitor the contents of the cerebrospinal fluid and secrete additional substances into it. (Butler, 1996, p. 330).

The diencephalon of gnathostomes share a number of features. In the ventral thalamus, gnathostomes possess a nucleus intermedius, ventromedialis, and ventrolateralis. (Butler, 1996, p. 262). In gnathostomes, the pineal projects to the habenular nuclei, although these connections are reduced in mammals. (Butler, 1996, p. 301-2). The hypothalamus is composed of a medial region surrounding the third ventricle and a pair of inferior lobes (Butler, 1996, p. 332). Gnathostomes also share features such as cerebellar tracts to the diencephalon, connections of the vestibulocochlear nerve projecting to the hypothalamus, a velum transversum, increased development of lateral geniculate nucleus, connections between ventral thalamus and hypothalamus to telencephalon, a tractus pallii, and a ganglion sacci vasculosi in hypothalamus (Ariens, p. 1192-4 ).

SHARK

The diencephalon of bony fish possess a number of characteristics which are shared in tetrapods such as connections of the hypothalamus to regions which regulate autonomic function (Ariens, p. 1195), a basal optic nucleus in the tegmentum, (Butler, 1996, p. 290), and a number of hypothalamic nuclei including a suprachiasmatic nucleus, and dorsal and ventral hypothalamic nuclei around third ventricle (Butler, 1996, p. 333). Bony fish and tetrapods possess a nucleus isthmi which projects to and from the optic tectum (the mammalian nucleus parabigeminalis is considered its homolog) (Wiggers, 1991). Prior to the evolution of amniotes, the dorsal thalamus was small and simple in contrast to other areas of the diencephalon (Butler, 1995).

PERCH

LUNGFISH

In tetrapods, the dorsal thalamus increased its size. Tetrapods share connections between the dorsal thalamus and telencephalon and olfactory pathways connecting the diencephalon to the telencephalon, such as the stria terminalis (Ariens, p. 1197). In tetrapods, the nucleus ventrolateralis is divided into dorsal and ventral regions and a lateral hypothalamic nucleus exists. (Butler, 1996).

Tetrapods evolved a pretectum (Butler, 1995).

FROG

In amniotes, the dorsal thalamus is more developed (Butler, 1995).

In amniotes, the central posterior nucleus and medial geniculate nucleus project to striatum and visual pathways from the midbrain project to both the striatum and pallium (Butler, 1995). The thalamic reticular nucleus projects to the dorsal thalamus and reciprocal circuits exist between the dorsal thalamus and cerebrum (Pritz, 1995). Most amniotes have a similar organization of the hypothalamus which includes the anterior nucleus, preoptic area (medial and lateral), supraoptic nucleus, suprachiasmatic nucleus, paraventricular nucleus, mamillary bodies (medial and lateral nuclei), posterior nucleus, lateral hypothalamic area, tuber cinereum, dorsomedial nucleus, and ventromedial nucleus (Butler, 1996, p. 336). In amniotes, the hypothalamus is involved in temperature regulation and the thalamus is an important sensory relay center for the cerebrum (Romer p. 586-7). Amniotes share a ventral lateral geniculate nucleus, a thalamic reticular nucleus, (Butler, 1996, p. 263) and a nucleus entopeduncularis in the ventral thalamus (Ariens, p. 1199).

TURTLE

ALLIGATOR

In mammals, the diencephalon is more complex than in reptiles. The mammalian diencephalon shares a common set of nuclei which include a dorsal lateral geniculate nucleus, ventral nuclear group, anterior nuclear group, medial nuclear group, intralaminar nuclear group, lateral posterior-pulvinar complex, medial geniculate body, limitans/suprageniculate complex, and lateral part of posterior nuclear group. Many nuclei exist in the diencephalon of non-mammals, especially birds and reptiles and many of these nuclei are homologous to those found in mammals to some degree (Butler, 1996, p. 318). In mammals, the medial geniculate nucleus of thalamus relays auditory information to auditory cortex, the lateral geniculate nucleus of thalamus relays visual information to visual cortex, the lateral thalamus relays information to cerebral cortex (Ariens, p. 1203-4, 3), the accessory optic nucleus is connected to the oculomotor complex rather than the cerebellum (Butler, 1996, p. 292), and lemnothalamic nuclei are more developed and move caudally (Butler 1995; Butler, 1996, p. 313). Mammals possess anterior and posterior paraventricular nuclei, medial and lateral habenular nuclei, and accessory optic nuclei (Butler, 1996, p. 304). The mammalian tectum is less important in visual processing and many visual fibers proceed to the thalamus (Romer p. 585). In mammals, all dorsal thalamic nuclei participate in relays to the cerebrum (Pritz, 1995). The hypothalamus performs roles in heartbeat, respiration, blood pressure, and sleep (Romer p. 586).

The following is an illustration of the pretectum of a tree shrew

OPOSSUM

In therian mammals, the nuclei of the dorsal thalamus are more separated (Butler 1994). Therian mammals possess cingulate and subcortical cortices, dorsal column nuclei, and parabrachial nuclei (Butler, 1995). The pretectum is more highly differentiated. (Butler, 1996, p. 288).

Placental mammals possess multiple nuclei of the anterior, medial, and intralaminar groups and a lateral posterior-pulvinar complex, unlike monotremes. (Butler, 1996, p. 320).

The pineal gland varies considerably in bats. Pteropus tonganus possess a greatly increased vascular supply to the pineal, forming a vascular cuff around it. The pineal gland is largest in Dobsonia praedatrix in which it reaches the brain surface and covers the anterior potion of the cerebellum. The reduction of the ependymal lining of the third ventricle allows the pineal to contact CSF. The pineal of D. praedatrix (in subfamily Pteropodinae) is .56% of the brain’s weight, which is 50 times larger than the expected value for the bats in the related subfamily Macroglossinae. Unlike most other bats, major pineal arteries actually enter the organ (Bhatnagar, 1990).

CAT

SHEEP

PIG

MONKEY

HUMAN