Diencephalon

Roof Plate and Epiphysis. The diencephalon, which develops from the median portion of the prosencephalon (Figs. 19.5 and 19.17), is thought to consist of a roof plate and two alar plates but to lack floor and basal plates (interestingly, sonic hedgehog, a ventral midline marker, is expressed in the floor of the diencephalon, suggesting that a floor plate does exist). The roof plate of the diencephalon consists of a single layer of ependymal cells covered by

Roof plate of diencephalon

Roof plate of diencephalon

Epithalamus

Pineal Lateral ventricle Hippocampus

Cerebral hemisphere

Foramen of Monro

Lamina terminalis

Optic

Transverse Brain Sections

Infundibulum

Figure 19.24 A. Medial surface of the right half of the prosencephalon in a 7-week embryo. B. Transverse section through the prosencephalon at the level of the broken line in A. The corpus striatum bulges out in the floor of the lateral ventricle and the foramen of Monro.

Epithalamus

Pineal Lateral ventricle Hippocampus

Cerebral hemisphere

Foramen of Monro

Lamina terminalis

Optic chiasma

Infundibulum

Figure 19.24 A. Medial surface of the right half of the prosencephalon in a 7-week embryo. B. Transverse section through the prosencephalon at the level of the broken line in A. The corpus striatum bulges out in the floor of the lateral ventricle and the foramen of Monro.

vascular mesenchyme. Together these layers give rise to the choroid plexus of the third ventricle (see Fig. 19.30). The most caudal part of the roof plate develops into the pineal body, or epiphysis. This body initially appears as an epithelial thickening in the midline, but by the seventh week it begins to evagi-nate (Figs. 19.24 and 19.25). Eventually it becomes a solid organ on the roof of the mesencephalon (see Fig. 19.30) that serves as a channel through which light and darkness affect endocrine and behavioral rhythms. In the adult, calcium is frequently deposited in the epiphysis and then serves as a landmark on radiographs of the skull.

Alar Plate, Thalamus, and Hypothalamus. The alar plates form the lateral walls of the diencephalon. A groove, the hypothalamic sulcus, divides the plate into a dorsal and a ventral region, the thalamus and hypothalamus, respectively (Figs. 19.24 and 19.25).

As a result of proliferative activity, the thalamus gradually projects into the lumen of the diencephalon. Frequently this expansion is so great that thalamic regions from the right and left sides fuse in the midline, forming the massa intermedia, or interthalamic connexus.

The hypothalamus, forming the lower portion of the alar plate, differentiates into a number of nuclear areas that regulate the visceral functions, including sleep, digestion, body temperature, and emotional behavior. One of these groups, the mamillary body, forms a distinct protuberance on the ventral surface of the hypothalamus on each side of the midline (Figs. 19.24A and 19.25 A).

Hypophysis or Pituitary Gland. The hypophysis, or pituitary gland, develops from two completely different parts: (a) an ectodermal outpocketing of the stomodeum immediately in front of the buccopharyngeal membrane, known

Radiography Hypophysis GlandEvagination
cephalon and diencephalon at the level of the broken lines in A.

as Rathke's pouch, and (b) a downward extension of the diencephalon, the infundibulum (Fig. 19.26, A and D).

When the embryo is approximately 3 weeks old, Rathke's pouch appears as an evagination of the oral cavity and subsequently grows dorsally toward the infundibulum. By the end of the second month it loses its connection with the oral cavity and is then in close contact with the infundibulum.

During further development, cells in the anterior wall of Rathke's pouch increase rapidly in number and form the anterior lobe of the hypophysis, or adenohypophysis (Fig. 19.26B). A small extension of this lobe,

Rathke Pouch Migration

Figure 19.26 A. Sagittal section through the cephalic part of a 6-week embryo showing Rathke's pouch as a dorsal outpocketing of the oral cavity and the infundibulum as a thickening in the floor of the diencephalon. B and C. Sagittal sections through the developing hypophysis in the 11th and 16th weeks of development, respectively. Note formation of the pars tuberalis encircling the stalk of the pars nervosa. D. High-magnification scanning electron micrograph ofthe region of the developing hypophysis similar to that in A. Rathke's pouch (arrow) and the infundibulum (arrowheads) are visible.

Figure 19.26 A. Sagittal section through the cephalic part of a 6-week embryo showing Rathke's pouch as a dorsal outpocketing of the oral cavity and the infundibulum as a thickening in the floor of the diencephalon. B and C. Sagittal sections through the developing hypophysis in the 11th and 16th weeks of development, respectively. Note formation of the pars tuberalis encircling the stalk of the pars nervosa. D. High-magnification scanning electron micrograph ofthe region of the developing hypophysis similar to that in A. Rathke's pouch (arrow) and the infundibulum (arrowheads) are visible.

the pars tuberalis, grows along the stalk of the infundibulum and eventually surrounds it (Fig. 19.26C). The posterior wall of Rathke's pouch develops into the pars intermedia, which in humans seems to have little significance.

The infundibulum gives rise to the stalk and the pars nervosa, or posterior lobe of the hypophysis (neurohypophysis) (Fig. 19.26C). It is composed of neuroglial cells. In addition, it contains a number of nerve fibers from the hypothalamic area.

CLINICAL CORRELATES Hypophyseal Defects

Occasionally a small portion of Rathke's pouch persists in the roof of the pharynx as a pharyngeal hypophysis. Craniopharyngiomas arise from remnants of Rathke's pouch. They may form within the sella turcica or along the stalk of the pituitary but usually lie above the sella. They may cause hydrocephalus and pituitary dysfunction (e.g., diabetes insipidus, growth failure).

Telencephalon

The telencephalon, the most rostral of the brain vesicles, consists of two lateral outpocketings, the cerebral hemispheres, and a median portion, the lamina terminales (Figs. 19.4, 19.5, 19.24, and 19.25). The cavities of the hemispheres, the lateral ventricles, communicate with the lumen of the dien-cephalon through the interventricular foramina of Monro (Fig. 19.24).

Cerebral Hemispheres. The cerebral hemispheres arise at the beginning of the fifth week of development as bilateral evaginations of the lateral wall of the prosencephalon (Fig. 19.24). By the middle of the second month the basal part of the hemispheres (i.e., the part that initially formed the forward extension of the thalamus) (Fig. 19.24A) begins to grow and bulges into the lumen of the lateral ventricle and into the floor of the foramen of Monro (Figs. 19.24B and 19.25, A and B). In transverse sections, the rapidly growing region has a striated appearance and is therefore known as the corpus striatum (Fig. 19.25 B).

In the region where the wall of the hemisphere is attached to the roof of the diencephalon, the wall fails to develop neuroblasts and remains very thin (Fig. 19.24B). Here the hemisphere wall consists of a single layer of ependymal cells covered by vascular mesenchyme, and together they form the choroid plexus. The choroid plexus should have formed the roof of the hemisphere, but as a result of the disproportionate growth of the various parts of the hemisphere, it protrudes into the lateral ventricle along the choroidal fissure (Figs. 19.25 and 19.27). Immediately above the choroidal fissure, the wall of the hemisphere thickens, forming the hippocampus (Figs. 19.24B and 19.25B). This structure, whose primary function is olfaction, bulges into the lateral ventricle.

With further expansion, the hemispheres cover the lateral aspect of the diencephalon, mesencephalon, and cephalic portion of the metencephalon (Figs. 19.27 and 19.28). The corpus striatum (Fig. 19.24B), being a part of the wall of the hemisphere, likewise expands posteriorly and is divided into two parts: (a) a dorsomedial portion, the caudate nucleus, and (b) a ventrolateral portion, the lentiform nucleus (Fig. 19.27B). This division is accomplished by axons passing to and from the cortex of the hemisphere and breaking through the nuclear mass of the corpus striatum. The fiber bundle thus formed is known as the internal capsule (Fig. 19.27B). At the same time, the medial wall of the

Diencephalon Surfaces
Figure 19.27 A. Medial surface of the right half ofthe telencephalon and diencephalon in a 10-week embryo. B. Transverse section through the hemisphere and diencephalon at the level of the broken line in A.

hemisphere and the lateral wall of the diencephalon fuse, and the caudate nucleus and thalamus come into close contact (Fig. 19.27B).

Continuous growth of the cerebral hemispheres in anterior, dorsal, and inferior directions results in the formation of frontal, temporal, and occipital lobes, respectively. As growth in the region overlying the corpus striatum slows,

Medial Surface Cerebral Hemisphere
Figure 19.28 Development of gyri and sulci on the lateral surface of the cerebral hemisphere. A. 7 months. B. 9 months.

however, the area between the frontal and temporal lobes becomes depressed and is known as the insula (Fig. 19.28 A). This region is later overgrown by the adjacent lobes and at the time of birth is almost completely covered. During the final part of fetal life, the surface of the cerebral hemispheres grows so rapidly that a great many convolutions (gyri) separated by fissures and sulci appear on its surface (Fig. 19.28B).

Cortex Development. The cerebral cortex develops from the pallium (Fig. 19.24), which has two regions: (a) the paleopallium, or archipallium, immediately lateral to the corpus striatum (Fig. 19.25B), and (b) the neopallium, between the hippocampus and the paleopallium (Figs. 19.25B and 19.27B).

In the neopallium, waves of neuroblasts migrate to a subpial position and then differentiate into fully mature neurons. When the next wave of neuroblasts arrives, they migrate through the earlier-formed layers of cells until they reach the subpial position. Hence the early-formed neuroblasts obtain a deep position in the cortex, while those formed later obtain a more superficial position.

At birth the cortex has a stratified appearance due to differentiation of the cells in layers. The motor cortex contains a large number of pyramidal cells, and the sensory areas are characterized by granular cells.

Olfactory Bulbs. Differentiation of the olfactory system is dependent upon epithelial-mesenchymal interactions. These occur between neural crest cells and ectoderm of the frontonasal prominence to form the olfactory placodes (Fig. 19.29) and between these same crest cells and the floor of the telencephalon to form the olfactory bulbs. Cells in the nasal placodes differentiate into primary sensory neurons of the nasal epithelium whose axons grow and make contact with secondary neurons in the developing olfactory bulbs (Fig. 19.29). By the seventh week, these contacts are well established. As growth of the brain continues, the olfactory bulbs and the olfactory tracts of the secondary neurons lengthen, and together they constitute the olfactory nerve (Fig. 19.30).

Olfactory PlacodeSagittal Section Diencephalon Region

choana

Figure 19.29 A. Sagittal section through the nasal pit and lower rim of the medial nasal prominence of a 6-week embryo. The primitive nasal cavity is separated from the oral cavity by the oronasal membrane. B. Similar section as in A toward the end of the sixth week showing breakdown ofthe oronasal membrane. C. At 7 weeks, neurons inthe nasal epithelium have extended processes that contact the floor of the telencephalon in the region of the developing olfactory bulbs. D. By 9 weeks, definitive oronasal structures have formed, neurons in the nasal epithelium are well differentiated, and secondary neurons from the olfactory bulbs to the brain begin to lengthen. Togther, the olfactory bulbs and tracts ofthe secondary neurons constitute the olfactory nerve (see Fig. 19.30).

Olfactory / bulb

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Responses

  • Tolman
    Is the floor of the diencephalon is formed by the Thalamus?
    7 years ago

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