18. Development of the diencephalon and the telencephalon. Development of the commissural pathways and the fornix

Last updated on August 23, 2020 at 21:54

Learning objectives

  • Which primary vesicle does the diencephalon develop from?
  • Describe the structure of the diencephalon
  • What is the fluid-filled structure of the diencephalon?
  • What arises from the roof plate of the diencephalon?
  • What arises from the alar plates of the diencephalon?
  • Describe the development of the pituitary gland
  • Which primary vesicle does the telencephalon develop from?
  • Describe the structure of the telencephalon
  • What is the fluid-filled structure of the telencephalon?
  • Describe the development of the hemispheres
  • What are the different types of pallium (cortex), and which is most prevalent in humans?
  • Describe the development of the basal ganglia
  • Describe the development of the commissural pathways
  • Which anatomical structure functions as a framework for several commissural pathways?

Development of diencephalon

The diencephalon develops from the prosencephalon. It consists of a roof plate and two alar plates, but no floor or basal plates.

Roof plate

The roof plate consists of a single layer of ependymal cells covered by vascular mesenchyme. These layers give rise to the choroid plexus of the third ventricle. The third ventricle is the fluid-filled structure of the diencephalon.

The most caudal part of the roof plate develops into the pineal body, also called the epiphysis.

Alar plates

The alar plates form the lateral walls of the diencephalon. Each plate is divided into a dorsal and ventral region by the hypothalamic sulcus. The dorsal region gives rise to the thalamus while the ventral region gives rise to the hypothalamus.

The different parts of the hypothalamus then give rise to the different hypothalamic nuclei, including the mamillary body, which protrudes from the ventral surface of the hypothalamus.

Pituitary gland

The pituitary gland, also called the hypophysis, has two parts, an anterior lobe and a posterior lobe. The anterior lobe develops from the Rathke’s pouch, an outpocketing of ectoderm of the stomodeum, while the posterior lobe develops from the infundibulum, an extension of the diencephalon which projects downward. Rathke’s pouch itself is derived from the adenohypophyseal placode, a local thickening of ectoderm which invaginates to form the pouch.

In addition to the posterior lobe itself, the infundibulum also gives rise to the pituitary stalk, which connects the posterior lobe to the hypothalamus. The posterior lobe does contain nerve fibres from the hypothalamus, after all.

Rathke’s pouch initially grows dorsally, towards the infundibulum. By the end of the seventh month it loses its connection with the oral cavity, remaining only in contact with the infundibulum, from which it will differentiate into the anterior lobe. An extension of the anterior lobe, called the pars tuberalis, grows along the stalk of the infundibulum and eventually surrounds it.

Development of telencephalon

The telencephalon develops from the prosencephalon and is the most rostral (anterior) of brain vesicles. It consists of two lateral outpocketings and one median portion. The two lateral outpocketings are the cerebral hemispheres, while the median portion is the lamina terminalis.

Cerebral hemispheres and hippocampus

The lateral outpocketings of the prosencephalon begin to extend into its lumen in week 5, eventually forming the hemispheres. In the region where the hemispheres come in contact with the underlying diencephalon a single layer of ependymal cells forms, covered by vascular mesenchyme. This forms the choroid plexuses of the lateral ventricles, the fluid filled structures of the telencephalon.

The hippocampus forms as a thickening of the hemispheres above the choroidal fissure.

The hemispheres continue to grow in all directions, forming the lobes. During the final part of foetal life, the surface of the hemispheres grows so rapidly that the surface invaginates, forming the characteristic gyri.

Cerebral cortex and the pallium

The cortex of the hemispheres develops from the pallium, a layer of grey and white matter which covers the hemispheres. There are three types of pallium, the paleopallium (paleocortex), archipallium (archicortex), and neopallium (neocortex).

These types have different phylogenetic ages, meaning that they developed at different times of evolution. The oldest one is the paleopallium, which existed in much earlier animals than the others. The youngest one is the neopallium. Each type of pallium has a different number of layers-.

In humans the neopallium accounts for 90% of the cortex, meaning that the human brain consists mostly of phylogenetically “young” cortex. It is what we think of when we think “cortex” in human brains, as this is the type of cortex which has six layers. The other types have fewer layers.

In humans the paleopallium is found on the floor of the hemispheres and comprises the rhinencephalon, a structure consisting of the olfactory bulb and surrounding structures.

In humans the archipallium is found on the medial surface of the hemispheres and comprises the hippocampus, indusium griseum, and the fornix.

Basal ganglia

As the hemispheres grow, a rapidly growing region called the striatal anlage (also called ganglion hill by POTE) forms, which gives rise to the (corpus) striatum. The striatum grows and extends posteriorly.

Axons passing to and from the cortex break through the corpus striatum, forming the internal capsule. The internal capsule separates the striatum into the caudate nucleus and the lentiform nucleus, which consists of the putamen and the globus pallidus.

According to our lecture the globus pallidus originates from the diencephalon instead of the telencephalon, but no other source corroborates this.

Development of commissural pathways and the fornix

At first the hemispheres grow separately, but by the end of the third month nerve fibres grow between them, forming the commissures, which are bundles of nerve fibres connecting the hemispheres. Most of them develop in the lamina terminalis and use it as a guide, while some develop outside it.

The first commissure to develop is the anterior commissure, which consists of fibres connecting brain areas related to olfaction of one hemisphere to the other.

The second commissure to develop is the hippocampal commissure, also called the fornix. These fibres connect the hippocampus of one hemisphere to the mamillary body and hypothalamus of the other, forming a characteristic C-shape.

The corpus callosum appears by week 10 and connects most of the cortexes of the two hemispheres with each other.

The posterior and habenular commissures, as well as the optic chiasm, develop outside the lamina terminalis.

Summary

  • Which primary vesicle does the diencephalon develop from?
    • The prosencephalon
  • Describe the structure of the diencephalon
    • It has a roof plate and two alar plates, but no floor or basal plates
  • What is the fluid-filled structure of the diencephalon?
    • The third ventricle
  • What arises from the roof plate of the diencephalon?
    • Choroid plexus of the third ventricle and the pineal body
  • What arises from the alar plates of the diencephalon?
    • Dorsal part gives rise to thalamus
    • Ventral part gives rise to hypothalamus and mamillary body
  • Describe the development of the pituitary gland
    • Posterior lobe develops from the infundibulum, a downward extension of the diencephalon
    • Anterior lobe develops from Rathke’s pouch, an outpocketing of ectoderm of the stomodeum, which grows toward the infundibulum, eventually losing contact with the oral cavity
    • Pars tuberalis develops from the anterior lobe and grows to surround the stalk of the infundibulum
  • Which primary vesicle does the telencephalon develop from?
    • The prosencephalon
  • Describe the structure of the telencephalon
    • It has two lateral outpocketings and one median portion (the lamina terminalis)
  • What is the fluid-filled structure of the telencephalon?
    • The lateral ventricles
  • Describe the development of the hemispheres
    • The lateral outpocketings of the prosencephalon grow into the lumen, forming the hemispheres
    • Where the hemispheres meet the diencephalon the choroid plexus of the lateral ventricles form
  • What are the different types of pallium (cortex), and which is most prevalent in humans?
    • From oldest to newest they are the paleopallium, archipallium, and neopallium
    • The neopallium is the most prevalent in humans, accounting for 90% of adult cortex
  • Describe the development of the basal ganglia
    • In the growing hemispheres the striatal anlage/ganglion hill forms, which gives rise to the striatum
    • Cortical axons form the internal capsule, separating the striatum into the caudate nucleus and lentiform nucleus
  • Describe the development of the commissural pathways
    • By the end of the third month nerve fibres begin to grow between the hemispheres, forming the commissures
    • The anterior commissure and fornix develop first, while the corpus callosum, optic chiasm and the posterior and habenular commissures develop later
  • Which anatomical structure functions as a framework for several commissural pathways?
    • The lamina terminalis, in which the anterior commissure, fornix, and corpus callosum develop in

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17. Development of the brainstem and the cerebellum

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19. The placodes, the neural crest and their derivatives. Development of the peripheral nervous system

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