Embryology

Here's a quick introduction and brief overview of the embryology of the brain. It will help give you an undertsanding of where the ventricles come from and how each part of the brain has developed.

= __Embryology of the brain and CSF__ =

The nervous system originates from the neural plate (beginning of 3rd week). Neural folds, lie lateral to the neural plate and as development continues they lengthen and fold in to meet in the midline and fuse, creating a tube called the neural tube. This process is called neurulation. The lumen of the neural tube communicates freely with the amniotic cavity. Fusion of the neural folds is initiated in the cervical region and then spreads in rostral and caudal directions, leaving an opening at either end of the tube called the rostral and caudal neuropores. Somite pairs from the 4th pair rostrally will develop to produce the future brain. The somites from the 4th pair caudally will become the spinal cord. Rostral and caudal neuropores gradually begin to close in the 3rd week, with the rostral neuropore closure occurs two days before the caudal. The rostral neuropore and fusion of the neural folds in the cranial region create expanded regions called brain vesicles that go on to form the:
 * Prosencephalon (forebrain)
 * Mesencephalon (midbrain)
 * Rhombencephalon (hind brain)

During the development of the spinal cord, further growth lateral to the neural tube forms a ventral and dorsal thickening, known as basal plates and alar plates, respectively. The basal plates contain ventral motor cells and gather in the ventral horn of the spinal cord, where as the alar plates found in the dorsal horn of the spinal cord have sensory cells. The sulcus limitans marks the junction between the two neural plates.

In the brain, alar and basal plates are only present in the Mesencephalon (midbrain) and the Rhombencephalon (hind brain). __ **Hindbrain consists of the:** __
 * **Metencephalon** – pons and cerebellum (Pontine flexure separates the two regions of the hind brain)

The Hindbrain consists of two regions, the pons and the cerebellum. The pons is a junction where nerve fibres between the spinal cord and the cerebrum and cerebellum pass and the cerebellum is where coordination and balance is controlled. Similar to the myelencephalon there are three groups of motor nuclei formed from the basal plates:


 * 1)  Somatic efferent - these form the nucleus of the abducens nerve.
 * 2)  Special visceral efferent - contribute to the trigeminal and facial nerves which supply muscles of the first and second pharyngeal arches
 * 3)  General visceral efferent - these innervate the submandicular and sublingual glands.

The alar plates form three groups of sensory nuclei:
 * 1) Somatic afferent - receives sensory information from the trigeminal nerve and part of the vestibulocochlear complex
 * 2) Special visceral afferent
 * 3) General visceral afferent


 * **Myelencephalon** – medulla oblongata

The basal plates develop into group of motor nerve cells (nuclei) and develop into columns. The group of nerve cells formed are as follows:
 * 1)  somatic efferent - This group extends rostrally into the mesencephalon (midbrain) forming the somatic efferent motor column. Motor neurones from this column contribute to the hypogloassal nerve which innervates the muscles of the tongue and the abducens, trochlear and oculmotor nerves which all innervate the muscles of the eye.
 * 2)  special visceral efferent - This group extends into the metencephalon forming the special visceral efferent motor column and neurones from this column supply striated muscles of the pharyngeal acrhes. They also contribute to the accessory, vagus and glossopharyngeal nerves.
 * 3)  general visceral efferent - contains neurones that innervate the muscles of respiratory tract, intestinal tract and the heart. This is involuntary, therefore can not be controlled consciously.

The alar plate, on the other hand, develops into sensory nuclei. The three groups are:


 * 1)  somatic afferent - information about the ear and surface of the head reach this group of nuclei, via the vestibulocochlear and trigeminal nerves.
 * 2)  Special visceral afferent - sensory information from the taste buds of the tongue, from the palate, oropharynx and epiglottis are relayed to this group of nuclei.
 * 3)  General visceral afferent - sensory information from the gastrointetinal tract and the heart is sent to these nuclei.

The ependymal cells of the roof plate of the myelencephalon and the vascular mesenchyme that lines them together form the pia mater. The vascular mesenchyme undergoes a series of divisions, forming processes that project into the ventricular cavity underneath. It is these processes that form the choroid plexus.

** The mesencephalon and the rhombencephalon are separated by the rhombencephalic flexure.** The alar plates of the mesencephalon from anterior and posterior colliculus which are nuclei of the midbrain. The Posterior colliculi receive input from auditory cortex and the anterior colliculi receive input from visual cortex. The colliculi are formed from neuroblasts ( which cells produced by stem cells that go on to develop into neurons or glia).

The basal plates contain two groups of motor nuclei. Somatic efferent- are oculmotor and trochlear nerves; visceral efferent- nucleus of Edinger- Westphal which innervates the spincter pupillary muscle.

__**The forebrain then forms two separate regions**__ : The telencephalon is made up of two outpouchings which later develop into the left and right cerebral hemispheres at the beginning of the 5th week of development. By the second month the base of the hemispheres expand into the lumen of the lateral ventricles ( cavities of the cerebral hemispheres). A connection exists between the two lateral ventricles called the interventricular foramen. The point of attachment of the hemispheres and the roof of the diencephalon is very think. There is a single layer of ependymal cells and vascular mesenchyme that form the choroid plexus of the lateral ventricles. The hippocampus which is involved in olfaction and it is located just above the choroid plexus of the lateral ventricles.
 * **Telencephalon** – which then develops into L and R cerebral hemispheres. (two outpouchings)[[image:edutoolanatomy/Capture_6.PNG width="604" height="261" align="right"]]

The corpus striatum is part of the hemisphere and during cerebral expansion it also expands forming two separate nuclei, called the caudate nucleus and the lentiform nucleus. Axons travelling back and forth through the corpus straitum create this divison.

Expansion of the cerebral hemispheres results in the formation of frontal, temporal, parietal and occitpital lobes. Growth occurs so rapidly causing the convoluted appearance of the cerebral hemispheres (gyri). The roof plate is made up of a single layer of ependymal cells covered by vascular mesenchyme that form the choroid plexus- cells that produce CSF. The lateral walls of the diencephalon are created by the alar plates. A hypothalamic sulcus divides the alar plates, forming a dorsal and ventral region named the thalamus and hypothamalmus, respectively. The cells of the thalamus proliferate and invade the lumen of the diencephalon. The diencephalon also contributes to the pituitary gland. A downward projection from the diencephalon called the infundibulum and an outpouching of the primordial mouth (stomodeum) called Rathke's pouch, together form the pituitary. Rathke's pouch disconnects from the oral cavity by the end of the second month.
 * **Diencephalon**- optic cup and stalk, pituitary, thalamus, hypothalamus, epiphysis

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