Development of

Rhombencephalon

or Hindbrain Dr. Muhammad RafiqueAnatomy, DIMC

31/08/2010

Objectives

Review the early development of Brain VesiclesKnow the Basic structures of Neural Tube Know the Nomenclatures Describe the development of Myelencephalon or

Medulla Oblongata Discuss the development of MetencephalonDiscuss the development of Cerebellum Discuss the development of Mesencephalon or

Midbrain

Development of Primary Brain Vesicles The neural tube cranial to the

fourth pair of somites develops into the brain. Fusion of the neural folds in the cranial region and closure of the rostral neuropore form three primary brain vesicles from which the brain develops .

The three primary brain vesicles form:

Forebrain (Prosencephalon) Midbrain (Mesencephalon) Hindbrain (Rhombencephalon)

Secondary Brain Vesicles During the fifth week, the

forebrain partly divides into two secondary brain vesicles

Telencephalon DiencephalonMidbrain does not divideHindbrain partly divides intoMetencephalon or cerebellum &

pons Myelencephalon or Medulla There are five secondary brain

vesicles.

Terminology Forebrain or ProsencephalonAgain subdivided into 2 regions Cerebral Hemisphere orTelencephalon Or DiencephalonMidbrain or Mesencephalon Hindbrain or

Rhombencephalon Again divided into 2 RegionsMetencephalon or cerebellum

& pons Myelencephalon of Medulla

Oblongata

Brain Flexures During the fourth week, the

embryonic brain grows rapidly and bends ventrally with the head fold. This produces the midbrain flexure in the midbrain region and the cervical flexure at the junction of the hindbrain and spinal cord. Later, unequal growth of the brain between these flexures produces the pontine flexure in the opposite direction. This flexure results in thinning of the roof of the hindbrain.

Basic Structures of Neural tube Initially, the primordial brain has the

same basic structure as the developing spinal cord; however, the brain flexures produce considerable variation in the outline of transverse sections at different levels of the brain and in the position of the gray and white matter (substance). The sulcus limitans extends cranially to the junction of the midbrain and forebrain, and the alar and basal plates are recognizable only in the midbrain and hindbrain

Basic Structures of Neural TubeWith the closure of neuropores,

the neuro-epithelium with in the neural tube proliferate rapidly and differentiate into a number of cells these are called as neuroblast. Now the neural tube consists of three zones with central cavity:

Ventricular zone surrounding central cavity

Intermediate zone or Mental Layer

Outer Zone or Marginal Layer

Ventricular Zone of Neural Tube The ventricular zone

surrounds the cavities of brain which are lined by the ependymal cells and responsible for the formation of cerebrospinal fluid.

Intermediate Zone of Neural Tube Also called as the Mantel

Layer, characterized by collection of neuroepithelial cells, which differentiated to form the Neurons. The cell bodies of neuron located within the marginal layer that’s why this layer appears as the gray on cross section and known as Gray Matter.

Outer Zone of Neural Tube Also known as the

Marginal Layer. The nerve fibers arising from the neuron in the mental layer they are passing through the marginal layer, these fibers are myelinated that’s why its appears white so called as White matter

Basal, Alar, Roof, and Floor Plates As a result of continuous

addition of neuroblasts to the mantle layer, each side of the neural tube shows a ventral and a dorsal thickening. The ventral thickenings, the Basal plates, which contain ventral motor horn cells, form the motor areas of the spinal cord;

Alar Plate Dorsal thickenings,

the Alar plates, form the sensory areas. A longitudinal groove, the sulcus limitans, marks the boundary between the two.

Roof & Floor Plates The dorsal and ventral

midline portions of the neural tube, known as the roof and floor plates, respectively, do not contain neuroblasts; they serve primarily as pathways for nerve fibers crossing from one side to the other.

Primary Structure of Neural Plate

Myelencephalon or Medulla Oblongata The myelencephalon is a

brain vesicle that gives rise to the medulla oblongata. It differs from the spinal cord in that its lateral walls are everted. Alar and basal plates separated by the sulcus limitans can be clearly distinguished.

Basal Plate of Medulla oblongata The basal plate, similar to

that of the spinal cord, contains motor nuclei. These nuclei are divided into three groups:

(a)A medial somatic efferent group,

(b)An intermediate special visceral efferent group

(c) A lateral general visceral efferent group.

Development of Motor Neurons of Medulla The first group is called

the somatic efferent motor column. In the myelencephalon, it includes:

Neurons of the hypoglossal nerve

In the metencephalon and the mesencephalon, the column contains neurons of abducens, trochlear, and oculomotor nerves respectively.

Motor Nuclei of Medulla The special visceral efferent

group extends into the metencephalon, forming the special visceral efferent motor column. Its motor neurons supply striated muscles of the pharyngeal arches. In the myelencephalon the column is represented by neurons of the accessory, vagus, and glossopharyngeal nerves.

Fourth Ventricle The roof plate of the

myelencephalon consists of a single layer of ependymal cells covered by vascular mesenchyme, the pia mater. The two combined are known as the tela choroidea.

Metencephalon

The metencephalon, similar to the myelencephalon, is characterized by basal and alar plates. Two new components form:

(a) The cerebellum(b) The pons

Basal Plate of Pons Each basal plate of the

metencephalon contains three groups of motor neurons

(a) the medial somatic efferent group, which gives rise to the nucleus of the abducens nerve

(b) the special visceral efferent group, containing nuclei of the trigeminal nerve

(c) the general visceral efferent group, whose axons supply the submandibular and sublingual glands.

Sensory Nuclei of Pons The alar plates of the

metencephalon contain three groups of sensory nuclei

(a) a lateral somatic afferent group, which contains neurons of the trigeminal nerve and a small portion of the vestibulocochlear complex

(b) the special visceral afferent group, and

(c) the general visceral afferent group.

Development of Cerebellum

The dorsolateral parts of the alar plates bend medially and form the rhombic lips. In the caudal portion of the metencephalon, the rhombic lips are widely separated, but immediately below the mesencephalon they approach each other in the midline.

Development of CerebellumAs a result of a further

deepening of the pontine flexure, the rhombic lips compress cephalocaudally and form the cerebellar plate.

In a 12-week embryo, this plate shows a small midline portion, the vermis, and two lateral portions, the hemispheres.

Development of Mesencephalon or Midbrain In the mesencephalon, each

basal plate contains two groups of motor nuclei:

(a) A medial somatic efferent group, represented by the oculomotor and trochlear nerves

(b) A small general visceral efferent group, represented by the nucleus of Edinger-Westphal

Development of Mesencephalon or

Midbrain

The marginal layer of each basal plate enlarges and forms the crus cerebri.

The alar plates of the mesencephalon appear as two longitudinal elevations separated by a shallow midline depression. With further development, a transverse groove divides each elevation into an superior and a posterior inferior colliculus. The colliculi are formed by waves of neuroblasts migrating into the overlying marginal zone.