UNSW Embryology

Neural System Development- Stage 22

© Dr Mark Hill (2008)

Acknowledgements

Stage 22 Neural 3D Model

The individual serial slices have also been incorporated into a 3D model of this embryo Neural System.

Serial Section Descriptions

Al-4 In the human embryo identify the large telencephalic vesicles and the choroid plexus. The cavity in these (the lateral ventricles) communicate with the ventricle of the diencephalon (3rd ventricle) through the interventricular foramen

A3: inside the lateral ventricle

Al-2: Distinguish the wall of the forebrain.

A3-4: The basal part of the telencephalon forms the basal ganglia, a solid mass. Posteromedially these basal ganglia are in contact with the diencephalon. The large masses in either side of the diencephalon form the thalami.

Al-7, B1: The diencephalon. In these levels the brain comes into section twice, because of the cephalic flexure, but in A2-4 the two parts are connected.

Al-5: Ventral to the thalamus you can identify the hypothalamus.

A6-7, B1: The hypothalamus

A6: the lamina terminalis, original anterior end of the neural tube. Notice the position of the optic nerves in relation to the hypothalamic part of the diencephalon (B1).

B2-3: Further caudally the pituitary gland is seen, the neural part of which is a derivative of the diencephalon.

Al-7: The midbrain (mesencephalon). Its dorsal part is the mesencephalic vesicle, a large, thin-walled structure (A2-6) which overlaps the initial part of the metencephalon (A6-7).( The ventral part of the mesencephalon is seen to be continuous with the diencephalon in A3-4. The tectal recess, an extension of the roof the mesencephalic vesicle is seen in A6.

A6: The junction between the mesencephalon and metencephalon is called the isthmus. The main metencephalic derivatives are the pons and cerebellum.

The ventricular lumen of the hind brain is the 4th ventricle (A7, B1-7, C1-3). The roof of the metencephalic part of the 4th ventricle is formed by the developing cerebellum (B2-3), beyond which the ventricle forms two large lateral recesses (B2-3). In more caudal sections the roof of the ventricle is seen as a thin membrane only, bearing choroid plexus.

B2-3: the root of the trigeminal nerve and its ganglion.

The myelencephalic part of the hind brain will form the medulla oblongata, the embryonic appearance of which is hardly different from the fully developed structure. The vagus nerve and its ganglion are seen to leave the base of the brain in B7, through the jugular foramen.

At all levels where the brain is present notice the meninges enveloping it and creating the large subarachnoid space which is filled with cerebrospinal fluid.

Study the histological appearance of the spinal cord. Note the alar and basal laminae, the dorsal root ganglia and the sympathetic trunk.

Stage 22 Embryo (Human)

A1	A2	A3	A4	A5	A6	A7
B1	B2	B3	B4	B5	B6	B7
C1	C2	C3	C4	C5	C6	C7
D1	D2	D3	D4	D5	D6	D7
E1	E2	E3	E4	E5	E6	E7
F1	F2	F3	F4	F5	F6	F7
G1	G2	G3	G4	G5	G6	G7

Terms

• 3rd ventricle- ventricular cavity within the diencephalon.
• 4th ventricle- ventricular cavity within the rhombencephalon.
• accessory nerve-
• adenohypophysis- anterior pituitary= 3 parts pars distalis, pars intermedia, pars tuberalis
• alar plate- afferent, dorsal horns
• anlage- (Ger. ) primordium, structure or cells which will form a future structure.
• arachnoid- (G.) spider web-like
• basal ganglia-
• basal plate- efferent, ventral horns
• brachial plexus- mixed spinal nerves innervating the upper limb form a complex meshwork (crossing).
• brain- general term for the central nervous system formed from 3 primary vesicles.
• buccopharyngeal membrane- (=oral membrane) at cranial (mouth) end of gastrointestinal tract (GIT) where surface ectoderm and GIT endoderm meet. (see also cloacal membrane)
• cauda equina- (=horse's tail) caudal extension of the mature spinal cord.
• central canal- lumen, cavity of neural tube within the spinal cord. Space is continuous with ventricular system of the brain.
• cerebral aqueduct- ventricular cavity within the mesencephalon.
• cervical flexure- most caudal brain flexure (of 3) between spinal cord and rhompencephalon.
  • ( sc-^V^ )
• choroid plexus- specialized vascular plexus responsible for secreting ventricular fluid that with further additions becomes cerebrospinal fluid (CSF).
• cloacal membrane- at caudal (anal) end of gastrointestinal tract (GIT) where surface ectoderm and GIT endoderm meet forms the openings for GIT, urinary, reproductive tracts. (see also buccopharyngeal membrane)
• cortex-
• cortical plate- outer neural tube region which post-mitotic neuroblasts migrate too along radial glia to form adult cortical layers.
• cranial flexure- (=midbrain flexure) most cranial brain flexure (of 3) between mesencephalon and prosencephalon.
  • ( sc-^V^ )
• diencephalon- the caudal portion of forebrain after it divides into 2 parts in the 5 secondary vesicle brain (week 5). (cavity- 3rd ventricle) Forms the thalmus and other nuclei in the adult brain.
  • (sc-My-Met-Mes-Di-Tel)
• dorsal root ganglia- (=spinal ganglia) sensory ganglia derived from the neural crest lying laterally paired and dorsally to the spinal cord (in the embryo found ventral to the spinal cord). Connects centrally with the dorsal horn of the spinal cord.
• dura mater-
• ectoderm- the germ layer which form the nervous system from the neural tube and neural crest.
• ependyma- epithelia of remnant cells after neurons and glia have been generated and left the ventricular zone
• floorplate- early forming thin region of neural tube closest to the notochord.
• ganglia- (pl. of ganglion) specialized neural cluster.
• glia- supporting, non-neuronal cells of the nervous system. Generated from neuroepithelial stem cells in ventricular zone of neural tube. Form astrocytes, oligodendrocytes.
• glossopharyngeal ganglion-
• grey matter- neural regions containing cell bodies (somas) of neurons. In the brain it is the outer layer, in the spinal cord it is inner layer. (see white matter)
• growth factor- usually a protein or peptide that will bind a cell membrane receptor and then activates an intracellular signaling pathway. The function of the pathway will be to alter the cell directly or indirectly by changing gene expression. (eg shh)
• hox- (=homeobox) family of transcription factors that bind DNA and activate gene expression. Expression of different Hox genes along neural tube defines rostral-caudal axis and segmental levels.
• intervertebral foramina-
• isthmus- (G. narrow passage)
• lamina terminalis- anterior region of brain where cranial neuropore closes.
• lumbar plexus- mixed spinal nerves innervating the lower limb form a complex meshwork (crossing).
• mantle layer- layer of cells generated by first neuroblasts migrating from the ventricular zone of the neural tube. Layers are rearranged during development of the brain and spinal cord.
  • (Ven-Man-Mar-CP)
• marginal zone- layer of processes from neuroblasts in mantle layer.
  • (Ven-Man-Mar-CP)
• mater- (L. mother)
• meninges- mesenchyme surrounding neural tube forms 3 layer (Dura-, pia-, arachnoid- mater) connective tissue sheath of nervous system.
  • (D-P-A-cns)
• mesencephalon- (=midbrain), the middle portion of the 3 primary vesicle brain (week 4).
  • (sc-R-M-P)
• metencephalon- the cranial portion of hindbrain after it divides into 2 parts in the 5 secondary vesicle brain (week 5). Forms the pons and cerebellum in the adult brain.
  • (sc-My-Met-Mes-Di-Tel)
• myelencephalon- the caudal portion of hindbrain after it divides into 2 parts in the 5 secondary vesicle brain (week 5). Forms the medulla in the adult brain.
  • (sc-My-Met-Mes-Di-Tel)
• neural tube- neural plate region of ectoderm pinched off to form hollow ectodermal tube above notochord in mesoderm.
• neural tube defect- (NTD) any developmental abnormality that affects neural tube development. Commonly failure of neural tube closure.
• neuroblast- undifferentiated neuron found in ventricular layer of neural tube.
• neurohypophysis- (=posterior pituitary=pas nervosa)
• neuron- The cellur "unit" of the nervous system, transmitting signals between neurons and other cells. The post-mitotic cells generated from neuroepithelial stem cells (neuroblasts) in ventricular zone of neural tube.
• neuropore- opening at either end of neural tube: cranial=rostral=anterior, caudal=posterior. The cranial neuropore closes (day 25) approx. 2 days (human) before caudal.
• notochord- rod of cells lying in mesoderm layer ventral to the neural tube, induces neural tube and secretes sonic hedgehog which "ventralizes" the neural tube.
• olfactory bulb- (=cranial nerve I, CN I) bipolar neurons from nasal epithelium project axons through cribiform palate into olfactory bulb of the brain.
• optic cup-
• optic nerve- (=cranial nerve II, CN II) retinal ganglion neurons project from the retina as a tract into the brain (at the level of the diencephalon).
• otocyst- (=otic vesicle) sensory placode which sinks into mesoderm to form spherical vesicle (stage 13/14 embryo) that will form components of the inner ear.
• pars- (L. part of)
• pharyngeal arches- (=branchial arches, Gk. gill) form structures of the head. Six arches form but only 4 form any structures. Each arch has a pouch, membrane and cleft.
• pharynx- uppermost end of GIT, beginning at the buccopharyngeal membrane and at the level of the pharyngeal arches.
• pia mater-
• placode- specialized regions of ectoderm which form components of the sensory apparatus.
• pontine flexure- middle brain flexure (of 3) between cervical and cranial flexure in opposite direction, also generates thin roof of rhombencephalon and divides it into myelencephalon and metencephalon. ( sc-^V^ )
• prosencephalon- (=forebrain), the most cranial portion of the 3 primary vesicle brain (week 4).
  • (sc-R-M-P)
• Rathke's pouch- a portion of the roof of the pharynx pushes upward towards the floor of the brain forming the anterior pituirary (adenohypophysis, pars distalis, pars tuberalis pars intermedia). Where it meets a portion of the brain pushing downward forming the posterior pituitary (neurohypophysis, pars nervosa). Rathke's pouch eventually looses its connection with the pharynx.
  • [Martin Heinrich Rathke 1973-1860, embryologist and anatomist]
• rhombencephalon- (=hindbrain), the most caudal portion of the 3 primary vesicle brain (week 4).
  • (sc-R-M-P)
• roofplate- early forming thin region of neural tube closest to the overlying ectoderm.
• spinal cord- caudal end of neural tube that does not contribute to brain. Note: the process of secondary neuralation contributes the caudal end of the spinal cord.
• spinal ganglia- (=dorsal root ganglia, drg) sensory ganglia derived from the neural crest lying laterally paired and dorsally to the spinal cord (in the embryo found ventral to the spinal cord). Connects centrally with the dorsal horn of the spinal cord.
• spinal nerve- mixed nerve (motor and sensory) arising as latera pairs at each vertebral segmental level.
• sonic hedgehog- (=shh) secreted growth factor that binds patched (ptc) receptor on cell membrane. SHH function is different for different tissues in the embryo. In the nervous system, it is secreted by the notochord, ventralizes the neural tube, inducing the floor plate and motor neurons.
• sulcus- (L. furrow) groove
• sulcus limitans- longitudinal lateral groove in neural tube approx. midway between roofplate and floorplate. Groove divides alar (dorsal) and basal (ventral) plate regions.
• sympathetic ganglia-
• telencephalon- the cranial portion of forebrain after it divides into 2 parts in the 5 secondary vesicle brain (week 5). (cavity- lateral ventricles and some of 3rd ventricle) Forms the cerebral hemispheres in the adult brain.
  • (sc-My-Met-Mes-Di-Tel)
• thalamus- (G. thalamos= bedchamber) cns nucleus, lateral to 3rd ventricle, paired (pl thalami).
• transcription factor- a factor (protein or protein with steroid) that binds to DNA to alter gene expression, usually to activate. (eg steroid hormone+receptor, Retinoic acid+Receptor, Hox, Pax, Lim, Nkx-2.2)
• trigeminal ganglion- (=cranial nerve V, CN V) first arch ganglion, very large and has 3 portions.
• vagal ganglion- (=cranial nerve X, CN X) fourth and sixth arch ganglion, innervates the viscera and heart.
• ventricles- the fluid-filled interconnected cavity system with the brain. Fluid (cerebrospinal fluid, CSF) is generated by the specialized vascular network, the choroid plexus. The ventricles are directly connected to the spinal canal (within the spinal cord).
• ventricular zone- Neuroepithelial cell layer of neural tube closest to lumen. Neuroepithelial cells generate neurons, glia and ependymal cells.
  • (Ven-Man-Mar-CP)
• vestibulocochlear nerve- (=cranial nerve VIII, CN VIII, also called statoacoustic)
• white matter- neural regions containing processes (axons) of neurons. In the brain it is the inner layer, in the spinal cord it is outer layer. (see grey matter)

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