Histology of Brain Stem, Cerebrum and Cerebellum

Brain stem

• Brain stem is structurally continuous with the spinal cord.

• It consists of;

– medulla oblangata

– pons

– mesencephalon

• The regions of gray and white matter are not clearly separated

• The nuclei of the cranial nerves appear as islands surrounded by white matter

Meninges • Connective tissue coverings of the brain and spinal cord

Dura Mater

• Dense, collagenous connective tissue

• Dura of the brain is composed of two layers;

– periosteal dura mater (outer); • attached to the inner surface of the skull

• serves as the periosteum

• osteoprogenitor cells, fibroblasts, collagen bundles, blood vessels

– meningeal dura mater (inner); • fibroblasts, fine collagen fibers, small blood vessels

• Dura of the spinal cord

– forms a continuous tube around the spinal cord

– does not adhere to the walls of the vertebral canal

– pierced by the spinal nerves

– epidural space; • between the dura mater and the periosteum of the vertebral canal

• filled with epidural fat and a venous plexus.

Arachnoid Mater

• Connective tissue without blood vessels, blood vessels course through it

• Composed of two regions: 1) sheet-like membrane in contact with dura 2) arachnoid trabeculae

loosely arranged cells (modified fibroblasts) with collagen fibers

contact the underlying pia

• Subarachnoid space; – cavities between the trabeculae – filled with cerebrospinal fluid (CSF)

• Arachnoid villi; – regions where arachnoid perforates the dura

for the passage of CSF into the dural venous sinuses

• Clinical significance: Subdural space is a “potential space”, it appears only after subdural hemorrhage, when blood forces two layers apart

Pia Mater

• Innermost layer of the meninges

• Follows all the surface irregularities of CNS

• Composed of a thin layer of flattened, modified fibroblasts and abundant blood vessels surrounded by fine connective tissue fibers

• Completely separated from the underlying neural tissue by basement membrane and neuroglial processes

Cerebrospinal Fluid

• Bathes, nourishes and protects the brain and spinal cord

• Produced by the choroid plexus

• %90 water, low protein and low density, rich in ions, few desquamated cells, 2-5 lymphocytes/ ml

• Reabsorbed through the thin cells of the arachnoid villi into the superior sagittal venous sinus

• Clinical correlations; – Hydrochephalus; reason is a decrease in absorption of the

fluid or a blockage within the ventricles which increases intracranial pressure.

– Congenital hydrocephalus; leads to enlargement of the head, followed by impairment of mental and muscular functions and death if left untreated.

Choroid Plexus • Fenestrated capillaries covered by the modified ependymal cells • Modified ependymal cells

– tight junction – infolding of the basal plasma membrane, – microvilli – mitochondrion

• Extend into the ventricles of the brain • Produces CSF

Ventricles • Cavities in the brain (4th, 3rd, lateral)

• Contain CSF

• Ependymal cells

• Tanycytes;

– special ependymal cells located in the floor of the 3rd ventricle.

– processes extend deep into the hypothalamus.

– Transfer chemical signals from CSF to CNS.

Cerebrum

• Cortex – gray matter (peripheral)

• Medulla – white matter

I. Molecular layer; parallel nerve fibers, horizontal cells of Cajal, neuroglia

II. External granular layer; small pyramidal cells, granule (stellate) cells, neuroglia

III. External pyramidal layer; large pyramidal cells, neuroglia

IV. Internal granular layer; thin layer composed of closely arranged small granule (stellate) cells, neuroglia.

V. Internal pyramidal layer; largest pyramidal cells (Betz cells), neuroglia. This layer has the lowest cell density of the cerebral cortex

VI. Multiform layer; cells of various shapes (Martinotti cells, fusiform cells, pyramidal etc.), neuroglia

Layers of the Cerebral Cortex

• Isocortex (neocortex) is the outer layer of the cerebral hemispheres made up of typical six layers

• Anisocortex (archicortex) (e.g. hippocampus)

Hippocampus (sea horse)

• hippocampus, dentate gyrus, temporal lobe gyrus

• polymorphic layer; nerve fibers, small cell bodies of interneurons • pyramidal cell layer; pyramidal cells • molecular layer; dendrites of the pyramidal cells

Dentate gyrus • polymorphic layer; nerve fibers, interneurons • granule cell layer; granule cells • molecular layer; dendrites of the granule cells

Hilus • region where the head of hippocampus join the dentate gyrus • contains multipolar neurons

Cerebellum • Outer gray matter “cortex”

– Molecular layer- Purkinje cell layer – Granular layer

• Inner white matter “medulla” • Surrounded by piamater

• lies directly below the pia mater

• contains – superficially located stellate cells

– basket cells

– Purkinje cells and their dendrites

– unmyelinated axons from the granular layer (parallel fibers)

• Purkinje cells – large pear-shaped cells

– arborized dendrites projecting into the molecular layer

– myelinated axons project into the white matter

– only cell of the cerebellar cortex that sends information (always inhibitory) to the outside

Molecular layer

• Contains – small granule cells – Golgi type II cells – glomeruli; synaptic regions between axons entering the cerebellum and the

granule cells.

• Axons of the granule cells extend to the molecular layer and synapse with the dendrites of Purkinje cells and basket cells (parallel fibers).

• Parallel fibers extend parallel to the longitudinal axis of the folium in the molecular layer.

Granular layer

Barriers in the CNS

• Glia limitans externa- interna

• Blood- brain barrier

• Blood- CSF barrier

Blood-Brain Barrier

• This barrier prevents the free passage of selective blood-borne substances into the neural tissue.

• O2, CO2, water, small lipid soluble materials can penetrate the barrier. • Transfer of glucose, amino acids, vitamins, nucleosides and ions require

transmembrane proteins.

1) endothelial cells lining the continuous capillaries linked by tight junctions 2) basal lamina 3) end-feet of astrocytes

References

1. Histology: A Text and Atlas by Michael H. Ross, Wojciech Pawlina (2010). 6th ed. Lippincott Williams & Wilkins, Philadelphia. ISBN: 978-0-7817-7200-6

2. Basic Histology: Text & Atlas by Luiz Junqueira, Jose Carneiro (2005). 11th ed. McGraw-Hill, New York. ISBN: 0-07-111888-8

3. Color Textbook of Histology by Leslie P. Gartner, James L. Hiatt (2001). 2nd ed. W.B. Saunders Company, Philadelphia. ISBN: 0-7216-8806-3

4. Histology and Cell Biology: An Introduction to Pathology by Abraham L Kierszenbaum, Laura Tres (2011). 3rd ed. Elsevier Saunders, Philadelphia. ISBN: 978-0-323-07842-9

5. Netter’s Essential Histology by William K. Ovalle, Patrick C. Nahirney (2007). 1st ed. Elsevier Saunders, Philadelphia. ISBN: 978-1-929007-86-8