Specimen:

SPECIMEN DETAILS:

Organ: Brain-Cerebral cortex – precentral gyrus
Origin: Human
Staining: Cresyl Violet (Nissl staining)

METHOD AND SPECIMEN DESCRIPTION:

Cresyl violet is a basic dye that binds to nucleic acids (RNA and DNA). It stains the chromatin, nucleolus, and rough endoplasmic reticulum (Nissl substance) blue-violet, making neuronal cell bodies and nuclei clearly visible. This staining technique is also known as Nissl staining.

OBJECTIVE OF THE EXAMINATION:

To study the laminar organization of the cerebral cortex, demonstrated in the precentral gyrus (primary motor cortex).

Special Features of the Specimen:

The precentral gyrus represents the primary motor cortex, which is classified as an agranular cortex. This means that layer IV (inner granular layer) is absent or poorly developed. A hallmark of this region is the presence of very large pyramidal neurons (Betz cells) in layer V, which project axons down to the spinal cord via the corticospinal tract.

The six-layered structure typical of the isocortex can be identified as follows:

Layer	Name (Latin)	Description
I	Lamina molecularis (Molecular layer)	Sparsely cellular; mainly neuropil and a few horizontal cells.
II	Lamina granularis externa (Outer granular layer)	Densely packed small pyramidal and stellate cells.
III	Lamina pyramidalis externa (Outer pyramidal layer)	Larger pyramidal cells; thicker than layer II.
IV	Lamina granularis interna (Inner granular layer)	Thin or absent in the motor cortex → hence “agranular cortex”.
V	Lamina pyramidalis interna (Inner pyramidal layer)	Very prominent; contains large Betz cells (giant pyramidal neurons). Divided into:
• Va: smaller pyramidal cells, merges with layer III.
• Vb: contains large Betz cells, easily recognizable.
VI	Lamina multiformis (Multiform layer)	Heterogeneous; small pyramidal and fusiform neurons. Axons project mainly to the thalamus.

TASKS:

• List the six layers of the isocortex and describe their basic cell types.
• Determine how the specimen can be identified as the precentral gyrus.
• Explain the meaning of the term “agranular cortex.”
• State the primary projection target of neurons from lamina multiformis (VI).
• Identify the inner pyramidal layer (V).
• Locate and label the Betz cells.
• Differentiate between layer Va and layer Vb.
• Determine the approximate boundary between layer III and layer V.

License