Human_Histology

gives this region its whitish appearance. Neuroglia and blood vessels are present in both grey and white matter.

The arrangement of the grey and white matter differs at different situations in the brain and spinal cord. In the spinal cord and brainstem the white matter is on the outside whereas the grey matter forms one or more masses embedded within the white matter. In the cerebrum and cerebellum there is an extensive, but thin, layer of grey matter on the surface. This layer is called the cortex. Deep to the cortex there is white matter, but within the latter several isolated masses of grey matter are present. Such isolated masses of grey matter present anywhere in the central nervous system are referred to as nuclei. As grey matter is made of cell bodies of neurons (and the processes arising from or terminating on them) nuclei can be defined as groups of cell bodies of neurons. The axons arising in one mass of grey matter very frequently terminate by synapsing with neurons in other masses of grey matter. The axons connecting two (or more) masses of grey matter are frequently numerous enough to form recognizable bundles. Such aggregations of fibers are called tracts. Larger collections of fibers are also referred to as funiculi, fasciculi, or lemnisci. Large bundles of fibers connecting the cerebral or cerebellar hemispheres to the brainstem are called peduncles.

Aggregations of processes of neurons outside the central nervous system constitute peripheral nerves.

THE SPINAL CORD

The spinal cord is the most important content of the vertebral canal. The upper end of the spinal cord becomes

continuous with the medulla oblongata. The lowest part of the spinal cord is conical and is called the conus medullaris. The conus is continuous, below, with a fibrous cord called the filum terminale (modification of pia mater).

When seen in transverse section the grey matter of the spinal cord forms an H-shaped mass (Fig. 10.15 and Plate 10.4). In each half of the cord the grey matter is divisible into a larger ventral mass, the anterior (or ventral) grey column, and a narrow elongated posterior (or dorsal) grey column. In some parts of the spinal cord a small lateral projection of grey matter is seen between the ventral and dorsal grey columns. This is the lateral grey column. The grey matter of the right and left halves of the spinal cord is connected across the middle line by the grey commissure that is traversed by the central canal. The central canal of the spinal cord contains cerebrospinal fluid. The canal is lined by ependyma.

The cell bodies of neurons differ in size and in prominence of Nissl substance in different regions of spinal grey matter. They are most prominent in the anterior grey column.

The white matter of the spinal cord is divided into right and left halves, in front by a deep anterior median fissure, and behind by the posterior median septum. In each half of the cord the white matter medial to the dorsal grey column forms the posterior funiculus (or posterior white column). The white matter medial and ventral to the anterior grey column forms the anterior funiculus (or anterior white column), while the white matter lateral to the anterior and posterior grey columns forms the lateral

funiculus (The anterior and lateral funiculi are collectively referred to as the anterolateral funiculus).

The white matter of the right and left halves of the spinal cord is continuous across the middle line through the ventral white commissure which lies anterior to the grey commissure. The white matter contains tracts (ascending or descending) that connect grey matter at different levels of the spinal cord. Some tracts ascend into (or descend from) the brainstem, the cerebellum or the cerebral cortex. Details of such tracts are given in books of neuroanatomy.

THE CEREBELLAR CORTEX

General Features

The cerebellum lies in the posterior cranial fossa. Like the cerebrum, the cerebellum has a superficial layer of grey matter, the cerebellar cortex. Because of the presence of numerous fissures, the cerebellar cortex is extensive.

Grey Matter of the Cerebellum

Most of the grey matter of the cerebellum is arranged as a thin layer covering the central core of white matter. This layer is the cerebellar cortex. The subdivisions of the cerebellar cortex correspond to the subdivisions of the cerebellum.

Embedded within the central core of white matter there are masses of grey matter that constitute the cerebellar nuclei. These are (Fig. 10.16).

The dentate nucleus lies in the center of each cerebellar hemisphere. It is made up of a thin lamina of grey matter that is folded upon itself so that it resembles a crumpled purse.

The emboliform nucleus lies on the medial side of the dentate nucleus.

The globose nucleus lies medial to the emboliform nucleus.

The fastigial nucleus lies close to the middle line in the anterior part of the superior vermis.

White Matter of the Cerebellum

The central core of each cerebellar hemisphere is formed by white matter. The peduncles are continued into this white matter. The white matter of the two sides is connected by a thin lamina of fibers that are closely related to the fourth ventricle. The upper part of this lamina forms the superior medullary velum, and its inferior part forms the inferior medullary velum. Both these take part in forming the roof of the fourth ventricle.

Structure of the Cerebellar Cortex

In striking contrast to the cortex of the cerebral hemispheres, the cerebellar cortex has a uniform structure in all parts of the cerebellum. It may be divided into three layers (Fig. 10.17 and Plate 10.5) as follows.

Molecular layer (most superficial)

Purkinje cell layer

Granular layer, which rests on white matter.

The neurons of the cerebellar cortex are of five main types:

Purkinje cells, forming the layer named after them

Granule cells, forming the granular layer

Outer (external) stellate cells, lying in molecular layer

Basket cells, lying in the molecular layer

Golgi cells, present in the granular layer.

Molecular Layer

The molecular layer is the superficial layer of the cortex and situated just below the pia mater. It stains lightly with hematoxylin eosin and is featureless as it consists of few cells and more of myelinated and unmyelinated fibers.

Two types of cells are found in this layer:

Stellate cells—situated in the superficial part of the molecular layer

Basket cells—situated in the deeper layer.

Outer Stellate Cells

These cells and their processes are confined to the molecular layer of the cerebellar cortex. Their dendrites (which are few) synapse with parallel fibers (of granule cells) while their axons synapse with dendrites of Purkinje cells (near their origin).

Basket Cells

These cells lie in the deeper part of the molecular layer of the cerebellar cortex. Their dendrites (which are few) ramify in the molecular layer and are intersected by parallel fibers with which they synapse. They also receive recurrent collaterals from Purkinje cells, climbing fibers, and mossy fibers. The

axons of these cells branch and form networks (or baskets) around the cell bodies of Purkinje cells. Their terminations synapse with Purkinje cells at the junction of the cell body and axon (preaxon).

Purkinje Cell Layer

The Purkinje cell layer contains flask-shaped cell bodies of Purkinje cells. This layer is unusual in that it contains only one layer of neurons. The Purkinje cells are evenly spaced. A dendrite arises from the “neck” of the “flask” and passes “upwards” into the molecular layer. Here, it divides and subdivides to form an elaborate dendritic tree. The branches of this “tree” all lie in one plane.

122 Textbook of Human Histology

Fig. 10.18:%

The axon of each Purkinje cell passes “downwards” through the granular layer to enter the white matter. As described later, these axons constitute the only efferents of the cerebellar cortex. They end predominantly by synapsing with neurons in cerebellar nuclei. They are inhibitory to these neurons.

Granular Layer

It is the innermost layer and consists of numerous granule cells and a few golgi cells and brush cells. The granular layer stains deeply with hematoxylin because it is densely packed with granular cell.

Granule Cells

These are very small, numerous, spherical neurons that occupy the greater part of the granular layer. The spaces not occupied by them are called cerebellar islands. These islands are occupied by special synaptic structures called glomeruli (Fig. 10.18).

Each granule cell gives off three to five short dendrites. These end in claw-like endings, which enter the glomeruli where they synapse with the terminals of mossy fibers (see below). The granule cells receive impulses from afferent mossy fibers. The dendrites of granule cells and axons of golgi cells synapse with terminals of mossy fibers to form lightly stained areas called glomeruli.

Golgi Neurons

These are large, stellate cells lying in the granular layer, just deep to the Purkinje cells. They are GABAergic inhibitory neurons. Their dendrites enter the molecular layer, where they branch profusely, and synapse with the parallel fibers.

Afferent Fibers Entering the Cerebellar Cortex

The afferent fibers to the cerebellar cortex are of two different types:

Mossy fibers

Climbing fibers.

Mossy Fibers

All fibers entering the cerebellum, other than olivocerebellar, end as mossy fibers. Mossy fibers originate from the vestibular nuclei (vestibulocerebellar), pontine nuclei (pontocerebellar), and spinal cord (spinocerebellar) and terminate in the granular layer of the cortex within glomeruli. Before terminating, they branch profusely within the granular layer, each branch ends in an expanded terminal called a rosette (Fig. 10.18).

Afferent inputs through mossy fibers pass through granule cells to reach the Purkinje cells.

Climbing Fibers

These fibers represent terminations of axons reaching the cerebellum from the inferior olivary complex (Fig. 10.17). They pass through the granular layer and the Purkinje cell layer to reach the molecular layer. Each climbing fiber exerts specific influence on one Purkinje cell.

Note: Both mossy and climbing fibers are excitatory.

Efferent Fibers

The efferent fibers from the cerebellar cortex are axons of Purkinje cells, which terminate in the cerebellar (central) nuclei (Fig. 10.17). Axons of the Purkinje cells are inhibitory to cerebellar nuclei.

The fibers from dentate, emboliform, and globose nuclei leave cerebellum through the superior cerebellar peduncle. The fibers from the fastigial nucleus leave the cerebellum through inferior cerebellar peduncle.

THE CEREBRAL CORTEX

General Features

The surface of the cerebral hemisphere is covered by a thin layer of grey matter called the cerebral cortex.

Like other masses of grey matter the cerebral cortex contains the cell bodies of an innumerable number of neurons along with their processes, neuroglia and blood vessels. The neurons are of various sizes and shapes. They establish extremely intricate connections with each other and with axons reaching the cortex from other masses of grey matter.

Neurons in the Cerebral Cortex

Cortical neurons vary in size, in the shape of their cell bodies, and in the lengths, branching patterns and orientation of their processes. Some of these are described below (Fig. 10.19 and Table 10.4).

Fig. 10.19:

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Table 10.4: Nerve cells in the cerebral cortex

The most abundant type of cortical neurons are the pyramidal cells. About two thirds of all cortical neurons are pyramidal. Their cell bodies are triangular, with the apex generally directed toward the surface of the cortex. A large dendrite arises from the apex. Other dendrites arise from basal angles. The axon arises from the base of the pyramid. The processes of pyramidal cells extend vertically through the entire thickness of cortex and establish numerous synapses.

The stellate neurons are relatively small and multipolar. They form about one-third of the total neuronal population of the cortex. Under low magnifications (and in preparations in which their processes are not demonstrated) these neurons look like granules. They have, therefore, been termed granular neurons by earlier workers. Their axons are short and end within the cortex. Their processes extend chiefly in a vertical direction within the cortex, but in some cases they may be oriented horizontally.

In addition to the stellate and pyramidal neurons, the cortex contains numerous other cell types like fusiform cells, horizontal cells, and cells of Martinoti.

Fig. 10.20:1

Laminae of Cerebral Cortex

On the basis of light microscopic preparations stained by methods in which the cell bodies are displayed (e.g. Nissl method) and those where myelinated fibers are stained (e.g. Weigert method), the cerebral cortex is described as having six layers or laminae (Fig. 10.20 and Plate 10.6).

From the superficial surface downwards these laminae are as follows.

Plexiform or molecular layer

External granular layer

Pyramidal cell layer

Internal granular layer

Ganglionic layer

Multiform layer

The plexiform layer is made up predominantly of fibers

although a few cells are present. All the remaining layers contain both stellate and pyramidal neurons as well as other types of neurons. The external and internal granular layers are made up predominantly of stellate (granular) cells. The predominant neurons in the pyramidal layer and in the ganglionic layer are pyramidal. The largest pyramidal cells (giant pyramidal cells of Betz) are found in the ganglionic layer. The multiform layer contains cells of various sizes and shapes.

In addition to the cell bodies of neurons the cortex contains abundant nerve fibers. Many of these are vertically oriented. In addition to the vertical fibers the cortex contains transversely running fibers that form

prominent aggregations in certain situations. One such aggregation, present in the internal granular layer is called the external band of Baillarger. Another, present in the ganglionic layer is called the internal band of Baillarger.

SKIN

The skin forms the external covering of the body. It is the largest organ constituting 15–20% of total body mass.

TYPES OF SKIN

There are two types of skin.

Thin or hairy skin: In this type of skin, epidermis is very thin. It contains hair and is found in all others parts of body except palms and soles (Plate 11.1).

Thick or glabrous skin: In this type of skin, epidermis is very thick with a thick layer of stratum corneum. It is found in palms of hands and soles of feet and has no hair (Plate 11.2).

STRUCTURE OF SKIN

The skin consists of two layers

A superficial layer the epidermis, made up of stratified squamous epithelium

A deeper layer, the dermis, made up of connective

tissue (Fig. 11.1).

The dermis rests on subcutaneous tissue (subcutis). This is sometimes described as a third layer of skin.

In sections through the skin the line of junction of the two layers is not straight, but is markedly wavy because of the presence of numerous finger-like projections of dermis upwards into the epidermis. These projections are called dermal papillae. The downward projections of the epidermis (in the intervals between the dermal papillae) are sometimes called epidermal papillae (Fig. 11.2).

Note: The surface of the epidermis is also often marked by elevations and depressions. These are most prominent on the palms and ventral surfaces of the fingers, and on the corresponding surfaces of the feet. Here the elevations form characteristic epidermal ridges or rete ridges (Fig. 11.3) that are responsible for the highly specific fingerprints of each individual.

The Epidermis

The epidermis consists of stratified squamous keratinized epithelium (Fig. 11.4).

Layers of Epidermis (Fig. 11.4)

Stratum basale: It is the deepest or basal layer of epidermis. It is made up of a single layer of columnar cells that rest on a basal lamina. The basal layer contains stem cells that undergo mitosis to give off cells called keratinocytes. Keratinocytes form the more superficial layers of the epidermis. The basal layer is, therefore, also called the germinal layer (stratum germinativum).

Stratum spinosum: Above the basal layer there are several layers of polygonal keratinocytes that constitute the stratum spinosum (or Malpighian layer). The cells of this layer are attached to one another by numerous desmosomes. During routine preparation of tissue for sectioning the cells often retract from each other except at the desmosomes. As a result the cells appear to have a number of “spines”: this is the reason for calling this layer the stratum spinosum (Fig. 11.5). For the same reason the keratinocytes of this layer are also called

prickle cells.

The cytoplasm of cells in the stratum spinosum is permeated with fibrils (made up of bundles of keratin filaments). The fibrils are attached to the cell wall at desmosomes. Some mitoses may be seen in the deeper cells of the stratum spinosum. Because of this fact the stratum spinosum is included, along with the basal cell layer, in the germinative zone of the epidermis.

Stratum granulosum: Overlying the stratum spinosum there are a few (1 to 5) layers of flattened cells that are characterized by the presence of deeply staining granules in their cytoplasm. These cells constitute the stratum granulosum. The granules in them consist of a protein called keratohyalin (precursor of keratin). The nuclei of cells in this layer are condensed and dark staining (pyknotic).

With the EM it is seen that, in the cells of this layer, keratin filaments are more numerous, and are arranged in the form of a thick layer.

Stratum lucidum: Superficial to the stratum granulosum there is the stratum lucidum (lucid = clear). This layer is so called because it appears homogeneous, the cell boundaries being extremely indistinct. Traces of flattened nuclei are seen in some cells.

Stratum corneum: It is most superficial layer of the epidermis. This layer is acellular. It is made up of flattened scale-like elements (squames) containing keratin

filaments embedded in protein. The squames are held together by a glue-like material which contains lipids and carbohydrates. The presence of lipid makes this layer highly resistant to permeation by water.

The thickness of the stratum corneum is greatest where the skin is exposed to maximal friction, e.g. on the palms and soles. The superficial layers of the epidermis are being constantly shed off, and are replaced by proliferation of cells in deeper layers.