Human_Histology

288 Textbook of Human Histology

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are polarized. Bending of stereocilia toward the apex of the “V” causes depolarization, while the reverse causes hyperpolarization. Ionic gradients associated with depolari­ zation and hyperpolarization are maintained because apices of hair cells and surrounding cells are tightly sealed by occluding junctions.

The Outer Phalangeal Cells and Reticular Lamina

These are the cells that support the outer hair cells. They lie lateral to the outer rod cells. Their bases rest on the basilar membrane. Their apical parts have a complicated configuration. The greater part of the apex forms a cup­ like depression into which the base of an outer hair cell fits. Arising from one side (of the apical part) of the cell there is a thin rod­like phalangeal process. This process passes “upwards”, in the interval between hair cells, to reach the level of the apices of hair cells. Here the phalan­ geal process expands to form a transverse plate called the phalanx. The edges of the phalanges of adjoining phalan­ geal cells unite with each other to form a membrane called the reticular lamina (The reticular lamina also receives contributions from the heads of hair cells). The apices of hair cells protrude through apertures in this lamina.

The cell edges forming the reticular lamina contain bundles of microtubules embedded in dense cytoplasm. Adjacent cell margins are united by desmosomes, occlud­ ing junctions and gap junctions. The reticular lamina forms a barrier impermeable to ions except through the cell mem­ branes. It also forms a rigid support between the apical parts of hair cells thus ensuring that the hair cells rub against the membrana tectoria when the basilar mem­ brane vibrates.

The Cochlear Duct

We have seen that the cochlear duct is a triangular canal lying between the basilar membrane and the vestibular membrane. We may now note some further details (Figs. 22.4 and 22.6).

The endosteum on the outer wall of the cochlear canal is thickened. This thickened endosteum forms the outer wall of the duct of the cochlea. The basilar and vesti­ bular membranes are attached to this endosteum. The thickened endosteum shows a projection in the region of attachment of the basilar membrane: this projection is called the spiral ligament. A little above the spiral ligament the thickened endosteum shows a much larger rounded projection into the cochlear duct: this is the spiral prominence. The spiral prominence forms the upper border of a concavity called the outer spiral sulcus.

Between the spiral prominence and the attachment of the vestibular membrane the thickened endosteum is covered by a specialized epithelium that is called the stria vascularis. The region is so called because there are capillaries within the thickness of the epi­ thelium (This is the only such epithelium in the whole body). The epithelium of the stria vascularis is made up of three layers of cells: marginal, intermediate, and basal. The cells of the marginal layer are called dark cells. They are in contact with the endolymph filling the duct of the cochlea. These cells have a structure and function similar to that of the dark cells already described in the planum semilunatum. These dark cells may be responsible for the formation of endolymph. The basal parts of the dark cells give off processes that come into intimate contact with the intraepithelial capillaries. The capillaries are also in contact with pro­ cesses arising from cells in the intermediate and basal layers of the stria vascularis.

We have seen that the spiral lamina is a bony projection into the cochlear canal. Near the attachment of the spiral lamina to the modiolus there is a spiral cavity in which the spiral ganglion is lodged. This ganglion is made up of bipolar cells. Central processes arising from these cells form the fibers of the cochlear nerve. Peri­ pheral processes of the ganglion cells pass through canals in the spiral lamina to reach the spiral organ of Corti (described below).

The periosteum on the upper surface of the spiral lamina is greatly thickened to form a mass called the

limbus lamina spiralis (or spiral limbus).

The limbus is roughly triangular in shape. It has a flat “lower” surface attached to the spiral lamina; a convex “upper” surface to which the vestibular membrane is attac­ hed; and a deeply concave “outer” surface. The concavity is called the internal spiral sulcus. This sulcus is bounded above by a sharp vestibular lip and below by a tympanic lip which is fused to the spiral lamina.

SPECIALIZED END ORGANS IN THE

MEMBRANOUS LABYRINTH

The internal ear is a highly specialized end organ that performs the dual functions of hearing and of providing information about the position and movements of the head. The impulses in question are converted into nerve impulses by a number of structures that act as transducers. These are spiral organ (of Corti) for hearing and maculae (singular = macula) present in the utricle and saccule for changes in position of the head (Fig. 22.10).

Chapter 22 Special Senses: Ear 289

Fig. 22.10: End organs in the membranous labyrinth

(Schematic representation)

Information about angular movements of the head is provided by end organs called the ampullary crests (or cristae ampullae). One such crest is present in each semi­ circular duct. One end of each semicircular duct is dilated to form an ampulla, and the end organ lies within this dilatation. These end organs are described below.

Ampullary Crests (Fig. 22.11)

One ampullary crest is present in the ampullated end of each of the three semicircular ducts. Each crest is an elonga­ ted ridge projecting into the ampulla, and reaching almost up to the opposite wall of the ampulla. The long axis of the crest lies at right angles to that of the semicircular duct. The crest is lined by a columnar epithelium in which two kinds of cells are present. These are hair cells which are specialized mechano­receptors, and supporting (or sus­ tentacular) cells.

The Hair Cells

The hair cells occupy only the upper half of the epithelium. The luminal surface of each hair cell bears “hairs”. When examined by EM the “hair” are seen to be of two types as follows:

There is one large kinocilium which is probably non­ motile.

There are a number of stereocilia (large microvilli). These “hair” extend into a gelatinous (protein polysac­

charide) material which covers the crest and is called the cupula. The hair processes of the hair cells are arranged in a definite pattern the orientation being specific for each semicircular duct. This orientation is of functional importance.

Fig. 22.11: Structure of an ampullary crest (Schematic representation)

Each hair cell is innervated by terminals of afferent fibers of the vestibular nerve. Efferent fibers that can alter the threshold of the receptors are also present.

Hair cells can be divided into two types depending on their shape and on the pattern of nerve endings around them. Type I hair cells (inner hair cell) are flask shaped. They have a rounded base and a short neck. The nucleus lies in the expanded basal part (outer hair cell). The basal part is surrounded by a goblet shaped nerve terminal (or calix). Type II hair cells are columnar. Both types of hair cells receive nerve terminals which are afferent (non­ granular) as well as efferent (granular).

Both in the ampullae of semicircular ducts, and in the maculae of the utricle and saccule, each hair cell is pola­ rized with regard to the position of the kinocilium rela­ tive to the stereocilia. Each hair cell (in an ampulla) can be said to have a side that is toward the utricle, and a side that faces in the opposite direction. In the lateral semi­ circular duct, the kinocilia lie on the side of the cells which are toward the utricle; while in the anterior and posterior semicircular ducts the kinocilia lie on the opposite side. When stereocilia are bent toward the kinocilium the cell is hyperpolarized. It is depolarized when bending is away from the kinocilium. Depolarization depends on the opening up of Ca++ channels.

The Supporting Cells

The supporting (or sustentacular) cells are elongated and may be shaped like hour glasses (narrow in the middle and wide at each end). They support the hair cells and provide them with nutrition. They may also modify the composition of endolymph.

Functioning of Ampullary Crests

The ampullary crests are stimulated by movements of the head (specially by acceleration). When the head moves,

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a current is produced in the endolymph of the semicircular ducts (by inertia). This movement causes deflection of the cupula to one side distorting the hair cells. It appears likely that distortion of the crest in one direction causes stimulation of nerve impulses, while distortion in the opposite direction produces inhibition. In any given movement the cristae of some semicircular ducts are stimulated while those of others are inhibited. Perception of the exact direction of movement of the head depends on the precise pattern formed by responses from the various cristae.

Added Information

Planum Semilunatum

On each side of each ampullary crest the epithelium of the semicircular duct shows an area of thickened epithelium that is called the planum semilunatum. The importance of this area is that (amongst other cells) it contains certain dark cells that have an ultrastructure similar to cells (elsewhere in the body) that are specialized for ionic transport. The cells bear microvilli and have deep infoldings of their basal plasma membrane. The areas between the folds are occu­ pied by elongated mitochondria (Compare with structure of cells of the distal convoluted tubules of the kidney). The dark cells are believed to control the ionic content of the endo­ lymph. Similar cells are also present elsewhere in the membranous labyrinth. The planum semilunatum may sec­ rete endolymph.

The Maculae

They lie in otolith organs (utricle and saccule). Macula of the utricle is situated in its floor in a horizontal plane in the dilated superior portion of the utricle. Macula of sac­ cule is situated in its medial wall in a vertical plane. The macula utriculi (approximately 33,000 hair cells) are larger than saccular macula (approximately 18,000 hair cells). The striola, which is a narrow curved line in center, divides the macula into two areas. They appreciate position of head in response to gravity and linear acceleration. A macula consists mainly of two parts: a sensory neuroepithelium and an otolith membrane.

Sensory neuroepithelium: It is made up of type 1 and type 2 cells, which are similar to the hair cells of the ampullary cristae. Type I cells are in higher concen­ tration in the area of striola and change orientation (mirror­shaped) along the line of striola with oppo­ site polarity. The kinocilia face striola in the utricular macula, whereas in saccule, they face away from the striola. The polarity and curvilinear shape of striola offer CNS wide range of neural information of angles in all the three dimensions for optimal perception and

Fig. 22.12: Macula of otolith organs utricle and saccule

(Schematic representation)

compensatory correction. During tilt, translational head movements and positioning, visual stimuli com­ bined with receptors of neck muscles, joint and liga­ ments play an important part.

Otolithic membrane: The otoconial membrane con­ sists of a gelatinous mass, a subgelatinous space and the crystals of calcium carbonate called otoliths (oto­ conia or statoconia) (Fig. 22.12). The otoconia, which are multitude of small cylindrical and hexagonally shaped bodies with pointed ends, consists of an organic protein matrix together with crystallized calcium carbonate. The otoconia (3–19 μm long) lie on the top of the gelatinous mass. The cilia of hair cells project into the gelatinous layer. The linear, gravitational and head tilt movements result into the displacement of otolithic membrane, which stimulate the hair cells lying in different planes. The maculae give information about the position of the

head and are organs of static balance. In contrast the ampullary crests are organs of kinetic balance. The macula of the saccule may be concerned with the reception of low frequencies of sound. Impulses arising from the ampullary crests and the maculae influence the position of the eyes. They also have an influence on body posture (through the vestibular nuclei).

SOME ELEMENTARY FACTS ABOUT THE MECHANISM OF HEARING

Sound waves travelling through air pass into the external acoustic meatus and produce vibrations in the tympanic membrane. These vibrations are transmitted through the chain of ossicles to perilymph in the vestibule. In this

process the force of vibration undergoes considerable amplification because (a) the chain of ossicles acts as a lever; and (b) the area of the tympanic membrane is much greater than that of the footplate of the stapes (increasing the force per unit area).

Movement of the stapes (toward the vestibule) sets up a pressure wave in the perilymph. This wave passes from the vestibule into the scala vestibuli, and travels through it to the apex of the cochlea. At this point (called the helicotrema) the scala vestibuli is continuous with the scala tympani. The pressure wave passes into the scala tympani and again traverses the whole length of the cochlea to end by causing an outward bulging of the secondary tympanic membrane. In this way vibrations are set up in the perilymph, and through it in the basilar membrane. Movements of the basilar membrane produces forces that result in friction between the “hairs” of hair cells against the membrana tectoria. This friction leads to bending of the “hairs”. This bending generates nerve impulses that travel through the cochlear nerve to the brain.

Chapter 22 Special Senses: Ear 291

The presence of efferent terminals on the hair cells probably controls the afferent impulses reaching the brain. It can also lead to sharpening of impulses emanating from particular segments of the spiral organ by suppressing impulses from adjoining areas.

It has to be remembered that the transverse length of the basilar membrane is not equal in different parts of the cochlear canal. The membrane is shortest in the basal turn of the cochlea, and longest in the apical turn (quite contrary to what one might expect). Different segments of the membrane vibrate most strongly in response to different frequencies of sound thus providing a mechanism for differentiation of sound frequencies. Low frequency sounds are detected by hair cells in the organ of Corti lying near the apex of the cochlea, while high frequency sounds are detected by hair cells placed near the base of the cochlea.

The intensity of sound depends on the amplitude of vibration. For further details of the mechanism of hearing consult a textbook on physiology.

Color Atlas

HISTOLOGY AND ITS STUDY

The study of histology is very important for the understanding of the normal functioning of the human body. It also forms the essential basis for the study of the changes in various tissues and organs in disease. (This is the science of pathology). From these points of view the study of histology is best done taking one organ system at a time. That is the approach most teachers prefer to take in practical classes of histology. It is also the basis on which the chapters of this book have been organized.

However, in practical examinations, the emphasis is on the ability of the student to recognize a tissue or organ that is being viewed through a microscope. Here it becomes necessary to know how to distinguish between similar looking tissues or organs belonging to different systems. This atlas has been organized to serve this objective. Tissues and organs that have a similar appearance are considered in one lot. For example, if a slide presents something that looks like a tube, whether it be an artery or the ureter or the ductus deferens, these are considered together. This makes the grouping unusual, but this is exactly what the student needs at the time of an examination.

At the same time it is true that an organ can be composed of several tissues (or layers), and the ability to recognize them can go a long way in arriving at a correct diagnosis of the organ being seen. We will, therefore, first try to study and identify the various tissues that make up different organs. We will then have a good basis for identifying any organ that we are required to recognize.

BASIC TISSUES THAT CAN BE RECOGNIZED IN HISTOLOGICAL SECTIONS

EPITHELIA

The outer surface of the body, and the luminal surfaces of cavities (big or small) lying within the body are lined by one or more layers of cells that completely cover them. Such layers of cells are called epithelia. Epithelial tissue forms the lining of the general body surfaces, passages and cavities within the body. Basement membrane connects the epithelium to the underline subepithelial tissues.

Classification of epithelial tissue is based on shape of the cells, number of cell layers and special modifications seen on the cells. Epithelia may be simple, when they consist of only one layer of cells, or stratified when there are several layers of cells. Epithelial cells may be flat (or squamous), cuboidal, or columnar.

Several types of epithelia can be recognized. Learning to identify an epithelium can be of considerable help in finding out what organ you are seeing.

A2 Textbook of Human Histology

Simple Squamous Epithelium

The cells of this epithelium are flattenedIn sections they appear so thin that bulgings are produced on the surface by

nuclei

In surface view (Fig. A1.3) the cells have polygonal outlines that interlock with those of adjoining cells

A simple squamous epithelium lines the alveoli of the lungs, the free surfaces of peritoneum, pleura, and pericardium. Here it is given the name mesothelium. It also lines the inside of blood vessels, where it is called endothelium, and of the heart where it is called endocardium.

Fig. A1.1: An alveolus of the lung showing a lining of simple squamous epithelium (arrows)

Fig. A1.2: A capillary lined by endothelium (arrow)

Fig. A1.3: Simple squamous epithelium (surface view)

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Simple Cuboidal Epithelium

The epithelium is made up of cells that look like squares (in which the length and breadth is equal)

Nuclei are rounded

A typical cuboidal epithelium lines follicles of the thyroid gland, kidney tubules, germinal layer of ovary and ducts of various glands.

Fig. A1.4: A thyroid follicle lined by simple cuboidal epithelium (arrow)

Simple Columnar Epithelium

In this epithelium the height of the cells is much greater than their width

The nuclei are oval being elongated in the same direction as the cells. They lie near the bases of the cells. Because of this we see a zone of clear cytoplasm above the nuclei

A simple columnar epithelium lines the mucous membrane of the stomach and of the large intestine.

Fig. A1.5: Simple columnar epithelium lining the mucosa of the stomach (arrow)

A4 Textbook of Human Histology

Columnar Epithelium Showing Striated Border

In some regions the free surfaces of the cells of columnar epithelium show a thickening with vertical striations in it: this is called a striated border

This is seen typically in the small intestine.

Fig. A1.6: Columnar epithelium with a striated border in the small intestine (arrow)

Pseudostratified Ciliated Columnar Epithelium

Pseudostratified epithelium differs from simple columnar epithelium in that it appears to be multi-layered. However, there is actually only one layer of cells. The multi-layered appearance is due to the fact that the nuclei lie at different levels in different cells. Such an epithelium is seen in the ductus deferens

In some situations, pseudostratified columnar epithelium bears hair-like projections called cilia

Pseudostratified ciliated columnar epithelium is seen in trachea and in large bronchi.

Fig. A1.7: epithelium in trachea (arrow)

Pseudostratified Columnar Epithelium with Stereocilia

In some situations, the pseudostratified columnar epithelium bears stereocilia as seen in epididymis

Stereocilia are actually long microvilli and not cilia.

Fig. A1.8: stereocilia in epididymis (arrow)

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Transitional Epithelium

In this type of epithelium we see several layers of cells with round nuclei

The deepest cells are columnar or cuboidal. The middle layers are made up of polyhedral or pear-shaped cells

The cells of the surface layer are large and often shaped like an umbrella

This epithelium lines many parts of the urinary tract.

Fig. A1.9:

Stratified Squamous Epithelium (Non-keratinized)

The stratified epithelium is made up of several layers of cells

The cells in the deepest (or basal) layer are columnar. In the middle layers they are polyhedral, while the more superficial layers show increasing degrees of flatteningThe nuclei are oval in the basal layer, rounded in the middle layer, and transversely

elongated in the superficial layers

There is no superficial keratinized zone; flattened nuclei are seen in the topmost layer

This kind of epithelium is seen lining some internal organs like the oesophagus and the vagina.

Fig. A1.10: seen in esophagus (arrow)

A6 Textbook of Human Histology

Stratified Squamous Epithelium (Keratinized)

Here the deeper layer are covered by additional layers that represent stages in the conversion of cells into non-living fibers. This process is called keratinization (or cornification)

The surface layer is made up of keratin which appears as fibers. No cellular outline or nuclei can be seen

It is seen typically in epidermis of the skin.

Key

Fig. A1.11:

CONNECTIVE TISSUE

In most organs there are areas filled in by fibers that are described as connective tissue. The main constituent of connective tissue is collagen fibers that stain pink. In stretch preparations they are seen as wavy bundles. Other fibers present (elastic, reticular) can be seen with special stains. Connective tissue also contains many cells but only their nuclei can be made out.

Irregular Connective Tissue

Irregular connective tissue is typically seen in dermis of skin

It consists of compactly packed bundles of collagen fibers that are not arranged in orderly fashion

Thin elastic fibers are present, but are not seen with H & E stain.

Fig. A2.1: !