General Properties of the BBB 2 страница

Specialized sensory cells are located in six regions in the wall of the membranous labyrinth.

-Three cristae ampullaris located in the ampullae of the semicircular ducts.

-Two maculae, one in the utricle (macula utriculi) and the other in the saccule (macula sacculi).

-The organ of Corti that project into the endolymph of the cochlear duct.

The three cristae ampullaris are sensitive to angular acceleration of the head (i.e., turning of the head). The maculae of utricle and saccule sense the position of the head and linear movement. The organ of Corti functions as the sound receptor.

The hair cell, a nonneuronal mechanoreceptor is the common receptor cell of the vestibulocochlear system.

Several important characteristics are common to hair cells:

-all are epithelial cells

-each possesses numerous stereocilia, modified microvilli, called sensory “hairs”

-in the vestibular system, each hair cell possesses a single true cilium called a kinocilium

-in the auditory system, hair cells lose their cilium during development but do have a residual basal body

-all hair cell are transducers, i.e., they convert mechanical energy to electrical energy that can be transmitted via the vestibulocochlear nerve to the brain.

Figure 17. Diagram of two types of sensory hair cells in the sensory areas

In the vestibular system, there are two types of hair cells and associated nerve endings. Type I hair cells are piriform in shape with a rounded base and a thin neck. Type II hair cells are cylindrical in shape.

All receptor (hair) cells of the inner ear appear to function by the bending or flexing of their stereocilia (sensory hairs). Stretching of the plasma membrane caused by the bending of the stereocilia generates transmembrane potential changes in the receptor cell that are conveyed to the afferent nerve ending(s) associated with each hair cell.

Cristae ampullaris: sensors of angular movement

Figure 18. Diagram of a typical semicircular duct within its canal

Each of the three cristae ampullaris is the sensory region of one of the semicircular ducts in the semicircular canals and lies in the ampulla of the semicircular canal. Each crista consists of hair cells with stereocilia and supporting epithelial cells.

A cupula is a gelatinous structure that is attached to the hair cells of each crista. The cupula projects into the lumen and is surrounded by endolymph. During rotational movement of the head, the walls of the semicircular canal and the membranous semicircular ducts move, but the endolymph contained within the ducts tends to lag behind because of inertia. The cupula, projecting into the endolymph, is swayed by the movement differential between the crista fixed to the wall of the duct and the endolymph. Bending of the stereocilia in the narrow space between the hair cells and the cupula leads to the generation of nerve impulses in the associated nerve endings.

Macula sacculi and macula utriculi: sensors of gravity and linear movement

Figure 19. Diagram of a cross section of the utricle

The maculae are innervated sensory thickenings of the epithelium facing the endolymph in the saccule and utricle of the vestibule. Each macula consists of hair cells of both types, supporting cells, and nerve endings associated with the hair cells. The maculae of the utricle and saccule are oriented at right angles to one another. When a person is standing, the macula utriculi is in a horizontal plane, and the macula sacculi is in vertical plane.

The gelatinous material that overlies the maculae is called the otolithic membrane. It contains the otoliths (otoconia) on its outer surface. The otolithic membrane moves on the macula in a manner analogous to that by which the cupula moves on the crista.

Organ of Corti: sensor of sound vibration

The cochlear duct divides the cochlear canal into three parallel compartments or scalae:

-scala media, the middle compartment in the cochlear canal

-scala vestibuli

-scala tympani

The cochlear duct, itself, is the scala media. The scala vestibuli and scala tympani are the spaces above and below, respectively, the scala media. The scala media is an endolymph-containing space that is continuous with the lumen of the saccule and contains the organ of Corti, which rests on its lower wall.

The scala vestibuli and the scala tympani are perilymph-containing spaces and communicate with each other at the apex of the cochlea through a small channel called the helicotrema. The scala vestibuli is described as beginning at the oval window and scala tympani is described as ending at the round window.

The scala media appears as a triangular space with its most acute angle attached to a bony extension of the modiolus, the osseous spiral lamina. The upper wall of the scala media, which separates it from the scala vestibuli, is the vestibular membrane. The lateral or outer wall of the scala media is the stria vascularis. It is lined with a pseudostratified epithelium that may be the site of synthesis of endolymph. The lower wall or floor of the scala media is the basilar membrane.

Figure 20. A. Schematic diagram of a midmodiolar section of the cochlea

B. Cross section of basal cochlear duct

C. Diagram of the sensory and supporting cells of the organ of Corti

c

The organ of Corti rests on the basilar membrane and is overlain by the tectorial membrane.

The organ of Corti is a complex epithelial layer on the floor of the scala media.

It is formed by

-inner (close to the spiral lamina) and outer (farther from the spiral lamina) hair cells

-inner and outer phalangeal (supporting) cells

-pillar cells

Several other named cell types of unknown function are also presented in the organ of Corti.

The hair cells are arranged in an inner and outer row of cells.

Figure 21. Scanning electron micrograph of the stereocilia of cochlear sensory hair cells (left) and the outer phalangeal cells (right)

The inner hair cells form a single row of cells throughout all 2,5 turns of the cochlear duct. Three ranks of outer hair cells are found in the basal part of the coil and five ranks of the cells at the apex of the cochlea.

The phalangeal and pillar cells provide support for the hair cells. The phalangeal cells associated with the inner hair cells surround the cell completely. The phalangeal cells associated with outer hair cells surround only the basal portion of the hair cell completely and send apical processes toward the endolymphatic space.

The apical ends of the phalangeal cells form the reticular lamina that seals the endolymphatic compartment from the true intercellular spaces of the organ of Corti. The extracellular fluid in this intercellular space is cortilymph.

Pillar cells have broad apical and basal surfaces that form plates and a narrowed cytoplasm. The inner and outer pillar cells between them form a triangular shaped tunnel, the tunnel of Corti.

The tectorial membrane extends from the spiral limbus over the cells of the organ of Corti. Its lateral free edges attaches to the organ of Corti by the stereocilia of the hair cells.

Sound perception

Figure 22. Schematic diagram that illustrates dynamics of the three divisions of the

ear

Sound waves striking the tympanic membrane are translated into simple mechanical vibration. The ossicles of the middle ear convey these vibrations to the cochlea. In the inner ear the vibrations of the ossicles are transformed into waves in the perilymph of the scala vestibuli. The vibrations are transmitted through the vestibular membrane to the scala media, which contains endolymph, and are also propagated to the perilymph of the scala tympani. Pressure changes in this closed Perilymphatic-endolymphatic system are reflected in movements of the membrane that covers the round window in the base of the cochlea. As a result of sound vibrations entering the inner ear, a traveling wave is set up in the basilar membrane. Hair cells are attached, through the phalangeal cells, to the basilar membrane, which vibrates during sound perception. The stereocilia of these hair cells are, in turn, attached to the tectorial membrane, which also vibrates. The tectorial membrane and the basilar membrane are, however, hinged at different points. The shearing effect between the basilar membrane and the tectorial membrane distorts the stereocilia of the hair cells and, thus, the apical portion of the hair cells, and this distortion generates membrane potentional that are conveyed to the brain via the cochlear nerve.

Integumentary system (skin)

The skin and its derivates constitute the integumentary system. The skin consists of 2 layers:

- epidermis, composed of a keratinized stratified squamous epithelium

- dermis, composed of a dense connective tissue

The hypodermis, the subcutaneous connective tissue, is a looser connective tissue than the dermis. It lies deep to the dermis. The hypodermis contains variable amounts of adipose tissue.

The epidermal derivates of the skin contain the following organ structures:

-hair follicles and hair

-sweat (sudoriferous) glands

-sebaceous glands

-nails

-mammary glands

Functions of integumentary system include:

-homeostatic function

-barrier function

-sensory function

-''secretory'' function

-excretory function

Epidermis

The epidermis is composed of stratified squamous epithelium in which 5 distinct layers are observed.

Beginning with the deepest layer, these are

-stratum basale, also called stratum germinativum

-stratum spinosum

-stratum granulosum

-stratum lucidum, only in the skin of the palms of the hands and of the soles of the feet

-stratum corneum

Figure 23. Diagram of the epidermis

The stratum basale provides for epidermal cell renewal. This stratum is represented by a single layer of cells that rests on the basal lamina. It contains the stem cells from which the new cells, the keratinocytes, arise.

The stratum spinosum is at least several cells thick. The cells of the stratum spinosum exhibit numerous spinous processes, which gives this layer its name. The processes are attached to each other by desmosomes.

The stratum granulosum is the most superficial layer of the nonkeratinized portion of the epidermis. This layer varies from one to a few cells thick. The cells contain numerous keratohyaline granules.

The stratum lucidum is a translucent, thin layer of extremely flattened eosinophilic cells.

The stratum corneum consists of 15-20 layers of flattened nonnucleated keratinized cells whose cytoplasm is filled with a birefringent filamentous scleroprotein, keratin. The plasma membrane of these cornified keratinized cells is thickened and is coated, with a glycolipid that forms the major constituent of the water barrier in the epidermis.

Dermis

The dermis is composed of two layers, a papillary layer and a reticular layer.

The papillary layer consists of loose connective tissue immediately beneath the epidermis. The papillary layer is relatively thin and includes the substance of the dermal papillae and dermal ridges. It contains blood vessels that serve the epidermis. The elastic fibers here are thread -like and form an irregular network.

The reticular layer lies deep to the papillary layer. It is characterized by thick, irregular bundles of collagen and by the presence of more coarse elastic fibers. The collagen and elastic fibers here form regular lines of tension in the skin.

Hypodermis

Deep to the reticular layer is a layer of adipose tissue and its associated loose connective tissue constitute the hypodermis. It is a major energy storage site and an important insulating layer.

Cells of the epidermis

Keratinocyte

Figure 24. Schematic diagram of keratinocytes of the epidermis

The keratinocyte is the predominate cell type of the epidermis. It has next functions:

-production of keratin

-creation of an extracellular water barrier

The keratinocytes in the basal layer contain numerous free ribosomes, most of which are engaged in the synthesis of intermediate filaments. The intermediate filaments, more commonly called tonofilaments in the case of the keratinocytes, represent the essential protein in keratin production.

As the cells enter and are moved through the stratum spinosum, the synthesis of tonofilaments continues, and the filaments become grouped into bundles. These bundles are called tonofibrils.

In the upper part of the stratum spinosum the keratinocytes begin to synthesize keratohyaline granules and lamellar bodies (membrane-coating granules). As the number of granules increases, they become the most distinctive feature of the cells that constitute the stratum granulosum.

Keratinization, i.e., the conversion of granular cells to cornified cells, this process also involves the breakdown of the nucleus and other organelles and the thickening of the plasma membrane. Finally, there is a regular desquamation of these keratinized cells.

Melanocytes Are Pigment-Producing Cells

The epidermal melanocyte is a dendritic cell found among the basal cells of the stratum basale. The ratio of melanocyte to keratinocyte in the basal layer ranges from 1:4 to 1:10 in different parts of the body. They are called dendritic cells because the rounded cell body resides in the basal layer and extends long processes be­tween the keratinocytes of the stratum spinosum. Melanocytes have developing and mature melanin granules in the cytoplasm.

The melanosomes are transferred to the keratinocytes by phagocytosis of the tips of the melanocyte processes by the keratinocytes.

Figure 25. EM of a melanocyte (left); M, melanocyte; P, melanocyte processes; K, keratinocytes and keratinocytes (right); Kg, keratohyaline granule

Figure 26. EM of a Merkel’s cell (left) and Langerhans cell (right)

Langerhans Cell

The Langerhans Cell Plays a Role in the Immune Response by Presenting Antigens to T Cells

In common with macrophages, the Langerhans cell pos­sesses surface receptors. At sites of allergic contact, lymphocytes are seen in close apposition to the Langerhans cell membrane shortly after antigenic chal­lenge. As an antigen-presenting cell, the Langerhans cell is involved in the initiation of cutaneous contact hypersensi­tivity reactions, i.e., contact allergic dermatitis, and in other cell-mediated immune responses in the skin.

Merkel Cell

The Merkel Cell Is an Epidermal Cell That Functions in Cutaneous Sensation

Merkel's cells are modified epidermal cells that are lo­cated in the stratum basale. The combination of the neuron and epidermal cell, called a Merkel's corpuscle, is a very sensitive mechanoreceptor.

SKIN APPENDAGES

Skin appendages include

Hair folliclesand their product, hair

Sebaceous glands and their product, sebum

Eccrine sweat glands and their product, sweat

Apocrine sweat glands and their product, serous secre­tion

HAIR

Hairs Are Composed of Keratinized Cells That Develop From Hair Follicles

Hairs are present over almost the entire body, being ab­sent only from the sides and palmar surfaces of the hands, from the sides and plantar surfaces of the feet, from the lips, and from the region around the urogenital orifices.

Structure of the Hair Follicle

Figure 27. Schematic diagram of a hair follicle and other skin appendages

The hair follicle is responsible for the production and growth of a hair.

The growing follicle is of nearly uni­form diameter except at its base, where it expands to form the bulb. The base of the bulb is invaginated by a tuft of vascularized loose connective tissue called, a dermal papilla. The outermost part of the hair fol­licle is the ex­ternal (outer) root sheath. Other cells forming the bulb, including those that surround the connective tissue papilla, are collectively referred to as the matrix, which consists simply of matrix cells. Scattered melanocytes are present in this germinative layer.

The dividing cells in the germinative layer of the matrix differentiate into the keratin-producing cells of the hair and the internal root sheath. Both the hair and the internal root sheath have three layers. The hair consists of a medulla, cortex, and cuticle, and the internal root sheath consists of a cuticle, Huxley's layer, and Henle's layer. Keratinization of the hair and internal root sheath occurs in a region called the keratogenous zone shortly after the cells leave the matrix. By the time the hair emerges from the follicle, it is entirely keratinized as hard kera­tin. The internal root sheath consists of the soft keratin. The follicle is surrounded by a connective tissue sheath to which the arrector pili muscle is attached.

SEBACEOUS GLANDS

Sebaceous Glands Secrete Sebum That Coats the Hair and Skin Surface

Sebaceous gland is simple, branched and acinar. The oily sub­stance that is produced in the gland, sebum, is the product of holocrine secretion. Ultimately, both secretory product and cell debris are discharged from the gland as sebum into the pilosebaceous canal. New cells are pro­duced by mitosis of the basal cells at the periphery of the gland. The basal lamina of these cells is continuous with that of the epidermis and the hair follicle.

The basal cells of the sebaceous gland contain smooth (sER) and few lipid droplets. As the cells move away from the basal layer and begin to pro­duce the lipid secretory product, the amount of sER in­creases, reflecting the role of the sER in lipid synthesis and secretion. The cells gradually become filled with numerous lipid droplets separated by thin strands of cytoplasm

SWEAT GLANDS

Sweat glands are classified on the basis of their structure and the nature of their secretion. Two types of sweat glands are recognized:

-Eccrine sweat glands, distributed over the entire body surface except for the lips and part of the external genitalia

-Apocrine sweat glands, limited to the axilla, areola, and nipple of the mammary gland, circumanal region, and the external genitalia. The ceruminous glands of the ear canal and the glands of Moll of the eyelid are also apocrine-type glands.

Eccrine Sweat Glands

It is arranged as a simple coiled tubular structure composed of a secretory segment located deep in the dermis or in the upper part of the hypodermis and a directly continuous duct segment that leads to the epidermal surface. It is evident that the secretion of eccrine sweat glands is a merocrine type.

Secretory and Myoepithelial Cells

Three cell types are present in the secretory segment of the gland: clear cells and dark cells, both of which are secretory epithelial cells; and myoepithelial cells, which are contractile epithelial cells. All of the cells rest on the basal lamina; their arrangement is that of a pseudostratified epithelium.

Clear cells are characterized by abundant glycogen and numerous mitochondria. They produce the watery component of sweat.

Dark cells are characterized by abundant rER and se­cretory granules. The Golgi com­plex is relatively large, a feature consistent with the glycoprotein secretion of these cells.

Myoepithelial cells are limited to the basal aspect of the tubule. They lie between the secretory cells. The cy­toplasm contains numerous contractile filaments (actin). Contraction of these cells is responsible for the expression of sweat from the gland.

The epithelium of the duct is stratified cuboidal, consisting of a basal cell layer and luminal cell layer.

The Eccrine Sweat Glands Principally Function in Regulating Body Temperature

The eccrine sweat glands play a major role in temper­ature regulation through the cooling that results from the evaporation of water from sweat on the body surface. This hypotonic watery solution is low in protein and contains varying amounts of sodium chloride, urea, uric acid, and ammonia. Thus, the eccrine sweat gland also serves, in part, as an excretory organ.

Normally, the body loses about 600 mL of water a day through evaporation from the lungs and skin.

Apocrine Sweat Glands

Apocrine Glands Are Large Lumen Glands Associated With Hair Follicles

Apocrine sweat glands develop from the same downgrowths of epidermis that give rise to the hair follicles.

Apocrine glands are coiled tu­bular glands. The secre­tory portion of the gland is deep in the dermis or, more often, in the upper region of the hypodermis.

Secretory and Myoepithelial Cells

Unlike the eccrine glands, the apo­crine glands store their secretory product in the lumen, which is very wide. The apical part of the cell pinched off and discharged into the lumen as an apocrine secretion, thus the name of the gland. The apical cytoplasm contains numerous small granules, the secretory component within the cells. Other features of the cell include the presence of numerous lysosomes and lipofuscin pigment granules.

Myoepithelial cells are also present in the secretory por­tion of the gland, situated between the secretory cells and adjacent to the basal lamina. As in the eccrine glands, con­traction of the processes of the myoid cell facilitates the ex­pulsion of the secretory product from the gland.

Apocrine Duct

The duct of the apocrine gland is sim­ilar to that of the eccrine duct; it has a narrow lumen. The duct epithelium is stratified cuboidal. In contrast to the eccrine gland, myoepithelial cells are absent in the duct.

Apocrine Secretions

Apocrine glands produce a se­cretion that contains protein, carbohydrate, ammonia, lipid, and certain organic compounds that may color the secre­tion. When secreted, the fluid is odorless, but through bacterial action on the skin surface it develops an acrid odor.

NAILS

Figure 28. Photomicrograph of a sagittal section of distal phalanx with a nail

Nails Are Plates of Keratinized Cells Containing Hard Keratin

The slightly arched fingernail or toenail, more properly referred to as the nail plate, rests on the nail bed. The bed consists of epithelial cells that are continuous with the stra­tum basale and stratum spinosum of the epidermis.

The proximal part of the nail, the nail root, is buried in a fold of epidermis and covers the cells of the germinative zone, the matrix. The cells of the matrix produce the keratin of the nail. Nail keratin is a hard keratin and it does not desquamate. It consists of densely packed keratin filaments embedded in a ma­trix of amorphous keratin.

The crescent-shaped white area near the root of the nail, the lunula, derives its color from the thick, opaque layer of partially keratinized matrix cells in this region. When the nail plate becomes fully keratinized, it is more trans­parent and takes on its coloring from the underlying vas­cular bed. The edge of the skin fold covering the root of the nail is the eponychium or cuticle. This is also composed of hard keratin, and for this reason it does not desquamate. A thickened epidermal layer, the hyponychium, secures the free edge of the nail plate at the fingertip.

Cardiovascular system

The cardiovascular system is a transport system that carries blood and lymph to and from the tissues of the body.

The cardiovascular system includes the heart, blood vessels, and lymphatic vessels.

Blood vessels provide the route by which blood circulates to and from all parts of the body. The heart pumps the blood. Lymphatic vessels carry tissue derived fluid, called lymph, back to the blood vascular system.

The blood vessels are arranged so that blood delivered from the heart quickly reaches a network of narrow, thin-walled vessels, the blood capillaries, within or in proximity to the tissues in every part of the body. Fluid usually leaves the blood vessels in the capillaries, whereas many of the white blood cells leave the blood vessels to enter the tissues in the post capillary venules.

The vessels that deliver blood to the capillaries are the arteries. The smallest arteries, called arterioles, are functionally associated with net works of capillaries into which they deliver blood.

Together, the arterioles, associated capillary network, and post capillary venules form a functional unit referred to as the microcirculatory or microvascular bed of that tissue.

Veins collect blood from the microvascular bed and carry it toward the heart.

General features of arteries and veins

The walls of arteries and veins are composed of three layers called tunics.

The three layers of the vascular wall, from the lumen outward, are

-tunica intima, the innermost layer that includes the endothelial lining

-tunica media, the muscular middle layer

-tunica adventitia, the outermost connective tissue layer

Histologically, the various types of arteries and veins are distinguished from each other on the basis of the thickness of the vascular wall and differences in the composition of the various layers, especially the tunica media.

In addition to the three tunics, large arteries and veins may have:

-a system of vessels, called vasa vasorum, to supply the vessels them selves

-a network of autonomic nerves, called nervi vascularis, to control contraction of the smooth muscle in the walls of the vessels.

Arteries

Traditionally, arteries are classified into three types on the basis of size and characteristics of the tunica media:

-large or elastic arteries

-medium or muscular arteries

-small arteries and arterioles

Elastic arteries

Figure 29. Schematic diagram of an elastic artery

Elastic arteries have sheets of elastic tissue in their wall and are the largest diameter arteries.

The largest of elastic arteries, the aorta and pulmonary arteries, convey blood from the heart to the systemic and pulmonary circulations, respectively.

These arteries, as well as their main branches, the brachiocephalic, common carotid, subclavian and common iliac arteries are classified as elastic arteries.

Their main function is to transport blood away from the heart. These arteries also serve to smooth out the large fluctuations in pressure created by the heartbeat.

Tunica intima

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