Integumentary System

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TEGUMENTARY SYSTEM

The integument or skin covers the body surface protecting it from harmful environmental influences. Because skin is easily accessible, it is important in physical examinations.

The skin provides:

 Protection of the body against the environment, abrasions, loss of liquid, harmful substances and invading microorganisms.

 Heat regulation through sweat glands and blood vessels.

 Sensitivity through the superficial nerves and their sensory terminations.

The skin forms a wrap for body structures and vital substances (liquids), thus forming the largest organ in the body.

Skin layersThe skin is composed of:

 Epidermis: superficial cell layer.
 Dermis: deep connective tissue layer.

Epidermis

The epidermis, or cuticle, is not vascularized, consists of stratified epithelium, fits perfectly over the papillary layer of the dermis, and varies in thickness in different parts. In some places, such as the palm and sole, it is thick, hard, and horny in texture. The stratified epithelium of the epidermis consists of several layers named according to various categories, such as cell appearance, texture, composition and position. These layers are from superficial to deep: stratum corneum, lucid stratum, granular stratum, spinous stratum and basal stratum. The stratum corneum is remnant of cells that contain a fibrous protein, keratin.

EpidermisEpidermis

Skin coloration is due to pigments in the epidermis cells. This pigment is most distinct in the cells of the basal layer. The pigment (melanin) consists of very small, dark brown or black granules, closely grouped within the cells.

Dermis

The dermis, chorion, true cutis or true skin is hard, flexible and elastic. It is thicker on the dorsal surface of the body than on the ventral side and on the side rather than the medial limbs. In the eyelids, scrotum and penis is excessively thin and delicate.

The skin consists of connective tissue with varying amounts of elastic fibers and numerous nerves, blood vessels, and lymph. The connective tissue is arranged in two layers: one deep or reticular and the other superficial or papillary.

The reticular layer consists of fibroelastic connective tissue, composed mainly of collagen bundles. The cells of this layer are mainly fibroblasts and histiocytes. In the deepest layers of the reticular layer are sweat glands, sebaceous glands, hair follicles, and small accumulations of cells.

Dermis

The papillary layer consists of numerous highly sensitive vascular eminences, the papillae. The papillae are small conical eminences with rounded or dilated ends.

Subcutaneous Tissue

The dermis is situated over the subcutaneous mesh. This last layer is not considered to belong to the skin and is therefore called mesh or subcutaneous tissue or hypodermis. Subcutaneous tissue is mainly composed of loose connective tissue and adipose tissue. It performs two main functions: it helps to isolate the body from extreme environmental variations and attaches the skin to the underlying structures. Few areas of the body do not have this tissue; In these places, the skin is fixed directly to the bone. The skin of the joints and fingers is creased and wrinkled because it is attached to the bone.

Subcutaneous Tissue

Skin Attachments

The appendages of the skin are the nails, the hair and the sweat and sebaceous glands with their respective ducts.

 NailsNails: These are flat, elastic, corneal textured structures applied to the dorsal surface of the distal phalanges. Each nail is implanted by a portion called root in a furrow; the exposed portion is called the body and the distal end free edge.

The nail is firmly attached to the chorion and exactly molded to the surface; The underside of the body and root of the nail is called the nail matrix because it is the nail that produces it. Near the root of the nail the tissue is not firmly adhered to the connective tissue, but only in contact with it; This is why this portion of the nail is whitish and called lunula because of its shape.

 

 By: They are found on almost every surface of the body. They vary greatly in length, thickness and color in different parts of the body and in the various human races. A hair consists of root (the part implanted in the skin) and stem (the protruding portion of the surface).

ByThe root of the hair ends in the hair bulb, which is whitish and softer in texture than the stem and is housed in a canaliculus of the surrounding epidermis called the hair follicle. At the bottom of each follicle is a small vascular or papilla conical eminence. It is continuous with the dermal layer of the follicle and supplied with nerve fibrils. The hair follicle consists of two tunics: external and internal or epidermal. The hair bulb is molded over the papilla and is made up of polyhedral epithelial cells that, when passed into the hair root, lengthen into spindles. The hair shaft consists of three parts from the inside out. : the medulla, cortex and cuticle. The medulla is usually absent in thin hair covering the body surface and commonly in the head. It consists of rows of polyhedral cells containing eleidine granules and often air spaces. The cortex forms the part of the stem; its cells are elongated and joined to form spindle-to-flat fibers containing pigment granules in dark hair and air in whites. The cuticle consists of a simple layer of flat scales that overlap from depth to surface.

Correlated with the hair follicles is a set of tiny involuntary smooth muscle fiber bundles called hair erectors. They emerge from the superficial layer of the dermis and insert into the follicle. They are placed on the side where the hair bends, and by its action reduce the obliquity of the follicle, making it straight.

 Sebaceous GlandsSweat Glands (Gl. Sweat): They are found in almost every part of the skin. They consist of a simple tube whose deep part constitutes a spherical or oval pocket called the body of the gland, while the upper portion or duct runs through the dermis and epidermis, opening on the surface of the skin through a tapered opening. In the superficial layers of the dermis the duct is rectilinear, but in the deep layers the duct is coiled or even twisted. They are very abundant in the palms and soles.

Sudoriparous Glands

Sebaceous Glands: These are small, saculiform glandular organs housed in the dermis, found in many parts of the skin, but abundant in the scalp and face. Each gland consists of a simple duct that emerges from an oval or bottle-shaped cluster - the alveoli, which are usually two to five, and may even reach twenty. Each well is composed of a transparent basement membrane containing a number of epithelial cells.

Sebaceous GlandsSebaceous Glands

Sensitive Receptors Found on the Skin

 Free Nervous Terminations: are found in all connective tissues. They are myelinated or unmyelinated, but always of small diameter and low conduction velocity (Group III or Group IV). They may be polymodal or unipodal (nociceptors). They are sensitive to mechanical, thermal and especially painful stimuli. They are formed by a branched axon surrounded by Schwann cells, both of which are surrounded by a basement membrane.

Free Nervous Terminations

 Epidermal Terminations: Associated with hair follicles (myelinated fibers):

Epidermal TerminationsPalisade Finishes & #8211; The fibers approach the follicle in different directions, just below the sebaceous duct, where it divides and runs parallel with the hair on the outer follicular layer. They are characterized as free nerve endings.

Tactile Menisci (Merkel Cels.) & #8211; An afferent fiber is usually branched with several terminal discs of these nerve branches. These discs are enclosed in a specialized cell whose distal surface is fixed to the epidermal cells by an extension of their protoplasm and interdigitated with adjacent keratinocytes.
Thus, the pressure and traction movements on the epidermis trigger the stimulus. They are mechanoreceptors (Type I) and slow adaptation, receptive to vertical pressure and served by large myelinated afferents (A alpha). They are found on the distal parts of the extremities and on the skin of the lips and external genitals.

 Encapsulated Nervous Terminations

Tactile Body (Meissner) & #8211; Found in the dermal papillae of the hand and foot, anterior part of the forearm, lips, eyelid and tongue. It is cylindrical in shape and has a connective tissue capsule and a central heartwood with myelinic nerve fibers. They are fast-adapting mechanoreceptors, providing information about rapidly fluctuating mechanical forces.

Vater-Paccini Large Laminated Corpuscles & #8211; Found on the ventral surfaces of the hand and foot, genitals, arm, neck, mammary papilla, periosteum and close to the joints. They are ovoid, spherical and spiral and each has a capsule (30 lamellae), an intermediate growth zone and a central heartwood (60 lamellae) containing an axon terminal. Each corpuscle is supplied by one or rarely two myelinated fibers (A alpha).

 Encapsulated Nervous Terminations

Tactile Body (Meissner) & #8211; Found in the dermal papillae of the hand and foot, anterior part of the forearm, lips, eyelid and tongue. It is cylindrical in shape and has a connective tissue capsule and a central heartwood with myelinic nerve fibers. They are fast-adapting mechanoreceptors, providing information about rapidly fluctuating mechanical forces.

Vater-Paccini Large Laminated CorpusclesVater-Paccini Large Laminated Corpuscles & #8211; Found on the ventral surfaces of the hand and foot, genitals, arm, neck, mammary papilla, periosteum and close to the joints. They are ovoid, spherical and spiral and each has a capsule (30 lamellae), an intermediate growth zone and a central heartwood (60 lamellae) containing an axon terminal. Each corpuscle is supplied by one or rarely two myelinated fibers (A alpha).

This fiber loses the myelin sheath and at the junction with the heartwood loses the Schwann cell. They are very fast adapting mechanoceptors, responding only to sudden disturbances and especially sensitive to vibration. They can reach a length of 1 to 4 mm, visible to the naked eye, like oval white bodies. When cut microscopically, it looks like an onion.

Glees and Marsland Silver Technique Electron micrograph
Glees and Marsland Silver Technique   Electron micrograph

 Special Skin Arrangements & #8211; Arrangements that inform the mechanical and thermal state of the body surface, including harmful stimuli.

They are subdivided into: mechanoceptors, thermoceptors and nociceptors. The activity of isolated sensory nerve fibers is activated only by certain types of stimuli applied to the area of the skin it innervates, which shows their high degree of specificity, making a close correlation between morphology and function difficult.


Skin

Summary Scheme of Sensitive Receptors Found in the Skin:

SURFACE RECEIVERS PERCEIVED FEELING
Ruffini Receivers Heat
Merkel Records Touch and pressure
Vater-Pacini Receivers Pressure
Meissner Receivers Tact
Free nerve endings Mostly pain