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THE INTEGUMENTARY SYSTEM

Ahmad Aulia Jusuf, MD, PhD

Department of Histology Faculty of Medicine University of Indonesia

2008

INTRODUCTION

Like Borobudur temple the skin (Fig-1) is the largest organ in our body, both in weight and surface area. The skin shows variation in structure at different sites in the body surface. The thickness of skin varies in any place in our body. The skin is thick on the upper back while the thinnest skin is on the upper and lower eyelids.

Fig-1 Skin is the largest organ in the body

The general aim of this module is to study the histological structure of the skin and its accessories in the correlation with their functions. In this module we study:

  1. Histological structure and its relation to skin function
  2. Role of 4 types epidermal cells in skin function
  3. Melanocyte structure
  4. Melanogenesis
  5. Vascularization of the skin and skin immune cells
  6. Histological structure of the skin in aging process
  7. Histological structure of thick and thin skin

The integumentary (inte=whole, -gument=body covering) is the largest organ in our body. It constitutes 15-20% of total body mass. The integumentary system consists of skin (cutis, integument) and its derivates, such hair, nails and multicellular exocrine glands. The function of skin are

  1. Protection

The skin provides protection against a wide variety of external damaging stimuli, including ultra violet light, chemical, thermal, and mechanical insult. The skin also provides a barrier against the excessive wetting (Waterproof) and also against bacterial and fungal invasion. Some of bacteria and fungi are present normally in the surface of skin, but they can not penetrate into underlying tissue.

  1. Sensation

Skin contains many different receptors for touch, pressure, pain and temperature. These sensory receptors present in most numerous in the skin which has most physical contact with solid objects in the enviroment, such as in the soles and palm and in the ventral surface of finger and toes

  1. Thermoregulation

Skin has an important role in heat conservation through the thick pelt of hairs on the surface and subcutaneous adipose tissue. There are several mechanisms for heat to be lost :

  1. increasing the blood flow through the rich vascular network in the skin
  2. secreation of sweat from the eccrine glands to the surface of skin
  3. evaporation
  1. Metabolic function

The most important metabolic function of skin is the synthesis of vitamin D3 (cholecalciferol) by the action of ultraviolet light on he precursor 7-dehydrocholesterol. The cholecalciferol is further processed in liver and kidney to produce the active 1,25-dihydroxycholecalciferol which is important in calcium metabolism and bone formation.

  1. Sexual Attractant

This greatly underestimated function has spawned an enormous industry in products claiming to improve the texture and appearance of skin, hair, and nails and other products which hide or minimise defects.

Histological structure and its relation to skin function as body protector

Integument (Fig-2) (Inte=whole; gument= body weight) composed of skin and its accessoriy structure: hair, nails and exocrine glands such as sweat glands and sebaceous glands.

Fig-2 The integument system : skin,

Skin consists of two layers (Fig-3) ; an outer epidermis and a deeper connective tissue layer, the dermis.

Fig-3 Epidermis and dermis

The epidermis, the surface layer of skin, is derived from ectoderm and is composed of stratified squamous keratinized epithelium. The thickness of epidermis varies over most of body. The thick skin is present on the palm of the hands and soles of the feet, while the thin skin covers most of the remainder of the body. Epidermis makes invagination into its underlying connective tissue (dermis) called as epidermal ridge which interdigitate with the dermal ridges (papillae), the ridges of the dermis.

The dermis, the layer of skin immediately deep to the epidermis, is derived from mesoderm and it consists of a loose papillary layer and a deeper denser reticular layer. Except of connective tissue, this mesoderm derived tissue contains sweat glands, sebaceous glands and hair follicles. The dermis forms the ridges, called as dermal ridges (papillae) which interdigitate with epidermal ridges.

Fig-4 The Epidermis

The hypodermis is a loose connective tissue containing varying amounts of fat , underlies the skin. The hypodermis is not part of the skin but is the superficial fascia of gross anatomical dissection that covers the entire body, immediately deep to the skin. Individuals who live in cold climates posses a large amount of fat deposited in the superficial fascia named panniculus adiposus.

EPIDERMIS

Epidermis (Fig-4) is composed of stratified squamous keratinizied epithelium. This layer has no blood vessel and lymph vessel. The nutrient is received by diffusion process in which the nutrient pass through the extracellular fluid from the blood capillaries in the dermis to the epidermis layer. Because of the cytomorphogenesis of keratinocytes during their migration from the basal layer of the epidermis to its surface, epidermis divided into 5 morphologically distinct zones:

  1. stratum basale (germinativum)
  2. stratum spinosum
  3. stratum granulosum
  4. stratum lucidum
  5. startum corneum.

Skin is classified into thick and thin skin (Fig-5) according to the thickness of epidermis. However these two classification are also distinguished by the presence or absence of certain epidermal layers and the presence or absence of hair.

Fig-5 The Thick Skin (left side) and Thin Skin (right side)

The thick skin is characterized by the presence of all five layers. Thick skin lacks hair follicles, arrector pili muscles and sebaceous glands but has the sweat glands. The thick skin covers the palms and soles.

The thin skin is characterized by the presence of thin stratum corneum and lacks the defined stratum lucidum and stratum granulosum, although individual cells of these layers are present in their proper locations. Thin skin has hair follicles, arrector pili muscles, sebaceous glands and sweat glands. The thin skin covers most of the remainder of the body.

KERATINOCYTES

Keratinocytes (Fig-6 and 7A) which are derivated from surface ectoderm of embrionic layers form the largest population of the cells (85-95% of cell population) and are arranged in all five recognizable layers while the remaining 3 other cell types are interspersed among keratinocytes in their specific location.

Keratinocytes undergo mitosis actively (Fig-7B) at night in the basal layers of epidermis. As the new cells are forming the cells above continue to be pushed toward the surface. Along their way to the surface, the cells differentiate and begin to accumulate keratin filaments in their cytoplasm. When they near to the surface, the cells die and are sloughed off (desquamation). This process takes 20-30 days.

Figure-6 The types of cells in the Epidermis

STRATUM BASALE

Stratum basale (Fig-6 and 7A), also known as the germinal layer is the deepest layer of the epidermis. It is supported by a basement membrane and sits on the dermis. The stratum basale consists of single layer of mitotically active, cuboidal to low columnar cells containing basophilic cytoplasm and a large nucleus. Many desmosomes are located on the lateral cell membrane attaching stratum basale cells to each other and to cells of the stratum spinosum. Hemidesmosomes attachs the cells to the basal lamina. Mitotic figures should be common in this layer because it is partially responsible for cell renewalin the epithelium. When new cells are formed via mitosis, the previous layer of cells is pushed surfaceward to join the startum spinosum.

Fig-7A. Keratinocytes in stratum basale
and stratum spinosum / Fig-7B. The process of keratinization

STRATUM SPINOSUM

The thickest layer of the epidermis is the stratum spinosum (Fig-7A) which is composed by polyhedral to flattened cells. The basally located keratinocytes in the stratum spinosum also are mitotically active, thus the stratum germinativum and stratum spinosum are responsible for the turnover of epidermal keratinocytes. The stratum spinosum and stratum germinativum are known as Malpighian layer. The cells in the stratum spinosum are richer in bundles of intermediate filaments (tonofilament) representing cytokeratin than in cells in stratum basale. These bundles radiate outward from perinuclear region toward highly interdigitated cellular processus. These cellular processes attach adjacent cells to each other by desmosomes. These processes, also called as intercellular bridges give cells of the stratum spinosum a“ prickle cell” appearance (Fig-8)

Figure-8 The prickle cell appearance in the keratinocyte cells in stratum spinosum

STRATUM GRANULOSUM

Fig-9 Stratum Granulosum

The stratum granulosum (Fig-9) consists of three to five layers of flattened keratinocytes which still have nuclei. The cytoplasm of these keratinocytes contains the keratohyalin granules, the large irregularly shaped and basophilic granules. Cells of stratum granulosum contain membrane-coating granules, which are released by exocytosis into extracellular space, forming sheets of lipid-rich substance that acts as a waterproof barrier, one of the functions of skin.

STRATUM LUCIDUM

Stratum lucidum (Fig-9) is a thin layer of cells immediately superficial to the stratum granulosum and is present only in the thick skin (i.e. palms of the hands and soles of the feet). Although the flattened cells of stratum lucidum lack organelles and nuclei, they contain densely packed keratin filament.

STRATUM CORNEUM

The stratum corneum (Fig-9) is composed of numerous layers of flattened, keratinized cells with a thickened plasmalemma. These cells lack nuclei and organelles but are filled with keratin filaments embedded in an amorphous matrix. The cells near the surface of the skin called squames or horny cells, lose their desmosomes and become desquamated (sloughed off).

MELANOCYTES

Melanocytes (Fig-10), derivated from neural crest are located among the cells of stratum basale and in the superficial portions of dermis. These cells are responsible for producing the pigment melanin which is responsibel for skin coloration. The pigment exists in various forms from yellowish brown to black. This skin pigment is thought to have protective function against damage from excessive ultraviolet light.

Fig-10 The melanocytes (indicated by arrow)

Fig-11 The synthesis of melanin pigment

Melanocytes are round to columnar cells whose long undulating processus extend from superficial surface of the cells and penetrate the intercellular spaces of the stratum spinosum. Under light microscope melanocytes is pale stainned with their cytoplasm contains scanty tiny round or oval dark staining bodies known as melanosomes which are responsible for the synthesis of melanin.

Melanocyte are present as scatter cells in the basal layer and are more numerous in areas which are exposed to light , for example they are more numerous on the face than on buttocks. There is no great difference in numbers of melanocytes between white and dark-skinned races, but they are considerably more synthetically active in darker skinned people. In pale skinned people the melanocytes can be stimulated into producing more melanin by gradually increasing exposure to UV light.

Fig-12 Cytocrine secretion of melanosomes

Tyrosinase produced by the RER of melanocytes (Fig-11) is packed by Golgi apapratus into oval granules known as melanosom. The amino acid tyrosine is transported into melanosom. The tyrosinase coverts tyrosine into melanin by a series of reactions through 3,4-dihydroxyphenylalanine (dopa, methyldopa) and dopaquinone. The enzyme tyrosinase is activated by ultraviolet light.

Melanosomes leave the cell body of the melanocytes (Fig-12) and travel to the tips of their processes penetrate the cytoplasm of keratinocytes and become pinched off via a special secretory process called as cytocrine secretion. Melanosomes are transported to the supranuclear region. So the melanosomes forms a protective barrier between the nucleus and the impinging ultraviolet rays from the sun. The melanin pigment is attacked and degraded by lysosome of the keratinocytes. This process occures over a period of several days.

LANGERHANS CELLS

Langerhans cells (Fig-13) are antigen presenting cells located among the cells of the stratum spinosum. This cell originate from precursor in the bone marrow and are a part of the mononuclear phagocyte system. Normally the Langerhnas cells represent 2% to 4% of the epidermal cell population. Langerhans cells also known as dendritic cells because of their numerous long processes.

Histologically these cells are pale stainned (Fig-13) with irregularly lobulated nuclei and almost clear cytoplasm. Cytoplasmic processes extend from the cell and insinuate between keratinocytes of all layers. Under electron microscope the Langerhans cells showthe present of unique feature of granules known as Birbeck granuleswhich in section resemble Pingpong paddles; however theire function is not known.

Figure-13 Langerhans cells, indicated by L and its processus indicated by CP, HE
staining (left side) and immunohistochemical stainning (right side)

These cells function in the immune response that will phagocytose and process foreign antigens. Then they migrate to lymph nodes in where they present epitopes of processed foreign antigen to T lymphocytes, thus Langerhans cells are antigen presenting cells.

MERKEL CELLS

Figure-14

Merkel cells (Fig- 14) are intra epidermal touch receptors and contain neuroendocrine type membrane-bound vesicles in their cytoplasm, particularly near theis base where they make synaptic junctions with unmyelinated sensory nerve twigs in upper dermis. They are very scanty in adult skin and are difficult to find.

DERMIS

Dermis (Fig-15) is the layer of skin immediately deep to the epidermis is derived from mesoder and composed of dense irregular collagenous connective tissue containing mostly type I collagen fibers and networks of elastic fibers which support the epidermis and bind the skin to the underlying hypodermis (superficial fascia).

Dermis is responsible the tone and texture of the skin. In the young the skin is tight and firm because of the quality of the collagen and elastin, but with increasing of age and particulary exposure to sunlight the collagen and elastin in upper dermis progressively degenerate, and the skin loses much of its texture and wrinkle.

Figure-15

Dermis contains the vascular supply and innervation of the skin and has two layer, a superficial papillary dermis benetah the epidermis and a deeper reticular dermis which borders the subcutis.

The superficial papillary layer of dermis interdigitates with the epidermis forming the dermal ridges (papillae). It is composed of a loose connective tissue containing type III collagen fibers, elastic fibers, and type VII collagen fibers which extend from the basal lamina into papillary layer, binding the epidermis to the dermis. This layer also contains fibroblast, macrophages, plasma cells, mast cells, and other cells common to connective tissue. The papillary layer (Fig 16) posses

Figure-16
  1. the capillary loops, called as vascular papillae which are rich with capillar vessel. These capillaries regulate the body temperature and nourish the cells of avascular epidermis
  2. the nerve ending receptor loops, also called as nerve papillae which are rich with nerve ending receptor such as Meissner corpuscle, the mechanoreceptor specialized to slight deformations of the epidermis. This receptor are most common in area of the skin that are sensitive to tactile stimulation (eg. Lips, external genitalia and nipples). Another mechanoreceptor present in the papillary layer is the Krauseend bulb, which once thought to respond to cold, but its actual function is currently unclear.

The reticular layer of dermis (Fig-15) is composed of dense irregular collagenous connective tissue (type I collagen fibers), elastic fibers, proteoglycan, fibroblast, macrophage, mast cells, lymphocytes, and fat cells in the deeper aspect of the reticular layer. The other structures present in this layer are sweat glands, sebaceous glands, hair follicle, smooth muscle (eg. arrector pili muscles) and 2 mechanoreceptor, the paccinian corpuscle which respond to pressure and vibrations and Ruffni corpuscles which functions still unclear.

SKIN CIRCULATION

Figure-17

The circulation of the skin (Fig-17) has an unusual arrangement which has correlation with its function :

  1. Nutrition of skin and appendages
  2. increased the blood flow to facilitate heat loss in hot condition
  3. decreased blood flow to minimise heat loss in cold condition

The artery supplying the skin are located deep in the subcutis from which they give rise to branches passing upwards to form two plexuses of anastomosing vessels. The deeper plexus lies at the junction of the subcutis and dermis, known as the cutaneous plexus. The more superficial plexus lies at the junction between papillary and reticular dermis, knowns as subpapillary plexus. This subpapillary plexus also gives rise to a capillary loop in each dermal papilla.