The Integument System 皮被 YC Wong, PhD 王雲川 The University of Hong Kong

Scope 內容 / 範圍 Overview on function of skin Components of skin Concept of keratinocyte and keratinisation Other cell types in epidermis and functions Glands in skin: Sweat and sebaceous glands Nerve endings and functions Hair follicle and hair growth Nail

Overview 概觀 Interface between body and external world Largest “organ” Protection: barrier and immunologic Regulation of temperature and excretion Sensation Prevention of loss of water Synthesis; i.e. vitamin D

Components 組成部份 Skin: Epidermis and dermis Sweat gland Hair and its associated structures Nail

Skin 皮膚 Thick and thin skin Determined not by the overall thickness of skin but by thickness of epidermis Epidermis –Cells in epidermis are collectively known as keratinocytes 角化細胞 Dermis

Epidermis (thick skin) 表皮 Typically stratified squamous keratinized epithelium Has several layers Stratum corneum 角質層 Stratum lucidum 透明層 S. granulosum 顆粒層 S. spinosum 棘狀層 S. basalis 基底層 Thick skin: found in palm and sole, also non-pigmented

Thin skin Only has four layers: 1.Stratum corneum 2.Stratum granulosum 3.Stratum spinosum 4.Stratum basalis Cells in spinosum with spiny processes Dermis

Keratinization Differentiation of epithelial cells from basal cuboidal or low columnar to squamous cornified cell on the surface with accumulation of keratin filaments, keratohyalin granules and lamellated granules Release of keratohyalin granule content to cytoplasm triggers the final aggregation of keratin filaments to form tonofilament bundles thus transforming the granular cell to cornified cells Keratin in cornified surface cells is known as soft keratin; it is hydrophobic thus prevent loss of water from body Stratum basalis Stratum spinosum Stratum granulosum Stratum corneum Karatohyalin granule Lamellated bodies

Other cell types in epidermis Melanocytes: pigmentation of skin Langerhans cells: Immune cells Merkel’s cells: receptor cells

Melanocytes 黑素細胞 Melanocytes Dermis Basal cells contain brown pigment

Melanocytes are basally located and with long cytoplasmic processes

Melanocytes Melanocytes have relatively clear cytoplasm and small melanin granules Located between basal cells

Melanocyte Melanin granules

Melanocytes and pigmentation of skin 黑素細胞与膚色 Colour of skin is determined not by number of melanocytes but by amount of melanin granules Number of melanocytes is essentially the same in all races In darker skin, more melanin granules are accumulated in epidermal cells exposure to UV in sun accelerates the rate of melanin production thus darker colour (to protect further UV damage to cells). Hair turns grey in old age is due to reduction in number of melanocytes, thus melanin produced

Langerhans cells 郎格漢斯 melanocyte

Langerhas cell With clear cytoplasm, often indented nucleus, and with unique cigar shape granules (not obvious in this graph)

Langerhans cell A form of antigen- presenting cell Typical cigar shape granules (arrow)

Merkel cell 梅克爾 Modified epidermal cells to perform sensory function Nerve terminal is seen making contact with Merkel’s cell.

Dermis 真皮 Dense irregularly arranged connective tissue Layer adjacent to epidermis: –papillary layer –reticular layer

Papillary layer 乳頭層 Papillary layer (dermal papilla), contains fine collagen fibres, capillary network and nerves Spiraling duct of sweat gland through thick epidermis Dermal ridges determine the patterns of fingerprints; scientific basis of dermatoglyphics Epidermal ridge Dermal ridge

Dermis (reticular layer) 网組層 Dense bundles of connective tissue Sweat gland

Dermis (hypodermis layer) Fat cells strictly speaking not part of dermis

Types of nerve endings in skin Free endingMeissner’s corpuscle Merkel’s ending Paccinian corpuscle Krause’s end organ

Meissner’s corpuscle

Paccinian corpuscle Located deep in dermis Sensation of pressure Characterized by nerve terminal (arrowhead) surrounded by concentric layers of connective tissue cells

Sweat glands 汗腺 Two categories –Eccrine sweat gland 外泌汗腺 Distributed throughout the body Duct open directly to skin surface –Apocrine sweat gland 頂漿分泌汗腺 Ducts do not open directly to skin surface, they open into hair follicles Found in axilla, genital region etc

Sweat gland Secretory portion Duct portion An eccrine type Typically of simple tubular coiled gland Containing the terminal secretory portion [single layer of cells] and ductal portion [with two layers of cells]

Hair and hair follicles 毛与毛囊

Schematic diagram to show various parts and associated structures of hair follicle A B C D

Structure of hair follicle Connective tissue sheath Cortex of hair Outer epithelial root sheath Inner epi root sheath

Hairy skin (scalp) Epidermis Hair follicle Sweat gland Sebaceous gland Dermis

Hair follicle Dermis Sebaceous gland Hair follicle Hair

Hair follicle Dermal papilla Inner epi root sheath External epi root sheath matrix

Types of hairs Terminal hairs 終毛 –Course, long hairs in scalp and beard in males –Long growing cycle –Could reach more than a meter in length Vellus [downy] hairs 毫毛 –Small, fine that may be visible only with magnifier –Shorter growing cycle, thus relatively short –Hairs of the forehead etc Replacement of terminal hairs gradually with vellus hairs resulting in baldness

Nail 甲

Nail Nail plate Nail bed Eponychium 甲上皮 Epidermis

Summary General features of integument Structure of skin and function Keratinization and Pigmentation Types of sweat glands Hair and hair follicles Nail