Human Skin

Objectives

At the end of this lecture, student will be able to

• Describe the layers of the epidermis and the cells that
compose them

• Describe various accessory structures of the skin

• Distinguish between the accessory structures and the main
components of skin

• Explain the functions of skin

Content

• Skin

– Layers of epidermis

– Accessory structures

– Function

 The Skin

• Also known as the cutaneous membrane or integument

• Covers the external surface of the body

• Largest organ of the body in both surface area and weight

Structurally, the skin consists of two main parts

1. Epidermis – superficial, thin epithelial tissue portion

2. Dermis – deep, thicker connective tissue portion

Subcutaneous layer (Hypodermis), attaches the dermis to underlying fascia

Functions of subcutaneous layer

Subcutaneous layer serves

• As a storage depot for fat

• Contains large blood vessels that supply to the skin

• Also contains nerve ending called pacinian (lamellated) corpuscles that are sensitive to pressure

Components of the integumentary system

Epidermis

• Composed of keratinized stratified squamous epithelium

• Contains four principal types of cells

 Keratinocytes

 Melanocytes

 Langerhans cells

 Merkel cells

Keratinocytes – 90% of epidermal cells

• Arranged in four or five layers

• Produce the protein keratin

• Protect the skin and underlying tissues from heat, microbes, and chemicals

Melanocytes

• Develop from the ectoderm of a developing embryo

•   Produce the pigment melanin

• Contributes to skin color and absorbs damaging UV light

• Long, slender projections extend between the keratinocytes

• Transfer melanin granules to keratinocyte

Langerhans cells

• Arise from red bone marrow and migrate to the epidermis

• Constitute a small fraction of the epidermal cells

• Participate in immune responses

• Recognize an invading microbe and destroy it

Merkel cells

• Least numerous of the epidermal cell

• Deepest layer of the epidermis

• Contact the flattened process of a sensory neuron, a merkel (tactile) disc

• Detect touch sensations

Layer of Epidermis

4 strata layers in most region	5 layers where friction is greatest (fingertips, palms, & Soles)
Stratum basale Stratum spinosum Stratum granulosum Thin stratum corneum (Thin skin)	Stratum basale Stratum spinosum Stratum granulosum Thick stratum corneum (Thick skin) Stratum lucidum

Stratum basale 

• Deepest layer of the epidermis

• Single row of cuboidal or columnar tinocytes

• Stem cells that undergo cell division to continually produce new keratinocytes

• Contain scattered tono filament (intemediate filaments)

• Melanocytes and merkel cells associated with merkel discs are scattered among the keratinocytes

Stratum spinosum

• Eight to ten rows of many-sided keratinocytes

• Bundles of tono filaments; includes arm like processes of melanocytes and langerhans cells

• Arrangement provides both strength and flexibility to the skin

Stratum granulosum

• Three to five rows of flattened keratinocytes

• Organelles are beginning to degenerate

• Cells contain the protein keratohyalin, converts tono filaments into keratin

• Lamellar granules, which release a lipid-rich, water-repellent secretion

Stratum lucidum

 • Present only in skin of fingertips, palms, and soles

• Consists of 3-5 rows of clear, flat, dead keratinocytes with large amounts of keratin

Stratum corneum

• Twenty-five to thirty rows of dead, flat keratinocytes that contain mostly keratin

Dermis

• Deeper part of the skin

• Composed of a strong connective tissue containing collagen and elastic fibers

• Fibers has great tensile strength

• Ability to stretch and recoil easily

• Based on its tissue structure, the dermis is divided into

a)   A superficial papillary region

b)  A deeper reticular region

Papillary region

• The superficial portion of the dermis

• Consists of areolar connective tissue with thin collagen and fine elastic fibers

• Contains dermal ridges that has capillaries, Meissner corpuscles, and free nerve endings

Reticular region

• Deeper portion of the dermis

• Consists of dense irregular connective tissue with bundles of thick collagen and coarse elastic fibers

• Spaces between fibers contain some adipose cells, hair follicles, nerves, sebaceous glands, and sudoriferous glands

Accessory structures of the skin

• Hair

• Skin glands

• Nails—develop from the embryonic epidermis

Hair

• Formed by the down growth of epidermal cells into dermis or subcutaneous tissue, Hair follicles

• At the base of the follicle is a cluster of cells, bulb

• Hair – formed by the multiplication of cells of bulb

• Pushed upward away from the source of nutrition, the cells die and get keratinised

• The part of hair above the skin, Shaft, the remainder is the root

• Arrector pili – little bundles of smooth muscle fibres attached to the hair follicles

• Contraction makes the hair stand erect and raise the skin around the hair causing goose flesh

Skin glands

Exocrine glands are associated with the skin:

• Sebaceous (oil) glands

• Sudoriferous (sweat) glands

• Ceruminous glands

Sebaceous (oil) glands

• Simple, branched acinar glands

• Connected to hair follicles; absent from the palms and soles

• Produce sebum, moistens hairs and waterproofs the skin

• Clogged sebaceous glands may produce acne

Sudoriferous (sweat) glands

Release sweat, or perspiration, into hair follicles or onto the skin surface through pores

Two types of sweat glands

– Eccrine glands

– Apocrine glands

Ceruminous glands

• Nodified sudoriferous glands

• Secrete cerumen, found in the external auditory canal (ear canal)

Types of sweat glands

Nails

Plates of tightly packed, hard, dead, keratinized epidermal cells

Each nail consists of

• A nail body (plate) – exposed part grown out from the germinative zone of epidermis

• A free edge – part of the nail body that may extend past the distal end of the digit

• A nail root – portion of the nail that is buried in a fold of skin

Nail and its internal details

Functions of the Skin

• Body temperature regulation – liberating sweat at its surface and by adjusting the flow of blood in the dermis

• Blood storage

• Excretion and absorption, and synthesis of vitamin D

• Provides physical, chemical, and biological barriers that help protect the body

• Cutaneous sensations include tactile sensations, thermal sensations, and pain

Summary

• Skin is the largest organ of the body in surface area and
weight

• Principal parts of the skin are the epidermis
(superficial) and dermis (deep)

• Types of cells in the epidermis are keratinocytes,
melanocytes, Langerhans cells, and Merkel cells

• Epidermal ridges provide the basis for fingerprints and
footprints

• The color of skin is due to melanin, carotene, and
hemoglobin

• Accessory structures of the skin—hair, skin glands, and
nails—develop from the embryonic epidermis

• Skin functions include body temperature regulation, blood
storage, protection, sensation, excretion and absorption, and synthesis of
vitamin D