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ANATOMY OF THE SKIN
The skin, which makes up the integumentary system, is also known as the cutaneous membrane
and is the largest organ of the body in both surface area and weight
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There is thin skin and thick skin, a main difference between the two being that thick skin has hair
whereas thin skin does not
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5mm, and at its
thickest at 4mm on the soles of the feet
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2
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4
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6
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It is the outermost layer of skin, and
although it itself is avascular, the epidermis is supplied by nutrients from dermis layer
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The 4 Epidermal Cells:
95% of epidermal cells are keratinocytes, cells that produce keratin, the amount of which
increases with age, so at the top of the skin they contain the greatest amount of keratin
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Keratinocytes also produce lamellar granules which secrete a sealant that decreases water entry
and loss, and inhibits the entry of foreign bodies into the skin
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These cells
transfer melanin granules to keratinocytes via long, slender processes that texted between the
keratinocytes
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However, the melanin granules themselves are particularly susceptible to
DNA damage from UV light
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Eumelanin produces brown/black colouring, whilst Pheomelanin
produces a yellow/red colouring
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Langerhans cells arise from red bone marrow and migrate to the epidermis where they have a
role in the immune response
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Langerhans cells are extremely susceptible to damage from UV light
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These cells are responsible for helping humans to feel light touches
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The 5 Epidermal Layers:
Stratum Basale (Basal layer):
This is composed of a single layer of columnar/cuboidal
keratinocytes, as well as some stem cells that differentiate into new keratinocytes, and is the
deepest of the epidermal layers
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Keratin intermediate filaments also
attach to desmosomes, which bind Stratum Basale cells together and then to the cells of the
Stratum Spinosum
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Finally, Merkel cells and their corresponding Merkel discs
are scattered throughout the Stratum Basale
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Keratinocytes in this layer retain their ability to divide but produce coarser bundles of keratin in
intermediate filaments than their Basal counterparts
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This cellular
arrangement is what provides skin with its flexibility and strength
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Stratum Granulosum: Lies around the middle of the epidermis and consists of three to five layers
of flattened keratinocytes that are undergoing apoptosis, and therefore cannot divide or be
metabolically active
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Cells contain the protein
keratohyalin, which converts keratin intermediate filaments into keratin, and lamellar granules
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It
consists of four to six rows of clear, flat, dead keratinocytes containing large amounts of keratin,
which is thought to provide extra protection against wear and tear and abrasion in these areas of
the body
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The thickness of this layer varies greatly between thin and thick skin and the cells here are
extremely thin as they have no organelles, simply packages of keratin enclosed by the plasma
membrane
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They are also constantly undergoing replacement from cells deeper within the strata
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A callus occurs when the skin is frequently exposed to friction and so excess
cell and keratin production occurs, leading to an abnormal thickening of the stratum corneum
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It is far thicker than the
epidermis and predominantly contains fibroblasts, along with some macrophages and a few
adipocytes near its boundary with the subcutaneous fatty layer
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There are two regions of dermis: the papillary region and
the reticular region
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It is comprised of
thin collagen and fine elastic fibres
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Dermal

papillae contain capillary loops, and some have receptorrs called Meissner corpuscles, which are
nerve endings that are sensitive to touch
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These give sensations of temperature, pain, itching and
tickling
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Reticular
region fibres are arranged in a more regular, ‘net-like’ manner than those in the papillary region,
and this helps the skin to resist being stretched
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In the spaces between fibres
are blood vessels, nerves, hair follicles, sebaceous glands and sweat glands
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The lateral bonding between
adjacent collagen fibres is compromised and small dermal blood vessels rupture
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In regions of high mechanical stress e
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the palms of the hands, epidermal ridges occur
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Epidermal ridges create strong bonds between the epidermis and dermis
because the ridges and the dermal papillae fit together in a complementary fashion
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This allows for a greater number of
Meissner corpuscles to be present, and thus increases tactile sensitivity
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The papillary surface of the dermis also increases the surface contact between the epidermis,
which is useful as the vascular network of the dermis provides the nutrition for the above
epidermis
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THE SUBCUTANEOUS LAYER
The subcutaneous layer (subQ) is also called the hypodermis layer, and it is not actually a part of
the cutaneous layer otherwise known as the skin
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Fibres that extend from the dermis attach the skin to the
subQ layer, which in turn is anchored toe underlying fascia, which is the connective tissue that
surrounds muscles and bones
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This region (and sometimes the dermis) also contains
pacinian corpuscles, nerve endings that are pressure-sensitive
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The shaft is the superficial portion of the hair that lies above the skin
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Both of these portions of the hair are
made up of three concentric layers of cells: medulla, context and the hair cuticle
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Sometimes, the inner medulla cells are
lacking in thin hair
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The hair's cuticle is its outermost layer, consisting of a single layer of thin, flat cells that are very
heavily keratinised
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This is made up of an epithelial root sheath, which has
both an internal and an external sheath
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There is then dense dermis surrounding the hair follicle which is known
as the dermal root sheath
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This papilla
contains areolar connective tissue and many blood vessels which serve to nourish the growing hair
follicle
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These arise from the stratum basale and are
responsible for the growth of hair and the production of new hair
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Arrector pili is smooth muscle that (when stimulated by autonomic nerve endings) contracts under
cold conditions to make hair stand on end
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The arrector pili extends from the superficial dermis of the skin to
the dermal root sheath
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e
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GLANDS
Sebaceous glands lie either side of a hair follicle, near the root
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Eccrine glands are found in most areas of the skin and are the how majority of sweat is produced
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Apocrine glands occur on the axilla, pubic region, and the bearded region on males
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They also secrete
pheromones
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They produce
cerumen (ear wax), with their secretory portions lying in the subQ layer and their excretory ducts
opening onto the surface of the ear canal or into the ducts of sebaceous glands, which lie
superficially
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Cerumen also waterproofs the canal
and presents bacteria and fungi from entering the cells there
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Each nail has a nail body, a
free edge and a nail root, which is embedded within a fold of skin
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The basal epithelial cells divide,
eventually becoming detached and enlarged
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No scarring occurs because the epidermis is avascular and thus no blood
vessels are damaged
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Once the would is closed, there can be scarring and loss of tissue function
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DEVELOPMENT OF SKIN
At 4 weeks: The epidermis is a single layer of ectoderm overlying a base of mesenchyme
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The hair bulb develops and mesenchyme in the surrounding dermis becomes
derma root sheath and arrestor pili muscle
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The papillae of the hair begins
to form but does not yet have a blood supply
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AGEING
The epidermis becomes damaged but is continually replaced through skin renewal, keeping skin
looking fresh
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Rate of ageing is largely determined by genetic factors, as gene control several natural processes
that lead to skin ageing
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The collagen and elastin fibre network also begins to break
down, resulting in fine lines and wrinkles, and then sagging skin
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Water can then escape the skin, causing it to
become drier with age
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UV also causes collagen and
elastin breakdown in the dermis, lines and wrinkles, skin looseness, and eventually skin cancer
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Mechanical factors include having
an expressive face and the work of gravity

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