The skin is the largest organ of the human body and serves as a protective barrier between the internal organs and the external environment. It is a complex and dynamic organ with various functions, including protection, sensation, regulation of temperature, and synthesis of vitamin D. Maintaining skin health is essential for overall well-being, and proper skincare practices, protection from UV radiation, and early detection of skin conditions contribute to skin health.

1. Dr / Ahmed Salah Ashour(Ph.D.)
Associate professor of human anatomy
Dr.Ahmedashour@gmu.ac.ae
USMLE Clinical Anatomy

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SKIN

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The skin is the largest organ of the
human body and serves as a
protective barrier between the
internal organs and the external
environment. It is a complex and
dynamic organ with various
functions, including protection,
sensation, regulation of temperature,
and synthesis of vitamin D.
Maintaining skin health is essential
for overall well-being, and proper
skincare practices, protection from
UV radiation, and early detection of
skin conditions contribute to skin
health.

5. In average, the total surface area of
the skin for an adult is estimated to be
around 1 million square milli-meters

6. • Layers:
1-Epidermis:
• Outermost layer.
• Epithelium.
2-Dermis:
• Innermost layer.
• C.T

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8. N.B:
Beneath the two layers lies the Hypodermis (Loose areolar connective tissue) ,
Not considered part of skin

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10. • It is avascular.
• Rich in nerve endings.
• It is Keratinized Stratified
Squamous Epithelium.
• Composed of 4 types of cells:
• Keratinocytes (85% of cells)
• Melanocytes
• Langerhans cells
• Merkel cells

11. • 1-Keratinocytes
üMost abundant population.
üUndergo a process of keratinization(accumulate keratin filaments until replace the
organelles → cells die)

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Keratinocytes are Arranged in 5 layers according to the degree of maturation:

13. Stratum Basale
(germinativum)
layers Single layer
Cell shape low columnar
Cytoplasm Organelles are healthy
Nucleus Basal

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Stratum Spinosum
layers Several layers
Cell shape polyhedral with spines
Cytoplasm Organelles are healthy
Nucleus central

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Stratum Granulosum
(Granular cell layer)
layers 2-3
Cell shape flat
Cytoplasm Organelles are degenerating
Keratin granules
Nucleus flat

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Stratum Lucidum
(Clear Zone)
layers 2-3
Cell shape Flat
Cytoplasm organelles are degenerating
Im-mature keratin filaments
Nucleus NO nucleus

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Stratum Corneum
layers Several layers
Cell shape Flat
Cytoplasm NO organelles
Mature keratin filaments
Nucleus NO nucleus

18. On average, the process of skin
renewal takes about 28 days.

19. • 2-Melanocytes
ü Found in basal cell layer
ü The cell form melanin and transfer their
pigment into keratinocytes

20. It is estimated that the average person
has between 1,000 and 2,000
melanocytes per square millimeter of
skin

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Vitiligo
is a localized skin condition in which, over time, there is a loss of pigmentation.
Although the precise cause of vitiligo is unknown, it is an autoimmune disease that
results in the destruction of melanocytes and thus skin pigment.

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Albinism
is a genetic condition characterized by a generalized lack of melanin, the pigment
responsible for the color of the skin, hair, and eyes. People with albinism have little
to no melanin production, which can result in a range of physical characteristics and
health issues.

23. • 3-Merkel cells
ü Found in basal cell layer
ü sensory nerve fibers terminate as disc-
shaped expansions beneath Merkel’s cells.

24. • 4-Langerhan’s cells
ü Between cells in Stratum Spinosum
ü Star shaped with multiple processes
ü Antigen presenting cells

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Cutaneous burns
First-degree (or superficial) burns are
limited to epidermis, in which the skin
presents with erythema and may peel;
mild sunburn is a common example.

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Cutaneous burns
Second-degree (or partial-thickness)
burns, often caused by scalding, extend
into deep (reticular) dermis, leading to
inflammation, severe pain, and blister
formation.

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Cutaneous burns
More serious third-degree (or full-
thickness) burns extend through the
entire dermis that may reach deeper
subcutaneous layers. Because these burns
are so deep, they cause little or no pain
because of destruction of nerves.

28. The entire surface of skin is
replaced every month, which
put another way means
human body have about
1,000 different skins in life!

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30. • Thicker than epidermis.
• Consists of 2 layers:

31. Papillary Layer
Thin
Loose connective tissue
Blood vessels, nerves & lymphatics
Reticular Layer
Thick
dense connective tissue
Less vascular

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Scleroderma
Of the many connective tissue
diseases that primarily target collagen
in the body, scleroderma (or systemic
sclerosis) is a chronic, degenerative
disorder that leads to overproduction of
collagen as a result of
an autoimmune dysfunction.

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Skin cancer
is the most common malignant
disease in North America. The three
major types are
• Basal cell carcinoma
• Squamous cell carcinoma (arise
from keratinocytes)
• Melanoma (originates from
melanocytes).

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35. • Irregular projections of the dermis called Papillae.
• Invaginations of the epidermis known as Epidermal ridges.
PAPILLAE
RIDGES

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PAPILLAE
RIDGES

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• Definition:
exocrine glands found in the skin that produce sweat. Sweat is a watery fluid that
plays a crucial role in regulating body temperature and is composed primarily of
water, electrolytes, and small amounts of other substances.
Sweat glands

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Eccrine Sweat Glands:
• Distribution:
Distributed all over the body, with a higher concentration on
[palms of the hands, soles of the feet, and forehead].

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• Function:
The primary function of eccrine sweat
glands is thermoregulation. When the
body temperature rises due to factors
such as exercise, heat, or stress, eccrine
glands release sweat onto the skin
surface. As the sweat evaporates, it cools
the body.

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Apocrine Sweat Glands:
• Distribution:
Are found in areas with dense hair follicles, such as the
[armpits and genital region].

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• Function:
Apocrine glands produce a thicker, milky
secretion that contains proteins and
lipids. These glands become active
during puberty and are influenced by
hormonal changes.

43. The human body has 2.5
million sweat pores

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Sweat Composition:
Sweat is primarily composed of
water, electrolytes (such as sodium
and chloride), urea, and small
amounts of other substances.
The composition of sweat can vary,
and different types of sweat glands
may produce sweat with slightly
different compositions.

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Disorders related to sweat glands can include conditions like
Hyperhidrosis (excessive sweating)
Anhidrosis (lack of sweating)
Hyperhidrosis
Anhidrosis

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Are microscopic exocrine glands in the
skin that are responsible for producing
and secreting sebum, an oily substance
that lubricates and waterproofs the skin
and hair.
Sebaceous glands

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• Location:
Sebaceous glands are found
throughout the skin. Most numerous
on the face and scalp but are also
found on other parts of the body.
Sebaceous glands are found
throughout the skin, Except on the
palms of the hands and the soles of
the feet.

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Most sebaceous glands are associated
with hair follicles.
The ducts of the sebaceous glands
usually open into the hair follicles, and
sebum is released onto the hair shaft
and then onto the skin surface.

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• Function:
The primary function of sebaceous
glands is to produce sebum, an oily
substance, sebum [fats, cholesterol,
proteins, and electrolytes, triglycerides,
wax esters, squalene] which helps to
keep the skin and hair moisturized,
supple, and protected from
environmental factors.

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Acne
Sebaceous glands can be involved in the development
of acne.
Excessive sebum production, combined with the
shedding of skin cells and the presence of bacteria,
can lead to the formation of acne lesions.

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Holocrine Secretion:
Sebaceous glands use a holocrine mode of secretion.
This means that the entire cell, filled with sebum, disintegrates and releases its
contents into the duct. The disintegrated cell remnants, along with the sebum, form
the substance that is eventually released onto the skin.

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Nails
• Are hard, protective plates that cover the tips of fingers and toes in humans and
some other animals.
• They are composed of a protein called keratin, which is the same protein found
in the skin and hair.

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• Histological arrangement
A- Nail Plate:
This is the visible part of the nail. It is composed of tightly packed, dead,
keratinized cells.
B- Nail Bed:
The skin beneath the nail plate.

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C- Cuticle:
The thin layer of tissue that overlaps the
base of the nail plate and protects the
new nail as it grows.
D- Lunula:
The white, crescent-shaped area at the
base of the nail. It is a visible part of the
matrix, where new nail cells are formed.

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• Nail Growth:
Nails grow from the germinal matrix, the
base of the nail beneath the cuticle.
The rate of nail growth varies but is
generally about 3 mm per month.

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Nail Disorders
Fungal infections onychomycosis: Fungi, including
dermatophytes, yeasts, and molds, can invade the nail
bed and cause infection.
Ingrown nails: occur when the edges or corners of a
nail grow into the surrounding skin, leading to pain,
redness, swelling, and, in some cases, infection.
Psoriasis: Psoriasis is a chronic autoimmune condition
characterized by inflammation and the rapid turnover of
skin cells. It involves the nails [nail psoriasis].

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Are small, tube-like structures in the skin that produce and
house hair.
They are found all over the body, except for areas like the
palms of the hands and the soles of the feet.
Hair follicles

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• Histological arrangement
A- Hair Bulb:
The base of the hair follicle is called the hair bulb. This
is where the hair originates and where cells divide
rapidly, contributing to hair growth.
The hair bulb surrounds the papilla, a structure with
blood vessels that nourish the growing hair.
Hair Bulb
Papilla

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B- Hair Shaft:
The hair shaft is the visible part of the hair that extends
above the skin's surface.
It is composed of dead, keratinized cells that have been
pushed up through the hair follicle.
Hair Shaft

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C- Root:
The part of the hair below the skin surface is called the root.
It is embedded in the hair follicle.
Hair Shaft
Hair root

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D- Follicle Wall:
The hair follicle is surrounded by several layers of
tissue, forming the follicle wall.
These layers include the internal root sheath and
external root sheath
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Hair Bulb
Papilla
Hai medulla
Internal root sheath
External root sheath.

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E- Arrector Pili Muscle:
Attached to the hair follicle is the arrector pili
muscle, a tiny muscle responsible for causing hair
to stand up when contracted.
This response is often triggered by cold or
emotional factors.

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• Location:
Associated with each hair follicle is a
sebaceous gland, which produces sebum,
an oily substance that helps keep the hair
and skin moisturized.

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• Hair Color:
The color of the hair is determined by pigment-producing cells called melanocytes,
which are located in the hair bulb. The type and amount of melanin produced
influence hair color.

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67. medulla
internal root sheath
external root sheath.
dermal papilla

68. Questions

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Q1 Which of the following is NOT considered an epidermal appendage?
Sweat gland
Hair
Hypodermis
Nails

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Q2 A 45-year-old patient presents with a newly discovered pigmented lesion on
their back. On examination, irregular borders and color variation are noted. The
patient reports a history of sun exposure during outdoor activities. A biopsy is
performed, revealing abnormal melanocytes. Further investigation reveals that these
melanocytes differentiate from which embryonic cell type?
A) Ectoderm
B) Mesoderm
C) Endoderm
D) Neural crest cells
E) Ectomesenchyme

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Q3 A 35-year-old female presents to the clinic with concerns about her skin texture
and thickness. Upon examination, the clinician observes that her skin has lost some
of its firmness and elasticity, particularly in the thighs and buttocks area. Further
assessment reveals a decrease in subcutaneous tissue in these regions. Which of the
following characteristics best describes the hypodermis, the layer primarily affected
in this condition?
A) Rich in nerve endings and sensory receptors
B) Abundant in sweat glands and hair follicles
C) High content of blood vessels and lymphatics
D) Predominantly composed of fibrous connective tissue and collagen
E) Dense with melanocytes and keratinocytes

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Q4 A 45-year-old male presents to the dermatology clinic with a complaint of
persistent dry, itchy skin. On examination, he has areas of erythema and scaling
distributed over his arms and legs. Upon further evaluation, a skin biopsy is
performed, revealing abnormalities in the epidermis. Which of the following
statements regarding the epidermis is correct?
A) The epidermis is highly vascular, allowing for efficient nutrient exchange.
B) It contains numerous blood vessels necessary for thermoregulation.
C) The epidermis is avascular, receiving its nutrients through diffusion from the
underlying dermis.
D) It lacks sensory nerve endings, contributing to its insensitivity to touch and pain.

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Q5 A dermatologist is examining a skin biopsy sample under the microscope. She
observes a layer of the epidermis that is actively involved in cell division and is
responsible for replenishing the outer layers of the skin. This layer, often referred to
as the site of skin renewal, is also known as the:
Answer Options:
A) Stratum corneum
B) Stratum granulosum
C) Stratum spinosum
D) Stratum germinativum

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Q6 All are true concerning stratum spinosum EXCEPT:
Present above the basal cell layer
Considered bags of keratin
Langerhans cells are present in this layer
Rich in desmosomes

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Q1 Hypodermis
Q2 Neural crest cells
Q3 High content of blood vessels and lymphatics
Q4 The epidermis is avascular, receiving its nutrients through diffusion from the
underlying dermis.
Q5 Stratum germinativum
Q6 Considered bags of keratin

76. List of Texts and Recommended Readings
• Last's Anatomy, Regional and Applied. Chummy S. Sinnatamby. 12th edition 2011, ISBN:13 - 978 0 7020 3394 0 (Available in ClinicalKey:
https://www.clinicalkey.com/#!/browse/book/3-s2.0- C2009060533X)
• Estomih Mtui, Gregory Gruener and Peter Dockery. Fitzgerald's Clinical Neuroanatomy and Neuroscience. 7th edition; 2016, ISBN: 13 - 978-0-7020-
6727-3 (Available in ClinicalKey: https://www.clinicalkey.com/#!/browse/book/3-s2.0- C20130134113
• Drake, Richard L. Gray's Anatomy for Students, Third Edition, Elsevier Saunders 2015. ISBN-13: 978-0702051319 (Available in ClinicalKey:
https://www.clinicalkey.com/#!/browse/book/3- s2.0-C20110061707).
• Sobotta Atlas of Human Anatomy. F. Paulsen. Vol.1, 15th Edition; 2013, ISBN: 9780702052514 (Available in ClinicalKey:
https://www.clinicalkey.com/#!/content/book/3- s2.0-B9780702052514500067)
• Sobotta Atlas of Human Anatomy. F. Paulsen. Vol.2, 15th Edition; 2013, ISBN:13 - 978-0-7020-5252-1 (Available in ClinicalKey:
https://www.clinicalkey.com/#!/browse/book/3- s2.0-C20130046919)

77. Recap