Skin

بسم هللا الرحمن الرحيم

Prof. Dr. Huda Al-khateeb 

Head of Histology Section/Dept. of Anatomy 
Head of Quality Assurance & Academic Performance 
College of Medicine/Univ. of Baghdad 

SKIN

By the end of this lecture the student should be able to: 

1.

List  the skin appendages  

2.

Describe the histology of hair and hair follicle 

3.

Discuss the histological features of the nail 

4.

List type of glands of skin 

5.

Recognize the structure and function of sebaceous gland 

6.

Differentiate between the eccrine and apocrine sweat glands 

7.

Realize some pathological conditions related to skin 

appendages 

objectives

Hair is an elongated 

keratinized

 structure 

derived from invaginations of the epidermal epithelium called 

hair follicles

. 

that of the palms, soles, 

except

has at least minimal hair 

All skin 

lips, glans penis, clitoris, and labia minora.

Hair has 

root

 (within the skin) and 

shaft

 (outside the skin).

Hair 

Hairs grow 

discontinuously,

 with periods of growth followed by 

periods of rest (peroid of growth depends on area of the body).

may last for several years, 

) 

anagen

(

growth periods 

In the scalp, 

inactivity

and 

) 

catagen

(

follicle regression 

whereas the periods of 

(telogen) 

may together last only 3 to 4 months.

sex 

Hair growth on the face and pubis is strongly influenced by 

.

, especially androgens

hormones

Hair

growth

Hirsutism – (Cushing syndrome)

Hair follicle

Hair follicle wall 
(1)Connective tissue sheath 
(2) Glassy membrane 
(3) External (outer)root sheath 
(4)Internal (inner) root sheath 
Hair 
(1) Cuticle    (2) Cortex         (3) Medulla 

Hair follicle

During anagen the hair follicle has a terminal dilatation called a 

hair bulb

.

 A 

dermal papilla 

inserts into the base of the hair bulb and 

contains a capillary network required to sustain the hair 
follicle.

 Loss of this blood flow results in death of the follicle.

Hair bulb

Allopecia and Allopecia Areata

A small bundle of smooth muscle fibers that run from the 
connective tissue sheath of hair follicle to the dermal papillary 
layer

. 

Contraction of these muscles pulls the hair shafts to a more 
erect position, usually when it is cold to produce tiny bumps on 
the skin surface ("

goose bumps

").

Arrector pili muscle 

Nails 

  hard, flexible plates of keratin on the dorsal surface of each 
distal phalanx.  
The proximal part of the nail is the 

nail root 

and is covered by 

the proximal skin fold which is thin and lacks both hair and 
glands. The epidermal stratum corneum extending from the 
proximal nail fold forms the 

cuticle

, or 

eponychium

.  

The keratinized nail plate is bound to a bed of epidermis called 
the 

nail bed

, which contains only the basal and spinous layers. 

 The 

nail plate

 extends from the nail root.  

The nail plate continues growth and pushes distally over the 
nail bed (which makes no contribution to the plate) at about 3 
mm/month for fingernails and 1 mm/month for toenails.  
The distal end of the plate becomes free of the nail bed at the 
epidermal fold called the 

hyponychium

 and is worn away or cut 

off. 

Nails 

paronychia

Sebaceous glands are embedded in the dermis over most 
of the body surface, 

except

 the thick skin of the 

palms and soles. 

 Sebaceous glands are 

branched acinar 

glands with 

several acini converging at a short duct which usually 

empties into the upper portion of a 

hair follicle

. 

Sebaceous

 Glands

The acini consist of a 

basal layer 

of undifferentiated 

flattened epithelial cells on the basal lamina. These cells 

proliferate and are displaced toward the middle of the 

acinus, undergoing terminal differentiation as distinctly 

large, lipid-producing 

sebocytes

.

Sebaceous

 Glands

The cytoplasm of 

Sebocytes  is 

filled with small fat droplets. 

Their nuclei shrink and undergo autophagy along with 
other organelles and near the duct the cells disintegrate 
and release the lipids via 

holocrine secretion

. The product 

of this process is 

sebum

, which is gradually moved to the 

surface of the skin along the hair follicle.

Sebaceous

 Glands

Sebum

 is a complex mixture of lipids that includes wax esters, 

squalene, cholesterol and triglycerides which are hydrolyzed by 

bacterial enzymes after secretion.

puberty,

from sebaceous glands greatly increases at 

Secretion

stimulated primarily by 

testosterone

 in men and by 

ovarian

 and 

adrenal

androgens

 in women.

the 

maintain

of sebum appear to include helping 

functions

Specific 

stratum corneum and hair, as well as exerting weak 

antibacterial

 and 

antifungal

 properties on the skin surface.

Sebaceous

 Glands

The flow of sebum is continuous, and a disturbance in the 
normal secretion and flow of sebum is one of the reasons for the 
development of 

acne

, a chronic inflammation of obstructed 

sebaceous glands common during and after puberty.

MEDICAL APPLICATION

Sweat glands are epithelial derivatives embedded in the dermis.

There are 2 types:

Eccrine

 sweat glands 

apocrine

 sweat glands

* have different distributions, functions, and structural details

Sweat Glands

* Eccrine sweat glands are widely distributed in the skin and 
are most numerous and most well developed on the 

palm

 of the 

hand and 

sole

 of the foot.

* Sweat gland is a 

simple coiled tubular 

gland.

 * It has a rule in body 

temperature regulation

.

* It is innervated by cholinergic nerves.

Eccrine sweat

 glands

Secretory part

: (3 types of cells)

1.Clear cells (basal) – secret NaCl and water ( sweat)

2.Dark cells (luminal) – secret bacricidal substances 

3.Myoepithelial cells (peripheral)- contraction helps discharge 

of secretion

:

Duct

Double layer s of cuboidal epithelium-reabsorb NaCl and water 

(under influence of aldosterone)

Eccrine sweat

 glands

 The cells of the secretory part has lightly 
stained cytoplasm as compared to that of ducts

Sweat pores- on surface of epidermis

, 

evaporates

After its release on the surface of the skin sweat 

cooling

 the skin. Besides its important cooling role, sweat 

organ, eliminating small 

excretory

glands also function as an 
amounts of 

nitrogenous

 waste and excess 

salts

.

(1)Apocrine sweat glands are found in  

axillary

 and 

perineal

 skin.

 (2)Their development (but not functional activity) depends on

sex hormones 

and is not complete until puberty. 

(3)Their secretory acini are 

larger

 than that of eccrine sweat glands 

and contain 2-3 layers of myoepithelial cells

(4) Their ducts opens to the 

hair follicle

nerve 

adrenergic

sweat glands are innervated by 

Apocrine

)

5

(

fibers

cholinergic

sweat glands receive 

eccrine

endings, whereas 

Apocrine 

sweat glands

eosinophilic

, 

cuboidal

consist of simple 

portions 

secretory

The 

cells with numerous apical secretory granules that undergo 

exocytosis

. Thus the glands are 

misnamed

: their cells 

show merocrine, not apocrine, secretion. 

glands.

eccrine

is similar to that of the 

the ducts 

The wall of 

..

Apocrine 

sweat glands

The slightly viscous secretion is initially odorless 
but may acquire a 

distinctive  body odor 

as a result 

of bacterial activity

. 

Apocrine 

sweat glands

Any question?