The Histology of Skin

Question 1

State the functions of melanocyte and Merkel cells found within the epidermis, as well as their embryonic origin.

Answer 1

1. melanocytes: produce melanin which protects against UV
2. Merkel cells: they are mechanoreceptors

Both of these cell types are derived from the neural crest cells.

Question 2

a) The skin is composed of: (2 layers)
b) Is the hypodermis part of the skin?

Answer 2

a) epidermis - epithelial layer of ectodermal origin; dermis/cornium - connective tissue layer of mesodermal origin

b) No. It is a subcutaneous tissue beneath the dermis made up of connective tissue containing many adipose cells that form the panniculus adiposus.
[Histological slide: hypodermis]

Question 3

How is the skin classified?

Answer 3

As thick or thin
As haired or glabrous (hairless)

Question 4

a) The epidermis consists of a __________________ epithelium composed of keratinocytes.
b) There are also 3 much less abundant epidermal cell types, namely:

Answer 4

a) stratified squamous keratinized
b) pigment - producing: melanocytes, antigen-presenting: Langerhans’ cells, tactile epithelial: Merkel cells

Question 5

The epidermis consists of five sub layers. Name them from the dermis outwards.

Answer 5

1. stratum basale
2. stratum spinosum
3. stratum granulosum
4. stratum lucidum
5. stratum corneum

(BSGLC) [Histological slide 1] [Histological slide 2]

Question 6

Discuss stratum basale.

Answer 6

• single layer of basophilic cuboidal or columnar cells on the basement membrane at the dermal-epidermal junction.
• heavily impregnated with melanin granules, and melanocytes are scattered in this layer
• cells are connected to each other and to keratinocytes by desmosomes, and hemidesmosomes attach these cells to the basement membrane.
• characterized by intense mitotic activity and contains, along with the deepest part of the next layer, progenitor cells for all the epidermal layers
• [Diagram: melanocyte within stratum basale]

Question 7

What is worth noting about stratum spinosum?

Answer 7

• thickest layer especially in the epidermal ridges
• also known as squamous or spinous layer
• Langerhans cells are predominantly found here
• consists of generally polyhedral cells having central nuclei with nucleoli and cytoplasm actively synthesizing keratins
• keratin filaments assemble here into microscopically visible bundles called tonofibrils, which converge and terminate at the numerous desmosomes holding the cell layers together.

Question 8

Just above stratum basale, some cells (in stratum spinosum) may still divide and this combined zone is sometimes called the ______________________________.

Answer 8

stratum germinativum

Question 9

What is worth noting about stratum granulosum?

Answer 9

• cells in this layer contain keratohyalin granules (intensely basophilic)
• some cells in this layer towards the surface have lost their nuclei but their outline is clear
• consists of three to five layers of flattened cells, now undergoing the terminal differentiation process of keratinization.

NB: Keratohyalin granules are dense non-membrane bound masses of filaggrin and other proteins associated with the keratins of tonofibrils, linking them further into large cytoplasmic structures.

Question 10

What is worth noting about stratum lucidum?

Answer 10

• relatively thin translucent and marked eosinophilic layer
• made up of flattened cells whose margins are still clear
• eleiden, a clear protein rich in lipids, derived from keratohyalin, gives cells in this layer a lucid appearance

Question 11

Why is stratum lucidum an inconsistent layer of the epidermis?

Answer 11

This is because it is found only in thick skin.

Question 12

What are friction blisters?

Answer 12

They are lymph-filled spaces created between the epidermis and dermis of thick skin by excessive rubbing, as with ill-fitting shoes or hard use of the hands. If continued, such activity produces protective thickening and hardening of the outer cornified epidermal layers, seen as corns and calluses.

Question 13

Make short notes on keratinocytes.

Answer 13

• they separate the organism from its external environment
• they develop from the surface ectoderm embryologically
• cytoplasm is filled with tonofilaments and contain keratohyalin granules
• keratohyalin granules contain intermediate filament associated proteins that aid in the aggregation of keratin filaments
• desquamation of surface keratinocytes from the stratum corneum is regulated by proteolytic degradation of the cells’ desmosomes
• they synthesize and exocytose lamellar bodies that contribute in the formation of the intercellular epidermal water barrier

Question 14

State the function of dermal papillae. Where are they most developed?

Answer 14

They enhance anchorage/tethering of the epidermis to the dermis. They are most developed where exposure to shearing forces is most constant e.g. sole and palm.

Question 15

What are three specializations of the cells of the stratum basalis layer?

Answer 15

1. highly indented and folded basal membrane
2. numerous hemidesmosomes
3. desmosomes (between adjacent basal cells and between overlying cells of stratum spinosum layer)

Question 16

Name the condition whereby the skin presents with opaque, flakey, white scales overlying thickened red epidermis. What causes formation of the scales?

Answer 16

(a) psoriasis
(b) the turnover time of keratinocytes is greatly reduced hence the maturation sequence of the epidermal layers is incomplete. Psoriasis is caused by overactive T lymphocytes triggering an autoimmune reaction in the skin, which can also lead to inflammation with redness, irritation, itching, and scaling, with a defective skin barrier.

Note: Histologically, parakeratosis will manifest in psoriasis. Parakeratosis refers to incomplete maturation of epidermal keratinocytes, resulting in abnormal retention of nuclei in the stratum corneum.

Question 17

“____(1)____ cells are involved in signaling in the immune system. These cells of ____(2)____ origin [what is their embryonic origin] are predominantly found in stratum ____(3)____ and possess dendritic processes. They cytoplasm in ____(1)____ cells contains typical tennis racket granules - ____(4)____ granules (vermiform granules). ____(1)____ cells are antigen-presenting cells of the epidermis.”

Answer 17

1. Langerhans cells
2. mesodermal
3. spinosum
4. Birbeck

Question 18

“Melanocytes are scattered within the stratum ____(1)____. They produce the pigment melanin which is responsible for skin and hair colour. Melanin is synthesized from the amino acid ____(2)____ within specific cytoplasmic organelles called ____(3)____. Melanin is produced by oxidation of ____(2)____ to ____(4)____ (DOPA) by ____(5)____ and the subsequent transformation of DOPA into melanin.”

Answer 18

1. basale
2. tyrosine
3. melanosomes (vesicles in the melanocyte)
4. 3,4-dihydroxyphenylalanine
5. tyrosinase

Question 19

“Melanocytes can normally proliferate in skin to produce ____(A)____, or benign melanocytic nevi of various types. Changes in the size or appearance of ____(A)____ are sometimes indicative of dysplasia (abnormal growth or development of cells) that can progress further to malignant ____(B)____. Dividing rapidly, malignantly transformed melanocytes often penetrate the basal lamina, enter the dermis, and metastasize by invading blood and lymphatic vessels.”

Answer 19

(A) moles
(B) melanoma

Question 20

a) An _______ is the number of melanocytes per unit area of epidermis.
b) List 2 melanocyte markers.

Answer 20

a) epidermal melanin unit
b) S 100, Melan A/MART - 1

Question 21

__________ (MCC) is a rare but highly aggressive type of skin cancer that develops when Merkel cells undergo uncontrolled proliferation. It is 40 times less common than malignant melanoma but has twice the mortality of that disease.

Answer 21

Merkel cell carcinoma

Question 22

_________ is characterized by localized absence of melanocytes. The disease destroys all the melanocytes in the affected skin and the skin becomes glaringly white. It is due to an autoimmune destruction of melanocytes.

Answer 22

Vitiligo

Question 23

Albinism is associated with absence of ________ enzyme. This produces skin hypopigmentation.

Answer 23

tyrosinase

Question 24

“_______(1)________cells, or epithelial tactile cells, are low-threshold mechanoreceptors associated with sensory nerve endings located in the stratum ____(2)____. They develop from the ________(3)_________ and these cells are essential for sensing gentle touch. They are bound to _________(4)____ by desmosomes.”

Answer 24

(1) Merkel
(2) basale
(3) neural crest
(4) keratinocytes

Question 25

Differentiate between eccrine and apocrine sweat glands.

Answer 25

(1) Eccrine glands are distributed over the entire surface of the body except for the lips and part of the external genitalia, whereas apocrine sweat glands are limited to the axilla, areola and nipple of the mammary gland, skin around the anus, and the external genitalia.
(2) Eccrine sweat glands open directly onto the skin surface, whereas apocrine sweat glands open into the hair follicle.
(3) Apocrine sweat glands are innervated by adrenergic nerve endings, whereas eccrine sweat glands receive cholinergic fibers.
(4) Eccrine sweat glands respond to heat and stress whereas apocrine glands respond to emotional and sensory stimuli but not to heat.
(5) [4-minute video]

Question 26

The glands of the external acoustic meatus canal are called ____(a)____ glands, and those of the margin of the eyelid are called ____(b)____. They are apocrine-type glands.

Answer 26

(a) ceruminous
(b) glands of Moll

Question 27

Name the types of cells present in the secretory portion of eccrine sweat glands.

Answer 27

clear cells, dark cells, myoepithelial cells

Question 28

What type of glands are sebaceous glands? What is the mode of secretion in sebaceous glands?

Answer 28

1. simple branched acinar glands
2. holocrine secretion

Question 29

Name and explain the three segments of the hair follicle.

Answer 29

1. Infundibulum: extends from the surface opening of the follicle to the level of the opening of its sebaceous gland.
2. Isthmus: extends from infundibulum to the level of insertion of arrector pili muscle
3. Inferior segment: extends from the bulge to the base of the follicle
   [Diagram: Segment of a hair follicle]

Further notes:
The infundibulum is a part of the pilosebaceous canal, which is used as a route for the discharge of sebum.

Question 30

State two adaptations of skin tissue to its function.

Answer 30

1. Keratinisation
2. Stratification

Question 31

State the functions of vernix caseosa.

Answer 31

In utero:
1. It protects the skin of the fetus against the macerating action of amniotic fluid.
2. During a vaginal delivery, it acts as a lubricant and antimicrobial cover against bacteria in the genital tract.

After birth:
1. It acts as a natural moisturizer, keeping the newborn’s skin hydrated. It helps prevent excessive water loss from the skin.

Question 32

Briefly discuss the following disorders of the skin:
a) Ichthyosis vulgaris
b) hypertrichosis
c) alopecia
d) congenital anonychia
e) albinism
f) Port-wine stain/syndrome (nevus flammeus)

Answer 32

a) Ichthyosis vulgaris: congenital condition characterized by dry and scaly skin due to excessive keratinization

b) hypertrichosis: excessive hair growth over a particular area (localized hypertrichosis) of the body or over the entire body (generalized hypertrichosis)

c) alopecia: autoimmune disease causing destruction of hair follicles (several forms: alopecia areata - bald patches on scalp, alopecia totalis - absence of hair on scalp, alopecia universalis - complete hair loss over scalp and entire body)

d) congenital anonychia: absence of fingernails and toenails

e) alibinism: little or no melanin; caused by genetic mutations that result in absence of enzyme tyrosinase

f) Port-wine stain/syndrome: type of birthmark characterized by swollen blood vessels in the skin which create a reddish-purplish discoloration

Question 33

Stratum basale together with stratum spinosum form the ____________ layer.

Answer 33

malpighian

Question 34

The principle GAG in the reticular dermis is ____________.

Answer 34

dermatan sulfate

Question 35

Here are some slides of skin showing sweat glands. Click on Answer to view them.

Answer 35

[Slide 1] [Slide 2] [Slide 3] [Slide 4] [Slide 5] [Slide 6]

Question 36

Here are some slides of the skin showing the pilosebaceous unit. Click on Answer to view them.

Answer 36

[Slide 1] [Slide 2] [Slide 3] [Slide 4]

Question 37

Here are some slides of the skin showing the nail. Click on Answer to view them.

Answer 37

[Slide 1] [Slide 2] [Diagram]

Question 38

Name two hormones secreted by adipose tissue indicating their functions (4 marks).

Answer 38

(1) Leptin regulates appetite and body energy expenditure
(2) Apelin increases cardiac muscle contractility
(3) Adiponectin stimulates fatty-acid oxidation in liver and muscles