The Histology of Skin Flashcards
State the functions of melanocyte and Merkel cells found within the epidermis, as well as their embryonic origin.
- melanocytes: produce melanin which protects against UV
- Merkel cells: they are mechanoreceptors
Both of these cell types are derived from the neural crest cells.
a) The skin is composed of: (2 layers)
b) Is the hypodermis part of the skin?
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]
How is the skin classified?
As thick or thin
As haired or glabrous (hairless)
a) The epidermis consists of a __________________ epithelium composed of keratinocytes.
b) There are also 3 much less abundant epidermal cell types, namely:
a) stratified squamous keratinized
b) pigment - producing: melanocytes, antigen-presenting: Langerhans’ cells, tactile epithelial: Merkel cells
The epidermis consists of five sub layers. Name them from the dermis outwards.
- stratum basale
- stratum spinosum
- stratum granulosum
- stratum lucidum
- stratum corneum
(BSGLC) [Histological slide 1] [Histological slide 2]
Discuss stratum basale.
- 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]
What is worth noting about stratum spinosum?
- 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.
Just above stratum basale, some cells (in stratum spinosum) may still divide and this combined zone is sometimes called the ______________________________.
stratum germinativum
What is worth noting about stratum granulosum?
- 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.
What is worth noting about stratum lucidum?
- 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
Why is stratum lucidum an inconsistent layer of the epidermis?
This is because it is found only in thick skin.
What are friction blisters?
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.
Make short notes on keratinocytes.
- 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
State the function of dermal papillae. Where are they most developed?
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.
What are three specializations of the cells of the stratum basalis layer?
- highly indented and folded basal membrane
- numerous hemidesmosomes
- desmosomes (between adjacent basal cells and between overlying cells of stratum spinosum layer)
Name the condition whereby the skin presents with opaque, flakey, white scales overlying thickened red epidermis. What causes formation of the scales?
(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.
“____(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.”
- Langerhans cells
- mesodermal
- spinosum
- Birbeck
“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.”
- basale
- tyrosine
- melanosomes (vesicles in the melanocyte)
- 3,4-dihydroxyphenylalanine
- tyrosinase
“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.”
(A) moles
(B) melanoma
a) An _______ is the number of melanocytes per unit area of epidermis.
b) List 2 melanocyte markers.
a) epidermal melanin unit
b) S 100, Melan A/MART - 1
__________ (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.
Merkel cell carcinoma
_________ 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.
Vitiligo
Albinism is associated with absence of ________ enzyme. This produces skin hypopigmentation.
tyrosinase
“_______(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.”
(1) Merkel
(2) basale
(3) neural crest
(4) keratinocytes
