Epidermis Flashcards
Epidermis
- layer of specialised epithelium
- composed of stratified squamous epithelium
2 main cells found in the epidermis
1) Epithelial cells/keratinocytes- form the bulk of the epidermis
2) Melanocytes- produce melanin, which is a skin pigment
Which cells are present in the epidermis that have temporarily migrated there ?
- Langerhans cells
- Lymphocytes
- Monocytes
- Macrophages
- they help to make the epidermis an active part of the immune system of the body
How many layers is the epidermis composed of ?
- 6
Germinating zone
- 2 inner layers
- living part
- cells that originate here pass upwards towards the surface, all while undergoing the process of keratinisation and finally end up as the superficial layer of the skin, which is constantly being shed (every 28-30 days)
Horny zone
- 3 outer layers
- dead part
- the superficial horny cells are continuously lost from the surface of the skin and must be replaced by cells that are actively multiplying in the living layers
Keratinisation
- process by which keratin replaces the normal living cytoplasm of the cell, killing the cell in the process
- keratin in the outer layer of the epidermis is the end result of transformation of epithelial cells into scales
Keratin
- tough, fibrous protein
- you get soft and hard keratin
- keratin in general is protective and helps waterproof the skin
What makes keratin hard or soft?
- amount of sulphur
Difference between hard and soft keratin
soft keratin (less sulphur) and hard keratin (more sulphur)
- soft keratin covers the skin as a whole, while hard keratin occurs in nails and hair
- hard keratin is solid, does not desquamate, a more permanent material and contains more cystine + sulphur
Layers of the epidermis
(surface downwards)
Phrase to remember layers: Daring Captains Lead Great Ships Going Boldly
1) Stratum Disjunctivum
2) Stratum Corneum
3) Stratum Lucidum
4) Stratum Granulosum
5) Stratum Spinosum
6) Stratum Germinativum
Basement Membrane/Basal Lamina
*2-4- horny layer/zone
*5-6- germinating layer/zone
Drawing of the layers of the epidermis
- in notes
Stratum germinativum
- innermost layer
- consists of a single layer of columnar shaped cells, with nuclei
- mitosis occurs frequently in this layer and is the source of cells for the whole keratinisation process
- cells here to be anchored to the underlying basement membrane which interfaces between the epidermis and dermis
Stratum spinosum
- composed of several cells which are irregularly shaped and slightly separated from each other
- there are intercellular bridges between these cells, which appear as spiny processes
- no continuity of the cell cytoplasm across these bridges and because of this appearance, this layer is also called the prickle cell layer
- the cytoplasm of these cells contain bundles of filaments (tonofibrils)
Tonofibrils
- bundles of filaments found in the cytoplasm which are thought to be precursors of keratin
Stratum granulosum
- composed of 1-4 rows of diamond-shaped cells, lying parallel to the skin surface
- the cytoplasm of these cells contain irregular shaped keratohyalin granules, which are involved in the process of formation of soft keratin
- as the granules increase in size and number, the cell nucleus becomes paler and indistinct and degenerates
- it is in this layer that the epidermal cells die
Stratum lucidum
- this layer is not always visible
- seen most clearly in the skin of the palms and soles
- it is a clear, translucent layer, 3-5 cells thick and are being flattened and densely packed
- no nuclei
- the cytoplasm contains eleidin, which is a gel-like substance resumed to be the transformation product of keratohyalin
- forms the waterproofing barrier of the skin
Stratum corneum
- outermost layer and is composed of clear, dead, scale-like cells, which are flattened and fused
- the cytoplasm has been replaced by soft keratin
- the most superficial layers are flat horny plates that are constantly shed
Melanocytes
- cells that produce the pigment melanin
- found beneath and within the basal zone of the epidermis
- melanin contributes to the colour of the skin, hair and eyes
- the skin colour is influenced by the amount of melanin the melanocytes produce, rather than by the number of melanocytes present
- melanin is mainly produced on exposure to UV light
- acts as a natural sun filter to reduce exposure of deeper tissues to the harmful UV rays
Langerhans cells
- a large lymphocyte which is present between epithelial cells
- migrate via the lymphatics and lymph nodes and are significant in skin allergy
- they are susceptible to UV light which reduce their numbers and function, thus suppressing the skin’s immune defense system
Dermis
- composed of connective tissue
Hypodermis
- composed of loose connective tissue
- fatty and is for protection + cushioning
Where is the skin the thickest ?
- palms
- soles
Where is the skin the thinnest?
- eyelids
