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Questions and Answers
What primarily composes the papillary layer of the dermis?
Which type of sensory receptor is primarily associated with light touch?
Which layer of the skin contains both blood vessels and the subpapillary plexus?
What is a primary function of the skin in addition to protection?
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Which layer of the epidermis is responsible for the high mitotic activity?
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What is the primary role of the A-V anastomosis in the dermis?
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Which type of skin cell is primarily involved in the immune response?
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What characteristic does the stratum granulosum have that aids in keratinization?
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What is the function of Langerhans cells in the skin?
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What defines the stratum lucidum's unique presence in thick skin?
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What primarily distinguishes the reticular layer of the dermis?
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What is a defining characteristic of the hypodermis?
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How does the epidermis renew itself?
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Which sensory receptor is associated with light touch sensations?
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What is the role of the hypodermis in the skin structure?
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In which epidermal layer are Merkel cells primarily found?
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Which sensory receptors are found in the hypodermis?
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What provides autonomic innervation to sweat glands?
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What structure in the hair follicle contains a capillary network?
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Where are melanocytes located in relation to the hair root?
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Which layer of hair is composed of the most heavily keratinized cells?
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What is the primary function of the hypodermis?
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Which part of the hair follicle degenerates above the level of the attached sebaceous gland?
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What type of cells form the matrix of the hair root?
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Study Notes
Melanin Synthesis
- Tyrosinase enzyme in Golgi derived vesicles is responsible for the synthesis of melanin.
- Melanin is deposited in vesicles called melanosomes.
- Melanosomes are transported to the tip of cytoplasmic extensions and then phagocytosed by keratinocytes in the stratum basale and spinosum.
- The supranuclear cap of keratinocytes absorbs and scatters sunlight, protecting DNA.
- Keratinocytes are the melanin depot.
- The epidermal melanin unit consists of one melanocyte and the keratinocytes it transfers melanosomes to.
- Euomelanin is a brown or black pigment, while pheomelanin is a red pigment.
- Melanin and carotene (skin color) contribute to the color of skin.
Cells of the Epidermis
- Keratinocytes form the keratinized stratified squamous epithelium. They are arranged in five layers and are responsible for keratin formation.
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Melanocytes produce melanin and are found in the stratum basale and hair follicles. They are derived from the neural crest and migrate to the epidermis.
- Melanocytes have irregular cytoplasmic processes, are rich in mitochondria, have a well-developed Golgi apparatus (site of melanin synthesis), and short cisternae of RER.
- Langerhans cells are antigen-presenting cells (APCs) found in the stratum spinosum. They play a role in the immune system.
- Merkel cells are mechanoreceptors for light touch sensation and are located in the stratum basale.
Layers of the Epidermis
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Stratum basale: A single layer of basophilic cuboidal or columnar cells with desmosomes attached to the basement membrane (hemidesmosomes).
- It has high mitotic activity.
- Cells in this layer are responsible for the renewal of the epidermis (15-30 days).
- Contains keratin filaments, melanocytes, and Merkel cells.
- As cells differentiate and move upward, the amount and type of keratin filaments increase.
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Stratum spinosum: The thickest layer. It consists of polyhedral cells with central nuclei and nucleoli.
- Contains keratin filaments that assemble into bundles called tonofibrils.
- Features cytoplasmic extensions (spines).
- Contains Langerhans cells.
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Stratum granulosum: Contains 3 to 5 layers of flattened cells undergoing terminal keratinization.
- Contains keratohyaline granules (filaggrin and keratin).
- Has membranous Golgi-derived lamellar granules. These contain layers of lipids that form an impermeable layer around the cells, acting as a barrier against water loss.
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Stratum lucidum: Only found in thick skin. Flattened, eosinophilic keratinocytes held by desmosomes.
- Nuclei and organelles are lost.
- The cytoplasm consists of packed keratin in an electron-dense matrix.
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Stratum corneum: 15 to 20 layers of squamous keratinized cells filled with keratin filaments.
- Cells lose all organelles.
- By the end of keratinization, these cells contain only amorphous fibrillar protein.
- This layer is known as the cornified layer and is constantly shed from the epidermal surface.
Dermis
- Composed of connective tissue rich in elastic fibers, blood supply, and lymphatic supply.
- Contains epidermal derivatives like hair follicles, sweat glands, and sebaceous glands.
- Divided into the papillary layer and the reticular layer.
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Papillary layer:
- Contains dermal papillae, loose connective tissue, and connective tissue cells.
- Primarily collagen I and III, elastic fibers, blood vessels, lymphatic vessels, tactile corpuscles, and anchoring fibrils (VII).
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Reticular layer:
- Contains dense irregular connective tissue, collagen I and elastic fibers, blood vessels, nerves, sensory receptors, and ducts of sweat glands and hair follicles.
- Responsible for blood and lymphatic supply.
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Blood and Lymphatic Supply:
- Subpapillary plexus: Located between the papillary and reticular layers.
- Cutaneous plexus: Lies between the dermis and hypodermis.
- Venous Plexuses: The skin has three venous plexuses: subpapillary, cutaneous, and in the middle of the dermis.
- A-V Anastomosis: Plays a major role in the control of heat loss.
- Lymphatic vessels: Start at dermal papillae and accompany arterial cutaneous and subpapillary plexuses.
Sensory Receptors
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Unencapsulated Receptors:
- Merkel cells: Associated with expanded nerve endings. Responsible for light touch, also known as Merkel discs.
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Free nerve endings: Found in the papillary dermis and extend into lower epidermal layers.
- Responsible for high and low temperature sensation, pain, and itching.
- Function as tactile receptors.
- Root hair plexus: Sensory fibers around hair follicles in the reticular dermis. Detect hair movement.
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Encapsulated Receptors: (All are mechanoreceptors)
- Meissner corpuscles: Found in dermal papillae. Abundant in fingertips, palms, and soles.
- Pacinian corpuscles: Found in the reticular dermis and hypodermis.
- Ruffini corpuscles: Found in the hypodermis.
Skin Functions
- Protective
- Thermoregulatory
- Metabolic
- Sensory
Thick and Thin Skin
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Thick skin: Found on the palms of the hands and soles of the feet.
- Contains a thick stratum corneum, stratum granulosum, and stratum spinosum.
- Has a stratum lucidum.
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Thin skin: Found on most of the body.
- Stratum corneum, granulosum, and spinosum are thinner than those of thick skin.
- Lacks a stratum lucidum.
Subcutaneous Tissue
- Also known as the hypodermis or superficial fascia.
- Loose connective tissue that binds the skin to the subcutaneous organs.
- Contains adipocytes, the number of which varies depending on the region of the body.
Hair
- Elongated, keratinized structure that forms within epidermal invaginations called hair follicles.
- Hair color, size, shape, and texture vary based on age, genetic background, and the region of the body.
- Minimal hair is found on the palms, soles, lips, glans penis, clitoris, and labia minora.
- Hair growth is discontinuous, with periods of growth followed by periods of rest.
Structure of Hair
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Hair follicle:
- Contains a terminal dilation called a hair bulb.
- A dermal papilla inserts into the base of the hair bulb and contains a capillary network.
- Keratinocytes continuous with those of the stratum basale cover the dermal papilla. These cells form the matrix of the hair root.
- Hair shaft: The part of the hair that extends beyond the skin surface.
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Hair root:
- Medulla: Formed by moderate keratinized cells, found in the center of the hair root.
- Cortex: Made up of heavily keratinized, densely packed cells.
- Cuticle: The most peripheral cells of the hair root, consisting of a thin layer of heavily keratinized squamous cells covering the cortex.
Hair Follicle Structure
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Epithelial root sheath: Consists of two layers.
- Internal root sheath: Completely surrounds the initial part of the hair root but degenerates above the level of the attached sebaceous gland.
- External root sheath: Covers the internal sheath and extends all the way to the epidermis. Continuous with the basal and spinous layers.
- Glassy membrane: A non-cellular hyaline layer that separates the hair follicle from the dermis. Formed by the thickening of the basal lamina.
- Connective tissue sheath: Surround the dermis, forming a connective tissue sheath.
Other Hair Follicle Structures
- Melanocytes: Located between the papilla and epithelial cells of the hair root.
- Sebaceous gland: Associated with the hair follicle and secretes an oily substance called sebum.
Krause end bulb and Pacinian corpuscles
- Krause end bulb: Found in the dermis.
- Pacinian corpuscles: Found in the reticular dermis and hypodermis.
Autonomic innervation
- Autonomic innervation to sweat glands is present.
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Description
Explore the fascinating processes of melanin synthesis and the various cells present in the epidermis. This quiz covers the role of tyrosinase in melanin production, the function of keratinocytes, and the significance of different melanin types. Test your knowledge of skin biology and the mechanisms behind skin color.
