PSIO110 Module 4 Lecture 1 Notes
Lecture Overview
Structure of the skin: Epidermis and Dermis
Accessory structures: Hair, Sebaceous glands, Sweat glands, Ceruminous gland, Nails
Structural basis of skin colour
Sensory Receptors
The Skin
The skin is the largest organ, weighing approximately kg and accounting for of body weight. Also known as the cutaneous layer, it covers the external surface of the body with a thickness varying from mm. It is composed of epithelial tissue (epidermis) and a deeper connective tissue portion (dermis).
The epidermis is avascular, while the dermis is vascular.
Deep to the dermis is the subcutaneous layer, which is not a part of the skin. It anchors the dermis and serves for fat storage and contains blood vessels.
Epidermis
The epidermis is comprised of keratinized stratified squamous epithelium. Its primary function is protection and it consists of four main cell types: Keratinocytes, which make up of the cells and produce keratin; Melanocytes, which constitute of the cells and produce melanin for UV light absorption; Intraepidermal macrophages for immune function; and Tactile epithelial cells which connect with sensory neurons forming tactile disks.
Epidermis Layers
The epidermis consists of multiple layers that differ in structure and function. Thin skin has four layers: Stratum corneum, granulosum, spinosum, and basale.
Thick skin, found in areas subject to high wear-and-tear, has five layers: Stratum corneum, lucidum, granulosum, spinosum, and basale.
Epidermis – Stratum Basale
The stratum basale is the deepest layer of the epidermis. It is a single row of cuboidal or columnar keratinocytes, some of which are stem cells. These cells form new cells from bottom-to-top within the skin.
Melanocytes and tactile epithelial cells are scattered throughout this layer.
Epidermis – Stratum Spinosum
The stratum spinosum is superficial to the stratum basale and composed of 8-10 layers of keratinocytes. These are living cells with intraepidermal macrophages and melanocyte projections in this layer.
The cells have projections (thorn-like spines) where keratin intermediate filaments connect them together.
Epidermis – Stratum Granulosum
The stratum granulosum is the middle layer in thick skin, composed of 3–5 layers of flattened keratinocytes that undergo apoptosis due to a lack of nutrition. This layer contains darkly staining protein granules, known as keratohyalin, and releases membrane enclosed lipid-rich granules called lamellar granules.
It acts as a water repellent, retarding the loss and entry of water and foreign material through the skin.
Epidermis – Stratum Lucidum
The stratum lucidum is only found in thick skin, such as that on the palms, soles, and fingertips. It is composed of 4–6 layers of clear, flattened, dead keratinocytes, providing additional toughness to the skin.
Epidermis – Stratum Corneum
The stratum corneum is the outermost layer of the epidermis, composed of 25–30 layers of flattened dead keratinocytes with no internal organelles. The cell layers have strong inter-layer connectivity.
Cells are shed by friction and injury and replaced by cells in the deeper layers, protecting against microbial invasion.
Keratinization and Growth of Epidermis
Cells in the stratum basale are slowly pushed to the surface, accumulating more keratin through a process called keratinization. Cells undergo apoptosis due to less nutrition in the upper cell layers.
Keratinized cells slough off and are replaced by new cells underneath.
Dermis
The dermis is the deep part of the skin, made of irregular connective tissues including collagen and elastin, allowing it to stretch and recoil easily. Its thickness varies from region to region.
It contains few cells, predominantly fibroblasts, macrophages, and adipocytes. Blood vessels, nerves, glands, and hair follicles are embedded in two regions: the thin, superficial papillary region and the thick, deeper reticular region.
Dermis - Papillary Region
The papillary region contains collagen and elastin fibres, increasing surface area through dermal papillae. It houses tactile receptors, known as Meissner corpuscles, and free nerve endings, responsible for pain, warmth, and itching sensations.
Dermis - Reticular region
The reticular region is attached to the subcutaneous layer and contains collagen and elastin fibres, along with fibroblasts and macrophages. Blood vessels, hair follicles, nerves, sebaceous glands, and sweat glands are arranged in between the fibers.
Accessory Skin Structures - Hair
Hair is present in most of the skin except palms and soles, and is abundant in areas such as the scalp, eyebrows, axillae, and pubic areas. Its growth is under hormonal and genetic influence.
Hair reduces heat loss, limits injuries to the scalp, protects the eyes (eyebrows and eyelashes), and protects against foreign particles in the ear canal and nostrils. Touch receptors associated with hair follicles detect light touch.
Hairs are columns of dead keratinized epidermal cells bound by extracellular proteins, consisting of a shaft (above the skin surface) and a root (under the skin). The shaft and root consist of three concentric layers: the inner medulla, composed of 2–3 rows of irregular shaped cells containing pigments; the middle cortex, composed of elongated cells; and the outer cuticle, a single layer of thin, flat keratinized cells.
Hair Follicle
The hair follicle is the root of the hair, composed of external and internal root sheaths. The external root sheath is a downward continuation of the epidermis, while the internal root sheath is epithelium between the external root sheath and the hair.
Dense dermis surrounding the hair follicle is known as the dermal root sheath. The base of the follicle is an onion-shaped bulb with a nipple-shaped indentation at the bottom called the papilla of the hair, which contains blood vessels and connective tissues. The bulb contains a germinal layer of cells (hair matrix) arising from the stratum basale and leads to hair growth.
Hair
Sebaceous glands (oil) and smooth muscle (arrector pili) cells are associated with hairs. The arrector pili muscle extends from the superficial dermis to the dermal root sheath.
Normally, hair emerges at an angle less than 90°. Emotional stress, cold, or fright, through autonomic control, causes the arrector pili to contract, leading to “goose bumps.” The hair follicle is surrounded by a hair root plexus which is sensitive to touch, generating nerve impulses if the hair root is moved.
Skin Glands
The skin contains several exocrine glands, including: Sebaceous glands (oil/sebum), Sudoriferous glands (sweat), Ceruminous glands (cerumen/earwax), and Mammary glands (specialised sudoriferous glands).
Sebaceous Glands
Sebaceous glands are simple, branched acinar glands connected to hair follicles. The secreting portion lies in the dermis, mostly opening into the neck of a hair follicle, and also open directly into skin in areas such as the lips, glans penis, labia minora, and eyelid glands.
They are absent in palms and soles. Sebaceous glands secrete an oily substance called sebum, which prevents hair from becoming dry and brittle, prevents excessive evaporation of water, keeps skin soft and pliable, and inhibits some bacterial growth.
Sudoriferous Glands
Sudoriferous glands release sweat (perspiration) into hair follicles or onto the skin. There are two main types: Eccrine and Apocrine.
Eccrine sweat glands
Eccrine sweat glands are simple, coiled tubular glands that are more common than apocrine glands. They are distributed throughout the body, mostly in the forehead, palms, and soles.
Their secretory portion is in the deep dermis, with ducts opening at the surface of the epidermis.
Apocrine (Merocrine) Sweat Glands
Apocrine sweat glands are simple, coiled tubular glands with larger ducts and lumens. Their ducts open into hair follicles, and are found mainly on the skin of the axilla, groin, areole of breasts, and bearded area of the face.
The secretory portion is located in the lower dermis or upper subcutaneous layer. These glands produce sweat that appears milky or yellowish in colour. The sweat interacts with bacteria, producing a musky odour.
Ceruminous Glands
Ceruminous glands are modified sweat glands in the external ear that produce waxy substances for lubrication. Their ducts directly open into the external auditory canal or ducts of sebaceous glands.
The combined secretion of ceruminous and sebaceous glands produces yellowish cerumen (earwax), which acts as a sticky barrier for foreign bodies and insects, and as a waterproof barrier against bacterial and fungal entry into cells.
Nails
Nails are plates of tightly packed, hard, dead, keratinized epidermal cells that form a clear solid covering over the distal part of the digits. They consist of a nail body (plate), free edge, and nail root.
The nail body is the visible portion and is comparable to the stratum corneum of the epidermis, with capillaries in the underlining dermis. The free edge is the extended part of the nail body with no underlining capillaries. The nail root is the portion of the nail buried in a fold of skin. The whitish crescent-shaped area is called the lunula, where vascular tissue underneath does not show through due to thick epithelium. The nail bed is the skin below the nail plate that extends from the lunula to the hyponychium.
Structural Basis of Skin Colour
Skin colour is dependent on melanin, haemoglobin, and carotene pigments in the skin. Red to pink skin colour is due to haemoglobin.
Beta Carotene, a yellow/orange pigment found in many fruits and vegetables, is stored in the stratum corneum. Melanin ranges from pale yellow to reddish-brown. Melanin is produced in melanocytes from the amino acid tyrosine, and UV light exposure increases melanin production, causing darkness.
Melanin absorbs UV radiation and prevents DNA damage. Small amounts of UV radiation are needed for vitamin D synthesis, while large amounts of UV exposure may lead to skin cancer formation.
Examples
Albinism
Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair, and eyes due to a defect of an enzyme involved in the production of melanin.
Vitiligo
Vitiligo is a chronic disorder that causes depigmentation patches in the skin. The precise cause is not known but is most likely a combination of genetic factors coupled with a disorder of the immune system (autoimmune disease).
Sensory Receptors
The sensory receptors in the skin belong to 3 major classes: Mechanoreceptors, Thermoreceptors (free nerve endings), and Nociceptors (free nerve endings).
Mechanoreceptors:
Mechanoreceptors include: Meissner corpuscles for fine touch, pressure, and slow vibration; Tactile discs for fine touch and pressure; Ruffini corpuscles for crude touch and stretching; Hair root plexus for crude touch; and Pacinian corpuscles for pressure and fast vibration.