Subcutaneous Tissue and Accessory Skin Structures: Comprehensive Anatomy, Physiology, and Diagnostics

Anatomy and Physiology of Subcutaneous Tissue

• General Organization and Terminology:

  • Deep to the skin lies the subcutaneous tissue, also known as the hypodermis.
  • The subcutaneous tissue is not part of the skin, but it attaches the skin to underlying bone and muscle.
  • It provides the skin with blood vessels and nerves.

• Histological Composition:

  • Consists of loose connective tissue containing collagen fibers and elastic fibers.
  • Cellular elements within the subcutaneous tissue include:
  • Fibroblasts: Synthesize extracellular matrix components and fibers.
  • Adipocytes: Fat-storing cells.
  • Macrophages: Phagocytic immune cells.

• Lipid Storage and Functions:

  • Contains approximately $50\%$ of the body's stored lipids.
  • Functions in insulation against heat loss, padding against physical trauma, and as a source of energy.

• Estimation of Total Body Fat:

  • Total body fat can be estimated by pinching the skin at selected locations and measuring the thickness of the skin fold and underlying subcutaneous tissue.
  • A thicker fold indicates a greater amount of total body fat.

• Variations by Age, Sex, and Diet:

  • Age: Most infants have a chubby appearance because they have proportionately more adipose tissue than adults.
  • Sex: Females have proportionately more adipose tissue than males, particularly over the thighs, buttocks, and breasts. This accounts for differences in body shape between the sexes and between individuals of the same sex.
  • Diet: Dietary intake directly affects the amount of stored subcutaneous adipose tissue.

• Administration of Injections:

  • Intradermal Injection:
  • Target: Extends into the dermis.
  • Method: Administered by drawing the skin taut and inserting a small needle at a shallow angle.
  • Example: Tuberculin skin test.
  • Subcutaneous Injection:
  • Target: Extends into the subcutaneous tissue.
  • Method: Administered by pinching the skin to form a "tent," into which a short needle is inserted.
  • Example: Insulin injection.
  • Intramuscular Injection:
  • Target: Reaches a muscle deep to the subcutaneous tissue.
  • Method: Administered by inserting a long needle at a $90^\circ$ angle to the skin.
  • Example: Most vaccines and certain antibiotics.

Hair Anatomy, Growth, and Physiology

• Distribution and Mammalian Characteristics:

  • Hair is a characteristic of all mammals. Dense hair covering most of the body surface is called fur.
  • In humans, hair is present on all skin surfaces except the palms, soles, lips, nipples, parts of the external genitalia, and distal segments of the fingers and toes.

• Developmental Hair Types:

  • Lanugo: Delicate, unpigmented hair covering the fetus by the $5\text{th to }6\text{th}$ month of fetal development.
  • Terminal Hairs: Long, coarse, and pigmented hairs.
  • Near birth, terminal hairs replace lanugo on the scalp, eyelids, and eyebrows.
  • At puberty, terminal hair replaces much of the vellus hair in the pubic and axillary regions.
  • In males, terminal hairs replace facial vellus hair to form the beard.
  • Body hair on the chest, legs, and arms is approximately $90\%$ terminal hair in males and approximately $35\%$ in females.
  • Vellus Hairs: Short, fine, and usually unpigmented hairs that replace lanugo on the rest of the body near birth.

• Functional Significance of Pubic and Axillary Hair:

  • Serves as a visible sign of sexual maturity.
  • Acts as wicks for dispersing odors produced by specialized glands in the pubic and axillary regions, serving as olfactory signals.
  • Pubic hair protects against abrasion during intercourse.
  • Axillary hair reduces friction when the arms move against the body.

• Microscopic Structure of Hair:

  • Anatomical divisions:
  • Shaft: Protrudes above the surface of the skin.
  • Root: Located below the surface of the skin.
  • Hair Bulb: The expanded base of the hair root.
  • Concentric layers of dead, keratinized epithelial cells:
  • Medulla: The central axis of the hair, composed of $2\text{ to }3$ layers of cells containing soft keratin.
  • Cortex: Surrounds the medulla and forms the bulk of the hair; cells contain hard keratin.
  • Cuticle: Single layer of cells containing hard keratin covering the cortex; cell edges overlap like shingles on a roof.

• Hair Follicle Anatomy and Skin Repair:

  • Structure: Tubelike invagination of the epidermis extending into the dermis.
  • Dermal Root Sheath: Portion of the dermis surrounding the epithelial root sheath.
  • Epithelial Root Sheath:
  • Divided into external and internal parts.
  • At the opening, the external epithelial root sheath contains all strata found in thin skin. Deeper in the follicle, cell layers decrease until only the stratum basale is present at the hair bulb.
  • Repair role: If the epidermis and superficial dermis are damaged, keratinocyte stem cells in the stratum basale of the undamaged hair follicle serve as a source of new epithelium.
  • Internal Epithelial Root Sheath Attachment: Has raised edges that mesh closely with raised edges of the hair cuticle to hold the hair in place. When hair is pulled out, the internal epithelial root sheath comes out with it as visible whitish tissue around the root.

• Hair Bulb and Matrix:

  • Matrix: Mass of undifferentiated epithelial cells inside the hair bulb that produces both the hair and the internal epithelial root sheath.
  • Hair Papilla: Projection of the dermis into the hair bulb containing blood vessels that supply nutrients to matrix cells.

• Phases and Kinetics of Hair Growth:

  • Growth occurs in cycles featuring a growth stage and a resting stage.
  • Growth Stage: New matrix cells are produced, differentiate, become keratinized, and die, adding length to the base of the hair root.
  • Resting Stage: Growth stops, the follicle shortens and holds the hair. Afterwards, a new cycle begins; a new hair forms and pushes the old hair out of the follicle.
  • Durations:
  • Eyelashes: Grow for approximately $30\,\text{days}$ and rest for $105\,\text{days}$.
  • Scalp hair: Grows for $3\,\text{years}$ and rests for $1\text{--}2\,\text{years}$.
  • Quantitative Data:
  • An estimated $90\%$ of scalp hairs are in the growing stage at any given time.
  • Normal scalp hair loss is approximately $100$ hairs per day.
  • Average hair growth rate is approximately $0.3\,\text{mm/day}$.
  • Cutting, shaving, or plucking does not alter growth rate or hair character (shaved hair feels coarse only because short hair is less flexible).
  • Maximum hair length is determined by the growth rate and duration of the growing phase.

• Hair Loss (Alopecia):

  • Chemotherapy can cause extensive hair loss that may revert after treatment.
  • Alopecia Areata (spot baldness): Autoimmune condition causing varying degrees of hair loss across the body due to immune destruction of healthy hair follicles; regrowth occurs in many, though some experience permanent loss.
  • Pattern Baldness: Most common permanent hair loss. Hair follicles shrink and revert to producing short, transparent vellus hair, and may eventually cease production entirely. Driven by genetic factors and testosterone.

• Hair Color Determination:

  • Caused by melanocytes in the hair bulb matrix passing melanin to keratinocytes in the hair cortex and medulla.
  • Blonde hair contains little black-brown melanin; jet black hair contains the most; brown shades contain intermediate amounts.
  • Red hair contains varying amounts of a red melanin variant.
  • Hair color is controlled by multiple genes, and dark hair color is not strictly dominant over light.
  • Decreased melanin with age causes color to fade or turn white (absence of melanin). Gray hair is a mixture of unfaded, faded, and white hairs.

• Arrector Pili Muscle Dynamics:

  • Bands of smooth muscle extending from the dermal root sheath to the papillary layer of the dermis.
  • Follicles naturally lie at an oblique angle. Contraction of the arrector pili pulls the follicle perpendicular to the skin surface, causing hair to stand on end and forming "goose bumps."
  • Triggered by cold temperatures and strong emotional responses. In other mammals, contraction provides thermal insulation and communicates aggression or fear.

Cutaneous Glands

• Sebaceous Glands:

  • Simple or compound alveolar glands located in the dermis.
  • Classified as holocrine glands because sebum is released by the lysis and death of secretory cells.
  • Secrete sebum, an oily white substance rich in lipids.
  • Most open into the upper part of hair follicles via ducts to prevent drying and protect against certain bacteria.
  • Sebaceous glands not associated with hair follicles open directly onto the skin surface at the lips, eyelids (tarsal glands), and genitalia.

• Sweat Glands (Sudoriferous Glands):

  • Eccrine (Merocrine) Sweat Glands:
  • Most common type of sweat gland; simple, coiled, tubular glands opening directly to the skin surface via sweat pores.
  • Divided into a deep coiled portion in the dermis and a duct passing to the surface.
  • Deep portion secretes an isotonic fluid containing water, sodium chloride, ammonia, urea, uric acid, and lactic acid.
  • As fluid travels through the duct, sodium chloride is reabsorbed into the body by active transport, leaving a hyposmotic fluid called sweat.
  • Functions in evaporative body cooling when core temperature rises. Released on palms, soles, and axillae during emotional stress.
  • Distributed over most of the body, most numerous on palms and soles. Absent on the margins of the lips, labia minora, and glans of the penis and clitoris.
  • Apocrine Sweat Glands:
  • Simple, coiled, tubular glands opening into hair follicles superficial to the openings of sebaceous glands.
  • Located in the axillae, genitalia (scrotum, labia majora), and around the anus.
  • Do not participate in temperature regulation in humans.
  • Become active at puberty under the influence of sex hormones.
  • Secretions contain organic substances that are odorless when released but metabolized by bacteria to create body odor. May signal sexual maturity.

• Other Specialized Skin Glands:

  • Ceruminous Glands: Modified eccrine sweat glands located in the ear canal (external acoustic meatus). Secretions combine with sebaceous secretions to form cerumen (earwax). Protects the tympanic membrane from dirt and insects; accumulation can block the ear canal and impair hearing.
  • Mammary Glands: Modified apocrine sweat glands located in the breasts that produce milk in lactating females.

Anatomy and Physiology of Nails

• Structure of the Nail Unit:

  • Thin plate consisting of layers of dead stratum corneum cells containing hard keratin, located on the distal ends of the digits.
  • Nail Root: Proximal buried portion covered by skin.
  • Nail Body: Visible attached distal portion of the nail.
  • Nail Fold and Nail Groove: Skin covering the lateral and proximal edges, holding the edges in place.
  • Cuticle (Eponychium): Stratum corneum of the nail fold growing onto the nail body.
  • Hyponychium: Thickened region of stratum corneum beneath the free edge of the nail body.
  • Nail Matrix: Thickened proximal portion of epithelial tissue containing a stratum basale that gives rise to cells forming nearly all of the nail.
  • Nail Bed: Epithelial tissue located between the matrix and hyponychium; appears pink through the clear nail due to blood vessels in the underlying dermis.
  • Lunula: Whitish crescent-shaped area at the base of the nail body (best seen on the thumb). Appears white because the thicker underlying nail matrix obscures dermal blood vessels.

• Nail Growth Kinetics:

  • Nails form in the matrix and bed, sliding over the nail bed toward the distal end of the digit.
  • Average growth rate is $0.5\text{--}1.2\,\text{mm/day}$.
  • Fingernails grow faster than toenails.
  • Nails grow continuously throughout life without a resting phase.

Clinical Diagnostics and Systemic Associations

• Diagnostic Skin Color Variations:

  • Cyanosis: Bluish skin color caused by decreased blood $O_2$ content, indicating impaired cardiovascular or respiratory function.
  • Jaundice: Yellowish skin color caused by excess bile pigment accumulation in the blood. Worn-out red blood cells are broken down, and their contents are excreted as bile pigments by the liver into the small intestine. Liver damage (such as viral hepatitis) prevents excretion, causing blood accumulation.

• Rashes, Lesions, and Hypersensitivity:

  • Scarlet Fever: Bacterial throat infection where bacterial toxins released into the blood produce a characteristic pink-red skin rash.
  • Allergic Reactions: Histamine release into tissues causes swelling and reddening. Development of hives can indicate allergies to foods or drugs such as penicillin.

• Nutritional Deficiencies:

  • Vitamin A Deficiency: Causes excess keratin production, resulting in skin with a sandpaper texture.
  • Iron-Deficiency Anemia: Causes nails to lose normal contour and become flat or concave (spoon-shaped).

• Diagnostic Hair Analysis:

  • Hair concentrates substances for laboratory testing. Lead poisoning causes elevated lead levels in hair. Forensic analysis tests hair samples for drug use and poisoning. Hair analysis is not reliable for determining general health or overall nutritional status.

Skin Microbiome and Pathogenesis of Acne

• Acne Vulgaris Epidemiology:

  • Most common skin condition in the United States, affecting $80\%$ of American adolescents and $40\text{ to }50\text{ million}$ total Americans across all age groups.

• Role of Cutibacterium acnes:

  • Cutibacterium acnes (C. acnes, formerly Propionibacterium acnes) lives in sebum-rich areas (forehead, nose side, back) and feeds on sebum lipids.
  • Three unique strains of C. acnes exist:
  • Good Strain: Dominates in acne-free skin. Breaks down lipids to lower skin pH to levels that inhibit invading pathogenic bacteria. Prevents colonization by pathogenic strains.
  • Bad Strains: Two strains are pathogenic to humans and trigger skin eruptions.
  • Skin Probiotics: Proposed treatment applying non-pathogenic C. acnes to the skin to prevent pathogenic strains from establishing.

Mechanisms of Mechanical Nail Trauma

• Subungual Hematoma Formation:

  • Direct impact to the nail body distal to the lunula and proximal to the hyponychium damages blood vessels in the underlying nail bed.
  • Blood leaks into the localized region between the rigid nail plate and the nail bed, forming a dark blood clot (hematoma).
  • Fluid accumulation creates pressure within the tight space beneath the nail, causing severe pain.

• Decompression and Progression of Recovery:

  • Drilling a hole through the nail plate releases trapped bloody fluid, instantly reducing pressure and relieving pain.
  • Over a period of approximately $2\,\text{months}$, as new nail cells are continuously produced by the stratum basale of the nail matrix, the nail plate slides distally, carrying the trapped dark blood area forward until it reaches the free distal edge.