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Chapter 5: The Integumentary System - Accessory Structures

The Integumentary System: Accessory Structures of the Skin

Hair (Pili)

  • General Characteristics:

    • Hair is associated with the word "pili."

    • Present on most skin surfaces, with notable exceptions being the palms, anterior surfaces of the fingers, and the soles of the feet.

    • Composed of dead, keratinized epidermal cells.

    • The thickness and distribution of hair are determined by genetic factors.

  • Functions:

    • Contributes to touch sensations.

    • Provides protection to the body against the harmful effects of the sun.

    • Aids in protection against heat loss from the body.

  • Parts of a Hair:

    • Shaft: The portion of the hair that extends above the surface of the skin.

    • Follicle: The structure that lies below the level of the skin, anchoring the hair.

    • Root: The portion of the hair that penetrates into the dermis. It includes:

      • An epithelial root sheath.

      • A dermal root sheath.

  • **Internal Structure (Detailed Components):

    • Hair matrix

    • Hair root

    • Medulla

    • Cortex

    • Cuticle of the hair

    • Melanocyte

    • Papilla of the hair

    • Hair follicle: Internal root sheath, External root sheath

    • Epithelial root sheath

    • Dermal root sheath

  • **Associated Structures (Diagram Components):

    • Sebaceous gland

    • Hair root plexus

    • Arrector pili muscle (causes hair to stand erect)

    • Eccrine sweat gland

    • Bulb

    • Apocrine sweat gland

    • Blood vessels supplying the papilla

  • Types of Hairs:

    • Lanugo: Fine, nonpigmented, downy hairs that typically cover the body of a fetus.

    • Vellus hairs: Short, fine, pale hairs that are barely visible to the naked eye.

    • Terminal hairs: Long, coarse, heavily pigmented hairs (e.g., scalp hair, eyebrows, pubic hair).

  • Hair Color:

    • Determined by the amount and type of melanin present in the hair matrix.

    • Dark hair: Contains mostly eumelanin.

    • Light/Blonde/Red hair: Contains largely pheomelanin.

    • Gray hair: Results from the cessation of melanin production by melanocytes in the hair matrix.

Hair Growth and Loss

  • Growth Process: Hair cells in the matrix at the base of the follicle divide and push older cells upward. As these cells move away from the blood supply, they undergo keratinization and die, forming the hair shaft that appears on the surface.

  • Hair Cycle: Hair growth follows a cyclical pattern with periods of growth and rest.

    • Growth Phase: Typically lasts for 2-6 years.

    • Resting Phase: Growth stops, and the old hair eventually falls out. This phase usually lasts for a few months.

  • Daily Hair Loss: It is normal to lose approximately 100 scalp hairs per day.

  • Alopecia: Refers to the partial or complete lack of hair.

    • Causes: Can result from a variety of factors including genetic predispositions, natural aging processes, endocrine disorders, side effects of chemotherapy, or certain skin diseases.

Skin Glands

  • General Overview: Recall from Chapter 4 that glands are epithelial cells specialized to secrete various substances. They operate via different modalities, such as merocrine or holocrine secretion.

Sebaceous (Oil) Glands

  • Mode of Secretion: Holocrine mode of secretion, meaning the entire cell ruptures to release its contents, and the cell is replaced.

  • Association: Primarily connected to hair follicles.

  • Secretion: Produce an oily substance called sebum.

  • Functions of Sebum:

    • Prevents Dehydration: Helps to prevent the hair and skin from drying out, keeping them supple.

    • Inhibits Bacterial Growth: Contains substances that inhibit the growth of certain types of bacteria on the skin surface, providing a protective barrier.

Sudoriferous (Sweat) Glands

  • General Characteristics: There are two main types of sweat glands, both of which are simple, coiled tubular glands.

    • The body contains an extensive network of 3-4 million sweat glands.

Eccrine Sweat Glands

  • Abundance: These are the most numerous type of sweat gland found throughout most of the body.

  • Mode of Secretion: Merocrine mode of secretion, where secretions are released by exocytosis from vesicles.

  • Secretion: Release a watery solution.

    • Amount: Can secrete approximately 600 mL of sweat per day.

  • Functions:

    • Thermoregulation: Primarily helps to cool the body through evaporation of sweat from the skin surface.

    • Waste Elimination: Eliminates small amounts of metabolic wastes, including H2O, Na^{+}, Cl^{-}, urea, uric acid, and amino acids (AA's).

  • Emotional Sweating: These glands also release sweat in response to emotional stress, such as fear or embarrassment. This phenomenon is commonly referred to as emotional sweating or a "cold sweat."

Apocrine Sweat Glands

  • Mode of Secretion: Merocrine mode of secretion.

  • Location: Mainly concentrated in specific areas of the body, including:

    • The axilla (armpit).

    • The groin region.

    • The areolae (around the nipples).

    • The bearded facial regions of adult males.

  • Secretory Portion: The secretory portion of these glands is largely situated in the subcutaneous layer of the skin.

  • Excretory Duct: Unlike eccrine glands which open directly to the skin surface, the excretory ducts of apocrine glands typically open into hair follicles.

  • Secretion: Produce a slightly viscous (thicker) sweat.

  • Stimuli for Secretion: Sweat is primarily secreted during periods of emotional stress and sexual excitement.

  • Body Odor: A significant portion of characteristic body odor is attributed to apocrine sweat. This is because the organic molecules in apocrine sweat are metabolized by bacteria on the skin surface, producing volatile aromatic compounds.

Ceruminous Glands

  • Type: These are modified sweat glands.

  • Location: Found specifically within the external auditory canal (ear canal).

  • Function: In conjunction with nearby sebaceous glands, ceruminous glands are involved in producing a waxy secretion known as cerumen (earwax).

  • Role of Cerumen: Cerumen provides a vital function by creating a sticky barrier that effectively prevents the entry of foreign bodies, such as dust, insects, and microbes, into the delicate ear canal.

Nails

  • Composition: Composed of hard, tightly packed, keratinized epidermal cells.

  • Location: Situated over the dorsal surfaces of the distal ends of the fingers and toes.

  • Nail Structures:

    • Free edge: The portion of the nail that extends past the end of the digit.

    • Transparent nail body (plate): The main visible part of the nail, covering the nail bed.

    • Lunula: A whitish, crescent-shaped area at the base of the nail body, representing the visible part of the thick nail matrix.

    • Nail root: The portion of the nail embedded in a fold of skin at the proximal end.

    • Eponychium (cuticle): The narrow band of epidermis extending from the skin fold over the nail root.

    • Hyponychium (nail bed): The thickened region of stratum corneum beneath the free edge of the nail.

    • Nail matrix: The deep portion of the nail root where cell division actively produces new nail cells, leading to nail growth.

    • Phalanx (finger bone): The underlying bone that supports the nail structure.