MCAT - Organ Systems:: Skin System

The skin (integument) consists of two primary layers; from superficial to deep, these are:

- **A)** Dermis, then epidermis
- **B)** Epidermis, then dermis (with the hypodermis/subcutaneous layer beneath, technically not part of the skin proper)
- **C)** Hypodermis, then epidermis
- **D)** A single unified layer with no meaningful subdivision

The epidermis is composed primarily of which cell type, organized into progressively differentiating layers from the basal layer to the surface?

- **A)** Keratinocytes, which progressively differentiate and accumulate keratin as they migrate from the basal layer toward the skin surface, eventually becoming the dead, flattened, keratin-filled cells of the outermost stratum corneum
- **B)** Osteocytes
- **C)** Hepatocytes
- **D)** Cardiomyocytes

The stratum corneum, the outermost epidermal layer, provides the skin's primary barrier function primarily because it consists of:

- **A)** Living, metabolically active cells with intact nuclei
- **B)** A single layer of simple squamous epithelium, identical to capillary endothelium
- **C)** Multiple layers of dead, flattened, anucleate, keratin-filled corneocytes embedded in a lipid-rich extracellular matrix, forming a tough, water-resistant physical barrier often described with a 'bricks and mortar' analogy (corneocytes as bricks, intercellular lipids as mortar)
- **D)** Actively dividing stem cells

Melanocytes, located primarily within the basal layer of the epidermis, function to:

- **A)** Produce keratin, identical to keratinocytes
- **B)** Detect touch and pressure sensation
- **C)** Mount immune responses against skin pathogens, identical to Langerhans cells
- **D)** Synthesize melanin pigment (within specialized organelles called melanosomes) and transfer it to surrounding keratinocytes, providing photoprotection by absorbing/scattering ultraviolet radiation and helping prevent UV-induced DNA damage

Langerhans cells, specialized dendritic immune cells residing within the epidermis, primarily function to:

- **A)** Produce melanin pigment, identical to melanocytes
- **B)** Capture and process antigens encountered at the skin surface, then migrate to regional lymph nodes to present this antigen to T lymphocytes, initiating adaptive immune responses against skin-encountered pathogens or allergens
- **C)** Detect light touch sensation
- **D)** Produce keratin

The dermis, located beneath the epidermis, is composed primarily of which type of connective tissue, providing the skin's strength and elasticity?

- **A)** Adipose tissue exclusively
- **B)** Bone tissue
- **C)** Dense irregular connective tissue, rich in collagen (providing tensile strength) and elastin fibers (providing elasticity/recoil), along with blood vessels, nerves, and various skin appendages
- **D)** Simple squamous epithelium, identical to the epidermal surface

Eccrine sweat glands, distributed over most of the body surface, function primarily to:

- **A)** Produce a primarily watery, hypotonic secretion (via active NaCl reabsorption in the duct) that evaporates from the skin surface, providing the body's primary mechanism for evaporative heat loss/thermoregulation
- **B)** Produce an oily, lipid-rich secretion (sebum) to lubricate skin and hair
- **C)** Only become active at puberty, being nonfunctional before that time
- **D)** Secrete milk for infant nutrition

Apocrine sweat glands differ from eccrine glands in that apocrine glands:

- **A)** Are distributed over the entire body surface, identical to eccrine glands
- **B)** Are the primary contributor to thermoregulatory sweating
- **C)** Are functional from birth, unlike eccrine glands
- **D)** Are concentrated in specific regions (axillae, groin, areolae), become functional at puberty (under androgen influence), secrete a thicker, protein/lipid-rich fluid into the hair follicle (rather than directly onto the skin surface), and are primarily associated with body odor production when their secretions are metabolized by resident skin bacteria