Sebaceous glands:

Correct Answer: D

Rationale: Sebaceous glands are integral to skin function, producing sebum, an oily substance. They do not secrete sebum directly to the skin's surface; instead, they release it into hair follicles, which then reach the surface, making the first option inaccurate. They derive from the epidermis (ectoderm), not mesoderm, which forms the dermis, ruling out the second. They are exocrine glands, secreting sebum externally via ducts, not endocrine glands, which release hormones into the blood, eliminating the third. The correct description is that they are compound saccular (or alveolar) glands, with multiple acini (sac-like structures) branching from a duct, a histological classification fitting their structure and holocrine secretion mode (cells disintegrate to release sebum). This aligns with their role in lubricating skin and hair, their epidermal origin, and their anatomical complexity, distinguishing them from simpler gland types.

Correct Answer: C

Rationale: Langerhans cells, found mainly in the stratum spinosum, are dendritic immune cells that detect and engulf bacteria, presenting antigens to trigger an immune response. Merkel cells sense touch, not infection. Keratinocytes form the skin barrier and produce keratin, but they don't directly fight bacteria. Melanocytes produce melanin for pigment, not immunity. When bacteria breach a wound, Langerhans cells act as the epidermis's first line of immune defense, migrating to lymph nodes to alert the immune system, making them the critical cells in this scenario.

Correct Answer: C

Rationale: Hair's waterproofing property stems from keratin, a tough, fibrous protein produced by keratinocytes in the hair follicle. Keratin forms the hair shaft's structure, creating a hard, insoluble layer that repels water and protects against environmental damage. This protein's high sulfur content, from cysteine amino acids, forms disulfide bonds, enhancing its durability and water resistance. Collagen, a major protein in the dermis, provides strength and support to skin but does not waterproof hair, as it's not a component of the hair shaft. Elastin, also in the dermis, offers flexibility, not waterproofing, and is absent in hair. Carotene is a pigment contributing to skin and hair color, not a protein or waterproofing agent. Keratin's role extends beyond hair to the epidermis, where it similarly prevents water loss, but in hair, its dense packing in the cuticle layer ensures a hydrophobic surface. This adaptation is critical for mammals, protecting hair from moisture-related degradation, making keratin the clear answer.

Correct Answer: A

Rationale: During embryonic development, the integumentary system's layers arise from distinct germ layers. The epidermis, the outer skin layer, forms from the ectoderm, the outermost embryonic layer responsible for external structures like skin, hair, and nails. Ectodermal cells differentiate into stratified squamous epithelium, eventually keratinizing for protection. The dermis, beneath the epidermis, develops from the mesoderm, the middle germ layer that generates connective tissues, muscles, and blood vessels. Mesodermal cells form the dermis's collagen, elastic fibers, and vascular network, supporting the epidermis. The endoderm, forming internal linings like the gut, contributes neither to the epidermis nor dermis. Reversing the layers (mesoderm for epidermis, ectoderm for dermis) contradicts embryology, as does involving endoderm. This ectoderm-mesoderm pairing reflects their specialized roles ectoderm for barrier formation, mesoderm for structural support consistent with developmental biology and the integument's dual-layer functionality.

Correct Answer: B

Rationale: The waterproof coating in epidermal cells is keratin, a fibrous protein forming the stratum corneum, the epidermis's outermost layer. Produced during keratinization, keratin fills dead keratinocytes, creating a tough, insoluble barrier that prevents water loss and entry. This hydrophobicity, due to keratin's sulfur-rich structure and tight packing, protects against dehydration and environmental damage, a key adaptation for terrestrial life. Myelin, a lipid sheath, insulates nerves, not skin cells. Melanin, a pigment from melanocytes, absorbs UV light but doesn't waterproof. Albumin, a blood protein, maintains osmotic pressure, unrelated to epidermal coating. Keratin's dominance in the corneum up to 85% of its dry weight ensures the skin's impermeability, a fact confirmed by studies of skin barrier function, setting it apart from pigments or unrelated proteins.