The hair matrix contains

Correct Answer: D

Rationale: The hair matrix is the actively growing part of the hair bulb, located at the base of the hair follicle, and it contains a layer of basal cells (stem cells) that divide to produce the hair shaft and inner root sheath. The hair follicle is the entire structure surrounding the hair, not a component of the matrix. The hair shaft is the visible, dead portion of the hair, formed by the matrix but not part of it. The glassy membrane is a basement membrane-like structure around the follicle, not within the matrix. The basal cells in the matrix are critical for hair growth, as they differentiate and keratinize to form the hair, making this the accurate description.

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.

Correct Answer: D

Rationale: Hypodermic administration means delivering a drug 'below the dermis' (from Greek 'hypo' meaning under), typically via injection into the subcutaneous fat layer (hypodermis) beneath the skin, as with insulin shots. Wiping onto the skin is topical, not hypodermic. A patch is transdermal, penetrating through the skin to the bloodstream. Injection into the dermis is intradermal, a shallower method used for tests like TB screening. Hypodermic injections target the fatty subcutaneous layer for slower absorption into circulation, a common medical practice, making this the precise definition.