Hair, nails and skin glands

Basal cells in the stratum basale divide throughout life. Hair and nails grow from matrix cells that work the same way.

• Stratum corneum:
• Correct: Stratum basale:
• Stratum granulosum:

In holocrine secretion a cell fills with product and breaks apart, so the gland needs new cells from dividing stem cells. The skin's oil glands work this way.

• Merocrine:
• Apocrine:
• Correct: Holocrine:

Smooth muscle has no striations and contracts without conscious control. The tiny muscle that raises each hair is smooth muscle.

• Correct: Smooth muscle:
• Skeletal muscle:
• Cardiac muscle:

The hair matrix is the layer of rapidly dividing cells in the bulb, just above the papilla. Its new cells are pushed upward, fill with hard keratin and die, forming the hair.

• Hair papilla: The hair papilla is the knob of connective tissue with capillaries beneath the matrix. It supplies the matrix but does not make the hair.
• Correct: Hair matrix: Correct. The dividing cells that make the hair form the hair matrix.
• Inner root sheath: The inner root sheath lines the follicle around the hair; it is not the growing layer at the base.
• Hair cortex: The cortex is the middle layer of the hair itself, made of dead cells, not a layer of dividing cells in the bulb.

During pregnancy, high hormone levels keep extra scalp follicles in anagen. After the birth those levels fall abruptly, and the held-over follicles enter catagen and telogen at the same time. Telogen lasts about 3 months; when those follicles start a new anagen, the new hairs push the old club hairs out together. The follicles are intact, so the hair regrows.

• Childbirth destroyed many hair matrix cells, which cannot be replaced: The matrix cells are not destroyed; the follicles are resting. The hair grows back once they re-enter anagen.
• Her sebaceous glands stopped making sebum at delivery: Sebum oils the hair but does not hold it in the follicle, and loss of sebum would not cause shedding three months later.
• Her arrector pili muscles are pulling hairs out: The arrector pili raises a hair by tilting it; it cannot pull hairs out.
• Correct: Follicles that rested together after the birth now shed together: Correct. Follicles pushed into the resting phase together shed together about three months later.

Apocrine sweat glands, in the armpits, groin and around the nipples, empty into hair follicles, start working at puberty, and make a thicker secretion with proteins and lipids that bacteria turn into body odor. Eccrine glands are the densest on palms and soles and the ones that release heat.

• Correct: Their ducts empty into hair follicles: Yes. Apocrine ducts open into hair follicles above the sebaceous glands.
• Correct: They start working at puberty: Yes. They become active at puberty.
• They are densest on the palms and soles: No. The palms and soles have no hair follicles and are packed with eccrine glands instead.
• Correct: Their secretion contains proteins and lipids: Yes. Proteins and lipids make apocrine secretion thicker than eccrine sweat.
• They are the main glands that release heat through sweat: No. Heat loss through sweat is the job of eccrine glands; apocrine glands respond mainly to emotional stress.

The secreting coil makes a watery fluid like protein-free plasma. As it flows up the duct, duct cells take back sodium and chloride. The faster the flow, the less time for that, so more salt stays in the sweat that reaches the skin.

• Correct: Fast-flowing sweat leaves the duct less time to take back sodium and chloride: Correct. Less time in the duct means less salt taken back, so fast sweat is saltier.
• Her apocrine glands take over at high sweat rates: Apocrine glands respond to emotion, not heat, and they are few; heavy sweating in a race comes from eccrine glands.
• Her sebaceous glands add salt to the sweat during hard work: Sebaceous glands make an oily lipid product, not salt.
• The secreting coil adds extra salt when it works hard: The coil's fluid starts at about the same salt level whatever the rate. What changes with flow is how much the duct takes back.

Shaving cuts the hair shaft at the surface. The shaft is dead and the matrix sits deep in the bulb, untouched, so the hair's width, color and growth rate do not change. The cut end is blunt instead of tapered, which feels stubbly, and short new hair has not yet been lightened by sun.

• Shaving stimulates the hair matrix to make wider hairs: The matrix is deep in the follicle and receives no signal from a cut at the surface.
• Shaving removes the cuticle, so regrown hair looks darker: The cuticle is part of each hair and is made fresh by the matrix; shaving the shaft does not change the new hair's layers or pigment.
• Correct: Shaving cuts only dead shaft; blunt tips feel coarser: Correct. Only dead shaft is removed, so the new hair is the same; it only feels coarser.
• Shaving moves follicles from telogen into anagen: The growth cycle is set inside the follicle, not by cutting the shaft above the skin.

The nail matrix, under the nail root beneath the proximal nail fold, makes the nail. Scarring there changes the plate the matrix produces, leaving it ridged or split for good. The nail bed only supports the plate as it slides forward, so injury there usually heals without a lasting deformity.

• The nail bed patient, because the nail bed makes the nail: The nail bed supports the plate; the matrix, not the bed, makes it.
• Neither, because nails are dead and regrow from the free edge: Nails grow from the matrix at their base, not from the free edge.
• Both equally, because the lunula covers the whole nail bed: The lunula is only the visible front of the matrix at the base of the nail; it does not cover the nail bed.
• Correct: The proximal fold patient, because the nail matrix lies there: Correct. Damage to the matrix changes the nail it makes.

A hair's maximum length is its growth rate times the length of anagen. Eyebrow hairs grow only about half as fast as scalp hairs, but scalp follicles stay in anagen for years and eyebrow follicles for only a few months, so the hair enters catagen and telogen, and is shed, while still short.

• Eyebrow hairs are worn down by friction against the skin of the brow: Uncut eyebrows reach the same short length whether or not they are rubbed; the limit is set in the follicle.
• Eyebrow follicles have no hair matrix: Every growing hair needs a matrix; without one, eyebrows would not grow at all.
• Eyebrow hairs lack a cortex, so they break off: Eyebrow hairs are terminal hairs with a normal cortex.
• Correct: Eyebrow follicles stay in anagen for only a few months: Correct. A short anagen phase limits how long the hair gets.