CDS (I) 2022 General Knowledge · Question 15 of 120

Correct — C, Squamous epithelium. The lining of the mouth and the oesophagus is stratified squamous epithelium — flat, plate-like cells laid down in several layers. The flatness is what makes the lining delicate, and the layering is what makes it survive being scraped by food, because the cells at the surface are rubbed away continuously and replaced from below. Since the injury described is to the lining, the tissue damaged is that epithelium, and squamous is the shape its cells take. The same kind of tissue lines the inside of the cheek — the source of the cells in any classroom slide made by gently scraping the inner cheek — as well as the alveoli of the lungs and the walls of blood vessels, where a single flat layer is thin enough for substances to diffuse across.

• (a)Smooth muscle — Present in the oesophagus, but in the wall rather than the lining. Smooth muscle in the lower part of the tube produces the waves of peristalsis that push food down; it is not the delicate surface layer the accident has injured.
• (b)Cuboidal epithelium — The right family, the wrong shape and the wrong place. Cube-shaped epithelial cells line the tubules of the kidney and the ducts of glands, where secretion and absorption are the work being done.
• (d)Striated muscle — Also genuinely present — the upper part of the oesophagus is striated muscle, which is why the beginning of a swallow is under voluntary control. Like smooth muscle, it belongs to the wall of the tube and not to its lining.

Epithelial tissue covers surfaces and lines cavities, and it is classified by the shape of its cells and by how many layers there are. Squamous cells are thin and flat like plates; cuboidal cells are cube-shaped; columnar cells are tall. Where the surface has to withstand wear, the squamous type is stacked into layers and called stratified, as in the skin, the mouth and the oesophagus. Where thinness matters more than toughness, it stays a single layer, as in the alveoli of the lungs and the lining of blood vessels, so that gases and dissolved substances can cross easily.

The question hands you a location and asks for a tissue, so the useful habit is to separate the lining of an organ from the wall of it. Both muscle options are real components of the oesophagus, which is what makes them tempting, but neither faces the food. Working from the function is a reliable check as well: a surface that is abraded every time something is swallowed needs cells that can be lost and replaced cheaply, and flat cells in layers are the cheapest arrangement there is. That is the same reasoning that puts stratified squamous epithelium on the outside of the skin.

• The mouth, the oesophagus and the skin are lined by stratified squamous epithelium.
• Squamous cells are thin, flat and plate-like, and appear polygonal when viewed from above.
• A single layer of squamous epithelium lines the alveoli of the lungs and the walls of blood vessels, where thinness allows diffusion.
• Cuboidal epithelium lines kidney tubules and gland ducts; columnar epithelium lines much of the intestine.
• The upper oesophagus contains striated muscle and the lower part smooth muscle, but both lie in the wall, not in the lining.

• Confusing the lining of an organ with the muscular wall behind it.
• Attaching cuboidal epithelium to the digestive tract when it belongs to kidney tubules and gland ducts.
• Forgetting that squamous epithelium comes in both single-layered and stratified forms, doing different jobs.

As a location-to-tissue question like this one, or as a match-list pairing tissue types with the organs they are found in.

• practice — not a real PYQ

  A single layer of flat cells forms the wall of the alveoli in the lungs. This arrangement is best suited to

  • (a)resisting wear from air moving in and out
  • (b)allowing gases to diffuse rapidly across the surface
  • (c)secreting mucus into the air passages
  • (d)absorbing digested food into the blood

  Answer(b) allowing gases to diffuse rapidly across the surface — the thinner the barrier, the faster oxygen and carbon dioxide cross it.

• practice — not a real PYQ

  Cuboidal epithelium is characteristically found in the

  • (a)lining of the mouth
  • (b)walls of the alveoli
  • (c)tubules of the kidney
  • (d)outer layer of the skin

  Answer(c) tubules of the kidney — cube-shaped cells there carry out the secretion and absorption that shape the urine.