Cell Shapes and Sizes

There are about 200 types of cells in the human body, and they vary greatly in shape (fig. 3.1). Squamous3 (SQUAY-

3squam = scale + ous = characterized by

Squamous Spheroid

y\j

1

Polygonal

Discoid

  • N-f* rr ^ * 7 * v/'x^
  • XAVV Aj'JL~<>1 I - A'TTT-IH^T-V^

Cuboidal

Fusiform (spindle-shaped)

Columnar

Fibrous

J 1 1 1 > H t 1 i i ) j > H

Stellate

2proto = first + plasm = formed

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Text

Chapter 3 Cellular Form and Function 95

mus) cells are thin, flat, and often have angular contours when viewed from above. Such cells line the esophagus and cover the skin. Polygonal4 cells have irregularly angular shapes with four, five, or more sides. Some nerve cells have multiple extensions that give them a starlike, or stellate,5 shape. Cuboidal6 cells are squarish and approximately as tall as they are wide; liver cells are a good example. Columnar cells, such as those lining the intestines, are markedly taller than wide. Egg cells and fat cells are spheroid to ovoid (round to oval). Red blood cells are discoid (disc-shaped). Smooth muscle cells are fusiform7 (FEW-zih-form)—thick in the middle and tapered toward the ends. Skeletal muscle cells are described as fibrous because of their threadlike shape.

Most human cells range from 10 to 15 micrometers (^m) in diameter. (See the inside back book cover for units of measurement.) The human egg cell, an excep poly = many + gon = angles sstell = star + ate = characterized by 6cub = cube + oidal = like, resembling 7fusi = spindle + form = shape tionally large 100 ^m in diameter, is barely visible to the naked eye. The longest human cells are nerve cells (sometimes over a meter long) and muscle cells (up to 30 cm long), but both are too slender to be seen with the naked eye.

There is a limit to how large a cell can be, partly due to the relationship between its volume and surface area. The surface area of a cell is proportional to the square of its diameter, while volume is proportional to the cube of its diameter. Thus, for a given increase in diameter, cell volume increases much faster than surface area. Picture a cuboidal cell 10 ^m on each side (fig. 3.2). It would have a surface area of 600 ^m2 (10 ^m X 10 ^m X 6 sides) and a volume of 1,000 ^m3 (10 X 10 X 10 ^m). Now, suppose it grew by another 10 ^m on each side. Its new surface area would be 20 ^m X 20 ^m X 6 = 2,400 ^m2, and its volume would be 20 X 20 X 20 ^m = 8,000 ^m3. The 20 ^m cell has eight times as much protoplasm needing nourishment and waste removal, but only four times as much membrane surface through which wastes and nutrients can be exchanged. A cell that is too big cannot support itself.

poly = many + gon = angles sstell = star + ate = characterized by 6cub = cube + oidal = like, resembling 7fusi = spindle + form = shape

Cell Shapes And Sizes Human Physiology

Figure 3.2 The Relationship Between Cell Surface Area and Volume. As a cell doubles in diameter, its volume increases eightfold, but its surface area increases only fourfold. A cell that is too large may have too little plasma membrane to serve the metabolic needs of its increased volume of cytoplasm.

Figure 3.2 The Relationship Between Cell Surface Area and Volume. As a cell doubles in diameter, its volume increases eightfold, but its surface area increases only fourfold. A cell that is too large may have too little plasma membrane to serve the metabolic needs of its increased volume of cytoplasm.

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

96 Part One Organization of the Body

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