HUMAN BIOLOGY

312 Chapter 16

how sperm Form

Sperm form in seminiferous tubules

Packed inside each of a male’s testes are 125 meters—over

400 feet—of seminiferous tubules. As many as thirty

wedge-shaped lobes divide the inside of a testis and

each lobe holds two or three coiled tubules (Figure 16.6A

a nd 16.6B).

In the walls of seminiferous tubules are cells called

spermatogonia (singular: spermatogonium; Figure 16.6C).

Spermatogonia are the starting

point of spermatogenesis, the

formation of sperm. This pro-

cess requires several rounds of

cell division, including a type

called mitosis and a type called

meiosis. You’ll read more about

cell division in Chapter 18; here

the main thing to keep in mind

is that meiosis is necessary to

form sperm and eggs.

Spermatogonia develop into primary spermatocytes,

which become secondary spermatocytes after a first round

of meiosis (meiosis I). A second round (meiosis II) forms

spermatids. The spermatids develop into spermatozoa, or

simply sperm, the male gametes. The “tail” of each sperm,

a flagellum, forms at the end of the process, which takes

9 to 10 weeks. Meanwhile, the developing cells receive

n In his reproductive years, a male continually produces

sperm, which develop in a series of steps controlled by

hormones.

n Links to Flagella 3.9, Hormones from the hypothalamus and

pituitary 15.3

seminal

vesicle

prostate

gland

bulbourethral

gland

urethra

epididymis

vas

deferens

penis

testis

seminiferous
tubule

primary

spermatocyte

spermatogonium

(diploid)

Leydig cells

between tubules

wall of

seminiferous

tubule Sertoli cell secondary spermatocyte

early

spermatids

late spermatid

immature sperm

(haploid)

mitosis meiosis I meiosis II lumen

Figure 16.6 Animated! Sperm form inside seminiferous tubules in the testes. A The male reproductive tract from behind. B Cells in

three neighboring seminiferous tubules. Leydig cells in spaces between tubules make testosterone. C How sperm form, starting with a

diploid germ cell. (A: © Cengage Learning; B: © Ed Reschke/Peter Arnold; C: © Cengage Learning)

A

B

C

16.4

seminiferous tubules

Coiled tubules in the testes

that contain the cells from

which sperm develop.

sperm Male gametes.

spermatogenesis

Process in which sperm

form.

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