Human Physiology, 14th edition (2016)

Reproduction 707

estrogens. As a result, the person with testicular feminization

syndrome develops into a female with well-developed breasts

who never menstruates (and who, of course, can never become

pregnant).

Some male pseudohermaphrodites have normally

functioning testes and normal testosterone receptors, but

they genetically lack the ability to produce the enzyme

5 a -reductase. Individuals with 5 a - reductase deficiency

have normal epididymides, ductus (vasa) deferentia, semi-

nal vesicles, and ejaculatory ducts because the development

of these structures is stimulated directly by testosterone.

However, the external genitalia are poorly developed and

more female in appearance because DHT, which cannot be

produced in the absence of 5 a -reductase, is required for the

development of male external genitalia.

An interesting cause of male pseudohermaphroditism is tes-
ticular feminization syndrome. Individuals with this condition
have normally functioning testes but lack receptors for testoster-
one. Thus, although large amounts of testosterone are secreted,
the embryonic tissues cannot respond to this hormone. Female
genitalia therefore develop, but the vagina ends blindly (a uterus
and fallopian tubes do not develop because of the secretion of
müllerian inhibition factor). Male accessory sex organs likewise
cannot develop because the wolffian ducts lack testosterone
receptors. A child with this condition appears externally to be
a normal prepubertal girl, but she has testes in her body cavity
and no accessory sex organs. These testes secrete an exceedingly
large amount of testosterone at puberty because of the absence
of negative feedback inhibition. This abnormally large amount
of testosterone is converted by the liver and adipose tissue into

Figure 20.6 The

development of the

external genitalia

in the male and

female. ( a [ a 1 , sagittal

view]) At 6 weeks,

the urethral fold and

labioscrotal swelling have

differentiated from the

genital tubercle. ( b ) At

8 weeks, a distinct

phallus is present during

the indifferent stage. By

week 12, the genitalia

have become distinctly

male ( c ) or female

( d ), being derived from

homologous structures.

( e, f ) At 16 weeks, the

genitalia are formed.

Genital

tubercle

Urethral

folds

Labioscrotal

swellings

Cloacal

membrane

Tail

(a)

Genital

tubercle

Urethral fold

Labioscrotal

swelling

(a 1 )

Male

Developing

glans penis

(c)

(b)

Glans

Urethral fold

Labioscrotal

swelling

Phallus

Urogenital

membrane

Female

(d)

Fused urethral folds

Perineum

Anus

Prepuce

Urethral orifice

Perineum

Anus

(e)

Glans penis

Scrotum

Scrotal raphe

Developing

glans clitorus

Labia minora

Labia majora

Glans clitoris

Hymen

Vaginal orifice

Urethral groove

(f)