HOMOLOGUES OF EXTERNAL GENITALIA
Before 9 weeks of gestation, both sexes have
identical external genitalia, characterized by a urogenital sinus. At this
undifferentiated stage, the external genitalia consist of a genital tubercle
above a urethral groove. Lateral to this are urethral or urogenital folds and
even more lateral are the labioscrotal swellings or folds. The male and female
derivatives from these structures are shown.
The
bladder and genital ducts find a common opening in the urogenital sinus. This
sinus is formed from the earlier urogenital slit, which is a consequence of the
perineal membrane separating the urogenital ducts from the single cloacal
opening.
In male
development, the genital tubercle elongates, forming a long urethral groove.
The distal portion of the groove terminates in a solid epithelial plate
(urethral plate) that extends into the glans penis and later canalizes. The
midline fusion of the lateral urethral folds is the key step in forming a
penile urethra, but this fusion only occurs after the urethral plate canalizes
distally. In the female, the primitive structures do not lengthen and the
urethral folds do not fuse in the midline. Instead they become the labia
majora.
The
vagina develops as a diverticulum of the urogenital sinus near the müllerian
tubercle. It becomes contiguous with the distal end of the müllerian ducts.
Roughly four-fifths of the vagina originates from the urogenital sinus and
one-fifth is of müllerian origin. In the male, the vaginal remnant is usually extremely
small, as the müllerian structures atrophy before the vaginal diverticulum
develops. In intersex disorders (formerly called pseudohermaphroditism and most
recently termed disorders of sexual development [DSD]) such as androgen
insensitivity syndrome, however, an anatomic remnant of the vaginal
diverticulum may persist as a blind vaginal pouch.
In normal
female development, the vagina is pushed posteriorly by a down growth of
connective tissue. By the 12th week of gestation, it acquires its own, separate
opening. In female intersex disorders, the growth of this septum is incomplete,
thus leading to persistence of the urogenital sinus.
Male and
female external genitalia in the first trimester of development appear
remarkably similar. The principal distinctions between them are the location
and size of the vaginal diverticulum, the size of the phallus, and the degree
of fusion of the urethral folds and the labioscrotal swellings.
FACTORS DETERMINING
DIFFERENTIATION OF THE EXTERNAL GENITALIA
Similar
to the genital ducts, there is a tendency for the external genitalia to develop
along female lines. Masculinization
of the genital ducts is induced by androgenic hormones, principally
testosterone from Leydig cells in the fetal testis during the differentiation
process. More important than the source of androgens, however, is the timing
and amount of hormone. Examples of this include inappropriate androgen exposure
from congenital adrenal hyperplasia or from the maternal circulation, both of
which can induce various degrees
of masculinization of the female system characteristic of intersex disorders.
By the 12th week, androgenic exposure will no longer cause fusion of the
urethral and labioscrotal folds in the female, as the vagina has migrated fully
posteriorly. Clitoral hypertrophy, however, may still result from such
exposures at
any time
in fetal life or even after birth.
