| Introduction.
The purpose of this paper is to highlight the most important aspects
of the undescended testis (cryptorchidism) and its variants, and to
update the reader to what is new concerning pathogenesis, incidence,
evaluation and management.
Historical Background. The
term cryptorchidism originates from Latin where Crypto means hidden
and Orchid means testis. The earliest study on testicular descent
was published in 1786 by John Hunter, who found that the testis
is in the fetal abdomen during the seventh month of gestation and
in the scrotum in the ninth. He believed that the descent of the
testis is directed by a cord or ligament that he termed "the
gubernaculum" (1-3).
Embryology.
Testicular descent into the low-temperature environment of
the scrotum in mammals is a complex process. Up to the time of sexual
differentiation in the human fetus, at seven to eight weeks of gestation,
the fetal testis and ovary occupy similar positions in the posterior
abdominal wall. The gonadal positions then diverge: the testis remains
close to the future inguinal canal and the ovary moves away from
the groin (1). Descent of the testes occurs in two morphologically
and hormonally distinct phases. The key structure in controlling
the process is the gubernaculum which is the embryonic ligament
anchoring the testes to the inguinal region. The gubernaculum enlarges
in the first phase to anchor the testis near the inguinal region
as the embryo enlarges. This occurs between 10 and 15 weeks of gestation.
In the second phase which occurs between 28 and 35 weeks of gestation,
the gubernaculum migrates out of the inguinal canal across the pubic
region and into the scrotum forming a processus vaginalis and creating
an intraperitoneal space into which the testes can descend (1,2).
Hormonal control of the first phase of testicular descent is still
controversial although most evidence indicates that MIS (mullarian
inhibiting substance, also known as AMH, antimullarian hormone)
is the active factor and causes
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regression of the paramesonephric duct. In the second phase however,
testosterone acts as a stimulant for inguinoscrotal migration. There
is mounting evidence now that androgenic (testicular) control is
through the genitofemoral nerve. It has been postulated and it is
almost certain that the sensory nucleus of the genitofemoral nerve,
upon androgen stimulation, releases CGRP (calcitonin gene related
peptide) which, in turn, guides gubernacular migration (1,4).
Variants.
- True undescended testes: are testes that are
not located in the scrotum, however they exist along the normal
path of descent and have a normally located gubernaculum.
- Retractile (migratory) testes: are testes that
readily retract from the scrotum to the superficial inguinal pouch
(5).
- Ectopic testes: are testes that have an abnormal
gubernacular migration and thus are located outside the normal line
of descent.
- Ascending testes: are testes that are noted
to be normally located in the scrotum in infancy but later ascend
out of the scrotum and become truly undescended (6).
Pathogenesis.
Any anomaly disrupting normal testicular descent
leads to cryptorchidism. The complexity of the normal process of
descent suggests that causative factors for maldescent are multifactorial.
Abnormalities of the gubernacular migration may be related to defects
in the migratory mechanism itself or failure of genitofemoral nerve
function. Defects in the nerve may be caused by deficiency of androgen
secretion during the second and third trimesters as a result of
deficiency of gonadotropin production by the pituitary or the placenta.
Other endocrine disorders such as MIS deficiency or decreased testosterone
synthesis or decreased receptor function (testosterone resistance)
may also cause failure of testicular descent but are relatively
rare (1).
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