«A SUGGESTION AS TO THE CAUSE OF THE ASPER- MATIC CONDITION OF THE IMPERFECTLY DESCENDED TESTIS BY F. A. E. CREW, M.D., D.Sc. (Paper from the Animal ...»
A SUGGESTION AS TO THE CAUSE OF THE ASPER-
MATIC CONDITION OF THE IMPERFECTLY
BY F. A. E. CREW, M.D., D.Sc.
(Paper from the Animal Breeding Research Department,
The University, Edinburgh.)
IN all Vertebrates the testes are developed in contact with the ventral surface
of the kidneys, behind the peritoneum covering the body-cavity. In some they remain permanently in this situation, but it is characteristic of the majority of mammals that during the course of development they leave their primitive lodgment and migrate posteriorly and ventrally to the terminal periphery where they protrude at the surface of the body-wall. This protrusion constitutes the scrotum which varies in character from that of a pair of small ill-defined slightly elevated areas to that of a capacious, definite, pedunculated sac.
A survey of the Vertebrates will show that the testes in different cases are found in positions which mark the stages in a complete migration from the primitive position near the kidneys to the peripheral scrotum. It will also be seen that the scrotal situation of the testis is characteristic of the more im- pulsively active mammals. In the case of the human, in whom the migration has been most thoroughly studied, it provides a very complete example of the manner in which ontogeny repeats and condenses phylogeny in whole or in part.
There is much diversity of opinion as to the exact mechanism of the migra- tion and it is probable therefore that not one but several factors are involved.
The process possibly may be explained thus. Mechanical forces resulting from the changing methods of progression compelled the dense, compact, suspended testis to pass from the primitive position towards the inguinal region of the abdominal cavity (Woodland). There it naturally came to lie in the line of the lymph-sinuses described by Sabin. With increasing impulsiveness of movement and with increased intra-abdominal pressure consequent upon the development of the diaphragm, the testis was forced along the lymph-track so that it came to occupy a sub-integumental position in the groin (Bramann, Eberth, Keith). Here the testis found itself in a situation which in several ways was different from the interior of the abdomen and it was obliged to adapt itself to the new conditions. But, owing to its decreased mobility and to the injurious effects of active flexion of the thigh upon the abdomen, it suffered repeated attacks of inflammation. The local peritoneum with the mesorchium became involved in this inflammation with the result that ad- Aspermatic Condition of the Imperfectly Descended Testis 99 hesions and bands were produced. The overlying skin became thinned and stretched. Then further increase of impulsive activity and of intra-abdominal pressure produced a hernia of the testis. So the scrotum and the inguinal fold could have been produced. The position of the testis was equivalent to the sub-integumental one in the groin but the organ was now secure from injury by muscular movements since its mobility had been restored.
This process would be repeated in every generation until at last variation succeeded modification, or, by adaptation, the migration became incorporated in the life-processes of the individual and anticipated by the development of a mechanism which would produce the descent of the testis during foetal life.
"Originally, the descent of the testes did not occur until sexual maturity in all cases, but in many Mammalia (e.g. Marsupials, Ungulates, Carnivores, Primates), the process has gradually become shifted backwards ontogenetically to earlier periods, so that the formation of the scrotum takes place independently in the embryo in the form of the external genital folds."
In the modern mammals in which the migration occurs, the testis is united to a mammary area-supra-pubic, inguinal, perineal, or scrotal, at first by the inguinal fold and later by the gubernaculum-the canal-former, the guide of the testis-which is attached by its upper end to the Wolffian duct, the epididymis, and at the point where the globus minor and vas deferens meet, and by its lower end to the subcutaneous tissues in the groin, the scrotum, the root of the penis, and on the pubis.
The gubernaculum is an actively growing mass of fibro-muscular tissue which, starting from the muscular stratum in the mesorchium and inguinal fold in the inguinal fossa, invades the abdominal wall, every layer of which it carries with it as a prolongation within the scrotum. Upon the peritoneum thus drawn down the testis is dragged like a log upon a sledge. The gubernaculum forms the inguinal canal by the growth of its wedge-shaped end along the line of the lymph-sinuses, The canal is formed before the testis passes.
The final situation of the testis is decided in great part by the relative development of the different gubernacular insertions.
There is no physiological reason why the testis should not leave its primitive position. It does not stand in the same relation to the general economy of the individual as do the other organs of the body. It belongs to the race rather than to the individual, for though there cannot be an actual isolation yet there is distinctly an apartness of the germ-cells and the body is but the carrier of the testes. The migration to the periphery does not disturb the general economy for this reason and just as reproduction itself consists of a separating off of a portion of the organism so the organs of reproduction become separated, in consequence of their migration, from the viscera which belong entirely to the individual.
It is too difficult to conceive that the migration has imparted any advantage to the organism: the process bears no great relation to either advantage o F. A. E. Crew 100 disadvantage, though the scrotal position would appear to be one much exposed to danger, for the ease with which the scrotum is attacked and the devastating effects of contusion of the testes make of this region a veritable Achilles' heel.
"...there remain many unsolved problems. Take, as an instance, the descent of the testis in the Mammalia. Neither direct nor indirect equilibration accounts for this. We cannot consider it an adaptive change, since there seems no way in which the production of sperm-cells, internally carried on in a bird, is made external by adjustment to the changed requirements of mammalian life. Nor can we ascribe it to the survival of the fittest; for it is incredible that any mammal was over-advantaged in the struggle for life by this changed position of these organs. Contrariwise, the removal of them from a place of safety to a place of danger would seem to be negatived by natural selection. Nor can we regard the transposition as a concomitant of re-equilibration, since it can hardly be due to some change in the general physiological balance." HERBERT SPENCER.
There certainly appears to be no compensatory physiological advantage offering benefits which outweigh the physical disadvantage and it would seem that the migration is, as suggested, but the inevitable concomitant of some other constant feature of the animal's existence, and that it has not arisen in relation to ulterior ends.
A study of the conditions in which the testis fails to complete its migration will show that the testis has become so modified that while it will function perfectly when within the scrotum, it is incapable of producing spermatozoa when situated elsewhere. This would indicate that the conditions within the scrotum are different from those of the interior of the abdominal cavity.
"The descent of that testicle is very slow which is not complete before birth, often requiring years for that purpose; and it sometimes never reaches the scrotum, especially the lower part of it. There is oftener, I believe, an inequality in the situation of the two testicles than is commonly imagined;
and I am of the opinion that the lowest is the more vigorous, having taken the lead readily, and come to its place at once.
"It is not easy to ascertain the cause of this failure in the descent of the testicle; but I am inclined to suspect that the fault originates in the testicles themselves. This, however, is certain, that the testicle, which has completed its descent is the largest, which is more evident in the quadruped than in the human subject; as in these we can have the opportunity of examining the parts when we please, and can determine how small in comparison with the other that testicle is which has exceeded the usual time of coming down; it never descends so low as the other.
" When one or both testicles remain through life in the belly, I believe that they are exceedingly imperfect, and probably incapable of performing their natural function, and that this imperfection prevents the disposition for descent from taking place." JOHN HUNTER.
"Arrest of descent is commonly regarded as a symptom of arrest of testicular development. John Hunter regarded arrested descent of the testicle as due to an imperfection in its development; all recent observations support his opinion." KEITH.
Aspermatic Condition of the Imperfectly Descended Testis 101 "It is commonly believed that the imperfection of an undescended testicle is due to its failure to reach the scrotum. This I believe to be an error. An undescended testis fails to reach the scrotum because of its imperfection."
In spite of the great weight of these opinions it is difficult to accept them without question since it is generally understood that the testis plays but a passive role in the migratory process. Imperfection of the local peritoneum during the development of the inguinal fold and mesorchium; of the gubernaculum; of the inguinal canal; or of the scrotum, these conditions also must lead to imperfect descent.
The long mesorchium may allow the testis to hang too freely in the abdominal cavity; there may be a deficiency or abnormality of the upper attachment of the gubernaculum; intra-uterine peritonitis may have caused adhesions which limit the mobility of the testis; shortness of the vas deferens and of the blood-vessels, though more likely to be an effect, may also be a cause of imperfect descent; the inguinal canal may be imperfectly formed or the scrotum ill-developed so that passage thereinto may be hindered. Overaction of the cremaster may be another possible cause, for in infants and children the action of this muscle occasionally draws the testicle up even beyond the external ring. Murard records a case in which the testis periodically disappeared into the abdominal cavity.
Imperfection of the testis most certainly can be a potent cause of its non-descent, but unless it is of such a size or of such a shape that it cannot pass along the passage prepared for it, surely the fault must lie with the powers, or with the passage, but not with the passenger.
" It is certain that in the majority of cases the imperfectly or abnormally descended testicle is functionless, at any rate as regards spermatogenesis.
But though the function of spermatogenesis is absent, that of producing the internal secretion necessary for the development of the secondary sexual characters of the male is generally, but not always, carried out. That the function of spermatogenesis is lost is shown by the fact that such persons are unable to beget offspring and is also confirmed by the histological examination of retained testes after removal.
" In rare cases the spermatogenetic function is not lost, even when there is a double imperfect descent with very small testes, or when both organs are arrested within the abdominal cavity; this has been proved both by the presence of normal tubules and active spermatozoa, and also by the fact that these persons have proved capable of procreation. Many examples of this are recorded in medical literature. There is, however, evidence that those in which spermatogenesis is normally carried out are young men mostly under thirty years of age. In men over this age the imperfectly descended testis is nearly always functionless. In the majority of cases under thirty the spermatogenetic function is absent and the proportion of functional organs is probably small." TURNER, P.
"After careful observations extending over many years I only once found spermatozoa in an undescended testis." BLAND-SUTTON.
F. A. E. Crew 102 "It is not rare to find spermatozoa in testicles which have remained in the lower part of the inguinal canal, but in those in the upper part, and in those taken from the abdomen, this is exceptional." HOBDAY.
" In mammals the testes fail at times to pass through the inguinal canal, and, in consequence of their retention in the body-cavity, the germ-cells fail to develop. On the other hand, the interstitial cells of the testis develop normally. Cryptorchid individuals show the normal secondary sexual characters of their species." MORGAN.
M'Fadyean, who examined a series of twenty-five imperfectly descended testes for Hobday, found that out of fourteen from the abdominal cavity, only two contained spermatozoa, and that of eleven from the inguinal canalthree being from the upper part-only five were capable of functioning. Gurlt failed to find spermatozoa in testes removed from the abdomen; Wesche found that testes from the inguinal canal were capable of producing functional spermatozoa; Paugoue records the case of a stallion in which both testes were undescended yet who sired many colts of which, however, five suffered from the same condition; Dollar states the opinion that testes retained within the abdomen contain degenerate spermatozoa or none at all.
As a result of the examination of a series of imperfectly descended testes from the horse, placed at my disposal by the courtesy of Mr Wm. Brown of the Veterinary Department, Marischal College, I am able to demonstrate the fact that the nearer the testis comes to lie to the normal scrotal position, the more likely it is that functional spermatozoa will be found therein. Testes removed from the lower part of the inguinal canal, though smaller in size than the normal and inclined to be of an unusual shape, are in the majority of cases imperfectly functional; testes removed from the abdominal cavity, so far as my experience goes, are invariably the seat of tumour growth, benign or malignant, even in the case of two-year old horses; testes removed from the upper part of the canal are usually atrophic and normal spermatozoa can be found therein only very exceptionally.