Down through history, the word "barren" has been a strictlyfeminine adjective. A male-dominated world took it for granted thatinfertility was the woman's fault, often reason enough for annullinga marriage.
Now the clinically correct fact is that both genders sharereproductive dysfunction equally. Forty percent of infertility arisesfrom various gynecological problems, and an equal percentage frommale sperm insufficiency. They share the remaining 20 percent,which are mutual problems.
Spermatic inadequacy in man (not to be confused with impotence)takes two forms: low sperm motility and low sperm count. In a cubiccentimeter of seminal fluid, 50 to 60 million spermatozoa representsan optimal count. About 20 million is high enough to supportpregnancy (given good motility); 10 to 20 is borderline.
Sperm cells arise and mature in the paired, compactly coiled,seminiferous tubules that make up the bulk of the testes. In a human,the tubules, if laid out end to end, would total some 500 meters inlength, filled with literally billions of spermatozoa.
In a mouse, said reproductive physiologist Ralph Brinster,seminiferous tubules have an inner diameter of 100 microns, andwalls about 25 microns thick. Brinster, a professor at the Universityof Pennsylvania School of Veterinary Medicine in Philadelphia, hasperfected a patent-pending technique for microinjecting sperm cellsfrom a fertile donor mouse into the seminiferous tubules of aninfertile recipient.
Body's Only Self-Renewing Cells
Two papers in today's Proceedings of the National Academy ofSciences (PNAS) report this work and its results. The most strikingoutcome is that some recipient male sired offspring carrying thegenes of the donor.
Sperm cells develop from precursor stem-cell spermatogonia, whichare "a unique and very fascinating population of cells," Brinster told BioWorld Today. "They are the only cell population in the body thatconstantly renews itself, and transmits its genes to the nextgeneration."
Testicles are thought to be suspended outside the body to keep theirsperm a degree or two below body temperature. When heat, trauma,irradiation, toxic drugs or illness diminish sperm count, Brinster said,the spermatogonia stem cells respond to the insult by dividing andrestoring the optimal population.
A rat's testes produce 10 million spermatozoa per day, per gram oftesticular tissue, PNAS noted. In contrast, a human female at pubertyhas a fixed lifetime total of about 400,000 oocytes (ova).
For their experiments, Brinster and his co-workers took testes frommice, removed their tunica, or sheaths, then digested theseminiferous tubules with enzymes to isolate their sperm cells.Suspended in solution, these averaged one to 10 million cells permilliliter.
Two kinds of infertile males received these sperm: One consisted ofalbino rodents, which are genetically unable to form spermatozoa.The other cohort they rendered sperm-free by injections of bisulfan,a chemical that destroys spermatogenic stem cells.
The donor sperm cells were genetically engineered to stain blue.They were introduced into the tubules via a glass pipette with a sharp40-micron tip.
It takes 35 days for a mouse stem cell to produce maturespermatozoa, so recipient males were kept between 48 and 230 daysbefore euthanasia.
Of 22 genetically sterile testes, four showed spermatogenesis. In oneexperiment, PNAS reported, of 104 chemically sterilized testes,"colonization of recipient testes was found in 38," In a second, of245, 173 (71 percent) "showed evidence that transplanted cells hadsurvived, colonized recipient seminiferous tubules, and initiatedspermatogenesis."
"The most striking results of these experiments," the paper observed,"was production of offspring from donor-cell-derived spermatozoa."Of three males left alive for breeding, one remained infertile; anotherone sired six of 39 transgenic progeny, the third, six of 15.
Research now in progress, Brinster said, "aims at understandingspermatogenesis, culturing the stem cells in vitro, and moving spermfrom other species into the mouse, "perhaps to enhance fertility in aninfertile bull." This potential, he added, goes beyond improvingcattle-breeding. "Sheep, swine, or any species you like," he foresees,"could be turned into models for important human diseases."
"Currently," he pointed out, "you can introduce new genes in manyspecies, but you cannot target gene sequence change, or knock out agene in any species but the mouse, because there aren't any stem-cellcultures available. So this is an alternative or complementaryapproach to deal with genetic modifications in all animals that have atestis _ not just mice, not just mammals, not just birds, but anyanimal that has a testis."
He summed up, "Theoretically, if you can get those cells to growoutside the body, then introduce them into the mouse, the mousebecomes the universal test tube _ just as it has for the humanimmune system, for studying AIDS and other human viruses."
The G-word _ Pro And Con
Cell biologist Martin Dym, of Georgetown University MedicalCenter in Washington, is a pioneer of spermatogenesis research. In acommentary accompanying the PNAS articles, Dym mentionedgermline gene therapy as a potential application of Brinster'sachievement.
"The development of the spermatogonial stem-cell transplantationtechnique," Dym wrote, "and the possibility of manipulating thegenetic makeup of the spermatogonia, provide many potentialopportunities for agricultural, biological and medical research."
He went on, "Although somatic cell gene therapy is a generallyaccepted means of preventing disease, this procedure affects only theindividual undergoing treatment. Germ-line gene therapy, on theother hand, affects not only the patient but subsequent generations aswell." But Dym cautioned, "the technology required to introduce acorrective gene to a specific site on a chromosome has not beenperfected, and the risk of error remains great."
He cited ethical arguments pro and con the g-word:
Pro: obligation of health professionals to use best availabletreatments; parental right to use all means to have a healthy child;germ-line therapy is more cost-effective than somatic.
Con: expensive; limited use; alternate strategies available; inherentrisks and mistakes; pressure to use germ-line for enhancement, nottherapy.
Brinster said he thinks "germ-line gene therapy in humans has to beworked out over the coming years. First, its practical aspects, thenwhat society wants to use.
"In practical terms," he continued, "if we or others get a culturesystem, there are some advantages to germ-line over the egg. Onceyou make a gene correction in an egg, it's done; there's no way toexamine it, and you don't remove the old gene. The spermatogonialstem cell can be cultured; you can select out the cell that carries theright change. So it has some potential long-term advantages over theegg, and vice versa, if people wish to use it down the line." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.