先前Cell杂志报道一篇由麻省综合医院被的研究成果：血液或骨髓中来源的间质干细胞能够形成小鼠的卵母细胞，最新的美国Joslin糖尿病研究的研究证实：成年雌性小鼠的卵母细胞不能由血液或骨髓中的生殖细胞形成！如果麻省的研究成果成立的话，雌性哺乳动物只能产生确定数量的卵母细胞，并且如果因损伤或疾病损失掉就不能再补充或再生。移植归宿或外围血细胞能够在受体小鼠的卵巢中产生新的卵母细胞。这可以产生源源不断的卵母细胞。
Joslin的这项新研究刊登在最新一期的Nature杂志上。麻省综合医院的研究曾引起了巨大的轰动，因为它意味着骨髓细胞可能成为恢复雌性生殖能力的一种新资源。他们的研究报道说移植细胞能够从骨髓或血液进入到遗传不孕或接受过化疗的雌性小鼠的卵巢中并产生了新的卵母细胞。但是他们并没有研究这些卵母细胞是否能够被排卵，或者骨髓细胞是否在正常情况下能迁移到卵巢中，作为卵巢再生的一个正常过程。
Joslin的研究人员利用一种人工联体（parabiotic）小鼠模型进行研究。这种模型的血管融合在一起，即两个小鼠利用一个共同的循环系统。这种模型使研究人员能够追擦正常情况下在血液中流通的细胞。
使用的小鼠在遗传上是等同的，除了其中一个小鼠的身体表达了绿色荧光蛋白。因此，一旦这个小鼠被联合在一起，任何从表达荧光蛋白小鼠的血液中运动到它的非荧光同伴的细胞都能被确定。
Wagers和他的研究组利用激素刺激这种联体动物的排卵作用并检测从两只小鼠是否排出了绿色的卵母细胞。结果，他们发现绿色荧光小鼠的所有卵母细胞都是绿色的，而它的联体搭档却没有一个绿色的卵母细胞。这证明排出的卵母细胞不是由通过血管达到卵巢的细胞产生的。
这些新数据表明循环系统的细胞通常不能形成能排出的卵母细胞，因此不可能受精。这项研究还检测了因化疗受损的卵巢是否在血液循环细胞存在下有不同的反应。结果他们再次发现没有证据表明血液细胞能通过循环作用在卵巢中生根以及促进成熟的、能被排出的卵母细胞的形成，而且血细胞也不能促进化疗后寄主卵母细胞的复原。
Joslin study refutes recent report that bone marrow can replenish female oocytes
Study shows that circulating bone-marrow derived cells do not contribute to egg formation
Ovulated egg cells, or oocytes, in adult female mice are not formed from germ cells in the blood or bone marrow. That's the conclusion of a new study led by investigators at Joslin Diabetes Center and Harvard University. These findings refute a controversial recent study conducted at Massachusetts General Hospital (MGH), which itself contradicted the long-held belief that female mammals are born with a finite number of oocytes that cannot be replenished or regenerated if lost to injury or disease by suggesting that transplanted bone marrow or peripheral blood cells were capable of generating new oocytes in the ovaries of recipient mice.
The Joslin study will appear in an upcoming issue of Nature and on the journal's Web site on June 14, 2006.
"It was a very important study to do," says Amy J. Wagers, Ph.D., Investigator in Developmental and Stem Cell Biology at Joslin Diabetes Center and Assistant Professor of Pathology at Harvard Medical School. "The suggestion that bone marrow cells might represent a previously unappreciated source of cells capable of restoring female fertility had significant implications for patients undergoing chemotherapy, which often leads to sterility, and for individuals donating or receiving bone marrow cells for transplant, as well as for women experiencing premature menopause."
The MGH study reported that transplanted cells from the bone marrow or blood could enter the ovaries of genetically infertile or chemically sterilized female mice and produce new oocytes but didn't study whether those oocytes could be ovulated, or whether bone marrow cells normally migrate to the ovary as part of a normal process of ovary regeneration. The Joslin study's goal was to find out if that was possible.
Joslin study researchers used a parabiotic mouse model in which pairs of mice are joined using a surgical procedure that enables blood vessels to fuse such that the mice develop a common circulatory system. "It's a very useful model," reports Wagers, "because it allows one to track cells that normally circulate in the blood under physiological conditions."
The mice used were almost genetically identical, except that one member of the pair expressed throughout its body green fluorescent protein (GFP), a gene from jellyfish that causes cells expressing it to glow green under certain wavelengths of light. Thus, once the mice were joined, any cells moving through the circulation from the GFP-expressing mouse to its non-fluorescent partner would be identifiable by their green marking, while cells moving from the non-fluorescent partner to the GFP partner would be identifiable by their lack of green fluorescence.
Wagers and the research team used hormones to stimulate ovulation in the parabiotic pairs after they had been joined for six to eight months and examined ovulated oocytes from both mice for the presence of green cells. They found that all of the oocytes in the GFP-expressing partners were green, while none of the oocytes collected from the non-fluorescent partners were green. This demonstrated that the ovulated oocytes had not been produced by cells that arrived in the ovary through the bloodstream.
"We didn't find any oocytes ovulated in the parabiotic mice that would have been derived from circulating cells. Our data argue that circulating cells don't normally contribute to oocytes that are ovulated and therefore available for fertilization," says Wagers.
The study also tested whether ovaries damaged by chemotherapy might respond differently to the presence of blood-circulating cells. The researchers treated non-fluorescent mice with two chemotherapy drugs that cause ovarian damage, cyclophosphamide and busulfan, and paired them with untreated GFP-expressing partners. However, again they found no evidence that blood cells seed the ovary through the circulation and contribute to mature, ovulated oocytes. In addition, they saw equivalent impairment in the ability of chemotherapy-treated animals to ovulate, whether they were joined to untreated partners or not, suggesting that blood-borne cells did not promote the recovery of host oocytes after chemotherapy.
The MGH study had raised hopes that bone marrow or peripheral blood cell transplants could be used to restore fertility in women having difficulty conceiving, or who were prematurely menopausal or whose eggs were damaged by illness or chemotherapy.
Yet, if bone marrow cells in fact generated ovulated oocytes, this might cause concern in settings of bone marrow transplant for other indications, as the transplanted germ cells would contain the donor's genetic contribution, analogous to a donor egg. However, the Joslin study suggests that this is unlikely to occur.
"Our data indicate that transplanted bone marrow cells or peripheral blood cells are unlikely to contribute to fertilizable oocytes in transplant recipients," says Wagers.
Wagers is a principal faculty member of the Harvard Stem Cell Institute, and her research is focused on the migration and functioning of blood-forming stem cells, found predominantly in the bone marrow, and in muscle-regenerative cells found in skeletal muscle. Her laboratory is broadly interested in adult stem cells and tissue regeneration. Wagers and her team are exploring whether transplants of blood-forming stem cells could be useful in the treatment of autoimmunity, by replacing reactive cells with non-reactive cells and potentially reversing the autoimmune process in diseases such as type 1 diabetes. "Some of the work we're doing may suggest ways to make transplants more effective and less toxic, which could one day have applications in treating diabetes," says Wagers.
原始出处：
Born or made? Debate on mouse eggs reignites p795
Doubts deepen over whether mammals can make new eggs. Kendall Powell
doi:10.1038/441795a
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