Adult Mammals Lack the Stem Cell Activity to Make New Eggs


Recent research in mice and humans have discovered a stem cell population in ovaries that can form eggs. However, this discovery begs a question: namely, why do adult female mammals run out of eggs in their lifetime if they have a stem cell population that can produce eggs?

New research from the Carnegie Institute for Science demonstrates that adult mice do not use stem to produce new eggs, thus answering this apparent conundrum.

Before birth, mouse and human ovaries contain an abundant supply of germ cells that originate from primordial germ cells that form from the inner layer of the primary umbilical vesicle (otherwise known as the yolk sac).  Between the time when the embryo is four to six weeks old, the primordial germ cells (PGCs) migrate from the wall of the primary umbilical vesicle to the gut tube.  From the gut tube, the PGCs migrate to the dorsal body wall by means of the mesentery that suspends the gut from the body wall.  Once in the body wall, the PGCs come to rest on either side of the midline in the loose mesenchymal tissue just inside the membranous lining of the body cavity (known more technically as the coelomic cavity).

PGC Migration Pathway2

Most of the PGCs populate the region of the body wall at the level that will form the gonads.  During their migration, PGCs continue to multiply by means of mitosis, which increase their numbers substantially.  Some PGCs may become stranded during their migration, coming to rest at extragonadal sites.  Occasionally, stray germ cells of this type may give rise to a type of tumor called a teratoma.

Teratoma
Teratoma

Once in their final location, the PGCs will stimulate the formation of the genital or gonadal ridge.

In females, PGCs (which are now called gonocytes) undergo a few more mitotic divisions after they are surrounded by the somatic support cells and become intimately associated with them.  The gonocytes differentiate into oogonia, and by the 5th month of fetal development all oogonia initiate meiosis.  After they initiate meiosis, the oogonia are called primary oocytes.  However, during an early phase of meiosis all sex cells enter a state of dormancy, and they remain in meiotic arrest as primary oocytes until sexual maturity.  Beginning at puberty, each month a few ovarian follicles resume development in response to the monthly surge of pituitary gonadotropic  hormones, but usually only one primary oocyte matures into a secondary oocyte and is ovulated. This oocyte enters a second phase of meiotic arrest and does not actually complete meiosis unless it is fertilized. These monthly cycles continue until the onset of menopause at approximately 50 years of age.

Near the time of birth, the ovaries of mice and humans contain an abundant supply of eggs that will be released from follicles during ovulation each menstrual cycle.  At the birth of the baby, she will possess a large reserve of primordial follicles that contain a single egg surrounded by supporting follicle cells.  Evidence of new follicle production is absent after birth.  Therefore, it has long been thought that the supply of follicles is fixed at birth and eventually is exhausted at menopause.

During the last decade, researchers have found primordial follicles in adult mouse ovaries that turn over and claimed that adult germ-line stem cells constantly resupply the follicle pool and sustain ovulation.  These claims were based on observations of ovarian tissue and one the behavior of extremely rare ovarian cells after these cells were cultured for some time in the laboratory.  Such criteria are subjective, especially in light of the fact that culturing cells for long periods of time in the laboratory can effectively reprogram them.

At Carnegie, Lei Lei and Allan Spradling used a technique that tracks individual cells and their progeny within living tissue over a specific time course.  The cells are marked with a gene, and this gene is inherited by the progeny of that cell, thus allowing the careful tracking of all the progeny of that cell or those cells.  This technique is called “lineage tracking” and it is a very popular technique in developmental and cell biology.

By subjecting primordial follicles to lineage tracking, Lei and Spradling showed that germ-line stem cell activity cannot be detected in mice.  Furthermore, primordial follicles are stable, and even if half the existing follicles die off, no germ-line stem cell activity is detectable.  This research does not prove that there are no germ-line stem cell divisions within the ovary of the mouse, but it does place an upper limit on the divisions of the germ-line stem cell population of one division every two weeks at the most, which is biologically insignificant.

What then can be said about the germ-line stem cell cultures isolated in the laboratory?  According to Alan Spradling, the cells “likely arise by dedifferentiation in culture,” and “the same safety and reliability concerns would apply as to any laboratory-generated cell type that lacks a normal counterpart” in the body.

This should be a warning to those conclusions that are solely derived from experiments conducted in culture alone and not in a living creature as well.

Neurons Made from the Skin Cells of Down Syndrome Patients Show Reduced Connectivity


The most common form of intellectual disability in the United States is caused by Down syndrome (DS). DS results when babies are born with an extra copy of an extra piece of chromosome 21. Individuals with DS show various types of intellectual deficits and other health problems as well, such as heart problems, poor muscle tone, an under-active thyroid, respiratory infections, hearing problems, celiac disease, eye conditions, depression or behavior problems associated with attention-deficit hyperactivity disorder or autism.

Even though Down syndrome patients have symptoms and health problems that are well described, how the extra chromosome causes such widespread effects is still largely mysterious.

In recently published research, Anita Bhattacharyya, who is a neuroscientist at the Waisman Center at the University of Wisconsin-Madison, reported that brain cells that were grown from skin cells taken from individuals with Down syndrome.

“Even though Down syndrome is very common, it’s surprising how little we know about what goes wrong in the brain,” says Bhattacharyya. “These new cells provide a way to look at early brain development.”

The skin cells taken from DS patients were grown in culture and genetically engineered to so that a fraction of them were transformed into induced pluripotent stem cells (iPSCs). Since iPSCs can be differentiated into any adult cell type, Bhattacharyya’s lab, working with collaboration with Su-Chun Zhang and Jason Weick, grew those iPSCs in culture and differentiated them into dorsal forebrain neurons, which they could test in the laboratory.

Neurophysiological tests of the DS neurons revealed that these neurons formed a reduced number of connections between them each other. Bhattacharyya says. “They communicate less, are quieter. This is new, but it fits with what little we know about the Down syndrome brain.” Brain cells communicate through connections called synapses, and the Down neurons had only about 60 percent of the usual number of synapses and synaptic activity. “This is enough to make a difference,” says Bhattacharyya. “Even if they recovered these synapses later on, you have missed this critical window of time during early development.”

Bhattacharyya and colleagues also examined the genes that were affected in the Down syndrome stem cells and neurons. They discovered that those genes on the extra chromosome were increased 150 percent, which is consistent with the contribution of the extra chromosome.

However, the output of about 1,500 genes elsewhere in the genome was strongly affected. “It’s not surprising to see changes, but the genes that changed were surprising,” says Bhattacharyya. The predominant increase was seen in genes that respond to oxidative stress, which occurs when molecules with unpaired electrons called free radicals damage a wide variety of tissues.

“We definitely found a high level of oxidative stress in the Down syndrome neurons,” says Bhattacharyya. “This has been suggested before from other studies, but we were pleased to find more evidence for that. We now have a system we can manipulate to study the effects of oxidative stress and possibly prevent them.”

DS includes a range of symptoms that might result from oxidative stress, Bhattacharyya says, including accelerated aging. “In their 40s, Down syndrome individuals age very quickly. They suddenly get gray hair; their skin wrinkles, there is rapid aging in many organs, and a quick appearance of Alzheimer’s disease. Many of these processes may be due to increased oxidative stress, but it remains to be directly tested.”

Oxidative stress could be especially significant, because it appears right from the start in the stem cells. “This suggests that these cells go through their whole life with oxidative stress,” Bhattacharyya adds, “and that might contribute to the death of neurons later on, or increase susceptibility to Alzheimer’s.”

Other researchers have created neurons with DS from induced pluripotent stem cells, Bhattacharyya notes. “However, we are the first to report this synaptic deficit, and to report the effects on genes on other chromosomes in neurons. We are also the first to use stem cells from the same person that either had or lacked the extra chromosome. This allowed us to look at the difference just caused by extra chromosome, not due to the genetic difference among people.”

The research, published the week of May 27 in the Proceedings of the National Academy of Sciences, was a basic exploration of the roots of Down syndrome. Bhattacharyya says that while she did not intend to explore treatments in this work, she did note that “we could potentially use these cells to test or intelligently design drugs to target symptoms of Down syndrome.”