Last night, I headed out to the beautiful Bell House, in the Gowanus section of Brooklyn, for this month's Secret Science Club lecture, featuring developmental biologist Ruth Lehmann of the NYU School of Medicine. Dr Lehmann's list of accomplishments is formidable- she is Director of the Skirball Institute of Biomolecular Medicine, Chair of the Department of Cell Biology, Laura and Isaac Perlmutter Professor of Cell Biology, an Investigator with the Howard Hughes Medical Institute, and Director of the Lehmann Lab. This month's lecture was presented in conjunction with the Albert and Mary Lasker Foundation.
The basic theme of Dr Lehmann's lecture was: Germ cells are forever. Germ cells are named that because they form something which can grow, they form the sperm and egg cells which propagate from generation to generation. While individuals grow older, germ cells house information from organism to organism- they pass this information to new organisms. The Soma, the non-germ cell body, is merely one body through generations of the germline. An antiquated view of reproduction held that each sperm cell contained a homonculus, that all of the information needed for reproduction was contained in the sperm. Now, it is known that the sperm and the egg each contain half of the genetic information needed for reproduction- a fertilized egg is a single cell incorporating DNA from the sperm and previously unfertilized egg cells. It undergoes rapid division, forming approximately two hundred adult cell types- nerves, muscles, blood cells (84% of the body's cells). Ultimately, the body contains approximately thirty trillion cells, plus an even greater amount of symbiotic microbes (a topic covered in other SSC lectures) A small percentage of these dividing cells are sperm and egg cells- in the early development of an embryo, the germ cells are 'set aside' from the somatic cells.
Dr Lehmann then showed us a diagram of the germline life cycle similar to this diagram- the sperm and egg combine to form a zygote, which develops into a morula, then a blastocyst. The germ cells are 'set aside' from the somatic cells, then migrate to the embryo's developing gonads- in humans, this takes place in the 5th and 6th weeks of embryonic development. The oocytes and spermatazoa remain immature in the gonads until puberty takes place. While most of the cells of the blastocyst form the embryo, some make supporting structures, such as the placenta.
The study of human embryos is difficult due to ethical considerations, and a paucity of materials. Human blastocysts used for in vitro studies cannot be more than fourteen days old, when nerve tissues start developing, so the study of germ cells' migration to the gonads cannot be studied. Much of what is known about the subject is derived from Drosophila and zebrafish studies. Dr Lehmann showed us a video of fluorescent-dyed germ cells migrating in a zebrafish, similar to this video:
At the five day stage, stem cells can be obtained from the Inner Cell Mass. Stem cells can replicate themselves in a culture, and can be forced to differentiate into other cell types. A cocktail of factors can be used to reprogram different cell types into Inner Cell Mass analogs- for example, skin cells can be transformed into 'stem' cells. This is important for medical research because patient-derived cell lines can recapitulate aspects of disease, so that what is really going wrong, the underlying mechanisms of a disease, can be studied.
Organoids, tissues analogous to small organs, can be grown in vitro from adult stem cells which self-organize into these structures. Tissues with multiple different cell types will form these organ-type structures, such as an optic cup, albeit one without a blood supply. Organoids provide good research models, and hint at the possibility of regenerative medicine.
During development, the blastocyst folds in a procedure known as gastrulation. Dr Lehmann quoted the biologist Lewis Wolpert: "It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life." During the onset of gastrulation, a structure known as the primitive streak forms- the precursors to germ cells can be identified 'set aside' at the end of the primitive streak. The somatic cells form into three germ layers in most animals: the ectoderm, the mesoderm, and the endoderm. Embryos in vitro are known as gastruloids- stem cells self-organize, but micropatterning in a dish can influence this differentiation. By 'setting aside' the germ line, the germ cells can be protected from such threats as somatic development and transposable elements, which jump locations and can prohibit other genes from expressing themselves, which would be deleterious to a germline. Germ cells behave differently from somatic cells- in women, with their XX genome, one X chromosome is inactive, but both X chromosomes are active in the germ cells.
Analogs of germ cells can be produced in vitro- skin cells can be transformed into pluripotent stem cell analogs which can then be transformed into primordial germ cells which can be implanted in gonads to become sperm or egg cells. If primordial germ cells didn't migrate to the gonads, which form from the mesoderm, they would die. In one form of childhood cancer, germ cells don't properly migrate and form tumors. In one exceptional case, the Caenorhabditis elegans nematode, the gonads migrate to the germ line. The germ cells migrate over and through other tissues, guided by signals both attractive and repulsive, then they stop and adhere to the gonads.
In humans, the oogenesis and spermatogenesis pathways diverge. If the SRY gene on the Y-chromosome is present, germ cells develop into sperm cells. If SRY is not present, the germ cells develop into ova. Germ cells in embryonic testes behave differently from germ cells in embryonic ovaries- the development of the sperm cells is arrested, while the germ cells in embryonic ovaries become immature ova. A female embryo contains about two million immature oocytes, a number which decreases to about twenty percent by age thirty. In males, the germ cells keep dividing to produce more sperm- about one hundred million sperm cells are released with each ejaculation.
Sexual reproduction involves a mix-and-match of genetic information from each parent. A mature sperm cell and a mature egg cell each contain half of the genetic information of all other body cells. The sperm and egg are produced through a cell division process known as meiosis, which Dr Lehmann jocularly referred to as 'a good thing'. Among gametes, chromosomes are not identical- the offspring will not be carbon copies of their parents. Among humans, somatic cells typically have 46 chromosomes, while gametes typically have 23 each. When a sperm cell fertilizes an egg, the 23 chromosomes of each gamete make up the full chromosome complement and the cycle of the germline development begins anew. Among male humans the process of meiosis produces four sperm cells, each with 23 chromosomes, while among females one oocyte and three polar bodies, which are 'thrown away' are produced. The final polar body is ejected when the oocyte is fertilized. With aging, mistakes involving polar bodies, such as trisomy, can occur.
Structures in the egg cell, such as the zona pellucida and the corona radiata attract sperm but prevent more than one sperm cell from fertilizing the egg. The sperm also 'prepares' for fertilization- because they move, they are packed with mitochondria. The sperm cell is 'capped' with a structure known as the acrosome, which is packed with enzymes which allow penetration of the egg cell wall. Out of millions of sperm, there can be only one winner- fertilization by more than one sperm cell would result in aberrant cell divisions.
Among fish, which typically use external fertilization, it is important that sperm from fish of other species are not recognized. In zebrafish, a 'bouncer' molecule ensures that sperm cannot enter from everywhere, but only one site- the bouncer molecule also recognizes zebrafish sperm and keeps foreign sperm out. A matchmaking molecule has to be recognized for fertilization to occur. If the bouncer molecule is removed, foreign sperm from closely related fish species can fertilize eggs, but the resultant embryos die during gastrulation. The bouncer molecule is sufficient to form the species barrier.
Dr Lehmann ended her lecture by referring the audience to a Radiolab series concerning gonads, then proceeded to hold a Q&A session. Some bastard in the audience asked her to touch upon the topic of cloning, wherein somatic cell nuclei are inserted into ova. She characterized cloning as cell reprogramming, in which the pluripotency factors of the ovum are used to develop tissues with the somatic cell chromosomes. A lot of oocytes are necessary for successful cloning, which is a lot more complicated than reprogramming somatic cells with a cocktail of factors. She indicated that cloning was not feasible in analyzing diseases, and noted that cloning is unnecessary, but interesting to study. Regarding the segregation of germ cells, Dr Lehmann noted that germ cells are dangerous- they never lose their pluripotency and have the potential to metastasize if not contained. Regarding determination of sex in intersex individuals, Dr Lehmann noted that, once the gonads develop, hormones take over to guide development- perhaps, in intersex individuals, a hormonal signal is not recognized, perhaps an XY germ cell can occur in a gonad acting like an ovary. Cells are not autonomously on their own while they develop. Regarding the production of better organoids, Dr Lehmann noted that micropatterning and other techniques could guide development, and more reliable organoids could be produced.
Dr Lehmann imparted a ton of knowledge in her lecture- it was a thorough introduction to reproduction and embryology, accessible to the layperson. Once again, the Secret Science Club, in conjunction with the Lasker Foundation, delivered a fantastic lecture. Kudos to Dr Lehmann, Dorian and Margaret, the good people of the Lasker Foundation, and the staff of the beautiful Bell House... high fives to all!
For an in-depth introduction to this topic, Dr Lehmann has a multi-part lecture about germlines:
Settle down and give thanks for SCIENCE!