Fertilization And Early Embryonic Development

Fertilization and Early Embryonic Development: Introduction and Index Fertilization in vertebrates is, of course, the union of two haploid gametes to reconstitute a diploid cell - a cell with the potential to become a new individual. Knowing this simple fact does not, however, impart an appreciation for the "beauty" of fertilization which comes from a more detailed understanding of this process. Fertilization is a not a single event. Rather, it is a series of steps that might be said to begin when egg and sperm first come into contact and end with the intermingling of haploid genomes. Prior to fertilization, the two gametes must become fully mature and be transported to a site within the female - the oviduct - that will support their interactions with one another. The first of many challenges following fertilization is to become multicellular, and the one-cell embryo rapidly cleaves into 2, 4, 8 and more cells. It then starts to do some interesting things like develop a discrete inside and an outside. Finally, the embryos of many species start to secrete hormones that ensure their survival - a process called maternal recognition of pregnancy.

Gamete Transport Fertilization depends upon the two gametes bumping into one another. In species with internal fertilization, which includes all mammals and birds, both sperm and egg must be transported into the oviduct, which serves as the site of fertilization.

Sperm Transport Semen is ejaculated and deposited initially into one of two sites: the vagina (e.g. humans, cattle, rabbits) or the uterus (e.g. horses, pigs, rodents). In species such as dogs, semen is probably deposited largely into the vagina, but also forced into the uterus. Despite these differences in deposition site and significant differences in the number of sperm ejacuated, there is remarkably little variation among species in the total number of sperm that reach the oviducts. Typicially, a few hundred to a few thousand sperm reach the oviducts following a single mating, which usually represents far less than one percent of the sperm in the ejaculate.

The vagina represents a hostile environment for sperm, and their continued survival depends on getting into more hospitable regions of the female tract. In their journey from vagina to oviduct, sperm must overcome a series of barriers, each of which eliminates a substantial proportion of the original population of sperm: The cervix connects the vagina to the uterus. The cervical canal follows an irregular, tortuous route, and the epithelium contains many deep crypts. The cervical epithelium is richly endowed with mucus-secreting cells, and, as a consequence, the lumen is filled with mucus. Interestingly, the consistency and viscosity of cervical mucus is under endocrine control. When estrogen levels are high and progesterone levels low, as occurs prior to ovulation, cervical mucus becomes watery and its mucin strands assume a parallel orientation. This state apparently greatly facilitates passage of sperm through the cervical canal. Conversely, when progesterone concentrations are high, as in the luteal phase of the cycle, cervical mucus becomes exceptionally viscous and disorganized, which largely precludes entry of sperm into the uterus. The uterus does not present an active barrier, but sperm must somehow be transported directionally along its length. Studies in several species have shown that sperm are able to get from the distal uterus to the oviducts in times as short as a few minutes, which is much too fast to be explained by sperm motility. Moreover, dead sperm and inanimate sperm-sized particles are rather efficiently transported upward through the uterine lumen. The conclusion from these types of studies is that sperm transport in the uterus is largely a result of uterine contractions, and that sperm motility plays a minor if any role in the process. In most, but not all species, the uterus is also a site hostile to sperm. In many animals, sperm within the uterus are rapidly phagocytosed. In other cases, sperm can remain viable in the uterus for several days, but their fertility rapidly declines. There are some dramatic exceptions to these general observations. The uterotubal junction is the region joining the tip of the uterine horn to the oviduct. The morphology of this region varies considerably among species, and and this structure appears to be a significant barrier to sperm especially in animals like rodents and pigs where huge numbers of sperm are deposited directly in the lumen of the uterus. In summary, the vast majority of ejaculated sperm are lost are various points between the cervix and oviduct. A few exhausted semifinalists make it to the site of fertilization. Of those, of course, there can be only one "winner" for each egg. Without meaning to, John Wayne provided a good synopsis of the life a sperm.

Egg Transport Mammalian eggs are ovulated from ovarian follicles as cumulus-oocyte complexes, which consist of the oocyte embedded in a cluster of follicle cells. The image to the right shows such a structure from a cow - the oocyte is encased in its zona pellucida, which is somewhat obscured by a cloud of follicle cells. In order to reach the site of fertilization, the ovulated egg must be picked up and transported into oviduct through an opening called the ostium. In most mammals the ovarian end of the oviduct flares into a funnel-shaped structure called the fimbria, which is positioned to partially cover the ovary. The fimbria is densely covered with ciliated epithelial cells, which beat toward the ostium and propel the cumulus-oocyte complex into the oviduct. In species such as the rodents and dogs, the ovary is enclosed completely or nearly completely in a thin membrane called the bursa. Because the ostium of the oviduct is inside the bursa, the eggs are essentially trapped after ovulation with no where to go except into the oviduct. Once an oocyte enters the oviduct, it is propelled by ciliary motion down into the ampulla, where fertilization takes place. The oviduct provides the appropriate environment not only for fertilization, but for early embryonic development, and it is important that the embryo remain there for a period of about three days.

The Fertilizable Lifespan of Gametes In most species, both sperm and egg have a short fertilizable lifespan, and once they are delivered into the female tract, the clock starts ticking. What this means, of course, if that mating or insemination must coincide closely with ovulation. If sperm are deposited many days before the egg reaches the oviduct, there is little chance that they will survive to fertilize. Conversely, if sperm reach the oviduct several days after ovulation, they will certainly find an egg that has long since degenerated. One of many demonstrations of this concept is depicted with data from humans in the graph to the right, which presents data on the probability of conception in healthy women that had sexual intercourse a single time within several days of ovulation (adapted from Wilcox et al. New Eng J Med 333:1517, 1995). A group of 221 women were in the trial and data were collected from a total of 625 menstrual cycles. Over the course of the study, 192 pregnancies were established, of which 129 resulted in delivery of a baby.

There were no term pregnancies established when a single intercourse took place greater than six days before ovulation or even a day after ovulation. The highest probability of becoming pregnant was seen when sex occurred during the two days preceeding ovulation. These and similar data from other studies indicate that in humans, the ovulated egg has a very short fertilizable lifespan. Cautionary note: In this study, no pregnancies were attained when sex took place after the day of ovulation - this does not mean that fertility is zero at that time, but rather, that it is low and was not observed in this sample of women.

Structure of the Gametes Before Fertilization Fertilization presents some major challenges to both sperm and egg: •

The fertilizing sperm must somehow recognize, bind to and ultimately traverse the zona pellucida surrounding the egg. It then must bind to the plasma membrane of the egg.

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The egg must not only respond to the fertilizing sperm in a number of ways, but actively prevent more than one sperm from fertilizing it. Fertilization by more than one sperm is bad.

In their mature form, both sperm and egg possess structures that allow them to fulfill these mission objectives.

Structure of the Sperm Mature sperm, know formally as spermatozoa, have a morphology that most people over the age of ten would recognize immediately. The nucleus is contained within the head, which, for most mammals, has a flattened, oval shape. During spermiogenesis, the haploid sperm cell develops a tail or flagellum, and all of its mitochondria become aligned in a helix around the first part of the tail, forming the midpiece. The entire cell is, of course, enveloped by a plasma membrane. The image to the right shows these structures at the light microscopic level with a bull sperm. The other structure in the mature sperm that plays a critical role in fertilization is the acrosome. The acrosome is, in essence, a gigantic lysosome that forms around the anterior portion of the nucleus. It is bounded by a membrane that is considered to have two faces - the inner acrosomal membrane faces the nucleus, while the outer acrosomal membrane is in close contact with the plasma membrane.

The image to the right shows the front end of a stallion sperm, viewed with an electron microscope. The ruffled appearance of the plasma membrane is an artifact of fixation. The acrosome is the dark band of material between the plasma membrane and nucleus inner and outer acrosomal membranes are not clearly visible at this magnification (image courtesy of Carol Moeller) The function and fate of the acrosome is discussed in the next section on fertilization.

Structure of the Egg Most mammals ovulate an "egg" that has matured into a secondary oocyte; it is always the secondary oocyte that is fertilized. The secondary oocyte is produced along with the first polar body as a result of the first meiotic division. Both of these cells are encased in a thick glycoprotein shell called the zona pellucida. The image to the right shows a secondary oocyte from a mouse; residual follicle cells have been stripped away. Genetically, the secondary oocyte that arrives in the oviduct is in metaphase of the second meiotic division. The metaphase plate is located inside the oocyte immediately below the first polar body. The final structural feature of the egg that serves a critical function during fertilization is a set of cortical granules. During oogenesis, the oocyte develops thousands of small membrane-bound granules that accumulate in the cortical cytoplasm, just beneath the plasma membrane.

Fertilization Fertilization is more a chain of events than a single, isolated phenomenon. Indeed, interruption of any step in the chain will almost certainly cause fertilization failure. The chain begins with a group of changes affecting the sperm, which prepares them for the task ahead. Successful fertilization requires not only that a sperm and egg fuse, but that not more than one sperm fuses with the egg. Fertilization by more than one sperm polyspermy - almost inevitably leads to early embryonic death. At the end of the chain are links that have evolved to efficiently prevent polyspermy. In overview, fertilization can be described as the following steps:

Sperm Capacitation Freshly ejaculated sperm are unable or poorly able to fertilize. Rather, they must first undergo a series of changes known collectively as capacitation. Capacitation is associated with removal of adherent seminal plasma proteins, reorganization of plasma membrane lipids and proteins. It also seems to involve an influx of extracellular calcium, increase in cyclic AMP, and decrease in intracellular pH. The molecular details of capacitation appear to vary somewhat among species. Capacitation occurs while sperm reside in the female reproductive tract for a period of time, as they normally do during gamete transport. The length of time required varies with species, but usually requires several hours. The sperm of many mammals, including humans, can also be capacitated by incubation in certain fertilization media. Sperm that have undergone capacitation are said to become hyperactiviated, and among other things, display hyperactivated motility. Most importantly however, capacitation appears to destabilize the sperm's membrane to prepare it for the acrosome reaction, as described below.

Sperm-Zona Pellucida Binding Binding of sperm to the zona pellucida is a receptor-ligand interaction with a high degree of species specificity. The carbohydrate groups on the zona pellucida glycoproteins function as sperm receptors. The sperm molecule that binds this receptor is not known with certainty, and indeed, there may be several proteins that can serve this function.

The Acrosome Reaction Binding of sperm to the zona pellucida is the easy part of fertilization. The sperm then faces the daunting task of penetrating the zona pellucida to get to the oocyte. Evolution's response to this challenge is the acrosome - a huge modified lysosome that is packed with zona-digesting enzymes and located around the anterior part of the sperm's head - just where it is needed. The acrosome reaction provides the sperm with an enzymatic drill to get throught the zona pellucida. The same zona pellucida protein that serves as a sperm receptor also stimulates a series of events that lead to many areas of fusion between the plasma membrane and outer acrosomal membrane. Membrane fusion (actually an exocytosis) and vesiculation expose the acrosomal contents, leading to leakage of acrosomal enzymes from the sperm's head.

As the acrosome reaction progresses and the sperm passes through the zona

Post-fertilization Events Following fusion of the fertilizing sperm with the oocyte, the sperm head is incorporated into the egg cytoplasm. The nuclear envelope of the sperm disperses, and the chromatin rapidly loosens from its tightly packed state in a process called decondensation. In vertebrates, other sperm components, including mitochondria, are degraded rather than incorporated into the embryo. Chromatin from both the sperm and egg are soon encapsulated in a nuclear membrane, forming pronuclei. The image to the right shows a one-cell rabbit embryo shortly after fertilization - this embryo was fertilized by two sperm, leading to formation of three pronuclei, and would likely die within a few days. Pass your mouse cursor over the image to identify pronuclei. Each pronucleus contains a haploid genome. They migrate together, their membranes break down, and the two genomes condense into chromosomes, thereby reconstituting a diploid organism.

Cleavage and Blastocyst Formation The product of fertilization is a one-cell embryo with a diploid complement of chromosomes. Over the next few days, the mammalian embryo undergoes a series of cell divisions, ultimately leading to formation of a hollow sphere of cells known as a blastocyst. At some point between fertilization and blastocyst formation, the embryo moves out of the oviduct, into the lumen of the uterus. The images below demonstrate major transitions in structure during early embryogenesis in cattle. Note that in all of the the early stages, the embryo is encased in its zona pellucida. Embryos from other mammals have a very similar appearance, and the general sequence of stages is seen in all mammals. Unfertilized oocytes typically fill the entire space inside the zona pellucida, but after fertilization, the one-cell embryo usually is somewhat retracted from the zona pellucida surrounding it. Although not visible in this image, one or two polar bodies are often visible in the perivitelline space, the area between the embryo and the zona pellucida.

The one cell embryo undergoes a series of cleavage divisions, progressing through 2-cell, 4-cell, 8-cell and 16 cell stages. A four cell embryo is shown here. The cells in cleavage stage embryos are known as blastomeres. Note that the blastomeres in this embryo, and the eight-cell embryo below, are distinctly round.

Early on, cleavage divisions occur quite synchronously. In other words, both blastomeres in a two-cell undergo mitosis and cytokinesis almost simultaneously. For this reason, recovered embryos are most commonly observed at the two, four or, and seen here, eight-cell stage. Embryos with an odd number of cells (e.g. 3, 5, 7) are less commonly observed, simply because those states last for a relatively short time. Soon after development of the 8-cell or 16-cell embryo (depending on the species), the blastomeres begin to form tight junctions with one another, leading to deformation of their round shape and formation of a mulberry-shaped mass of cells called a morula. This change in shape of the embryo is called compaction. It is difficult to count the cells in a morula; the embryo shown here probably has between 20 and 30 cells. Formation of junctional complexes between blastomeres gives the embryo and outside and an inside. The outer cells of the embryo also begin to express a variety of membrane transport molecules, including sodium pumps. One result of these changes is an accumulation of fluid inside the embryo, which signals formation of the blastocyst. An early blastocyst, containing a small amount of blastocoelic fluid, is shown to the right. As the blastocyst continues to accumulate blastocoelic fluid, it expands to form - you guessed it - an expanded blastocyst. The blastocyst stage is also a landmark in that this is the first time that two distinctive tissues are present. A blastocyst is composed of a hollow sphere of trophoblast cells, inside of which is a small cluster of cells called the inner cell mass. Trophoblast goes on to contribute to fetal membrane systems, while the inner cell mass is destined largely to become the embryo and fetus. In the expanded blastocyst shown here, the inner cell mass is the dense-looking area at the botton of the embryo.

Eventually, the stretched zona pellucida develops a crack and the blastocyst escapes by a process called hatching. This leaves an empty zona pellucida and a zona-free or hatched blastocyst lying in the lumen of the uterus. Depending on the species, the blastocyst then undergoes implantation or elongates rapidly to fill the uterine lumen.

As mentioned, the developmental progression depicted above for bovine embryos is essentially identical to what all mammalian embryos go through, including humans. For example, the image to the left shows an expanded blastocyst from a dog. This embryo was stained to accentuate the trophoblast and inner cell mass. The length of time required for preimplantation development varies somewhat, but not drastically, among species. The zona-intact bovine blastocysts shown above were collected 5-6 days after fertilization. The same stages would be seen in mice at about 3.5 days after fertilization. In addition to the morphological changes in the embryo described here, preimplantation development is associated with that might be called an awakening of the embryonic genome. There is, for instance, little transcription in the embryos of most species prior to the 8 cell stage, but as embryos develops into morulae, then blastocysts, a large number of genes become transcritionally active and the total level of transcription increases dramatically.

Maternal Recognition of Pregnancy For most species, a critical need arises early in gestation for the mother to "recognize" that she is pregnant. More specifically, the concentration of progesterone in maternal blood must be sustained at a high level in order that the endometrium be maintained in a state conducive to embryonic survival. This means that the corpus luteum must not die and regress, as it normally does just prior to the onset of the next cycle. Maternal recognition of pregnancy doesn't involve any type of conscious recognition by the mother. Rather, it is a process in which some type of signal prevents luteal regression, allowing the corpus luteum to persist and continue to secrete progesterone. This concept can be illustrated by looking at blood progesterone concentrations over time in a cycling sheep that becomes pregnant.

The pattern shown above for sheep is conceptually similar to what is seen other species with multiple cycles, including humans. Although the end result is the same, several different mechanisms for maternal recognition of pregnancy have evolved in different groups of mammals. Some of this diversity can be appreciated by looking at humans, cows and dogs: Blastocysts of humans and other primates secrete large quantities of a protein hormone called chorionic gonadotropin (CG), which is very similar to luteinizing hormone. CG binds to luteinizing hormone receptors in the corpus luteum and stimulates continued secretion of progesterone. It may also block signals in the corpus luteum that cause luteal regression. In cattle and other ruminants, the corpus luteum regresses at the end of the non-pregnant cycle as a result of secretion by the endometrium of prostaglandin F2-alpha (PGF). The early ruminant embryo secretes copious quantities of a protein called interferon tau. Exposure of the endometrium to this hormone dampens the secretion of PGF, thereby blocking the signal for luteolysis. As a result, the corpus luteum survives and progesterone levels are maintained. Dogs do not have multiple, sequential cycles like women or cows. Rather, they have a single cycle roughly every 4 to 6 months. Following ovulation, the pattern of progesterone secretion is essentially the same regardless of whether the bitch is pregnant or not. Consequently, dogs do not have a need for maternal recognition of pregnancy and apparently no mechanism for this process. Several other interesting variations in maternal recognition of pregnancy have been characterized among mammals. The common theme is that the early conceptus is the source of the signal that interferes with normal luteal regression at the end of the

cycle. This makes good biological sense - in essence, the embryo is shouting out its presence to the corpus luteum, saying please do not regress, I need your support! A final important point should be made. The window of time for maternal recognition of pregnancy to occur is narrow, and failures either in sending or receiving the signal may well be a significant cause of early embryonic death. If, for example, a human embryo fails to secrete adequate amounts of CG in time to rescue its mother's corpus luteum, it will die. Luckily, you didn't have this problem, or you wouldn't be reading this page!