Pre-conception and Conception
In the field of pediatrics, there are what are described "developmental milestones." These events signal certain accomplishments in the developing child, and with them, progress. Likewise, there are things the obstetrician welcomes from the baby developing inside the expectant mother. But since the patient is actually a wall hiding any direct visual observation of the baby, both she and the expectant father rely on the obstetrician to keep them abreast of obstetrical "milestones," fertilization, implantation, embryonic development, the first, second, and third trimesters.
I've always wondered why the recognized milestones should begin at birth, as labeled by a pediatrician. Certainly the developing child represents a continuum. And so I hope to describe "fetal milestones."
The First Fetal Milestone: Fertilization
All things, miraculous or otherwise, must have a beginning. It is a startling realization that your future child is actually as old as her mother. (I'll say "her" or "she" as a frame of reference, even though "she" may be destined to develop as either male or female later.) She began as a full representation of her mother's genetic architecture which sat in an egg in one of her mother's ovaries--in fact, sat there since her own mother's fetal development. Since we all began with an egg as old as our mother, we're all as old as our mothers! Unfortunately, this is why pregnancies in women over the age of forty can be more inclined to genetic defects, as these babies are beginning to develop from eggs over forty years old already.
The lucky egg in this discussion is indeed special, as it was only one of over six million that were present in Mom's ovary during her own fetal development. Of course, this six million dwindled down to about two million by the time of Mom's birth, and to only three hundred thousand by her puberty, at which time maturing follicles which contained an egg finally began to do their maturing near the surface of the ovary so that release, or ovulation, would be possible. Quietly "she" had incubated over the years as her little unbloomed blossom self until she rose to the occasion one menstrual cycle, that occasion being her random stimulation, along with several others in one of her mother's two ovaries. While she was just completing her own episode of maturity, she was witness to several other future brothers' or sisters' attempts to bail out free from their ovarian nursery in the process called ovulation. Of course, the bunch of competing follicles that didn't match her own timing of maturity fizzled away, leaving her the lone victor in the competition (unless the future involved twins!).
All of this (ovulation) depended, of course, on a very complicated hormonal sequence which is intricately repeated each month, but which is also even more miraculous considering the winning follicle, and within it, our heroine, must take advantage of the current cycle. By the time this happens, her chromasomes have been reduced by a couple of genetic divisions to an orderly one-half of the full genetic complement of her mother. That one-half is always female. After ovulation, she will be followed by new crops of competing follicles to follow her in the cycles to come. This is unless of course she does something rash which will for some time spoil it for all of the rest. I speak here of fertilization.
Waiting for your future daughter is the fallopian tube. This is a conduit which is much more than a hollow tube that "grabs" the egg at one end and dumps it into the uterus at the other end. It is a pathway lined with delicate cells that have hair-like structures called cilia on them which wave the egg toward the intrauterine cavity (where the baby will grow). They wave in rythmic motion toward the uterus (womb), which, by the way, is preparing a welcome mat in the form of thickened, vascular tissue hormonally stimulated in optimistic preparation for allowing a pregnancy to bed in. The welcome mat will be rudely snatched away, however, should the egg try to register there unfertilized. All of the lovely nurturing lining will be jettisoned in the phenomenon known as menses, or "the period." As the egg bumps and careens its way down the fallopian tube, going with the flow, i.e., cooperating with anatomy, it is assaulted by the swarm of sperm, each sperm of which carries one-half of the genetic architecture of its father, this half being either male or female in genetic persuasive ability. So in this way, the sperm, and through it, Dad, determines the sex of the union of sperm and egg that will make your baby.
One lucky sperm makes that crucial penetration which, almost explosively, sets off a chemical reaction all around the egg, sealing it off from any further penetration by other sperm, which could disastrously add more genetic material. And so at this exact moment your child's sex is determined. For our purposes, a sperm which carries the "X," or female, sex chromosome has fused with the egg. Of course, it could just as easily have been a sperm that carried a "Y," or male sex chromosome. Regardless, this spermatid's genetic package from Dad is introduced into the egg's interior so as to fuse with the genetic complement from the mother. At this exact point we are, in fact, dealing with an entity who will be, for continued reference, a "she."
Now the egg's and the sperm's stories are over. This is a new structure altogether--the "zygote," and because it is a new structure, "fertilized egg" is an unfair designation. A sperm has released its genetic material to combine with the egg's genes. In this way what is more properly titled the zygote is created in the midportion of the fallopian tube, which is still doing what it does best, waving this new package down the line to its new home, the uterus.
The Second Fetal Milestone: Implantation
The lining inside the uterus has planned for the zygote cyclically for all of the normal menstrual cycles in the past since puberty. In the past, however, the disappointment of an unfertilized egg has caused all of the hard work to be discarded as the menses, or period. When a fertilized egg does not implant into the lining of the womb, the actual absence of signals by an embedded zygote is in itself a signal that allows pituitary hormones in the bloodstream to drop off, and therefore turns off the hormones from the ovary (especially progesterone) that keep this uterine lining intact. But when there is a fertilized egg (zygote), things go much differently, and finally congratulations are in order.
The story goes like this:
The very end of each tube, near the ovary, has an opening rimmed by finger-like structures called fimbria, which many theorize have an adhesive effect on the just-liberated egg (from the ovary--ovulation). In this way, this "fimbriated" end secures the area around the ovulation site of the ovary and further channels the egg into the tube. The egg at this point has between twelve and twenty-four hours of susceptibility to fertilization. When fertilization takes place, the changes are swift, involving cell division upon cell division so that when the ultimate package is delivered to the womb it will be a complex structure about to be a fetus. And so coming down the pike is the star of the show, having been fertilized in the mid-portion of the fallopian tube. By this time the chromosomal contributions from both mother and father have joined and the zygote splits a few times (during the first two days after fertilization), converting itself from a zygote into a solid clump of only about sixteen cells called a "morula."
But it is solid for only a short time, as the pace within it is rapid: As an effect of continued specialized division of the cells in a certain pattern, a cavity appears in this morula which redefines it into what's called a blastocyst (the fourth and fifth days after fertilization). At this point, your future child may still be free-floating along the fallopian tube, pushed by cilia on the way to being spilled into the inside of the womb, or intrauterine cavity.
The blastocyst cavity separates two types of cells-- ones that will be developing baby and placenta (afterbirth), but also cells that will erode the lining of the womb with enzymes, allowing the blastocyst, which has been now floating free in the uterus for days, to implant and finally submerge into the uterine lining. In this way implantation takes place. At fertilization your future daughter was smaller than the period at the end of this sentence, and it took nearly three to five days for full, deep implantation to happen.
(Some feel that there are many silent miscarriages, the blastocyst failing to implant, with a resultant sloughing of the lining of the uterus seen as a seemingly normal period.)
The blastocyst containing its fluid-filled cavity is now completely submerged in the uterine lining. The cells at one border of this cavity, that border adjacent to the lining of the uterus, will be that part of the blastocyst that will develop into the fetus and placenta. For the conception to make the transition to fetus, all of the organ systems of your future daughter must arise and fit together as they progressively develop. This is well established by the end of the seventh week, and this is when the transition from embryo to fetus is made. Of course, pulling diverse organ systems out of that magician's hat that up until now has consisted of a microscopic group of cells makes one indeed think of magic; still, the field of embryology (the study of the embryo) has allowed us to learn some of the tricks behind the illusions.
This is not to say that the whole process isn't miraculous. Miraculous is still an appropriate description. But we have learned a lot, unfortunately only to be raised to higher degrees of wonder based on continuing revelations. After fertilization, the human race has truly begun, as a single cell has been contacted by another, a genetic package defining a new unique human has been assembled, and both growth and division of a mass of cells rush toward completion of a project that will have an impact on the world for the next seventy or eighty years of so. The blastocyst, as described earlier, moves her from the implantation milestone to the embryonic one. This is the structure that makes contact and thereby makes a relationship with the host. It is a non-malignant tumor that takes from her mother, the mother herself a somewhat passive participant in an exhausting association.
With the blastocyst successfully received by Mom, this human race can continue, the cells showing some differences from each other as trends of cell lines begin. Nine days after fertilization, the cells of the blastocyst that will develop into the placenta begin to develop as strands of cells, creating little lakes interspersed among these strands. These lakes ultimately connect up with each other and are infiltrated with blood and blood vessels from Mom, the whole complex destined to be the placenta, by which process a natural site is determined to easily transfer nutrients and oxygen from Mom to future daughter. As a blastocyst your daughter has been pushed into the womb, the intrauterine cavity. She won't be leaving until her birth, and by that time she will be amazingly different.
The Third Fetal Milestone: Embryonic Development
For the conception to make the transition to a fetus, all of the organ systems of your future daughter must arise and fit together as they progressively develop. This is well established by the end of the seventh week, and this is when the transition from embryo to fetus is made. With the blastocyst successfully received by Mom, the cells begin showing some differences from each other as trends of cell lines begin.
The cells of the blastocyst that are destined to be fetus have already developed three different types of cells, ectoderm (the back of the fetus), endoderm (the front of the fetus), and mesoderm (the remainder).
The very first structure associated with an actual embryo is something called "the primitive streak," which is made up of the ectoderm and endoderm, with the mesoderm in between. These three different types of cells are called the germ lines, or germ layers, as from them spring all of the beginnings of all of the organs of the developing embryo.
From the ectoderm develop the entire nervous system (brain, spinal cord, and nerves), and skin. From the endoderm come the entire gastrointestinal tract (from mouth to rectum) along with all of the associated inner organs related in some way with digestion (e.g., liver, pancreas, etc.). From the mesoderm are derived all of the muscles, ligaments, tendons, bones, and blood elements; as well as urinary organs such as kidneys and bladder; and the reproductive organs (including all of those eggs we've heard so much about!).
Extending from the primitive steak is the notocord, which is the earliest structure of support for the embryo. It's what makes us vertebrates within the entire animal world. It is still in you and me as material in the discs between the vertebrae of our backbones, those discs that "slip" from time to time in the less fortunate among us. Since the business end of any sentient life form is where the central nervous system resides, it is fitting that your daughter's development is preferential at the "head" end, with the rest of the embryo following behind. A lengthwise structure of ectoderm develops and its two edges of this long groove curl toward each other and finally close, creating a tube that will run the length of the fetus. It is associated with the notocord, and from it will develop the brain and spinal cord.
Mesoderm, flanking this tube on either side, will surround it as serial bones (or the vertebrae that make up the backbone), as well as the skull around the brain. Also important is a space that develops within the mesoderm, a cavity called the "coelum." This cavity, a space lined by the actively dividing cells which surround it, is very important, for it establishes your future baby as a tubular structure. A tubular structure, any physics teacher will tell you, will have its empty space within distorted when the tube itself is twisted. This is exactly what happens when different cells of the tube start growing and reproducing at differing rates. This is, thankfully, to our benefit, because as this tubular core splits and portions of it pinch off in ways consistent with the differing rates of growth outside of it, it allows us to have the digestive tract, central nervous system, cardiovascular system; and other quite necessary systems develop from their cell lines in positions that make us work as a correctly operating human machine. That is, that allow us to survive. Because alongside and interspersed among the several tubular spaces that are being so acted upon are the several tracts of cell types. And this cavity called the coelum is one of the tubular spaces that have been twisted this way and that. Within it are twisting tubes-within-tubes that will make the digestive tract from the growing endoderm, and alongside these tubes-within-tubes the previously liver and other numerous organs contained in the coelum, which will later be called the peritoneal cavity by doctors (and the belly or abdomen by everyone else).
There is an interesting thing about this peritoneal cavity. Besides developing organs essential to the baby for after she's born and for the rest of her life, it also contains structures that contain blood vessels that come and go from the embryo and fetus through the umbilical cord. It is ironic that there is as well a "coming and going" of function here: the organs are developing to prepare for life outside of the womb, while the umbilical cord structures are designed to maintain pre-birth life only, and will disintegrate (and perhaps remain as only wispy internal remnants) after birth.
The mesoderm-derived cardiovascular system is perhaps the most complicated tubes-within-a-tube system. It does acrobatic twists and gyrations as it grows, extensively coring every cubic millimeter of the body as well as bunching up into a multi-chambered convolution, the heart. There is muscle in this organ, creating a pump whose developmental tubular gyrations have miraculously coincided with another major development of tubes that also undergo their own gyrations to produce trachea (windpipe), bronchi, and lungs.
The twisting tubes-within-tubes phenomenon is common throughout the developing fetus which began as a solid clump of matter. The kidneys and bladder and the collecting tubes between; filament-like guides along which limb buds grow correctly to end up as arms, legs, fingers, and toes; lymphatic channels, bile ducts, milk ducts, even ducts that circulate fluid within the brain--it is beyond comprehension how it can all coincide perfectly so frequently. One would think that with so many interrelated steps and such interdependent synchrony to occur that mistakes would be common in our species. The true miracle of life is that it not only happens at all--that, too, is beyond comprehension--but that it happens so well so frequently.