Fertilization and Early embryo
01. Heredity
Our chromosomes carry building instructions, not for us but for the magnificent agglomeration of structures that enable us to become us. As the author E.B. White wrote: "Heredity is a strong factor, even in architecture. Necessity first mothered invention. Now invention has little ones of her own and they look just like grandma." All living cells are comprised mainly of two types of organic molecules - nucleic acids and proteins. Nucleic acids - genes - carry the instructions the cells needs to function and reproduce; in other words the overall plans for the building, as well as for its own small part. Proteins do the work, erecting scaffolding, chopping each other up, recombining, making more proteins. Indeed inside every cell teems a sub-universe - another skyscraper - comprised of a billion billion proteins conducting complex chemical reactions at rates of ten billions times per second. Meanwhile, other molecules on the cell's surface interact with - talk to - molecules on the surface of other cells. Such, literally, is life, which is distinguished from non-life by the profoundest activity of all: makingself-copies.
02. Sperm
Overview
Sperm production is the most prolific of the male human body's phenomenal production systems. Every second a healthy, young male body produces nearly 1,000 sperm. Over a lifetime, a man produces an astounding number of sperm, perhaps as many as twelve trillion. Each sperm, moreover, contains an entirely unique selection of genetic material. In humans, chromosomes come in pairs of twenty-three. The twenty-third pair consists of two X-shaped chromosomes in women, and one Y-shaped chromosome in men. The joining of an X sperm with the egg creates a female embryo; fertilization by a Y sperm creates a male. Because X and Y sperm are manufactured in equal number, mathematical odds would seem to dictate equal numbers of male and female embryos, but statistics reveal that 106 boys are born for every 100 girls. Scientists speculate that Y sperm swim faster than the X sperm because they carry slightly less genetic material. As a result, Y sperm are more likely to reach the egg first.
Detailed View
During Intercourse the sperm is ejaculated against the opening of the cervix at the far end of the vagina. Conditions in the vagina activate the sperm for about half an hour before they start moving up toward the uterus. The journey of 15-18 cm(about 6-7inches) from the vagina to the fallopian tube. Their journey takes many hours, even days. The long survival time of the sperm, especially in the recesses of the cervix, means that intercourse four or five days before ovulation can result in fertilization. Many more sperm are lost while trying to navigate through the mucus which blocks the passage through them cervix into the uterus. The sperm that make it this far continue to swim towards the fallopian tubes and the waiting ovum.
03. Egg
This process - ovulation - starts with a wave of hormone released from the pituitary, the eye-shaped gland at the base of the brain that directs the endocrine system. Inside the ovaries, each egg cell nestles in its own pod, or follicle. Spurred by the wash of stimulating hormone in the blood, between15 and 20 follicles begin secreting estrogen, the female sex hormone which, among other functions, awakens and nourishes undeveloped eggs.
04. Fertilization
The first barrier - though hardly the most daunting - is distance. Whipping their tails, sperm swim mightily, about a half-inch per minute. But the upper reaches of the Fallopian tubes are nearly a foot away - in fish terms, miles. In between, the sperm must navigate a deadly gauntlet of physical and chemical obstacles. Acid kills sperm, and though the normally acidic vagina becomes less so during ovulation, most male sex cells would die there if not for the protection of the semen, which is alkaline. Those that survive and enter the uterus soon face a choice that for half will mean certain failure and extinction. Of the hundreds of millions of sperm released during ejaculation, fewer than 500 - perhaps one in 500,000 - finally encounter the ovulated egg. Once the first sperm penetrates and fuses with the egg, the membrane rapidly changes electrical charge, in effect demagnetizing. All other competing sperm literally drop off.
05 Infertility
Many men do not produce sufficient sperm, or their cells are weak swimmers or lack "purpose" - a sense of where to go. Women commonly have problems ovulating, or their fallopian tubes are blocked, or their eggs are unhealthy. Infertility is a medical problem, not a sexual disorder. Among couples seeking medical help, most - 85 to 90 percent - receive conventional therapies such as drugs or surgery, the rest high-tech aids like in vitro fertilization or the use of donor eggs.
06 Division
One and a half to three days
Within a few hours after the nuclei of sperm and egg have fused, every gene synthesizes a copy of itself from available chemicals in the zygote. Then each chromosome splits lengthwise, creating two half-chromosomes that regroup into two distinct nuclei. To watch cells divide, or "cleave," is to witness a monumental improbability. One minute there is a spherical mass of protoplasm. The next, the ball pancakes on one side, surface tension gathers, a pinched line appears in the middle, and suddenly the mass parts as if pulled by a drawstring into two equal halves, each within its own membrane and with its own nucleus. By two days after conception, the cells cleave again; by the end of the third day, after splitting once or twice more, 16 to 32 cells huddle inside zona pellucida, like tiny soccer balls jammed in a clear sack.
07. Blastocysts
Four days
The morula first alters shape, its cells subtly shifting location. Instead of a dense cluster, they now form a single-layered hollow sphere around a fluid-filled cavity. Two types of cells start to form, with profound implications. Some of the cells around the hollow ball clump together on one side. These will become the child. The rest (the outer ring) will develop into the child's environment - the membranes that will protect, nourish and contain it, and connect to its mother.
08. Implantation
Five to six days
Since fertilization, each cell has lived off stored energy and newly minted DNA, there's been no further help from parents, and the tasks have rapidly become much more complicated. To survive the blastula must organize both to feed all its cells and be fed by its mother. When cells along the rim of the blastula, now secreting chemical enzymes designed to erode the already softened uterine lining, make contact, they latch on. The process is called implantation, with the ideal site located on the back wall of the uterus, near the support and protection of the spine.
3 to 8 Weeks
01. Overview
Growing from within, the embryo at 28 days curls in a characteristic C-shape, a tight comma. Its primitive heart and brain are active, in place and folding rapidly into complex - i.e., more functional - shapes. The top is enlarged, like the hasp of a safety pin, as the brain region thickens and begins organizing into sub regions. Although the embryo is still relatively featureless, a system of ridges - precursors of the face and neck - arch inward from the underside of the crown. Each ridge is already wired with a major artery and a cranial nerve - trunk lines, so to speak - and contains a rod of cells designed to grow into bone. Contained within the three arches are the essential apparatus for hearing, speaking, breathing, eating and facial expression.
02. Week Six
Daily, new shapes beget newer ones. The features of the face begin to assemble rapidly. A groove is laid down next to the nasal pit - the incipient lower jaw and lip. A day or two later, the upper and lower jaw parts, which have formed in symmetrical halves separated by a gap, fuse. Two days after that, with the jaws well formed, the teeth and facial muscles begin to grow and the vestigial gill arches recede.
Tiny mounds of tissue erupt where the whole of the external ear will grow. The eyes begin to show color, then within 24 hours eye muscles - among the body's most delicate - begin to form.
03. Week Seven
Upper and lower limb buds develop rapidly during this period, with arm buds leading leg buds by several days at each stage. This seems to anticipate a pattern later on: human infants grasp with their hands long before they can walk. The hands are now far apart, the fingers recognizable yet webbed. The arms are bent at the elbows. As neck muscles begin to form, the head, which has grown large, straightens slightly, and the embryo's original tight C-shape is more elongated. Its posture is that of a night watchman fallen asleep in his chair, head on chest. The main network for supplying blood to the growing brain appears above the eye, near the temple. Permanent kidney tubules replace the embryo's temporary apparatus for filtering blood. In girls, the clitoris appears, and the ovaries begin to descend.
3rd Trimester
01. Overview
As the baby grows large (crown-rump length 14 inches or more, average weight up to five or six pounds) the uterus becomes cramped (Imagine quadruplets). The legs draw up into the fetal position as, in most cases, the head moves down into the pit of the pelvis. Dimpling at the elbows and knees, creasing at the neck, soft bulges of subcutaneous fat - all give the baby a plump appearance. The rate of weight gain in both fetus and mother begins to slow in anticipation of birth. (If the fetus continued to grow at the same rate as previously, if would weigh an estimated 200 by its first birthday.) Still, because the fetal digestive track - stomach, liver, pancreas, intestines - remains immature, the baby stores nutrition taken from the mother against the risk of early birth. Meconium - dark green mucous and dead cells from the liver, pancreas and gall bladder accumulating in the intestines - forms in the intestines. All sense organs are fully functional, and the baby looks much as it will upon arrival, although most now have blue eyes regardless of permanent color. Final formation of eye pigmentation requires exposure to light and usually happens a few weeks after delivery.
02. Bone Develoement
By the sixth month the nostrils, which are fused shut like the eyelids, open, and the baby begins to make muscular breathing movements. The baby's brain wave patterns are now similar to those that would be seen in a full-term newborn. These waves are believed to originate in the more highly evolved part of the brain, the cortex. They reflect beginning activity in the baby's hearing and visual systems. In this month the fetus grows from 10inches and 1/2 pound to 14 inches and 2 1/2 pounds. The bones, now hard with calcium, show up well on an ultrasound. A critical point is reached in the baby's development: at twenty-four weeks olds, the fetus has a chance of surviving outside the uterus, although it requires special and intensive care to do so. The lungs have developed both the alveoli and the surrounding blood vessels necessary to allow the baby to exchange oxygen and carbon dioxide after it is born. The baby also begins to make muscular breathing movements in preparation for life outside the womb.
03. Cross Section
By the end of the forth week, the second set of kidneys appears: it is thought to function briefly. The arm and leg bunds are recognizable as small swellings on lateral body walls. The developing brain and spinal cord comes from the neural rube and develops rudimentary segments called sometimes which will become the vertebral column. Bronchial develop about the neck area resembling gills. The most precarious time of prenatal development is during the embryonic stage of development. For this reason, it is crucial that the mother to be take extremely good care of herself and even more so if she believes she is pregnant.
Birth
Once the baby has emerged, the respiratory apparatus kicks swiftly into action. In confinement, the fetus's lungs remained either collapsed or partially filled with amniotic fluid, its millions of air sacs compressed, soaked and untested. At birth, after the umbilical cord is tied and cut following a last surge of blood from the placenta, the baby's oxygen supply is abruptly terminated. As circulation continues in the baby's body, carbon dioxide builds up in the bloodstream, causing the respiratory control center in the brain stem to signal the muscles of the diaphragm and rib cage to begin the regulated pattern of movement first rehearsed months earlier. The muscles contract, expanding the ribcage, pulling the diaphragm downward, and allowing room for the lungs to fill like sails in a gale. Upon this first great inhalation - and the vigorous exhalation that follows - the baby cries out. No other behavior indicates more positively that the baby has adjusted to life outside the womb, and parents and others can be forgiven their tremendous relief and excitement if they cry out too.
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