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A New Life

bullet Fertilization - Blastulation
bullet Gastrulation - Embryo
bullet The Fetus
bullet Child Birth
bullet Breastfeeding
bullet Growth





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Fertilization - Blastulation.

    When the ovule in the fallopian tube is fertilized by sperm, it becomes an egg. That is the moment that marks the beginning of a new life. Half of the genetic heritage of the mother will merge with the half from the father's side, and a new cell will begin to multiply, for about nine months, to reach the 3 billion cells of this new human being. This is the miracle of life!



Sperm cells meet the ovule
The sperm cells meet the ovule.
Which one will claim paternity ?

    Once a sperm fuses with the egg, it changes the polarity of the ovule membrane thus preventing other sperm to fuse. During the next 12 hours, the 23 chromosomes of the sperm will join the 23 chromosomes of the egg and together form a new genetic code, a mixture of the genes from the father and the mother. It will program the development of a new and unique human being. This new cell will copy its genetic material and, in minutes, will divide into two apparently identical cells, each with its own nucleus containing a copy of all genes. Normally, these two cells adhere to each other and, synchronously, continue to divide to 4, 8, 16, 32, 64 cells. By the way, if, at the 2 cells stage, the cells got separated from each other, then there would be two distinct cells, with identical genetic material, that will give rise to two identical twins. After 3 days, the egg has 32 similar cells. It is called the morula because it looks like a blackberry. Moved by the hair cells lining the fallopian tube, the morula continues to float toward the uterus.

    The next stage of development is called blastulation. The cells stop dividing into similar cells (otherwise we would a ball of identical cells) and will begin to differentiate into cells with slightly different function. They will begin to change places, and instead of forming a compact mass, they form a hollow sphere composed of a single layer of cells surrounding a cavity filled with liquid. With differentiation, cells adopt specific functions: some will group at a point, the embryoblast, and will become the baby, while the rest of cells, the trophoblast, will form the placenta and the membranes that will connect the child to his mother. The blastocyst reaches the uterus within 6-10 days and the trophoblast cells secrete enzymes to erode the uterine lining and promote settlement of the fetus.


Division of the egg
The first cell divisions;
the morula.

Blastula
The blastocyte.
 

Gastrulation - Embryo.

    At this stage of development, a second important differentiation occurs. The cells of the embryonic bud, called embryoblast, grow faster and faster and differentiate into two distinct layers: the outer layer, the ectoderm, will produce the skin and the nervous system, and the inner layer, the endoderm, will form the digestive system. Subsequently, a third layer, the mesoderm, which will develop between the first two layer, will produce the skeleton, the muscles, and most of the viscera.


Implantation of the embryo
Implantation of the embryo.

Development of the neural plate
Development of the neural plate.

Development of the somites
Development of the somites.

The embryo at 26-28 days
The embryo at 26-28 days.

The embryo at 28-30 days
The embryo at 28-30 days.

The embryo at 32 days
The embryo at 32 days.

The embryo at 6 weeks
The embryo at 6 weeks.

    This is also the time that the trophoblast merge with the uterine lining. At the point of contact, blood vessels and other tissues are formed and interlace with those of the mother, still without any exchange of blood for oxygen, nutrients and metabolic wastes.

    As the cells continue to divide, the embryonic disc, consisting of the 3 layers, continues to thicken and a row of cells appears on the surface and forms a fold called the primitive streak. This line will determine the future direction of the fetus: left, right, front, back, top and bottom. Then the layers are folding, and migrating cells, following some chemical signals, start shaping the embryonic tissues. At this point, we can distinguish the nerve tissue that forms a long canal, the neural tube. The tube extends and closes on itself. At this stage, it only measures a pinhead.

Development of the neural plate
Schematic of the development of the neural plate.

    At around three weeks, a cephalic protuberance appears. Mesodermal cells gather together into somites on each side of the neural tube. They will mainly differentiate into muscle and dermis. In the vitellum, stem cells appear and form blood cells as well as some blood vessels.

    With the precision of a clock, and in a well defined pattern, the cells continue to divide and differentiate, increasing their number exponentially and creating newer cells with various functions. Twelve pairs of somites begin the draft of the spine. Two thick disks appear at the cephalic protuberance and will become the eyes. Some cells of the ear start to appear. In the middle region of the future chest, a S-shaped tube is formed. The upper part of S pumps, and the lower part aspirates, a primitive heart begins to beat.

    In the fourth week, the organs and structures of the embryo begin to be better defined. In the mother, hormonal changes, causing fatigue and nausea, and lack of menstruation inform about her new condition, being pregnant. Within a month, this little human being went from a one cell, to several million individual; from a single cell, to a complex organism with a nervous system, a circulatory system pumping blood to the placenta, and with the outline of internal organs such as liver, lung, stomach and pancreas. While it takes more and more of the characteristics of a human being, at this stage it still look more to a mouse or a chicken embryo; it has a tail, a yolk sac and even some primitive gills.

    The brain continues to develop and begins to organize; a primitive heart pumps the blood cells in a network of increasing complexity; cephalic crests curves to form the skull, the face and the neck; inside the brachial arcs hearing, swallowing, respiration and phonation structures appear; the heart begins to form cavities; 40 pairs of muscles and 33 vertebrae develop in the spinal area; a primitive kidney, the bladder and genitals appear; and on each side of the body 2 small buds (total 4), shaped like fins, begin to grow and will give the hands and the feet. At this stage, the embryo is not longer than a grain of rice.

    Then, the nervous system extends its ramifications to all muscle cells, skin receptors, glands, the 10,000 taste buds, 240,000 auditory receptors, and the 50 billion eye sensors (photoreceptors). The brain gets compartmentalized and each region will specialize. The eyes and ears are developing tissues with increasing specialization. The gills disappear and facial structures are shaping the nose and sinuses. The limb buds are elongating. The heart has now four chambers, forming a closed circuit that will not only flow to the umbilical cord, but also to the future lungs and the rest of the body. The heart and the brain are now occupying almost half of the total size of the fetus.

    In the sixth week, the development is accelerating and structures of increasing complexity are formed. The organs of the digestive system develop: the liver, the oesophagus and the stomach are taking shape. The upper limbs get segmented in arm, forearm and hand. The lower limbs get flattened, and the tail regresses. The face is changing rapidly with the ears, nose, mouth and facial muscles growing, while the 'gills' are regressing.

    At seven week the embryo is just over 1cm long. His brain and sensory organs shape more and more. His fingers begin to sprout. In boys, sexual organs begin to differentiate and a penis start to form. Cartilages, which will become bones, lengthen. And, the leg and foot are shaping.

    In the eighth week, arms and hands, then legs and feet, get better defined, then the fingers and toes start to separate. The cartilages begin to calcify and to form bone. Muscles continue to develop and the embryo straightens slightly. The main blood vessels are in place and kidneys develop to their final form. The ears, nostrils, mouth and lips become clearly visible. The well pigmented eyes move to the front of the head. The embryo looks more and more like a tiny human being.

The embryo at 7 weeks
The embryo at 7 weeks.

The embryo at 8 weeks
The embryo at 8 weeks.
 

The Fetus.

    After two months, the embryo becomes a fetus (could be spelled foetus). It measures 3cm from the head to what is left of the tail, and it only weighs a few grams. At this point, most of the organs are formed, the tissues are differentiated and the fetus begins a phase of growth where it will take nearly a thousand times its actual weight.


Size of the uterus during pregnancy
Size of the uterus during pregnancy.

    The liver, while waiting to be ready to filter the digested food, is, in the mean time, used to produce blood cells because it is only in the last months of growth than the bone marrow will fully perform its role. Bronchi and lungs continue to develop and will become functional in the sixth month, even though still imperfect. The intestines take their place in the abdominal cavity. The sexual organs get better defined and the boy's penis grows. The skin thickens and hair follicles appear below the surface. The spinal cord and the nerves continue to grow and make more and more connections; the first reflexes begin to appear. Muscles strengthen and begin to react to commands from the brain and, from that time on, the fetus will show movements. We can now hear the heartbeat with a stethoscope.

Growth of the fetus
Growth of the fetus.

    Subsequently, the organs and structures continue to grow and refine. The fetus becomes more and more independent. His glands are functional and secrete the necessary hormones for metabolism and physiological functions. Bones calcify and tooth buds appear. Mouth, larynx and vocal cords become defined. At eleven weeks, the fetus measures 6cm and weighs 10-12g. He only needs to grow up, gain strength and refine the functioning of its organs.

    At thirteen weeks, the fetus begins to move. We can easily distinguish whether it will be a boy or a girl. His body starts to grow more than its head; it lengthens and straightens. The cortex of the brain folds and convolutes. This increases its surface and its ability to process information. The skin shows wrinkles, at fingertips and toes, that form our fingerprints (unique to each individual, even in identical twins).

    At twenty weeks, the fetus measures about 20cm and weighs about 250g, it has the dimensions of a young adult rats. The bones have hardened and the muscles are stronger. Fetal movements are felt by the mother. Blood flow runs perfectly; the little heart beats strongly enough to pump about 150 litters of blood per day.

    Around six months, the fetus reaches 30cm and weighs more than 600g. It shows signs of alternating periods of wakefulness and sleep. The eyes begin to see light and shadows; they are blue because they will take their final pigmentation only after several weeks of exposure to light. The ears perceive sounds. And, the fetus can regulate its body temperature. Respiratory movements are evident and his lungs are capable to breathe. It is sufficiently autonomous that it would have a chance of survival if premature birth happens.

    At seven months, the cerebral cortex is better developed and the different functions get localized. The centres for vision, hearing, smell, touch and motor functions, each take their place, and this organization of the brain which is the same for everybody, will continue to refine for few years after birth. At the end of the seventh month, the baby will turn, head down, to have more room. And, I suppose, he instinctively knows that this is the best position to get out of this nest. If the fetus fails to turn, as it is in 5% of the cases, it will be delivered by the seat, which is more difficult for both, the baby and mother.

    At the end of the eighth month, the fetus measures more than 40cm and weighs about 2.5Kg. Mummy's tummy becomes increasingly tight and the fetus must bend his legs and bow his head, and to fit in this limited space, he also need to limit his movements. The growth rate also slow, and the fetus begins to accumulate fat as energy reserve. His gut stores food in the form of meconium, a compound made of green mucus and dead cells. The hemo-placental barrier, between the blood of the mother and the blood of the baby, starts to leak the mother's antibodies into the baby's blood. This is to protect them during the first weeks of independent living. Any time now, the baby could be born and be perfectly viable. But generally it is not until the end of the ninth month that delivery takes place.

    At nine months, the girl will weigh from 2.8 to 3.6Kg and measure 47 to 51cm, while the boy weighs 2.8 to 3.8kg and measure 48 to 52cm. The nails have grown so much that the baby may scratch himself. Growth is well advanced, the organs are functional, and there is not enough room in mom's tummy for further growth. The uterine muscle is stretched to the limit and secretion of oestrogen increases its contractility. The time of delivery has arrived.


Child Birth.

    The baby's head pushes on the cervix of the uterus, which causes small contractions of the smooth muscles. These contractions are infrequent (at 15 to 30min) and of short duration (40sec), pushing further the head of the baby onto the cervix. At the same time, the brain of the mother makes some oxytocin, a hormone that is released from the neurohypophysis (posterior pituitary) in the bloodstream, to stimulate further contractions of the uterus. Between each contraction, the uterus relaxes, but not completely, so that the following contractions will become more and more effective at expulsing the baby. At the same time, these contractions bring the cervix over the baby's head. Until delivery, the contractions will get closer (3 to 5 min between contractions) and will increase in intensity and duration (90sec). The cervix will dilate from a 5mm to a 10cm diameter before the baby's head could pass through. With the dilatation, the mucus plug, blocking the cervix conduit, will dislodge. Also, and fortunately, the bones of the baby's head are not yet fully formed, the bony plates are connected only by non-ossified areas. This allows the baby's head to deform slightly and better pass through this narrow passage. Usually, it is when the head engages in the cervix that the amniotic sac tears. It is said that the mother burst her water, and amniotic fluid escapes through the vulva. At that time, the birth is imminent.


Delivery
Child Birth.

Pelvic passage during child birth
Pelvic passage during child birth.

First cry of the new born
First cry of the new born.

    When the baby is expelled from the womb there is so much pressure on his chest that the mouth and upper airways are emptying of amniotic fluid. The ambient temperature decreases dramatically and the amount of oxygen in the blood decreases. Together, these factors stimulate the first breath of the newborn. Usually, this is when the child pushes his first cry. It now depends on his own brain to adjust his breathing rate in order to maintain appropriate oxygen blood levels necessary to fulfill the requirements of his new life.

    Since the newborn is now breathing on his own, the umbilical circulation is no longer necessary and communication between the atria of the baby's heart (the foramen ovale) closes. The cord can be clamped and cut. The placenta is no longer stretched and shrinks rapidly. Torn and deprived of blood flow, arterioles between the uterus and placenta contract and blood circulation is interrupted. The placenta separates from the uterine wall and will be expelled. This is usually accompanied by some bleeding, but rarely more than 300-400ml.

    Mom and baby are tired but are doing well. The next step will be to meet the nutritional needs of the baby.


Breastfeeding.

    Breastfeeding is the first meal of the baby, but it does much more than that. Breast milk is formed by the mammary glands located in the breasts of the women who have just given birth. Its composition corresponds exactly to the nutritional need of the newborn. In addition, it also contains antibodies that protect the baby against microbial infections, until it will develop its own immune system.


The mammary glands
The mammary glands.

    Men and women are born with identical breasts: a rudimentary network of 15-20 milk ducts (lactiferous ducts) connected to the nipple. But it is only in women, at the age of puberty, the onset of menstrual cycles, when oestrogen levels are higher, that the mammary glands develop partially and accumulate fat cells in the breast.

    Then, during pregnancy, under the influence of high levels of oestrogen and progesterone, the mammary glands fully develop. But at the same time, these high levels of oestrogen and progesterone inhibit milk production. It is only after childbirth, when these hormone levels drop significantly, and that the level of another hormone, the prolactin, increases that milk production begins. The graph below illustrates the hormonal changes that occur during pregnancy, childbirth and during recovery of the menstrual cycle.

Hormonal changes
Hormonal changes associated to pregnancy,
childbirth, and return of the menstrual cycle.

    At the beginning, breast milk is called the colostrum. It is first a thick milk, produced in small quantity (20-50ml/meal) but rich in nutrients and containing cells and antibodies that protect the baby against microbial. After a few days, the milk becomes more liquid, whitens and becomes much more abundant. As the amount of colostrum gradually decreases, the baby, by exposure to its new environment, will start building its own immune system. After a few weeks, the mother can produce up to 750ml of milk per day.


    The lactation reflex begins when the baby's mouth stimulates the nipple of the mother. This leads to the release, from the neurohypophysis (posterior pituitary), of oxytocin into the bloodstream. Then, in less than a minute, oxytocin will cause contraction of myoepithelial cells surrounding the glands and the lactogenic ducts, and thus milk ejection. In this regard, it is interesting to note that the feeding on one breast also causes a milky ejection of the other breast.

    The mother can breastfeed for several years if she wishes. But, after about 9 months of breastfeeding, milk production tends to decrease. It is recommended that infants be breastfeed only for the first four months of his life. Second, even if the mother continues to breastfeed, she should gradually introduce solid foods anyway, such as cereals and fruits and vegetables purees. When the mother decides to wean (stop breastfeeding) her child, the hypothalamus is no longer stimulated by suckling and prolactin levels decrease. In about a week, milk production will cease, and will not resume until after a subsequent pregnancy.


Growth.

    Boys and girls grow at about the same speed until the age of puberty. Under the influence of growth hormone, there is a burst of growth from birth to two years of age. After two years, the growth continues, albeit at a slower and regular pace. Then, at puberty, around age 11 years old in girls and 13 years old for boys, occurs one second burst of growth, especially a weight gain. Among boys, this second burst of growth is accentuated by the effect of testosterone which also stimulates growth. The chart below shows the growth curves, in size and weight, of boys and girls from 0 to 20 years of age. These graphs were constructed from some 2005 data of the World Health Organization (WHO: 0-2years) and those of the 'Center for Disease Control' (CDC: 2-20years).

Child growth
Child growth, in height and in weight, of the boys and girls from 0 to 20 years old.
 
I found an interesting paper on childbirth (in French): http://www.ordiecole.com/biol/8h_reproduction.pdf
     
     
 
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