HUMAN FERTILIZATION, DEVELOPMENT, AND BIRTH CONTROL

Fertilization – is defined as the union of gametes or the fusion of the egg cell nucleus and the sperm cell nucleus when a sperm penetrates through an egg membrane. 

 

· The release of sperms at one ejaculation there may be up to 600 million sperms.

· Of these only a few thousands make the journey from the vaginal canal to the oviducts. This may take 4-8 hours. The life of the sperm is 1-3 days after the ejaculation.

· Each sperm is a small cell of about 50um length and 2.5um diameter.

· It consists of a head, middle piece and the tail regions. The head consists of the nucleus and a structure called the acrosome at the tip. The acrosome secretes lytic substances that break down the walls of the egg for fertilization. The beating of the tail propels the sperm forward at the rate of 1-4 mm per minute. 

 

· Each egg or ovum is a spherical cell much bigger than the sperm. It mainly consists of a single nucleus and some reserve food made of lipid droplets.

· In the fallopian tubes, many sperms surround an egg. However, only one enters the egg leaving behind the tail. The enzymes of the acrosome digest the several layers of tissue to reach the egg cytoplasm. Once the sperm is inside, the male and female nuclei become lighter and are called pro-nuclei. The two pro-nuclei fuse forming a zygote.  A fertilized egg is called a zygote. It contains a full set of chromosomes, half from each gamete or sex cell used in its inception. 

 

· The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo.

· Embryo: the embryonic period starts at three weeks and continues until the end of the 8th week of pregnancy

· Once a sperm enters an egg, the thickening of the outer walls of the egg blocks the entry of other sperms.

· Once the egg is fertilized, ovulation and menstruation do not take place. The fertilized structure, now called the zygote is diploid. It starts dividing and forms a blastocyst, (the period between conception and embryonic stages) the first stage of an embryo. The blastocyst gets implanted on the uterine walls. It then grows into a fetus.

· The fetus remains attached to the mother through an umbilical cord which is embedded in a tissue called placenta at one end.

Human Embryo Development

Day 1: first cleavage - 1 cell becomes 2

· The first cell of a new human being, called a zygote, is formed when a sperm fertilizes an egg. In the egg cell, the nuclei of the sperm and egg appear as two yellow-brown, irregular shapes. When these two nuclei fuse, fertilization is accomplished.

· Cleavage refers to the cell divisions that occur in the zygote during the first 3 days after fertilization. The first cleavage happens around 30 hours following fertilization, producing two daughter cells known as blastomeres. These blastomeres simultaneously divide over and over again doubling the number of blastomeres each time. When the zygote reaches 16 or more cells, it is called a morula. The morula is not larger than the zygote, but keeps producing smaller and smaller blastomeres through cleavage. The morula divides into approximately 100 cells in 4-5 days, until it becomes a blastocyst, characterised by a hollow sphere with an outer layer of flattened cells called the trophoblast and an inner layer known as the inner cell mass. The trophoblast goes on to form the placenta while the inner cell mass forms the embryo itself.

 

 

Day 2: second cleavage - 4-cell stage

Day 3: 6-12 cell stage - can test at this stage for genetic diseases

· The zygote begins its journey down the Fallopian tube. About 36 hours after fertilization, cell division begins in the Fallopian tube, and continues to occur about twice a day. In three to four days, the cluster of dividing cells completes the 10 cm (4 in) journey through the Fallopian tube to the uterus.

Day 4: 16-32 cell stage - solid ball of cells – morula (When the embryo consists of a hundred or more cells it may form a solid mass, called a morula)

Day 5: Solid morula develops into hollow, fluid-filled blastula (an embryo at an early stage of development, consisting of a hollow ball of cells)

· The embryo will develop from the inner cell mass, or embryonic disc

Day 6: About the fifth day after fertilization, a cluster of about a hundred cells floats in the uterus. It soon takes the form of a fluid-filled ball of cells called a blastocyst. After one or two days in the uterus, the blastocyst burrows into the uterine lining, which grows over the blastocyst and provides it with nutrients. This process is known as implantation, and the blastocyst is now called an embryo.

Day 6 -7: Blastocyst attaches to the endometrium and burrows in: implantation.

· Roughly 6 days after fertilization, the blastocyst attaches to the lining of the uterus, beginning the process of implantation. As part of the process, the trophoblast cells thicken and separate into two layers called the syncytiotrophoblast and the cytotrophoblast. The syncytiotrophoblast buries into the uterus, digesting the cells of the uterine lining as it goes. The uterine lining then grows over the invading syncytiotrophoblast and encloses it. Implantation is therefore said to be invasive in humans, where the conceptus breaks through the outer lining to invade the underlying tissue. The trophoblast layer also plays roles in secreting several hormones of pregnancy. It secretes human chorionic gonadotrophin (hCG) from as early as 6-7 days after fertilisation, which is then passed into maternal circulation. It also secretes both estrogen and progesterone after 2 months when it develops into the chorion. This renders the ovaries, which usually secretes estrogen and progesterone, inactive for the rest of pregnancy.

· During the second week after fertilization, implantation is completed and the embryo forms two layers which are the forerunners that will eventually give rise to all the structures in the embryo. To do this, the inner cell mass differentiates into two layers called the epiblast and hypoblast. This gives rise to the bilaminar embryo, consisting of these two sheets of cells.

 

Days 7-10: Gastrulation: major cellular reorganization into 2 or 3 tissue (germ) layers

· The flat two layered embryo becomes a flat three layered embryo through a process of cell migration called gastrulation.

  Gastrula: Early embryo with 3 tissue layers. All cells have the same DNA; however, different cells now begin to "turn on" (or "express") different genes to become different organs.

Germ Layers

1. Endoderm that will form the lining of lungs, tongue, tonsils, urethra and associated glands, bladder and digestive tract.

2. Mesoderm that will form the muscles, bones, lymphatic tissue, spleen, blood cells, heart, lungs, and reproductive and excretory systems.

3. Ectoderm that will form the skin, nails, hair, lens of eye, lining of the internal and external ear, nose, sinuses, mouth, anus, tooth enamel, pituitary gland, mammary glands, and all parts of the nervous system.

 

Days 10 - 14: Pregnancy becomes established

• Fluid filled amniotic cavity starts to form
• Yolk sac starts to form (will make blood cells, germ cells)
• Embryo starts to form from embryonic disc
• Chorion (placenta) starts to form
• At the end of this stage, a woman will have just missed her period!

Days 15 - 21: Emergence of the vertebrate body parts

• Primitive streak starts to form - this is the site of gastrulation (formation of the 3 tissue layers – ecto, endo, and mesoderm
• Neural groove (future spinal cord and brain) begins to form
• Somites (bands of tissue that will become muscles and bones) begin to form
• Pharangeal arches (future face, neck, mouth, nose) begin to form

 

Days 21 on... Week 3 - Week 8 (Embryo) - Development of all organ systems

Day 22: Cardiac cells (early heart) begins to beat

Week 4: How you looked at 1 month past conception....

- After implantation, cells of the embryo begin to specialize and form primitive organs. In the four week-old embryo, the head can be distinguished. The heart beats, and limb buds of the arms and legs are visible. The placenta develops at this stage and provides the embryo with nutrients from the mother. The four-week-old embryo measures about 5mm (about 0.2in) from crown to rump.

Developing Embryo's First Month

• At the close of the first month, all major organs have begun their development. The eyes are  visible, the arms and legs begin to bud, and the four-chambered heart beats for the first time.
•  The embryo is now about 1/5" long. It looks something like a tadpole. The structure that will develop into a head is visible, as is a noticeable tail. The embryo has structures like the gills of a fish in the area that will later develop into a throat.

 

Week 5: Tiny arm and leg buds have formed. Hands with webs between the fingers have formed at the end of the arm buds. Fingerprints are detectable.

Week 6: The embryo is about 1/2" long. The face has two eyes on each side of its head; the front of the face has connected slits where the mouth and nose eventually will be.

Week 7 / Day 48: First brain waves can be detected

Week Eight

· After eight weeks of development, all the rudimentary structures of a human have formed, and the embryo passes into the fetal stage of development. The fetus floats in the watery amniotic fluid, which is contained by the transparent, tough, amniotic membrane. The amniotic fluid acts as a shock absorber, preventing damage to the developing fetus. The eight-week-old fetus measures about 30 mm (about 1.2 in) from crown to rump.

What the baby looks like at two months

• The baby is more than half an inch long crown to rump and has distinct, slightly webbed fingers. Her arms and legs may be long enough to meet in front of her body.
• Baby's spinal column are beginning to form around week 5
• Head, chest and abdomen are forming
• Heart is beating
• Arm and leg buds appear
• Beginnings of mouth and jaw are visible
• Beginnings of fingers and toes appear
• Lungs are formed
• Eyes and ears are developing
• Intestines are forming
• Spine is flexible and moveable by week 8
• All internal organs are in place by week 8
• Genital organs are visible
• Shoulders, elbows, knees and hips are developing

 

Week 9 – Week 40: ( Fetus )

• After 12 weeks or so, the baby's development is largely "finished" - except brain and lung development
• The fetus just spends much of the 2nd and 3rd trimesters just growing (and doing various flip-turns and kicks inside the amniotic fluid)
• Inner ear, external ears, eyes, mouth and nose are developing
• Arms, legs, hands and feet are growing quickly
• Heart is pumping blood to all parts of the body
• All internal organs are functioning

 

What the baby looks like at three months

The baby is about 2 inches long. His fingers and toes have separated and he has begun swallowing and kicking. The basic structures of all his organs are in place and beginning to function. The fetus has a recognizable human form by the 12th week of development

The Baby at Four Months

• The baby is more than 4 inches long and covered with a layer of fine, downy hair called lanugo.
• Approximately 6 inches long
• Baby completely formed
• Placenta is starting to work

The Baby at Five Months

• A creamy, white substance called vernix caseosa protects the baby from his long immersion in amniotic fluid. He's swallowing more and giving his digestive system a workout. In an ultrasound, you might see him sucking his thumb.
• Baby is covered in fine hair
• Muscles are developing rapidly
• Baby continues to grow and put on weight
• Teeth are beginning to form
• Hair is growing on baby's head
• Muscle strength is increasing
• Baby is very active 

The Baby at Six Months

• By six months, the mother starts to feel the baby moving and will soon start to put on some baby fat.
• Her facial features — lips, eyebrows, eyelids — are more distinct and her teeth are starting to develop under her gums.
• Baby can suck its thumb
• Baby can hiccup and cough
• Baby weighs just over 1 pound

The Baby at Seven Months

• At seven months of pregnancy, the baby weighs more than 2 pounds and looks more like a newborn.
• His body is well formed.
• Fingernails are starting to cover his fingertips.
• He can now open and close his eyes and may turn toward a source of bright light.
• Vernix coats the baby's skin
• Baby's lungs are mature
• Fat is starting to accumulate
• Baby weighs approximately 2 pounds

The Baby at Eight Months

• The baby may weigh nearly 4 pounds and is gaining about half a pound per week. Layers of fat are piling on under her skin. She has probably turned head-down in preparation for her coming birth.
• Most babies are in the head down position
• The placenta is mature
• Baby's head and body are now in proportion to each other
• Baby weighs approximately 3.5 pounds

The Baby at Nine Months

• Most of your baby's bones have hardened, although his skull is still pliable. He's shedding most of the downy covering of hair as well as the creamy vernix caseosa that covered his body. He'll spend the next few weeks just putting on weight. 
•  20" long and with an average weight of 7 pounds, a full-term fetus' is typically born about this time.

Stages of Labor

The usual length of labor for a first-time mother is about 13 to 14 hours.

1. Labor – as labor begins, rhythmic uterine contractions begin weakly and grow progressively stronger. Each contraction shortens the muscle fibers in the uterus, pulling the cervix wider. In this early stage of labor, the cervix dilates (opens) to about 4cm (1.5 in)

2. Cervical Dilation – as the first stage of labor ends, the labor pattern changes. Contractions become more painful and occur closer together. As labor progresses the cervix opens to its full width of 10cm (4 in). the baby’s head begins to rotate to fit through the birth canal.

3. Descent – after the cervix becomes fully dilated, contractions become fully intense less than 5 minutes apart, and usually last a minute or longer. The mother may push or bear down with her abdominal muscles during contractions. With each contraction the baby continues its descent through the birth canal.

4. Crowning – if a baby’s position is head first during delivery, the mother’s vagina fits like a crown around the baby’s head, making the head visible as it emerges from the birth canal.

5. Birth – as the head emerges the head flexes and the baby rotates to the side. This enables the shoulders to maneuver around the pubic bone. One shoulder emerges, quickly followed by the other shoulder and the rest of the body.

6. Delivery of Placenta – in the final stage of labor, the uterus continues to contract and the placenta detaches from the uterus and is expelled. And then before you know it a baby is born!

Issues Related to Fertility and Population Growth

Population growth can be limited to a certain limit by birth control and responsible parenthood. Birth control allows people to limit the size of their families. It can be done by avoiding sexual intercourse when fertilization is most likely to occur. Birth control is done by using contraception. Contraception is done either by preventing the egg and the sperm from meeting or by preventing implantation. Methods of contraception are classified into three: Natural, Artificial, and Mechanical.

Natural Methods:

Natural contraception depends on the natural physiological cycles occurring in a woman’s body.

1. Abstinence – is the total avoidance of sexual activity.

2. Rhythm Method – a method whereby couples do not engage in sexual intercourse when a fertilizable egg is present in the Fallopian tube, that is, just before and after an egg has been produced in the woman’s body.

3. Coitus interruptus – a method by which the penis is withdrawn from the vagina prior to ejaculation, thus preventing the sperm from being deposited inside.

Artificial Methods: 

1. Birth Control Pills – is an oral contraceptive taken by the woman to suppress ovulation. These artificial hormone pills are taken daily to stop ovaries from producing eggs.

2. Depo-Provera – this quarterly shot stop the egg from being released from the ovary and interferes with the sperm’s ability to enter the womb

3. Foam and Suppositories – these are sperm-killing chemicals placed into the vagina before having sex.

Mechanical Methods:

1. Condom – is a thin sheath of thin strong latex rubber which unrolled through the penis to catch all the sperm cells released by the male upon ejaculation. The sperm cells are trapped within the condom to avoid its entry into the woman’s vagina.

2. Diaphragm – a small rubber cup that fits inside the vagina where it holds sperm-killing jelly or cream over the opening of the womb.

3. IUD (intrauterine Device) – a small device made of plastic or stainless steel. It is inserted into the womb by a doctor to be 95 to 97 percent effective.

4. Norplant – these slim, hormone-filled capsules, placed under the skin by a doctor, stop egg from being released from the ovary and interfere with sperm’s ability to enter the womb.

5. Sponge – this single-use, small, soft sponge fits inside the vagina where it covers the opening of the womb to block, absorb, and kill sperm.

6. Sterilization – an operation for men and women which permanently makes it possible to have a baby. A vasectomy is much safer than a tubal ligation.

• Vasectomy – is an operation performed on males to ensure prevention of sperm release into the semen.

• Tubal Ligation – an operation performed on females whereby their Fallopian tubes are blocked or cut at the point where fertilization occurs.