Category: Biology

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Assisted Reproductive Technology (Art)

A collection of procedures, which includes the handling of gametes and/or embryos outside the body to achieve pregnancy is known as Assisted Reproductive Technology. It increases the chance of pregnancy in infertile couples. ART includes intra-uterine insemination (IUI), in vitro fertilization, (IVF) Embryo transfer (ET), Zygote intra-fallopian transfer (ZIFT), Gamete intrafallopian transfer (GIFT), Intra-cytoplasmic sperm injection (ICSI), Preimplantation genetic diagnosis, oocyte and sperm donation and surrogacy.

Intra-uterine insemination (IUI)

This is a procedure to treat infertile men with low sperm count. The semen is collected either from the husband or from a healthy donor and is introduced into the uterus through the vagina by a catheter after stimulating the ovaries to produce more ova. The sperms swim towards the fallopian tubes to fertilize the egg, resulting in normal pregnancy.

In vitro fertilization (IVF) or Test tube baby

In this technique, sperm and eggs are allowed to unite outside the body in a laboratory. One or more fertilized eggs may be transferred into the woman’s uterus, where they may implant in the uterine lining and develop. Excess embryos may be cryopreserved (frozen) for future use. Initially, IVF was used to treat women with blocked, damaged, or absent fallopian tubes. Today, IVF is used to treat many causes of infertility.

The basic steps in an IVF treatment cycle are ovarian stimulation, egg retrieval, fertilization, embryo culture, and embryo transfer. Egg retrieval is done by minor surgery under general anesthesia, using ultrasound guide aftr 34 to 37 hours of hCG (human chorionic gonadotropin) injection.

The eggs are prepared and stripped from the surrounding cells. At the same time, sperm preparation is done using a special media. After preparing the sperms, the eggs are brought together.

10,000-1,00,000 motile sperms are needed for each egg. Then the zygote is allowed to divide to form 8 celled blastomere and then transferred into the uterus for a successful pregnancy. The transfer of an embryo with more than 8 blastomeres stage into uterus is called Embryo transfer technique.

Zygote intra-fallopian transfer (ZIFT)

As in IVF, the zygote upto 8 blastomere stage is transferred to the fallopian tube by laparoscopy. The zygote continues its natural divisions and migrates towards the uterus where it gets implanted.

Intra uterine transfer (IUT)

Embryo with more than 8 blastomeres is inserted into uterus to complete its further development.

Gamete intra-fallopian transfer (GIFT)

Transfer of an ovum collected from a donor into the fallopian tube. In this the eggs are collected from the ovaries and placed with the sperms in one of the fallopian tubes. The zygote travels toward the uterus and gets implanted in the inner lining of the uterus.

Intra-cytoplasmic sperm injection (ICSI)

In this method only one sperm is injected into the focal point of the egg to fertilize. The sperm is carefully injected into the cytoplasm of the egg. Fertilization occurs in 75 – 85% of eggs injected with the sperms. The zygote is allowed to divide to form an 8 celled blastomere and then transferred to the uterus to develop a protective pregnancy.

Surrogacy

Surrogacy is a method of assisted reproduction or agreement whereby a woman agrees to carry a pregnancy for another person, who will become the newborn child’s parent after birth. Though in vitro fertilization (IVF), embryos are created in a lab and are transferred into the surrogate mother’s uterus.

Male infertility

Azoospermia is defined as the absence of spermatozoa in the ejaculate semen on atleast two occasions and is observed approximately in 1% of the population.

Micro-testicular sperm extraction (TESE)

Microsurgical sperm retrieval from the testicle involves a small midline incision in the scrotum, through which one or both testicles can be seen. Under the microscope, the seminiferous tubules are dilated and small amount of testicular tissue in areas of active sperm production are removed and improved for sperm yield compared to traditional biopsy techniques.

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Infertility and Its Causes of Infertility

Inability to conceive or produce children even after unprotected sexual cohabitation is called infertility. That is, the inability of a man to produce sufficient numbers or quality of sperm to impregnate a woman or inability of a woman to become pregnant or maintain a pregnancy.

The causes for infertility are tumours formed in the pituitary or reproductive organs, inherited mutations of genes responsible for the biosynthesis of sex hormones, malformation of the cervix or fallopian tubes and inadequate nutrition before adulthood. Long-term stress damages many aspects of health especially the menstrual cycle. Ingestion of toxins (heavy metal cadmium), heavy use of alcohol, tobacco and marijuana, injuries to the gonads and aging also cause infertility.

Other causes of infertility

• Pelvic inflmmatory disease (PID), uterine firoids and endometriosis are the most common causes of infertility in women.
• Low body fat or anorexia in women. i.e. a psychiatric eating disorder characterised by the fear of gaining weight.
• Undescended testes and swollen veins (varicocoele) in scrotum.
• Tight clothing in men may raise the temperature in the scrotum and affect sperm production.
• Under developed ovaries or testes.
• Female may develop antibodies against her partner’s sperm.
• Males may develop an autoimmune response to their own sperm.

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Sexually Transmitted Diseases (Std)

Sexually transmitted diseases (STD) or Venereal diseases (VD) or Reproductive tract infections (RTI) are called as Sexually transmitted infections (STI). Normally STI are transmitted from person to person during intimate sexual contact with an infected partner.

Infections like Hepatitis-B and HIV are transmitted sexually as well as by sharing of infusion needles, surgical instruments, etc with infected people, blood transfusion or from infected mother to baby.

People in the age of 15 to 24 years are prone to these infections. The bacterial STI are gonorrhoea, syphilis, chancroid, chlamydiasis and lymphogranuloma venereum. The viral STI are genital herpes, genital warts, Hepatitis-B and AIDS.

Trichomoniasis is a protozoan STI, and candidiasis is a fungal STI. STI caused by bacteria, fungi and protozoa or parasites, can be treated with antibiotics or other medicines, whereas STI caused by virus cannot be treated but the symptoms can be controlled by antiviral medications. Latex condoms usage greatly reduces the risk, but does not completely eliminate the risk of transmission of STI.

Prevention of STDs

• Avoid sex with unknown partner/multiple partners.
• Use condoms.
• In case of doubt, consult a doctor for diagnosis and get complete treatment.

Cervical cancer

Cervical cancer is caused by a sexually transmitted virus called Human Papilloma virus (HPV). HPV may cause abnormal growth of cervical cells or cervical dysplasia. The most common symptoms and signs of cervical cancer are pelvic pain, increased vaginal discharge and abnormal vaginal bleeding. The risk factors for cervical cancer include

• Having multiple sexual partners
• Prolonged use of contraceptive pills

Cervical cancer can be diagnosed by a Papanicolaou smear (PAP smear) combined with an HPV test. X-Ray, CT scan, MRI and a PET scan may also be used to determine the stage of cancer. The treatment options for cervical cancer include radiation therapy, surgery and chemotherapy. Modern screening techniques can detect precancerous changes in the cervix.

Therefore screening is recommended for women above 30 years once in a year. Cervical cancer can be prevented with vaccination. Primary prevention begins with HPV vaccination of girls aged 9 – 13 years, before they become sexually active. Modification in lifestyle can also help in preventing cervical cancer. Healthy diet, avoiding tobacco usage, preventing early marriages, practicing monogamy and regular exercise minimize the risk of cervical cancer.

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Medical Termination of Pregnancy (MTP)

Medical method of abortion is a voluntary or intentional termination of pregnancy in a non-surgical or non-invasive way. Early medical termination is extremely safe upto 12 weeks (the first trimester) of pregnancy and generally has no impact on a women’s fertility. Abortion during the second trimester is more risky as the foetus becomes intimately associated with the maternal tissue.

Government of India legalized MTP in 1971 for medical necessity and social consequences with certain restrictions like sex discrimination and illegal female foeticides to avoid its misuse. MTP performed illegally by unqualified quacks is unsafe and could be fatal. MTP of the first conception may have serious psychological consequences.

The medical termination of pregnancy can only be done (by law) if there is: A possible health risk to the physical/mental condition of the mother. A potential risk to the health of the growing foetus. If a woman gets pregnant as a result of rape.

Termination of pregnancy through medicine is an effective and safe method for very early pregnancies. It avoids the usage of anaesthesia, surgeries or vacuum aspiration unless it fails. It is done in more secrecy and has privacy. The procedure is non-invasive.

Notwithstanding anything contained in the Indian Penal Code (45 of 1860), the termination of pregnancy by a person who is not a registered medical practitioner shall be an offence punishable with rigorous imprisonment for a term which shall not be less than two years but which may extend to seven years under that give us medicine.

MTP Kit

Medical abortion with mifepristone and misoprostol is a very safe option for termination of pregnancy when consumed under medical supervision with a success rate of 92-97%. Clear guidelines have been formulated by organizations like WHO and in India by FOGSI regarding the use of abortion pills.

Some women describe the experience as being similar to having a heavy period and cramps. Others may experience more intense cramping. When someone has a medical abortion, they usually pass out the pregnancy tissue within 4-5 hours.

The Bill allows abortion to be done on the advice of one doctor up to 20 weeks, and two doctors in the case of certain categories of women between 20 and 24 weeks. The Bill sets up state level Medical Boards to decide if a pregnancy may be terminated after 24 weeks in cases of substantial foetal abnormalities.

Under the Medical Termination of Pregnancy Act, a pregnancy can be terminated until 20 weeks after conception if it will harm the mother, if the pregnancy was the result of rape, if the child will be born with serious physical or mental defects, or in case of contraceptive failure.

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Population Explosion and Birth Control

Increased health facilities and better living conditions have enhanced longevity. According to a recent report from the UN, India’s population has already reached 1.26 billion and is expected to become the largest country in population size, surpassing China around 2022. To overcome the problem of population explosion, birth control is the only available solution. People should be motivated to have smaller families by using various contraceptive devices.

Advertisements by the Government in the media as well as posters/bills, etc., with a slogan Naam iruvar namakku iruvar (we two, ours two) and Naam iruvar namakku oruvar (we two, ours one) have also motivated to control population growth in Tamilnadu. Statutory rising of marriageable age of the female to 18 years and that of males to 21 years and incentives given to couples with small families are the other measures taken to control population growth in our country.

Birth control methods

The voluntary use of contraceptive procedures to prevent fertilization or prevent implantation of a fertilized egg in the uterus is termed as birth control. An ideal contraceptive should be user friendly, easily available, with least side effects and should not interfere with sexual drive. The contraceptive methods are of two types – temporary and permanent. Natural, chemical, mechanical and hormonal barrier methods are the temporary birth control methods.

1. Natural method

Is used to prevent meeting of sperm with ovum. i.e., Rhythm method (safe period), coitus interruptus, continuous abstinence and lactational amenorrhoea.

a. Periodic abstinence/rhythm method

Ovulation occurs at about the 14^th day of the menstrual cycle. Ovum survives for about two days and sperm remains alive for about 72 hours in the female reproductive tract. Coitus is to be avoided during this time.

b. Continuous abstinence

Is the simplest and most reliable way to avoid pregnancy is not to have coitus for a defined period that facilitates conception.

c. Coitus interruptus

Is the oldest family planning method. The male partner withdraws his penis before ejaculation, thereby preventing deposition of semen into the vagina.

d. Lactational amenorrhoea

Menstrual cycles resume as early as 6 to 8 weeks from parturition. However, the reappearance of normal ovarian cycles may be delayed for six months during breastfeeding. This delay in ovarian cycles is called lactational amenorrhoea.

It serves as a natural, but an unreliable form of birth control. Suckling by the baby during breast-feeding stimulates the pituitary to secrete increased prolactin hormone in order to increase milk production.

This high prolactin concentration in the mother’s blood may prevent menstrual cycle by suppressing the release of GnRH (Gonadotropin Releasing Hormone) from hypothalamus and gonadotropin secretion from the pituitary.

2. Barrier methods

In these methods, the ovum and sperm are prevented from meeting so that fertilization does not occur.

a. Chemical barrier

Foaming tablets, melting suppositories, jellies and creams are used as chemical agents that inactivate the sperms in the vagina.

b. Mechanical barrier

Condoms are a thin sheath used to cover the penis in male whereas in female it is used to cover vagina and cervix just before coitus so as to prevent the entry of ejaculated semen into the female reproductive tract. This can prevent conception. Condoms should be discarded after a single use. Condom also safeguards the user from AIDS and STDs. Condoms are made of polyurethane, latex and lambskin.

Diaphragms, cervical caps and vaults

Are made of rubber and are inserted into the female reproductive tract to cover the cervix before coitus in order to prevent the sperms from entering the uterus.

c. Hormonal barrier

It prevents the ovaries from releasing the ova and thickens the cervical fluid which keeps the sperm away from ovum.

Oral contraceptives

Pills are used to prevent ovulation by inhibiting the secretion of FSH and LH hormones. A combined pill is the most commonly used birth control pill. It contains synthetic progesterone and estrogen hormones. Saheli, contraceptive pill by Central Drug Research Institute (CDRI) in Lucknow, India contains a non-steroidal preparation called Centchroman.

d. Intrauterine Devices (IUDs)

Intrauterine devices are inserted by medical experts in the uterus through the vagina. These devices are available as copper releasing IUDs, hormone releasing IUDs and non-medicated IUDs. IUDs increase phagocytosis of sperm within the uterus. IUDs are the ideal contraceptives for females who want to delay pregnancy. It is one of the popular methods of contraception in India and has a success rate of 95 to 99%.

Copper releasing IUDs

Differ from each other by the amount of copper. Copper IUDs such as Cu T-380 A, Nova T, Cu 7, Cu T 380 Ag, Multiload 375, etc. Release free copper and copper salts into the uterus and suppress sperm motility. They can remain in the uterus for five to ten years.

Hormone-releasing IUDs such as Progestasert and LNG – 20 are often called as intrauterine systems (IUS). They increase the viscosity of the cervical mucus and thereby prevent sperms from entering the cervix. Non-medicated IUDs are made of plastic or stainless steel. Lippes loop is a double S-shaped plastic device.

3. Permanent birth control methods

Are adopted by the individuals who do not want to have any more children.

Surgical sterilisation methods

Are the permanent contraception methods advised for male and female partners to prevent any more pregnancies. It blocks the transport of the gametes and prevents conception. Tubectomy is the surgical sterilisation in women.

In this procedure, a small portion of both fallopian tubes are cut and tied up through a small incision in the abdomen or through vagina. This prevents fertilization as well as the entry of the egg into the uterus. Vasectomy is the surgical procedure for male sterilisation. In this procedure, both vas deferens are cut and
tied through a small incision on the scrotum to prevent the entry of sperm into the urethra. Vasectomy prevents sperm from heading off to penis as the discharge has no sperms in it.

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Social Impact Of Sex Ratio, Female Foeticide and Infanticide

The sex ratio is the ratio of males to the females in a population. In India, the child sex ratio has decreased over the decade from 927 to 919 female for every 1000 males. To correct this ratio, steps are needed to change the mind set and attitudes of people, especially in the young adults. Female foeticide and infanticide is the manifestation of gender discrimination in our society.

Female foeticide refers to ‘aborting the female in the mother’s womb’; whereas female infanticide is ‘killing the female child after her birth’. These have resulted in imbalance in sex ratio. In UNDP’s GII 2018 (United nations developmental programmes gender inequality index) reflected that India was ranked at 135 out of 187 countries due to availability of very few economic opportunities to women as compared to men.

In order to prevent female foeticide and infanticide, Government of India has taken various steps like PCPNDT Act (Preconception and Prenatal diagnostic technique act-1994) enacted to ban the identification of sex and to prevent the use of prenatal diagnostic techniques for selective abortion.

Various measures are taken by the Government to ensure survival, provision of better nutrition, education, protection and empowerment of girls by eliminating the differences in the sex ratio, infant mortality rate and improving their nutritional and educational status.

POCSO Act (Prevention of children from sexual offences), Sexual harassment at workplace (Prevention, prohibition and redressal) Act and the changes in the Criminal law based on the recommendations of Justice Verma Committee, 2013 aims at creating a safe and secure environment for both females and males.

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Amniocentesis and Its Statutory Ban

Due to small family norms and the skewed choice for a male child, female population is decreasing at an alarming rate. Amniocentesis is a prenatal technique used to detect any chromosomal abnormalities in the foetus and it is being often misused to determine the sex of the foetus. Once the sex of the foetus is known, there may be a chance of female foeticide. Hence, a statutory ban on amniocentesis is imposed.

It is used to detect any chromosomal defect in the embryo. However, recently amniocentesis is being used to detect the gender of the foetus which results in several female foeticides. This declines the gender ratio.

Therefore, there is a statutory ban on amniocentesis to avoid female foeticides. The amniotic fluid contains cells from foetus skin and respiratory tract. Sex of the foetus is determined using amniocentesis and then if it turns out to been a female one, foetus is aborted. That is why amniocentesis has been banned in India.

Yes, the ban is necessary because amniocentesis is misused now-a-days. It is used to determine the sex of the foetus and in many cases it led to female foeticide. In such extreme cases that would be incurable, adecision to abort the foetus could be taken.

Amniocentesis is a procedure in which amniotic fluid is removed from the uterus for testing or treatment. Amniotic fluid is the fluid that surrounds and protects a baby during pregnancy. This fluid contains fetal cells and various proteins.

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Need For Reproductive Health Problems and Strategies

India is amongst the first few countries in the world to initiate the ‘Family planning programme’ since 1951 and is periodically assessed every decade. These programmes are popularly named as ‘Reproductive and Child Health Care (RCH). Major tasks carried out under these programmes are:

• Creating awareness and providing medical assistance to build a healthy society.
• Introducing sex education in schools to provide information about adolescence and adolescence related changes.
• Educating couples and those in the marriageable age groups about the available birth control methods and family planning norms.
• Creating awareness about care for pregnant women, post-natal care of mother and child and the importance of breast feeding.
• Encouraging and supporting governmental and non-governmental agencies to identify new methods and/or to improve upon the existing methods of birth control.

Family planning counselling, pre-natal care, safe delivery, post-natal care, appropriate treatment of infertility, prevention of abortion, treatment of sexually transmitted diseases, responsible parenthood, services against HIV/AIDS, breast cancer should be made available.

Sexually Transmitted Diseases. Ill health of both mother and her baby. Early marriages before attaining the puberty. An increased mortality rate of both mother and Infants. Reproductive health in a society helps to prevent the spread of various sexually transmitted diseases and impart the ability to produce offspring carrying better survival rates. Sex education helps in maintaining the population size and to avoid unwanted pregnancies.

The important conditions to maintain good reproductive health during adolescence are: 1)It is necessary to eat balanced diet during adolescence. 2)It is necessary to maintain personal hygiene during adolescence. 3)It is necessary to take adequate physical exercise during adolescence.

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Parturition and Lactation

Parturition is the completion of pregnancy and giving birth to the baby. The series of events that expels the infant from the uterus is collectively called “labour”. Thoughout pregnancy the uterus undergoes periodic episodes of weak and strong contractions. These contractions called Braxter-Hick’s contractions lead to false labour. As the pregnancy progresses, increase in the oestrogen concentration promotes uterine contractions.

These uterine contractions facilitate moulding of the foetus and downward movement of the foetus. The descent of the foetus causes dilation of cervix of the uterus and vaginal canal resulting in a neurohumoral reflex called Foetal ejection reflex or Ferguson reflex.

This initiates the secretion of oxytocin from the neurohypophysis which in turn brings about the powerful contraction of the uterine muscles and leads to the expulsion of the baby through the birth canal. This sequence of events is called as parturition or childbirth.

Relaxin is a hormone secreted by the placenta and also found in the corpus luteum. It promotes parturition by relaxing the pelvic joints and by dilatation of the cervix with continued powerful contractions. The amnion ruptures and the amniotic fluid flows out through the vagina, followed by the foetus. The placenta along with the remains of the umbilical cord called “after birth” is expelled out after delivery.

Lactation is the production of milk by mammary glands. The mammary glands show changes during every menstrual cycle, during pregnancy and lactation. Increased level of oestrogens, progesterone and human Placental Lactogen (hPL) towards the end of pregnancy stimulate the hypothalamus towards prolactin – releasing factors. The anterior pituitary responds by secreting prolactin which plays a major role in lactogenesis.

Oxytocin causes the “Let-Down” reflxthe actual ejection of milk from the alveoli of the mammary glands. During lactation, oxytocin also stimulates the recently emptied uterus to contract, helping it to return to pre – pregnancy size.

The mammary glands secrete a yellowish fluid called colostrum during the initial few days after parturition. It has less lactose than milk and almost no fat, but it contains more proteins, vitamin A and minerals. Colostrum is also rich in IgA antibodies.

This helps to protect the infant’s digestive tract against bacterial infection. Breast milk is the ideal food for infants as it contains all the constituents in suitable concentration and is easily digestible. It is fully sufficient till about 6 months of age and all infants must be breast fed by the mother to ensure the growth of a healthy baby.

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Maintenance Of Pregnancy and Embryonic Development

The inner cell mass in the blastula is differentiated into epiblast and hypoblast immediately after implantation. The hypoblast is the embryonic endoderm and the epiblast is the ectoderm. The cells remaining in between the epiblast and the endoderm form the mesoderm.

Thus the transformation of the blastocyst into a gastrula with the primary germ layers by the movement of the blastomeres is called gastrulation. Each germ layer gives rise to specific tissues, organs and organ systems during organogenesis.

The extra embryonic membranes namely the amnion, yolk sac, allantois and chorion protect the embryo from dessication, mechanical shock and help in the absorption of nutrients and exchange of gases (Fig. 2.12). The amnion is a double layered translucent membrane filled with the amniotic fluid.

It provides a buoyant environment to protect the developing embryo from injury, regulates the temperature of the foetus and provides a medium in which the foetus can move. The yolk sac forms a part of the gut and is the source of the earliest blood cells and blood vessels.

The allantois forms a small out pocketing of embryonic tissue at the caudal end of the yolk, sac. It is the structural base for the umbilical cord that links the embryo to the placenta and ultimately it becomes part of the urinary bladder. The chorion is the outermost membrane which encloses the embryo and all other membranes and also helps in the formation of the placenta.

The trophoblast cells in the blastocyst send out several finger like projections called chorionic villi carrying foetal blood and are surrounded by sinuses that contain maternal blood. The chorionic villi and the uterine tissues form the disc-shaped placenta.

Placenta is a temporary endocrine organ formed during pregnancy and it connects the foetus to the uterine wall through the umbilical cord. It is the organ by which the nutritive, respiratory and excretory functions are fulfilled.

The embryo’s heart develops during the fourth week of pregnancy and circulates blood through the umbilical cord and placenta as well as through its own tissues. The primary germ layers serve as the primitive tissues from which all body organs develop. The ectoderm gives rise to the central nervous system (brain and spinal cord), peripheral nervous system, epidermis and its derivatives and mammary glands.

The connective tissue, cartilage and bone, muscles, organs of urinogenital system (kidney, ureter and gonads) arise from the mesoderm. The endodermal derivatives are epithelium of gastrointestinal and respiratory tract, liver, pancreas, thyroid and parathyroids.

Human pregnancy lasts for about 280 days or 40 weeks and is called the gestation period. It can be divided for convenience into three trimesters of three months each. The first trimester is the main period of organogenesis, the body organs namely the heart, limbs, lungs, liver and external genital organs are well developed.

By the end of the second trimester, the face is well formed with features, eyelids and eyelashes, eyes blink, body is covered with fie hair, muscle tissue develops and bones become harder. The foetus is fully developed and is ready for delivery by the end of nine months (third trimester).

During pregnancy, the placenta acts as a temporary endocrine gland and produces large quantities of human Chorionic Gonadotropin (hCG), human Chorionic Somatomammotropin (hCS) or human Placental Lactogen (hPL), oestrogens and progesterone which are essential for a normal pregnancy.

A hormone called relaxin is also secreted during the later phase of pregnancy which helps in relaxation of the pelvic ligaments at the time of parturition. It should be noted that hCG, hPL and relaxin are produced only during pregnancy. In addition, during pregnancy the level of other hormones like oestrogen and progesterone, cortisol, prolactin, thyroxine, etc., is increased several folds in the maternal blood. These hormones are essential for supporting foetal growth.

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Fertilization and Implantation

Fertilization occurs when a haploid sperm fuses with a haploid ovum to form a fertilized egg or diploid zygote. The sperms deposited in the female reproductive tract undergo capacitation, which is a biochemical event that enables the sperm to penetrate and fertilise the egg. Fertilization occurs only if the ovum and sperms are transported simultaneously to the ampullary isthmic junction of the fallopian tube.

Before a sperm can enter the egg, it must penetrate the multiple layers of granulosa (follicular) cells which are around the ovum forming the corona radiata (Fig. 2.10). The follicular cells are held together by an adhesive cementing substance called hyaluronic acid. The acrosomal membrane disintegrates releasing the proteolytic enzyme, hyaluronidase during sperm entry through the corona radiata and zona pellucida.

This is called acrosomal reaction. Once Fertilization is accomplished, cortical granules from the cytoplasm of the ovum form a barrier called the Fertilization membrane around the ovum preventing further penetration of other sperms. This polyspermy is prevented.

The first cleavage produces two identical cells called blastomeres. These produce 4 cells, then 8 and so on. After 72 hours of Fertilization, a loose collection of cells forms a berry shaped cluster of 16 or more cells called the morula (Fig. 2.11).

Under the influence of progesterone, smooth muscles of the fallopian tube relax and the dividing embryo takes 4-5 days to move through the fallopian tube into the uterine cavity and finally gets implanted in the uterine wall.

At this point the embryo consists of a fluid filled hollow ball of about 100 cells, called the blastocyst. The blastocyst is composed of a single layer of large flattened cells called trophoblast and a small cluster of 20-30 rounded cells called the inner cell mass. The inner cell mass of the blastocyst develops into the embryo and becomes embedded in the endometrium of the uterus.

This process is called implantation and it results in pregnancy. If the fertilized ovum is implanted outside the uterus it results in ectopic pregnancy. About 95 percent of ectopic pregnancies occur in the fallopian tube. The growth of the embryo may cause internal bleeding, infection and in some cases even death due to rupture of the fallopian tube.