Two kinds of sex cells or gametes are involved in human reproduction: the male gamete or the Sperm cell and the female gamete or the Ovum. When semen is deposited in the vagina the spermatozoa travels through the cervix and body of uterus, into the fallopian tube. Fertilization of ovum usually takes place in the fallopian tube. The first sperm that penetrates fully into the egg, donates its genetic material (DNA). The egg then polarizes and therefore repels any additional sperm. The resulting combination is called zygote.
THE INITIAL STAGES OF HUMAN EMBRYO-GENESIS
The embryonic period in human beings at fertilization (penetration of the egg by the sperm) and continues until the end of 10th week of gestation (8th week by embryonic age). The embryo spends next few days traveling down the fallopian tube. It starts out as a single cell zygote and then divide several times to form a ball of cell called a Morula. Further cellular division is accompanied by the formation of small cavity between the cells. This stage is called a Blastocyst. Up to this point there is no growth in the overall size of the embryo, as it is confined within a cyclo-protein shell, known as Zona pellucida.
The Blastocyst reaches the uterus at roughly the fifth day after fertilization. It is here that lysis of pellucida occurs. This allows the trophectoderm cells of Blastocyst to come into contact with, and here to, the endometrial cells of uterus. The trophectoderm will eventually give rise to extra-embryonic structures, such as placenta. Placenta and membranes are collectively referred to as conceptus or “product of conception”.
PRE-NATAL DEVELOPMENT
Prenatal development takes place during three periods: Germinal, Embryonic and Fetal periods.
Germinal Period
It is a period between conception and implantation (attachment to uterine wall – about 14 days). The period occurs in the initial period of human embryogenesis
Embryonic Period
The period extends from 2 weeks to 8 weeks after conception. Fourteen days after conception, the Blastocyst implants itself in the uterine wall. The embryonic period begins at the end of 2nd week. The embryo develops from a round layer of cells across the centre of Blastocyst. At 18 days embryo is about 0.0625 (1/16)thof an inch long. During its early weeks, human embryo closely resembles those of other vertebrate animals. The embryo has a tail and traces of gills, both of which will soon disappear. The head develops before the rest of the body. Eyes, nose and ears are not yet visible at one month, but a backbone and vertebral canal have formed. Small buds that will develop into arms and legs appear. The heart form and start beating: other body systems begin to take shape.
The Fetal Period
The period extends from 8 weeks through remainder of pregnancy. By the end of the embryonic period (2 months), the fetus has developed the first bone structure and distinct limbs and digits that take in human form. Major blood vessels form and internal organs continue to develop. By the end of the first trimester (1/3rd length of pregnancy or 12.7 weeks).The fetus is about 3 inches long: most major organs are present, a large head and face are well developed, and heart beat can be detected with a stethoscope.
By the end of 4th or 5th period (month) can usually feel that movement. The skin of fetus is covered with fine hair, usually shed before birth. At the end of 5th month, fetus weighs about 1 pound and is about 12 inches long. It sleeps, wakes, sucks and moves its position. At the end of 6th month eyes, eyelids and eyelashes form. The fetus eyes are light sensitive and he/she can hear uterine sounds and respond to vibrations and acoustical stimulation.The head and body of the fetus become more proportionate during 3rd trimester .Fat layers form under the skin. By the end of 8th month, the fetus weighs about 5 pounds and is about 18 inches long.
By the end of the 9th month, the nails have grown to the end of fingers and toes. The skin becomes smoother and is covered with a protective waxy substance called Vernix Caseosa.The baby is ready for delivery.
THE INITIAL STAGES OF HUMAN EMBRYO-GENESIS
The embryonic period in human beings at fertilization (penetration of the egg by the sperm) and continues until the end of 10th week of gestation (8th week by embryonic age). The embryo spends next few days traveling down the fallopian tube. It starts out as a single cell zygote and then divide several times to form a ball of cell called a Morula. Further cellular division is accompanied by the formation of small cavity between the cells. This stage is called a Blastocyst. Up to this point there is no growth in the overall size of the embryo, as it is confined within a cyclo-protein shell, known as Zona pellucida.
The Blastocyst reaches the uterus at roughly the fifth day after fertilization. It is here that lysis of pellucida occurs. This allows the trophectoderm cells of Blastocyst to come into contact with, and here to, the endometrial cells of uterus. The trophectoderm will eventually give rise to extra-embryonic structures, such as placenta. Placenta and membranes are collectively referred to as conceptus or “product of conception”.
PRE-NATAL DEVELOPMENT
Prenatal development takes place during three periods: Germinal, Embryonic and Fetal periods.
Germinal Period
It is a period between conception and implantation (attachment to uterine wall – about 14 days). The period occurs in the initial period of human embryogenesis
Embryonic Period
The period extends from 2 weeks to 8 weeks after conception. Fourteen days after conception, the Blastocyst implants itself in the uterine wall. The embryonic period begins at the end of 2nd week. The embryo develops from a round layer of cells across the centre of Blastocyst. At 18 days embryo is about 0.0625 (1/16)thof an inch long. During its early weeks, human embryo closely resembles those of other vertebrate animals. The embryo has a tail and traces of gills, both of which will soon disappear. The head develops before the rest of the body. Eyes, nose and ears are not yet visible at one month, but a backbone and vertebral canal have formed. Small buds that will develop into arms and legs appear. The heart form and start beating: other body systems begin to take shape.
The Fetal Period
The period extends from 8 weeks through remainder of pregnancy. By the end of the embryonic period (2 months), the fetus has developed the first bone structure and distinct limbs and digits that take in human form. Major blood vessels form and internal organs continue to develop. By the end of the first trimester (1/3rd length of pregnancy or 12.7 weeks).The fetus is about 3 inches long: most major organs are present, a large head and face are well developed, and heart beat can be detected with a stethoscope.
By the end of 4th or 5th period (month) can usually feel that movement. The skin of fetus is covered with fine hair, usually shed before birth. At the end of 5th month, fetus weighs about 1 pound and is about 12 inches long. It sleeps, wakes, sucks and moves its position. At the end of 6th month eyes, eyelids and eyelashes form. The fetus eyes are light sensitive and he/she can hear uterine sounds and respond to vibrations and acoustical stimulation.The head and body of the fetus become more proportionate during 3rd trimester .Fat layers form under the skin. By the end of 8th month, the fetus weighs about 5 pounds and is about 18 inches long.
By the end of the 9th month, the nails have grown to the end of fingers and toes. The skin becomes smoother and is covered with a protective waxy substance called Vernix Caseosa.The baby is ready for delivery.
