RGD - Fertilization to Birth Lesson
Fertilization to Birth
Fertilization
Fertilization is the fusion of the male and female gametes. Sperm are typically deposited into the female reproductive system during sexual intercourse, a process that involves the insertion of an erect penis into a vagina. Thirty minutes after ejaculation, sperm will have traveled from the vagina through the uterus and into the fallopian tube where fertilization usually takes place. Here the sperm will wait for ovulation to occur. Sperm can live for several days in the fallopian tubes. A day or two after ovulation, the egg may unite with a sperm. Of the several hundred million sperm released during ejaculation, only a few thousand reach the egg, and only one sperm will fertilize the egg.
When a sperm penetrates the egg, it triggers a series of chemical changes in the outer membrane of the oocyte to prevent any other sperm from entering, and it initiates the secondary oocyte to begin meiosis II. The sperm cell itself also undergoes changes: its flagella, or tail, falls off, and its nucleus fuses with the nucleus of the egg. The resulting cell or fertilized egg is called a zygote. The fusion of the egg and sperm nuclei creates a diploid zygote with 23 pairs (46 total) of chromosomes.
Cleavage and Implantation
The zygote spends the next few days traveling down the fallopian tube toward the uterus. As it travels, it divides several times to form a solid ball of cells called a morula. The cell divisions are called cleavages because, unlike in normal mitosis, they increase the number of cells but not the overall size of the new organism.
As more cell divisions occur, a fluid-filled cavity forms inside the ball of cells. At this stage, the hollow ball of cells is called a blastocyst. The movie below is time-lapse microscopy of mouse eggs from fertilization to the blastocyst stage. Although it is not human, the human development at this stage would be almost identical. If you look closely at the beginning, you can even see the nuclei of the sperm and egg (pronuclei) coming together and the polar bodies from oogenesis.
The cells of the blastocyst form an inner cell mass and an outer cell layer. The inner cell mass will soon develop into an embryo, whereas the outer cell layer, or trophoblast, will develop into other structures needed to support and nourish the embryo such as the placenta and amniotic sac.
Implantation
The blastocyst reaches the uterus about 4 or 5 days after fertilization. When its outer cells touch the lining of the uterus, the blastocyst embeds into the endometrium. This process is called implantation and generally occurs about a week after fertilization.
Human chorionic gonadotropin (hCG) is a hormone produced by the embryo after implantation that promotes placental development. Home pregnancy tests work by testing for this hormone in the mother's urine. The placenta is a temporary organ that develops from the trophoblast layer of cells shortly after implantation. The placenta continues to develop and grow throughout pregnancy to meet the needs of the growing embryo/fetus. A fully developed placenta is made up of a large mass of blood vessels from both the mother and fetus. The maternal and fetal vessels are close together but separated by tiny spaces that allow the maternal and fetal blood to exchange substances across their capillary walls without the blood actually mixing.
The embryo/fetus is connected to the placenta through a series of blood vessels called the umbilical cord. Blood from the embryo/fetus enters the placenta through the umbilical arteries, exchanges gases, and other substances with the mother's blood, and travels back to the embryo/fetus through the umbilical vein.
Attached to the placenta is the amniotic sac, an enclosed membrane that surrounds and protects the embryo/fetus. It contains amniotic fluid, which consists of water and dissolved substances. The fluid allows the fetus to move freely until it grows to fill most of the available space. The fluid also cushions the fetus and helps protect it from injury.
Pregnancy
Pregnancy, or gestation, is the fertilization and development of an offspring (or more than one offspring in the case of multiples such as twins and triplets), in a woman's uterus. It is commonly divided into three trimesters periods (around 13 weeks each) that are used to reference the various stages of prenatal development.
If you were to look just at the human development aspect, however, there are actually two major phases of pregnancy: embryonic and fetal development.
Embryonic Development
After implantation occurs, the blastocyst is called an embryo. The embryonic stage lasts through the eighth week following fertilization. During this time, the embryo grows in size and becomes more complex. It develops specialized cells and tissues and starts to form most organs.
During the second week after fertilization, cells in the embryo migrate to form three distinct cell layers, called the ectoderm, mesoderm, and endoderm. Each layer will soon develop into different types of cells and tissues.
During the third week after fertilization, the embryo begins to undergo cellular differentiation. Differentiation is the process by which unspecialized cells become specialized.
Differentiation occurs as certain genes are expressed ("switched on") while other genes are suppressed ("switched off"). ecause of this process, cells develop unique structures and abilities that suit them for their specialized functions. The movie below is a great representation of this amazing process.
After cells differentiate, all the major organs begin to form during the remaining weeks of embryonic development. During this period, it also grows in size. By the eighth week of development, the embryo is just over 1 inch in length and may also have begun to move.
Fetal Development
From the end of the eighth week until birth, the developing human organism is referred to as a fetus. Birth typically occurs at about 38 weeks after fertilization, so the fetal period generally lasts about 30 weeks. During this time, the organs complete their development. The fetus also grows rapidly in length and weight. By the 38th week, the fetus is fully developed and ready to be born. A 38-week fetus normally ranges from 14-20 inches in length and weighs from 6 -10 pounds.
The video below serves as a good overview of the massive changes from fertilization to birth. It highlights the many complexities of embryonic and fetal development.
Childbirth
During childbirth, a baby passes from the uterus, through the vagina, and out of the mother's body. Childbirth usually starts when the amniotic sac breaks commonly known as the "water breaking."
During the first stage of birth, the cervix dilates to allow passage of the fetus. Full dilation is 10 cm.
The second stage of birth involves strong uterine contractions that propel the fetus through the birth canal, usually head first. Hormonal control of the birth process works in a positive feedback system and involves the release of oxytocin and prostaglandins. These hormones are stimulated by uterine contractions, which stimulate more hormones that cause more contractions, etc. As the second stage progresses, contractions get stronger and closer together. They may go on for hours. Eventually, the contractions squeeze the baby out of the uterus. Once the baby enters the vagina, the mother starts pushing. She soon pushes the baby through the vagina and out of her body.
As soon as the baby is born, the umbilical cord is cut. After the cord is cut, the baby can no longer get rid of carbon dioxide through the cord and placenta. As a result, carbon dioxide builds up in the baby's blood triggering the baby's first breath.
The third stage of birth occurs after the delivery of the baby. The amniotic sac and placenta (or afterbirth) pass through the vagina and out of the body shortly after the birth of the baby.
In some instances, it is safer for the baby to be surgically removed such as if the baby is breech (presenting bottom first), multiples, or in a medical emergency. This is done through a procedure known as a Caesarian section.
Lactation
After birth, a woman's breast produce milk to feed their baby. The breasts make milk in response to a baby's suckling which stimulates the production of oxytocin (the milk let-down hormone). Breastfeeding offers many benefits to the baby. Breast milk contains the right balance of nutrients to help an infant grow into a strong and healthy toddler. Some of the nutrients in breast milk also help protect an infant against some common childhood illnesses and infections. It also is beneficial to the mother. It helps to contract the uterus back to its normal size and certain types of cancer may occur less often in mothers who have breastfed their babies.
Gestational and Congenital Disorders
Pregnancy, embryonic development, and birth are complex processes that can be affected at any stage by a number of factors. Learn about gestational and congenital (present from birth) disorders in the learning object below.
Review embryonic and fetal development in the learning object below:
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