Transport of Sex Cells and Fertilization
Hormonal Changes During Pregnancy
Fetal Stage, Fetal Blood and Circulation
Pregnancy is defined as the condition of having a developing offspring in the uterus. Pregnancy results when a sperm cell unites with an egg; this union of egg and sperm is called fertilization.
Prior to fertilization, an egg is released from an ovary. The egg then travels down a uterine tube. During sexual intercourse, the male deposits semen into the vagina. The sperm cells must travel up through the uterus to the uterine tubes to fertilize the egg.
Prostaglandins in semen stimulate the flagella of the sperm cells to undulate causing the swimming action of sperm. Prostaglandins also stimulate muscles in the uterus and uterine tubes to contract. These contractions also help the sperm reach the egg.
Estrogen stimulates the uterus and cervix to secrete a thin watery fluid that also promotes the movement of sperm toward the egg.
Although many sperm may reach an egg, only one usually unites with the egg to fertilize it. The sperm cell that unites with the egg penetrates the follicular cells and zona pellucida which surround the cell membrane of the egg. The acrosome of the sperm releases enzymes that helps the sperm penetrate the membrane of the egg.
Once a sperm unites with an egg, the egg will release enzymes the prevent other sperms from invading it. The enzymes cause the zona pellucida to become hard and therefore inpenetrable.
The nucleus of the egg (with 23 chromosomes) and the nucleus of the sperm (with 23 chromosomes) eventually fuse together to make one nucleus that contains 46 chromosomes. The cell that is formed by this union is called a zygote.
The prenatal period is the time before the offspring is born. The prenatal period is divided into an embryonic period (weeks 2 through 8 of pregnancy) and a fetal period (weeks 9 to delivery of offspring).
About one day after the zygote forms it begins to undergo mitosis at a relatively rapid rate. This rapid cell division is called cleavage. The resulting ball of cells is called a morula. The morula travels down the uterine tube to the uterus. Fluid then invades the morula and it is now called a blastocyst. The blastocyst implants in the wall of the uterus. From zygote formation to implantation of the blastocyst takes about one week.
Once the blastocyst is implanted, a group of cells in the blastocyst, called the inner cell mass, gives rise to an embryo.
Other cells in the blastocyst along with cells of the uterus eventually form the placenta.
Click here to see the stages of embryonic development
Following implantation, cells of the embryo begin to secrete HCG (human chorionic gonadotropin). HCG maintains the corpus luteum in the ovary so it will continue to secrete estrogen and progesterone.
The placenta also secretes large amount of progesterone and estrogen. Progesterone and estrogen stimulate the uterine lining to thicken. They also inhibit the anterior pituitary from secreting FSH and LH which prevents ovulation during pregnancy. Estrogen and progesterone also stimulate the development of the mammary glands, inhibit uterine contractions, and stimulate the enlargement of female reproductive organs.
Another hormone called relaxin that comes from the corpus luteum inhibits uterine contractions and relaxes the ligaments of the pelvis in preparation for childbirth.
The placenta also secretes lactogen which stimulates enlargement of the mammary glands.
Aldosterone is secreted from the adrenal gland to increase sodium and water retention.
PTH secretion increases from the parathyroid gland to help maintain high calcium levels in the blood.
The embryonic stage extends from the second week of pregnancy to the end of the eighth week of development. During this stage, the placenta forms along with most internal organs and external structures.
The cells of the inner cell mass organize into layers called primary germ layers. All organs are formed from the primary germ layers. The two week old embryo is now called a gastrula.
There are three layers of the primary germ layers as follows:
1) Ectoderm - gives rise to nervous tissue and some epithelial tissue
2) Mesoderm - gives rise to connective tissues and some epithelial tissue
3) Endoderm - gives rise to epithelial tissues only.
The placenta develops during the embryonic period. The placenta allows nutrients and oxygen from maternal blood to pass to embryonic blood. It also allows waste products from the fetal blood to pass into maternal blood.
The amnion develops during this period. The amnion is a protective, fluid-filled sac that surrounds the embryo. The umbilical cord also forms during this period. The umbilical cord contains three blood vessels, one umbilical vein and two umbilical arteries. These blood vessels transport blood between the fetus and the placenta.
The yolk sac also forms during the embryonic stage. This structure makes new blood cells for the fetus as well as cells that eventually become sex cells of the baby.
By the end of the embryonic stage, the baby closely resembles a human as all external structures (arms, hands, legs, feet, etc.) have formed.
The fetal stage begins at the end of the eighth week of development and ends at birth. During this period, growth of the offspring is rapid. By the twelfth week ossification centers have appeared in bones and the external reproductive organs can be distinguishable as male or female.
The growth rate of the fetus slows down in the fifth month but skeletal muscles become active. In the sixth month, the fetus starts to gain substantial weight. In the seventh month the eyelids open. In the last three months of pregnancy, fetal brain cells divided rapidly and organs continue to grow. The testes of the male descend into the scrotum. The last organ systems to completely develop are the digestive and respiratory systems. By the end of the ninth month, the fetus is usually positioned upside down in the uterus.
Click here to see images of a 6 month old fetus
Throughout prenatal development, the placenta and umbilical blood vessels carry out the exchange of nutrients, oxygen and waste products between the maternal and fetal blood.
The hemoglobin of the fetus has a much higher affinity for oxygen than normal hemoglobin found in adults. Therefore, the fetus's blood is adapted to carry more oxygen.
In the fetal heart there is a hole called the foramen ovale between the right atrium and the left atrium. Remember that the fetus does not have to pump its blood to the lungs to get oxygen. It gets its oxygen from the placenta. So, in the fetal heart, most blood flows from the right atrium into the left atrium.
Some fetal blood does flow from the right atrium into the right ventricle. The right ventricle then delivers the blood to the pulmonary trunk. In the fetus, there is a connection between the pulmonary trunk and the aorta; this connection is called the ductus arteriosis. So, blood in the fetus flows from the pulmonary trunk into the aorta. Remember the blood does not need to go to the lungs in the fetus.
Click here to see a diagram of fetal circulation
The birth process ends pregnancy. The following factors start pregnancy:
1) Progesterone levels decrease. Remember progesterone inhibited uterine contractions.
2) Prostaglandin secretion promotes uterine contractions.
3) The posterior pituitary releases oxytocin. Oxytocin stimulates strong uterine contractions which begins labor.
Following the birth of the baby, the placenta is also expelled from the uterus.
The postnatal period is the period following birth.
During pregnancy, hormones stimulated the breasts to enlarge. After childbirth, prolactin causes the mammary glands to produce milk. The hormone oxytocin stimulates the ejection of milk from mammary gland ducts. As long as milk is removed from the mammary glands, milk production will continue. Once a female stops breast feeding the hypothalamus will inhibit the release of prolactin and oxytocin so milk production will stop.
The neonatal period extends from birth to the four weeks following birth. Several important facts should be noted about this period as follows:
1) The lungs must expand. This is why the first breath of a baby is forceful.
2) The liver is immature so the baby has to get most of its glucose from fat stores in the skin (baby fat).
3) The newborn urinates a lot as the kidneys are too immature to concentrate urine well.
4) Body temperature tends to be unstable for the enfant.
5) The umbilical vessels of the newborn must constrict.
6) The foramen ovale must close.
7) The ductus arteriosus must close.