Human Biology: Fertilization through Birth

Fertilization, the joining of sperm and egg, typically occurs in the fallopian tube, after sperm have made the long swim up through the vagina, past the cervix, and through the uterus. Fertilization brings together the chromosomes from each parent, creating the first cell, or zygote, of the new human.

Because a human egg lives no longer than 24 hours after ovulation and human sperm live no longer than 72 hours, intercourse that occurs in the three-day period prior to ovulation or within the day after ovulation is the only chance of fertilization during a given month.

The zygote divides by mitosis, beginning production of all the cells necessary for the human body. The development of a new organism occurs through the production and specialization of new cells. The development from zygote to newborn occurs in several stages.

Embryonic development occurs from the zygote through the eighth week of pregnancy. The zygote begins traveling down the fallopian tube, heading for the uterus so it can implant in the uterine lining. As it travels, the zygote undergoes cleavage, a rapid series of mitotic divisions that result in a multicellular embryo.

  • After cell division produces a solid ball of 16 cells, the developing human is called a morula.

  • The morula fills with liquid, forming a hollow ball of cells called a blastula.

  • In humans, a group of cells inside the blastula becomes specialized to form the embryo, and the blastula becomes a blastocyst. Different layers of cells become specialized within the blastocyst, taking the first step toward forming specialized tissues. (You can recognize the blastocyst by the flattened cells along its edge, called the trophoblast.)

After implanting itself, the developing mass of cells moves inward, forming a ball of cells called a gastrula that has three layers of cells; this process is referred to as gastrulation. Each layer of cells in the gastrula eventually becomes a different type of tissue:

  • The outer layer is the ectoderm, which develops into the skin and nervous systems.

  • The middle layer is the mesoderm, which develops into the muscular, skeletal, and circulatory systems.

  • The innermost layer is the endoderm, which gives rise to the linings of the digestive and respiratory tracts, as well as organs such as the liver and pancreas.

After gastrulation, the specialized cells of the ectoderm, mesoderm, and endoderm begin migrating toward other cells with the same specialty. This cellular migration is referred to as morphogenesis because it gives the embryo a shape. As cells differentiate, organ systems begin to form. At the end of the embryonic period, a human embryo is about an inch and a half long and has recognizable human body structures.

While the embryo is developing, structures form outside the embryo that support and nourish the embryo and fetus. Two specialized membranes develop outside the embryo. One, called the chorion, combines with tissues created by the mother to become the placenta. The other, called the amnion, surrounds the amniotic cavity, which fills with amniotic fluid that protects the developing embryo and fetus.

A tubular structure called the allantois forms off of the center cavity of the blastocyst. In humans, the allantois eventually becomes the body stalk and then the umbilical cord, which connects the fetus to the placenta.

After the eighth week of pregnancy, the developing human is a fetus. Fetuses are completely differentiated, meaning their cells have migrated and formed organ systems. All fetuses do in the uterus is continue to grow and develop features such as hair and nails. As the fetus gets stronger, longer, and heavier, it looks more and more like a newborn baby.

During the last weeks of pregnancy, the hormone estrogen reaches high levels in the mother's blood. Estrogen triggers the formation of receptors for another hormone, called oxytocin, on the uterine wall. Labor is triggered when the fetus starts producing oxytocin, which causes the mother's pituitary to produce even more oxytocin.

The uterus's receptors receive the oxytocin signals, causing uterine muscles to start labor contractions. Oxytocin also stimulates the placenta to produce prostaglandins, which increase uterine contractions.

Labor occurs in three stages:

  1. Dilation refers to the beginning of labor until the cervix has opened (dilated) to a diameter of 10 centimeters.

    Dilation is the longest period of labor, typically lasting from 6 to 12 hours but sometimes quite a bit longer.

  2. Expulsion is the delivery of the infant.

    The mother experiences strong uterine contractions and an urge to push. Expulsion usually lasts for 20 minutes to one hour.

  3. Delivery of the placenta usually occurs within 15 minutes of expulsion.

When the fetus is finally born, the infant is called a neonate, meaning newborn. The baby will continue to develop rapidly throughout the first few years of its life.

For questions 1 – 7, use the terms that follow to identify the correct stage of a developing human.

a. Blastula

b. Fetus

c. Morula

d. Embryo

e. Zygote

f. Blastocyst

g. Neonate

For questions 8 – 11, use the terms that follow to correctly identify the major events that occur during an embryo's development.

a. Differentiation of organ systems

b. Morphogenesis

c. Cleavage

d. Gastrulation

  1. A fluid-filled ball of cells.

  2. A multicellular human at four weeks.

  3. A single cell formed from the fusion of sperm and egg.

  4. A newborn baby.

  5. A solid ball of 16 cells.

  6. A developing human at five months.

  7. A ball of cells containing a trophoblast and a developing embryo.

  8. A ball of cells develops three specialized layers of cells.

  9. Specialized cells of the same type migrate toward one another.

  10. A zygote undergoes a rapid series of cell divisions.

  11. Organs begin to form.

The following are the answers to the practice questions.

  1. The answer is a. Blastula.

  2. The answer is d. Embryo.

  3. The answer is e. Zygote.

  4. The answer is g. Neonate.

  5. The answer is c. Morula.

  6. The answer is b. Fetus.

  7. The answer is f. Blastocyst.

  8. The answer is d. Gastrulation.

  9. The answer is b. Morphogenesis.

  10. The answer is c. Cleavage.

  11. The answer is a. Differentiation of organ systems.

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