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Multilingual Illustrated DVD [Tutorial]

The Biology of Prenatal Development



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The Embryonic Period (The First 8 Weeks)

Embryonic Development: The First 4 Weeks

Chapter 3   Fertilization

Biologically speaking, "human development begins at fertilization," when a woman and a man each combine 23 of their own chromosomes through the union of their reproductive cells.

A woman's reproductive cell is commonly called an "egg" but the correct term is oocyte.

Likewise, a man's reproductive cell is widely known as a "sperm" but the preferred term is spermatozoon.

Following the release of an oocyte from a woman's ovary in a process called ovulation, the oocyte and spermatozoon join within one of the uterine tubes, which are often referred to as Fallopian tubes.

The uterine tubes link a woman's ovaries to her uterus or womb.

The resulting single-celled embryo is called a zygote, meaning "yoked or joined together."

Chapter 4   DNA, Cell Division, and Early Pregnancy Factor (EPF)

The zygote's 46 chromosomes represent the unique first edition of a new individual's complete genetic blueprint. This master plan resides in tightly coiled molecules called DNA. They contain the instructions for the development of the entire body.

DNA molecules resemble a twisted ladder known as a double helix. The rungs of the ladder are made up of paired molecules, or bases, called guanine, cytosine, adenine, and thymine.

Guanine pairs only with cytosine, and adenine with thymine. Each human cell contains approximately 3 billion base pairs.

The DNA of a single cell contains so much information that if it were represented in printed words, simply listing the first letter of each base would require over 1.5 million pages of text!

If laid end-to-end, the DNA in a single human cell measures 3 1/3 feet or 1 meter.

If we could uncoil all of the DNA within an adult's 100 trillion cells, it would extend over 63 billion miles. This distance reaches from the earth to the sun and back 340 times.

Approximately 24 to 30 hours after fertilization, the zygote completes its first cell division. Through the process of mitosis, one cell splits into two, two into four, and so on.

As early as 24 to 48 hours after fertilization begins, pregnancy can be confirmed by detecting a hormone called "early pregnancy factor" in the mother's blood.

Chapter 5   Early Stages (Morula and Blastocyst) and Stem Cells

By 3 to 4 days after fertilization, the dividing cells of the embryo assume a spherical shape and the embryo is called a morula.

By 4 to 5 days, a cavity forms within this ball of cells and the embryo is then called a blastocyst.

The cells inside the blastocyst are called the inner cell mass and give rise to the head, body, and other structures vital to the developing human.

Cells within the inner cell mass are called embryonic stem cells because they have the ability to form each of the more than 200 cell types contained in the human body.

Chapter 6   1 to 1½ Weeks: Implantation and Human Chorionic Gonadotropin (hCG)

After traveling down the uterine tube, the early embryo embeds itself into the inner wall of the mother's uterus. This process, called implantation, begins 6 days and ends 10 to 12 days after fertilization.

Cells from the growing embryo begin to produce a hormone called human chorionic gonadotropin, or hCG, the substance detected by most pregnancy tests.

HCG directs maternal hormones to interrupt the normal menstrual cycle, allowing pregnancy to continue.

Chapter 7   The Placenta and Umbilical Cord

Following implantation, cells on the periphery of the blastocyst give rise to part of a structure called the placenta, which serves as an interface between the maternal and embryonic circulatory systems.

The placenta delivers maternal oxygen, nutrients, hormones, and medications to the developing human; removes all waste products; and prevents maternal blood from mixing with the blood of the embryo and fetus.

The placenta also produces hormones and maintains embryonic and fetal body temperature slightly above that of the mother's.

The placenta communicates with the developing human through the vessels of the umbilical cord.

The life support capabilities of the placenta rival those of intensive care units found in modern hospitals.

Chapter 8   Nutrition and Protection

By 1 week, cells of the inner cell mass form two layers called the hypoblast and epiblast.

The hypoblast gives rise to the yolk sac, which is one of the structures through which the mother supplies nutrients to the early embryo.

Cells from the epiblast form a membrane called the amnion, within which the embryo and later the fetus develop until birth.

Chapter 9   2 to 4 Weeks: Germ Layers and Organ Formation

By approximately 2 1/2 weeks, the epiblast has formed 3 specialized tissues, or germ layers, called ectoderm, endoderm, and mesoderm.

Ectoderm gives rise to numerous structures including the brain, spinal cord, nerves, skin, nails, and hair.

Endoderm produces the lining of the respiratory system and digestive tract, and generates portions of major organs such as the liver and pancreas.

Mesoderm forms the heart, kidneys, bones, cartilage, muscles, blood cells, and other structures.

By 3 weeks the brain is dividing into 3 primary sections called the forebrain, midbrain, and hindbrain.

Development of the respiratory and digestive systems is also underway.

As the first blood cells appear in the yolk sac, blood vessels form throughout the embryo, and the tubular heart emerges.

Almost immediately, the rapidly growing heart folds in upon itself as separate chambers begin to develop.

The heart begins beating 3 weeks and 1 day following fertilization.

The circulatory system is the first body system, or group of related organs, to achieve a functional state.

Chapter 10   3 to 4 Weeks: The Folding of the Embryo

Between 3 and 4 weeks, the body plan emerges as the brain, spinal cord, and heart of the embryo are easily identified alongside the yolk sac.

Rapid growth causes folding of the relatively flat embryo. This process incorporates part of the yolk sac into the lining of the digestive system and forms the chest and abdominal cavities of the developing human.


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