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Unit 14: Embryonic Development and its Regulation

Embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal developmentdescribe later stages. Embryogenesis starts with the fertilization of the egg cell (ovum) by a sperm cell, (spermatozoon).

  • 14.1: Embryonic Development
    The genome of the zygote contains all the genes needed to make the hundreds of different types of cells that will make up the complete animal. There are two major categories of these genes: "housekeeping" genes and tissue-specific genes. However, every cell descended from the zygote has been produced by mitosis and thus contains the complete genome of the organism (with a very few exceptions).
  • 14.2: Frog Embryology
    The frog egg is a huge cell; its volume is over 1.6 million times larger than a normal frog cell. During embryonic development, the egg will be converted into a tadpole containing millions of cells but containing the same amount of organic matter.
  • 14.3: Cleavage
    Cleavage refers to the early cell divisions that occur as a fertilized egg begins to develop into an embryo.
  • 14.4: The Organizer
    In the embryonic development of a zygote, gradients of mRNAs and proteins, deposited in the egg by the mother as she formed it, give rise to cells of diverse fates despite their identical genomes. But is the embryo fully patterned in the fertilized egg? It is difficult to imagine that the relatively simple gradients in the egg could account for all the complex migration and differentiation of cells during embryonic development. And, in fact, the answer is no.
  • 14.5: Segmentation - Organizing the Embryo
    Insects, like all arthropods, are segmented. The body of Drosophila melanogaster is built from 14 segments, but what signals guide segment formation? The process begins with the gradients of messenger RNA (mRNA) that the mother deposited in her egg before it was fertilized. Shortly after fertilization, these are translated into their proteins with a gradient of bicoid diminishing from anterior to posterior and a gradient of nanos diminishing from posterior to anterior.
  • 14.6: Homeobox Genes
    Insect (Drosophila) and frog (Xenopus) development passes through three rather different (although often overlapping) phases.
  • 14.7: Stem Cells
    Stem cells are cells that divide by mitosis to form either two stem cells, thus increasing the size of the stem cell "pool", or one daughter that goes on to differentiate, and one daughter that retains its stem-cell properties. How the choice is made is still unknown. However, several genes have been found whose activity prevents a daughter cell from differentiating.
  • 14.8: Embryonic Stem Cells
    a research team led by James Thomson of the University of Wisconsin reported (in the 6 November 1998 issue of Science) that they were able to grow human embryonic stem (ES) cells in culture. At the time of implantation, the mammalian embryo is a blastocyst. It consists of the trophoblast — a hollow sphere of cells that will go on to implant in the uterus and develop into the placenta and umbilical cord. inner cell mass (ICM) that will develop into the baby as well as the extraembryonic amnion
  • 14.9: Germline vs. Soma
    Could a mutation in one of your liver cells ever be passed on to your children? No! Why not? The fusion of one sperm cell and one egg cell represents the only genetic link between the bodies of parents and the body of their child and the cells destined to produce sperm and eggs are set aside very early in embryonic life.
  • 14.10: Regeneration
    Regeneration is the ability to replace lost or damaged body parts. This ability varies greatly among living things.

Thumbnail: Human embryo, 8-9 weeks, 38 mm. Image used with permission (CC BY-SA 3.0; Anatomist90).