Mammalian Development

This page is intended to give an introduction to mammalian development and provides an outline of other blogs on this website devoted to the development of mammals. The outline is at the bottom of the page.

Mammalian Developmental Biology

Mammals are really remarkable organisms. They show diverse morphologies, yet have some very conserved characteristics which allow scientists to develop model systems in more simple mammals, such as mice. This ability to use model organisms to study similar systems in more complicated animals is necessary because performing empirical studies is either very difficult or impossible with some animals (such as humans).

Mammalian Characteristics Video:

Mammals are some of the only life forms on the planet to be viviparous. Vivipary allows the embryo to develop in a safe environment, leads to live births, and generally yields high survival of neonates.

Embryo and Uterus Prior to Implantation

Unlike other vertebrates (such as amphibians), mammalian embryos must implant into the uterine wall for successful development. This requires additional extra-embryonic tissue to assist in the implantation. Therefore in early embryonic development, mammals spend time forming these extra-embryonic tissues, collectively called trophoblasts, (or trophectoderm after gastrulation), rather than establishing and organizing the body plan. (Graham, 2000; Beddington and Robertson, 1999)

Initial Human Embryonic Development Video:

A more detailed version of the stages of mammalian embryonic development is below. E#.# represents the very defined embryonic stage of “days post fertilization” where E1 would indicate 1 day after fertilization. (Marikawa, 2006) Something to keep in mind is that while mouse and human development are similar, the timescale is rather different between the two-humans take longer to develop and therefore have accomplished different stages on the same time post-fertilization.

At E6.0 in mouse development (about the third week in humans) cells of the epiblast differentiate into the three germ layers of the embryo including the endoderm, mesoderm and ectoderm. This process is called gastrulation.  Body patterning also starts at this stage and is directed by the signalling molecules made in the node and the anterior visceral endoderm, or the AVE .

Right after the formation of the node and the AVE, the primitive streak develops which directs bilateral symmetry of the embryo. The posterior cells of the epiblast start to move forward, made possible by enzymatic degradation of the basement membrane: migration is directed in between the epiblast and the visceral endoderm (see the purple arrows in the figure below). (Rivera-Pérez and Magnuson, 2005) This migration is vital to embryonic development because it is this tissue that forms the notochord and mesoderm. More information about early tissue differentiation can be found here.

Mouse Embryonic Development

By E8.0 in the mouse, the the primitive ectoderm of the blastocyst (postimplantation) has generated the ectoderm, mesoderm, and endoderm of the gastrula. Remember germ layers in general give rise to the following tissues:

Ectoderm (outer layer)

  • Central nervous system  (brain and spinal chord)
  • Peripheral nervous system
  • Skin (or outer surface of the organism)
  • Cornea and outer lens of eye
  • Epithelium that lines the mouth, nasal cavities, and the anal canal
  • Epithelium of the pineal gland and pituitary gland (structures of the brain)
  • Epithelium of the adrenal medulla
  • Cells of the neural crest

Mesoderm (middle layer)

  • Muscle (skeletal, smooth and cardiac tissues)
  • Urogenital structures (kidneys, ureters, gonads, and reproductive ducts)
  • Bone marrow
  • Blood
  • Fat cells
  • Cartilage (and other connective tissues)
  • Peritoneum

Endoderm (inner layer)

  • Epithelium of the digestive tract (except the mouth and anal canal)
  • Epithelium of the respiratory tract
  • Liver and pancreas
  • Thyroid, parathyroid, and thymus glands
  • Epithelium of the reproductive ducts and glands
  • Epithelium of the urethra and bladder

To form all of the tissues in a fully formed embryo requires the proper activation and inactivation of genes at specific times, cell-cell interactions, and interactions between cells and their enironments. At the links in the outline below, you can find several signalling mechanisms governing the movement and differentiation of tissues in a mammalian embryo.

More information about the study of embryo development, or embryology can be found on the National Institutes of Health, or NIH, website. This webpage by the NIH includes several chapters devoted to regenerative medicine including pages devoted to mouse embryonic stem cell research, identification of stem cells, and principles of gene therapy. Below is a link to a tutorial that uses SEM images to visually demonstrate different stages in mammalian development.

Embryo Images Normal and Abnormal Mammalian Development

Website of SEM Images of Mammalian Developmental Biology

Somiteogenesis

Somiteogenesis is an important process which forms somites in the developing embryo. This is crucial for the proper development of the spine. When disruptions occur during somiteogenesis, then the embryo can portray undetectable to severe forms of scoliosis.

 

Many genes have been identified as casual for scoliosis; however, most cases of scoliosis are not produced by these mutations through inheritance in offspring. The study by Sparrow et al ( 2012) determined the role of environmental factors as triggers of deformities of vertebral development in mice. They found candidate genes with disruptions of Notch expression which could be used to identify additional causal genes in humans.

Developmental Biology Interactive:

Outline of Mammalian Pages:

*  Embryology

*  Reproduction and Sex Determination

*  Stem Cells

*  Digit and Limb Differentiation

*  Skin System

*  Immune System Development

*  Brain Development

*Somiteogenesis

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References:

Beddington, R. S. P. and Robertson, E. J. Axis development and early asymmetry in mammals. 1999. Cell, 96, 195-209.

Graham, A. Mammalian development: New trick for an old dog. 2000. Current Biology, 10, R401-R403.

Marikawa, Y. Wnt/Βeta-catenin signaling and body plan formation in mouse embryos. 2006. Seminars in Cell & Developmental Biology, 17, 175-184.

Rivera-Pérez, J. A., and Magnuson, T. Primitive streak formation in mice is preceded by localized activation of Brachyury and Wnt3. 2005. Developmental Biology, 288, 363-371.

http://stemcells.nih.gov/info/scireport/pages/appendixa.aspx


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