Origami Embryo, Page 4
The Origami Embryo
The figure above is a scanning electron micrograph of a young embryo. The head is to the right. I removed the ectoderm over the back (dorsal side). A long cylinder at the midline of the embryo is the neural tube, the precursor of the spinal cord and brain; it is opened at the brain to show that the brain is mostly empty space at this early stage. In the trunk at each side (lateral to the neural tube) are rows of somites, blocks of mesodermal tissue that will form all the skeletal muscle of the body, the vertebrae and ribs, and the dermis of the back. The heart is the round structure ventrally, near the head. At this stage, the heart is still outside the body, since the body walls have yet to fold ventrally to form the gut tube and enclose the body. The beginning of body wall folds are visible dorsal to the heart, extending back toward the posterior (tail end).
© Kathryn Tosney
Not only do you get a take-home 3-dimensional model of an embryo that illustrates the basic human body plan that will guide our understanding of anatomical structure for the rest of the semester….but many see the way origami is done—a sequential series of simple instructions that eventually produce an elaborate final product as a good analogy for how genes interact with the tissues during the course of development…
Materials:
· Construction Paper
o 1 sheet blue (ectoderm)
o 2 sheets red (endoderm
o 1 sheet yellow (endoderm)
· Scissors, Scotch tape, Stapler
·
Transverse section through chick.
Step 1. The 3-layer embryo (the primary germ layers)
Insert two sheets of red paper (representing mesoderm) between a superficial blue sheet (ectoderm) and an underlying yellow sheet (endoderm). The two red sheets should be cut down and the middle and offset slightly to each side leaving a slight gap right down the middle (the mid-sagittal or median line). Remember that these colors are used universally by embryologists to track the eventual adult structures that derive from each of these “primary germ layers.”
Staple all four sheets together with two rows of staples, each row slightly off to one side as shown in the figure. Leave enough space down the middle to insert a pencil.
Step 2. The notochord
Push a pencil between the ectoderm and endoderm right down the mid-sagittal line between the two rows of staples. This represents the notochord (formed as Hensen’s node migrates down the long axis of the embryo, just as you are pushing in your pencil).
Remember that the notochord is a fluid-filled rod that forms early during the development of all vertebrates (fish, amphibians, reptiles, birds, mammals). It defines the dorsal long axis of the body and gets filled in with segmental pieces of bone to form the vertebral bodies.
Step 3. The neural tube.
The notochord stimulates the ectoderm (blue layer) to fold up and over into a tube. Make the neural tube and tape the folds together as shown in the picture.
The neural tube goes on to form the brain and spinal cord in the adult, situated along the dorsal midline (protected by the vertebral arches and braincase).
Step 4. The mesoderm.
Pinch or “scrunch” up the topmost pink sheet just lateral to the neural tube (as shown in the picture.) Then staple all four sheets down the side to hold in place.
This shows how the mesoderm differentiates:
Segmented somites: scrunched pieces just lateral to neural tube
Segmented intermediate mesoderm: staples
Flat sheets of lateral mesoderm:
somatic/parietal part—top sheet
visceral/splanchnic part—bottom sheet
The mesoderm goes on to form most of the connective tissues and muscles of the body, as well as the urogenital organs (intermediate mesoderm forms kidneys, gonads, etc.)
Step 5. The Inner Gut Tube
Leave the ectoderm (blue) and parietal mesoderm (top red sheet) loose on top. Fold the endoderm (yellow) and visceral mesoderm (bottom red sheet) into a tight tube that represents the gut tube. Staple it with two rows of staples along the bottom as indicated and. Then cut the staples back towards the middle leaving only a stalk that represents the yolk stalk and where the umbilical cord will form.
This is now the inner tube or gut tube of the body that will form most of the internal organs including the entire digestive tract. The opening at the front would represent the mouth and the opening at the rear the anus. Note that all of these organs have an internal endodermally derived lining with a mesodermal layer of muscle and connective tissue over that.
Step 6. The Outer or Parietal Tube
Now fold the ectoderm (blue sheet) and underlying parietal mesoderm (red sheet) into a larger tube that encompasses the gut, but leaving a space (celom) between the two tubes. Staple along the bottom and cut back to the yolk stalk/umbilical cord.
This outer tube (what we call the parietal or somatic part of the body) goes on to form the skin and underlying muscles, bones, connective tissues and skeletal muscles. The ectoderm principally forms the epidermal layer of the skin while most of the other tissues are formed by the underlying parietal mesoderm. The entire celomic space (or body cavity) is lined by a mesodermally derived layer called the peritoneum (in the abdomen) and pleura (in the chest). Note these linings of the body when you cut into your fetal pig or cat!!
Can you go back to the scanning electron micrographs on page 1 and label ectoderm, endoderm, mesoderm, neural tube, notochord?