We are members of the phylum "chordata", all of the members of which have a notochord at some stage of their development.|
In mammals and birds, the function of the notochord is replaced by the vertebral column (bones of the back-bone+intervertebral discs)
But tadpoles and larval fish actually depend on the notochord to allow them to swim by alternating contraction of muscles along each side.
The notochord mesoderm also induces formation of the neural tube.
(and it probably is also needed to elongate mammal embryos)
Anyway, our embryos can't develop without a notochord.
The structure of the notochord is a long stack of (not always) flattened cells, that often have very much the geometry of a stack of coins.
Wrapped tightly around the surface of this long cylinder of cells are layers of fibers of type I collagen in many spiraling layers, that alternate in direction (a clockwise layer, then a counterclockwise layer, then another clockwise layer, etc. etc. etc.)
The cells inside the notochord then form large vacuoles in their cytoplasm, which makes each cell swell in volume, pushing against the spiral layers of (strong & non-stretchable!) collagen fibers.
The result is that the notochord can bend side to side,
but can't be compressed along its anterior-posterior axis.
Tail of a living tadpole. The broad line just below the center
is the notochord, and you can see that it is filled with highly vacuolated
cells. The neural tube is above it. You can also see pigment cells and
mesenchymal cells scattered around.
Neural tube and notochord
Notochord of salamander
Section through chick embryo
Several odd facts about the notochord:
In reptiles, birds and mammals, the notochord cells are the last to leave the surface (epiblast) at the Hensen's Node.
* In some species (humans & turtles), Hensen's node is partly an invagination, & a hollow tube runs down the middle of the notochord
(this tube then disappears, as the cells push together)
* In sections of amphibian embryos, you can see pigment granules tend to accumulate down the middle of the notochord.
* The spiral layers of collagen fibers that are the sheath of the notochord are oriented at 90 degrees to each other.
This is one of several examples in which collagen fibers mysteriously become oriented in alternating layers, with the fibers of each layer
exactly perpendicular to the fibers on the layers above and below.
Also, there are many examples in which muscle cells become lined up in perpendicular alternating layers: Mammal tongues, elephant trunks,
squid tentacles, and in Hydra, both the body and the tentacles.
Prof Bill Kier of this department specializes in studying them.
Polarization micrograph of a section through a mouse tongue
Anybody who could figure out the mechanism that creates these perpendicular layer patterns, and prove their theory true, would go down in biological history.
Somites are segmental blocks that form beside the neural tube.
At first, there are continuous columns of "paraxial" mesoderm
The drawing above shows the geometrical rearrangements that some scientists have reported to casue somite segmentation in different classes of chordates. Top: Amphioxus, Middle: bird and mammal, Bottom: frogs. Anterior is to right side.
Then these columns spontaneously split apart, one pair at a time.
As if a quonset hut were to split into a row of igloos.
Or as if a long piece of french bread were to slice itself.
Longitudinal section of notochord and somites in a developing frog embryo
Somites forming in a fixed whole mount chicken embryo. Anterior to left.
Somites forming in a living chicken embryo. Anterior to left.
The physical mechanism of splitting apart could be active constriction, or could be decreases in cell-cell adhesion proteins.
(or could be the same as whatever forms feather papillae!)
In scanning EM photos, "somitomeres" can be seen, where a somite is going to form, apparently as the first stages of somite formation.
In most species, somites start as hollow epithelial balls,
and conversion of cells from being mesenchymal to epithelial
therefore seems to be part or the process of separation.
(These epithelial cells seem to have basal surfaces outward)
Much research has been done on the genes needed for somite formation
Suspected genes include "Notch", "Lunatic Fringe", and "Hairy1"
The most popular categories of theory are
1) That somite segmentation has the same mechanism as formation of segments in flies, using genes analogous to "even-skipped" etc.
2) The "Clock-and-Wavefront Hypothesis", according to which one quantity oscillates higher and lower in amount, while another variable forms a gradient that gradually increases in amount along its length.
A somite is supposed to be split off each time the oscillator increases.
Really, no one even knows whether we should think of the splits as being the real entities, as opposed to the somites themselves.
Links to papers on somite formation, including these theories
For most species, the same number of somites forms in each embryo.
(the number is 32-34 in humans, 50 in chicks, 65 in mice (many in tail)
more than 500 in some snakes, and varies with temperature in fish!
You form as many vertebrae as your embryo had formed pairs of somites.
Each somite subdivides into four parts:
The dermatome --> cells form the inner layer of skin (dermis)
The myotome ---> all the skeletal muscle cells of the body
The anterior scleroderm --> the posterior half of a vertebra!
The posterior scleroderm --> the anterior half of a vertebra!
Not a typographical error! Each somite gives cells to 2 vertebrae
And each vertebra develops from cells of two adjacent somites.
That way muscles run from one vertebra to the next,
and motor and sensory nerves come out between vertebrae.
On page 459 of the eighth edition of the Gilbert textbook, in figure 14.19, he advocates the existence of one more sub-part of somites, which he names the "Syndetome". Certain
researchers have postulated that such a thing exists, and that it is the cause of formation of tendons.
Gilbert admits you can't actually see this structure: "Because there is no obvious morphological distinction between the sclerotome and syndetome cells (they are both mesenchymal), our knowledge...had to wait until we had molecular markers...etc." What this amounts to is inventing a new name for a part of the somites based on discovery that messenger RNA for a certain protein is concentrated there. We all better hope there aren't too many more parts of somites where particular messenger RNAs can be detected, because if there are then somebody is going to invent names for every one of them, like Thisotome and Thatotome!
Somites only temporarily exist in the embryo:
but their spacing controls future segmentation of many tissues;
vertebrae, ribs, arterial branches
and also sensory ganglia (that develop from neural crest!!)
and also motor nerve bundles from the spinal cord.