Chick Embryo Somites

Today I got to study some more chick embryos. In particular, I wanted to get a good look at the somites, or the paired blocks of mesoderm which later give rise to muscle, bone and cartilage. Below is a chick embryo I dissected.

~96 hour Chick Embryo with somites (right lateral view)

Along the future spine you can faintly make out segmented blocks. The oldest somites are nearest to the neck (anterior/rostral), with the newest somites forming near the caudal (lower) end of the embryo. New somites progressively displace the older somites in an anterior direction, or from the tail end towards the neck.

~96 hour Chick Embryo

Below you can see some of the somites, which adorn either side of the neural tube. They are circular-like structures which condense after going through the determination front. A unique combination of oscillating genes, and oppositing gradients create the "determination front," where new somites form as mesoderm epithelializes into balls. A Mesenchymal to Epithelial (EMT) transition occurs, wherein the cell's morphology begins to change characteristics, from migratory to sessile.

In other words, a tangle of mesenchymal cells come together and begin to organize, then adhere to one another's cell surface.  The outer cells of the "ball" will be epithlial in nature, adhering closely with each other to create a circular membrane, while the inner core of the somite remains a collection of disorderly mesenchymal cells.

Chick somites and neural tube (dorsal view)

The individual cells can radically transform their shape by changes in gene expression. The mesenchymal cells, which are long and spindly will change (through changes in gene expression) into cuboidal cells. The cells, through expressing different cell surface proteins, like Cadherins, will begin to stick together, self-organize and an apical-basal polarity will emerge.


Below is the posterior tailbud of the chick embryo and the pre-somitic mesoderm (PSM). The PSM results in a temporo-spatial series due to the relationship between newly created somites and the axis elongation.

Chick embryo somites and tail bud (dorsal view)

In the posterior tailbud is a population of progenitor stem cells proliferate amongst a high concentration of Wnt3a and FGF. The Wnt gradient starts to decrease away from the tailbud. When the progenitor stem cells begin to migrate out of the tailbud, they will begin to express different genes and to eventually differentiate, as they make their way into the pre-somitic mesoderm (PSM).

Chick pre-somitic mesoderm (PSM) and tailbud (right lateral view)

Below you can see the tailbud and somites (from bottom to top). The cells which make new somites will move from the end of the tailbud upwards, contributing to axis elongation. 

Somites are conventionally numbered from 'S negative' to 'S positive' numbers, with S0 being the somite that is currently budding off. S1 is the newest formed somite. S5, as pictured below is the oldest somite, and will continue to progress anteriorly as it is displaced by newly forming somities. Between S-1 and S0 is the determination front. 

Chick embryo somites and tailbud (right lateral view)

The determination front is named so because the mesoderm undergoes changes due to gene expression. The Wnt and Fgf gradients are at the highest concentration at the tailbud. From the opposite (rostral) side, Retinoic Acid (RA) is highest in concentration. At the determination front, all of these gradients are at the lowest concentration. Lack of Wnt results in a loss of progenitor properties and the cells are able to differentiate. The unique conditions of the determination front include low FGF, low WNT, and low RA.

 PSM

Although the migration of progenitor cells from the tailbud into the PSM contributes to axis elongation, note that even when mitotic activity is blocked (in the tailbud), that axis elongation still occurs (in zebrafish), suggesting that cellular reorganization (like intercalation) can also contribute to the lengthening of the axis.

I partially split the axis in half and tried to isolate a row of somites. This was incredibly difficult, as the somites are very small compared to the size of my forceps and scalpel.

A row of chick embryo somites

Below you can see about 3 somites I isolated. 

Chick embryo somite

All embryos were treated ethically according to IUCAC protocol.





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