The non-homologous embryonic formation of vertebrae

It is well known that at a relatively early stage in embryonic development, the embryos of different types of vertebrate look quite similar – what is known as the vertebrate phylotypic stage – and this is seen as evidence of their common ancestry. However, contrary to the evolutionary explanation for this similarity, the vertebrate 'phylotypic stages' arise in very diverse ways, and this clearly refutes their common ancestry.

Vertebrae form after what is generally considered to be the phylotypic stage. What is remarkable is that despite the similarities of the (preceding) phylotypic stage and the obvious similarities of the (subsequent) vertebrae, these too form embryonically in very different ways, and hence are not homologous in an evolutionary sense. This is especially significant because vertebrae are of course the primary defining feature of vertebrates, yet even these are not consistent with evolution from a common ancestor.

Tetrapods (amphibians, reptiles, birds, mammals)

Early in the development of a tetrapod embryo a neural tube forms along the embryo’s back – this will develop into the spinal cord. Running alongside (ventral) this is the notochord which is a supporting structure in the early embryo and is the focus for formation of the spinal column. However, in tetrapods, the vertebrae do not form from the notochord itself; rather, these develop from somites which arise in pairs, either side of and along the length of the neural tube. So at an early stage of development we have the situation illustrated in Figure 1a.

In most tetrapods the cells of the somites differentiate into three distinct layers: on the outside is dermatome which will develop into connective tissue, then myotome which will develop into muscle, and innermost is sclerotome which develops into the vertebrae.

The sclerotome cells grow and migrate towards the notochord, cells from both sides of the neural tube, combining and surrounding the notochord to form what is called the perichordal tube (Fig. 1b); this then develops into cartilaginous vertebrae which subsequently ossify to give the bony vertebrae.

The notochord all but disappears: some contributes to the intervertebral discs, but none to the vertebrae themselves.

Resegmentation

A notable and somewhat unexpected feature of vertebrae formation in tetrapods is resegmentation. As the sclerotome cells move towards and envelop the notochord, the posterior (tail-ward) end of one combines with the anterior (head-ward) end of the adjacent one (Fig. 2b), and at the same time an intervertebral disc forms within each perichordal ring (Fig. 2c). The result is that each vertebra is formed from the posterior half of one perichordal ring and the anterior half of the adjacent one. That is, the vertebrae do not arise from the somites one-for-one, but each is derived from adjacent halves of neighbouring somites, illustrated in Fig. 2.

Chondrichthyans (cartilaginous fish)

Formation of vertebrae is somewhat different in cartilaginous fish such as sharks. In these sclerotome cells migrate towards the notochord, but rather than surrounding it they form into blocks of cartilaginous tissue called arcualia. Typically, each pair of somites produces four pairs of arcualia, with one of each pair on either side of the neural cord / notochord. The arcualia then transform into a vertebra, with each of the arcualia destined to become a specific part of it (see Fig. 3).

The differences from the tetrapods are so substantial that Earnest Williams concluded:

Thus there seems to be no simple way of comparing shark vertebrae and tetrapod vertebrae; there exists indeed a reasonable doubt that they are built to the same plan.[1]

Teleosts (most fish)

A completely different development route is used in teleosts. In these the innermost part of the vertebrae arises directly from the notochord (i.e. not from sclerotomes, although these contribute to outer parts of the vertebrae), see Figure 4. Nordvick gives a recent and detailed account.[2]

There are four distinct layers:

1. The development of the vertebral bodies is initiated within the notochord: within its sheath cartilaginous elements arise and mineralise to form the inner layer of each vertebra.
2. The notochord is then encased by a layer of collagen fibres; this extends the whole length of the notochord, becoming thickest between the vertebrae where it contributes to the intervertebral ligaments, whereas the vertebral regions become mineralised.
3. Outside of this, a layer of dense bone is deposited.

As the preceding steps proceed, undifferentiated cells (mesenchyme) on top and bottom of the notochord (dorsal and ventral) begin to form cartilaginous arch centres.

4. Sclerotome-derived cells deposit bone onto the outside of the inner three layers (and enclosing the arch centres).

Features that distinguish this from the formation of vertebrae in tetrapods and cartilaginous fish are: the role and persistence of the notochord which forms a central part of the vertebrae; the four distinct layers of bone of which the inner two form by mineralisation of a collagen matrix, and the outer two layers form by direct ossification (directly as bone), i.e. no part is preformed as cartilage; and there is no resegmentation.

Go to Vertebrae in the Overview of diverse early embryonic development of vertebrates.

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Page created 2017, revised October 2020.