Somites
Time period:
days 20–35
Mesoderm
In the
formation of the trilaminar disc we see the 3 layers of the cells of the embryo
becoming organised as ectoderm, mesoderm and endoderm (see Chapter 15). The
mesoderm layer is further organised into areas of paraxial mesoderm (medially),
interme- diate mesoderm and lateral mesoderm (laterally). These
areas of mesoderm will contribute to the formation of different structures (see
Figure 25.1).
A clumping
of cells and a thickening of the mesodermal layer on either side of the midline
of the embryo forms from paraxial mesoderm and gives the first pair of somitomeres.
Here we see the beginning of the characteristic segmentation of vertebrate
animals. In the cranial region, the first 7 somitomeres contribute to the
development of the musculature of the head, but the remaining somitomeres
become somites.
Somites are
cuboidal‐shaped condensations (groupings) of cells visible upon the surface of
the embryo (Figures 22.1 and 22.2). The organisation of cells here will give
rise to much of the axial musculoskeletal system and body wall of the embryo.
What signals
initiate somite formation? The answer to this is complex, but many signals come
from the overlying ectoderm. Notch signalling and Hox genes are certainly
involved here, amongst others (see Chapter 21).
The first
somite forms during day 20 and subsequent somites appear at a rate of 3 pairs a
day. Somites form in a cranial to caudal sequence, lying laterally to the
neural tube. By the end of week 5 a full complement of somites will have
formed, including 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and
8–10 coccygeal pairs. The number of visible somites is often used as a method
of dating (or staging) an embryo.
The first
occipital and last 5–7 coccygeal somites degenerate, so from the 42–44 pairs of
somites that form around 37 remain. The tightly packed cells of the remaining
somites develop a lumen in their centres, termed the somitocoele (Figure
22.3). The somitocoele cells are involved in many complex interactions
resulting in epithelialisation (layering) and polarity within the somite
(cells become organised).
The cells in
each somite differentiate and move to give ventral and dorsal groups of cells
called the sclerotome and dermomyotome, respectively (Figure
22.3).
The cells of
the ventromedial part of the somite form the sclerotome. When they lose their
tight bindings to one another they migrate to surround the notochord.
These cells
will form the vertebrae, the intervertebral discs, the ribs and connective
tissues (Figure 22.4). The caudal part of the sclerotome of one somite and the
dorsal part of its neighbouring somite’s sclerotome combine to form a single
vertebral bone (see Chapter 24).
The word
sclerotome is formed from the Greek words skleros, meaning ‘hard’, and tome,
meaning ‘a cutting’. Cells from the sclerotome form hard structures of the
axial skeleton.
A specific
dorsolateral region in the sclerotome has relatively recently been shown to
form the origins of tendons, termed the syndetome (see Chapter 25).
The
dermomyotome mass of cells in the dorsolateral part of the somite splits again
into 2 more groups: the myotome and the dermotome (Figure 22.3). The cells of
the myotome will become myoblasts and form the skeletal muscle of the body
wall.
Medially
positioned cells within the myotome form the epaxial muscles intrinsic
to the back (e.g. erector spinae). Lateral cells will form the hypaxial muscles
(the muscles of the ventrolateral body wall such as the intercostal muscles and
the abdominal oblique and transverse muscles). Laterally placed cells will also
migrate out to the limb buds and form the musculature of the limbs (Figure
22.4).
This is
covered in a little more detail in Chapters 25 and 26.
The other
part of the dermomyotome, the dermotome, is the most dorsal group of cells
within the somite. These cells will contribute to the dermis and subcutaneous
tissue of the skin of the neck and trunk (Figure 22.4).
The
integumentary system receives contributions from a variety of sources. The
epidermis, nails, hair and glands develop from ectoderm, the dermis (connective
tissue and blood vessels) develop from mesoderm and the dermotome, and
pigmented cells (melanocytes) differentiate from migrating neural crest cells.
It is
important to note that cell groups retain their innervation from their segment
of origin, no matter where the migrating cells end up. A spinal nerve develops
at the level of each somite and will comprise a collection of sensory and motor
axons.
The groups
of cells within each myotome and dermotome will migrate to their final
destinations trailing the axons of these neurons in their paths. In the adult
clear patterns of innervation segmenta- tion remain, commonly seen by medical
students in dermatome maps (Figure 22.5).
Not to be
confused with dermotomes, a dermatome is a region
of skin that is predominantly supplied by the sensory component of one spinal
nerve (Figure 22.5). The dermatomes are named according to the spinal nerve
that supplies them. In diagrams the dermatomes are shown as very specific
areas, but in reality there is significant overlap between dermatomes. Although
sensation may be affected by nerve damage it may not completely numb the area.
Also be aware that the overlap between dermatomes varies for the sensations of
tempera- ture, pain and touch.
Clinical relevance
The varicella
zoster virus that causes chickenpox can lie dormant in dorsal root ganglia
after the patient has recovered. Later in life the virus may follow the pathway
of a spinal nerve to travel to the skin, causing shingles (herpes
zoster; Figure 22.6). It manifests visibly as a rash restricted to a single
dermatome, amongst other symptoms. Sometimes, starkly delineated rashes show
the shape of the dermatome derived from a single somite’s dermotome.
By testing
for a loss of sensation in particular dermatomes your knowledge of somitic
embryology can also be used to find clues to help identify the level of spinal
cord damage in a patient to determine whether specific spinal nerves have been
injured.
