Time period: days 20–35
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.
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.