Intra-embryonic mesoderm has been described in at least one specimen e.g. Carnegie 7634 (Krafka) where it was thought to derive from extraembryonic mesoderm present between the endoderm and ectoderm at the lateral edges of the embryonic disk. However it is not known to what extent this is incorporated into definitive body mesoderm. This is distinct from the intra-embryonic mesoderm that originates via the process of gastrulation. During gastrulation, surface ectoderm migrates into the primitive groove and separates the overlying ectoderm from the underlying endoderm by an intermediate (ectoderm derived ) layer of cells, the intra-embryonic mesoderm which is later fully incorporated into the embryo.
The initial appearance of the primitive streak is the defining criteria for stage 6b. Brewer`s (1938) definition of a primitive streak is active proliferation of the cells, loss of the basement membrane separating the epiblast and subjacent endoderm, migration of epiblastic cells and intermingling of the epiblast and endoderm cells. The primitive groove and node also appear at approximately this stage. O'Rahilly R., Müller F. (1987) p.45
By this stage the mesoderm derived from gastrulation (mesenchyme) is beginning to form distinct bilateral condensations lengthways along the embryo, the cephalic end being most advanced. On either side of the neural tube, paraxial mesenchyme forms somites and laterally the mesenchyme forms a condensation that, with the appearance of the coelom, is divided into two layers; the splanchnopleure (splanchic mesenchyme) and somatopleure (somatic mesenchyme) which line the intra-embryonic coelom. The splanchnopleure in the future cardiac area forms the cardiogenic plate
The appearance of somites, paired wedge-shaped segmentationís in the paraxial mesenchyme on either side of the notochord, is the defining criteria of this stage. 1-3 pairs are distinguishable at the level of the rhombencephalon although the number may differ on the two sides of the body. Some somites possess cavities within them called myocoeles
It is difficult to estimate the somite count from study of histological sections, therefore, somite number in an embryo, as an indication of age, must be treated with caution.
Some mesoderm derived from the primitive streak forms the caudal eminence. It is situated caudal to the neurenteric canal in the tail bud region. Not described by any authors at this stage except Müller F., O'Rahilly R., (1983). It is the site of secondary neuralation and is illustrated in Larsen W.J.,(1993) pp.73-74 and figs 4.11, 3.10
The defining feature of this stage is the appearance of somitic pairs 4-12.
Differentiation of the somites has begun and sclerotomal cells can be seen migrating from the ventromedial side of the somites in Carnegie Embryo No. 5074. Corner G.W.(1929) Plate 6, Fig 23-26.
Intermediate mesenchyme (mesenchyme between the somites and the lateral plate mesenchyme) becomes apparent.
The first indication of the urinary system appears at about 8 paired somites when the intermediate mesenchyme at the level of the first two somites differentiates into discrete vesicular bodies enclosing cavities which communicate (via a primitive pronephric duct) with the coelom. These are interpreted as being nephrotomes enclosing nephrocoeles. e.g. Carnegie Embryo No. 6330.
By the 10 somite stage, a continuous, solid, pronephric ridge has developed in the dorsal mesenchyme in the region of the 8th and 9th somitic pairs. See Carnegie Embryo No. 5074 cited in Torrey T.W. (1954)
The defining characteristic of this stage is the development of somites 13-20. In the 20-somite embryo, Carnegie Embryo No. 2053, myocoeles are present from the 4 to the 16th somite. The somites has differentiated into myotome, dermamyotome and sclerotome up to the 10th somite and somite 13 shows differentiation of sclerotome.
The intermediate mesenchyme is differentiating into the urogenital system, which consists of the mesonephric duct, connected to mesonephric vesicles via tubules in some cases and a cord of nephrogenic tissue that will form vesicles in a rostrocaudal direction.
The defining characteristic of this stage is the development of somites 21-29. Myotome cells can be distinguished as cuboidal and eosinophilic.
Somite 1 is being transformed into loose mesenchyme but may retain the dermatome. None of the occipital somites possess a myocoele and their myotome has not yet migrated around the notochord.
An apparent ventromedial rotation of the somites takes place between stages 11 and 12. O'Rahilly R., Müller F., (1984)
The maxillomandibular condensation of cells appears in stage 11 but is recognizably compact in stage 12. Cranially it lies next to the dorsocaudal surface of the optic vesicle. and ventrally it is continuous with the mesenchyme of the mandibular arch. Gilbert P.W., (1957)
The defining characteristic of this stage is the presence of 30+ somites. The highest number recorded at this stage is 39 according to Müller F., O'Rahilly R., (1988b)
Four somites are occipital, eight cervical, twelve thoracic, five lumbar and a further eight to ten coccygeal somites will differentiate within the caudal eminence. However it is difficult to accurately count somites by the analysis of histological sections alone.
The first somite has virtually disappeared, four occipital myotomes being present. The epithelioid character of the dermatomes of the occipital somites is lost and the sclerotome has formed a continuous column on each side. O'Rahilly R., Müller F., (1984)
The maxillomandibular mesenchyme gives rise to a pre-muscle condensation recognized as the primordial lateral rectus in stage 13. At stage 14 another condensation appears, the primordial superior oblique. Gilbert P.W., (1957)
By stage 15 the embryonic mesoderm that formed from the primitive streak has almost all differentiated into a more definitive cell type.
The splanchnopleuric division of the lateral mesenchyme has differentiated into mesentery, visceral pleura, peritoneum and the heart wall. It also contributes cells to the adrenal cortex, and mesenchyme to the stroma of the gonadal ridges, connective tissue and smooth muscle of viscera and blood vessels as well as forming hemangioblastic tissue.
The intermediate mesenchyme first formed the mesonephros, and now contributes to the metanephros.
The paraxial mesenchyme which formed somites is still distinguishable as somites in the caudal most areas. Rostrally the somites have differentiated into sclerotome which migrates medially to form the axial skeleton, myotome which forms the muscular plates and appears to contribute to the limbs and finally dermatome which will form the connective layers of the skin. Gilbert S.F., (1994) p324
Eight cervical, twelve thoracic, five lumbar, five sacral and 5-6 coccygeal somites are present. O'Rahilly R., Müller F., (1984)
At this stage the cells derived from the sclerotome of the somites have migrated around the notochord and condensed in the area between the neural tube and the aorta and also projections into the body wall area. These condensations are the primordial intervertebral discs, the future arch neural arch and costal processes of the vertebra. The condensation only occurs in the caudal half of each segment. e.g. Carnegie Embryo No. 2
During this stage the maxillomandibular condensation extends and 'arm' of cells forward over the optic vesicle with which the expansion of the latter, is carried laterally and cranially. The caudal and medial portion then condenses and is thought to be the primordium of the lateral rectus. Gilbert P.W., (1957)