This review describes the evolutionary history of the mammalian skull vault as a basis for understanding its complex structure. Current information on the developmental tissue origins of the skull vault bones (mesoderm and neural crest) is assessed for mammals and other tetrapods. This information is discussed in the context of evolutionary changes in the proportions of the skull vault bones at the sarcopterygian-tetrapod transition. The dual tissue origin of the skull vault is considered in relation to the molecular mechanisms underlying osteogenic cell proliferation and differentiation in the sutural growth centres and in the proportionate contributions of different sutures to skull growth.

Mammalian Langerhans cells (LC) are epidermal dendritic cells which originate in bone marrow and migrate toward the T cell area of lymph nodes, where they act as professional antigen-presenting cells. A variety of cell surface markers, such as the ectoenzyme adenosine triphosphatase (ATPase), Ia and CD1a antigens, have been used extensively to identify LC. Ultrastructural identification of this cell type in the mammalian epidermis is made by the demonstration of a typical and unique cytoplasmic organelle, the Birbeck granule (BG). Although we had earlier demonstrated the coexpression of ATPase and Ia antigens on epidermal dendritic cells of the chicken epidermis, the presence of the BG has not previously been documented. The aim of the present study was to investigate whether chicken epidermal LC-like cells possess an organelle similar to the BG, and thus to complete their identification. Our findings are the first demonstration of characteristic rod-shaped, racket-shaped and disc-shaped intracytoplasmic organelles, morphologically similar to the mammalian BG, in avian LC.

A separate supernumerary muscle in the lateral cubital fossa originating from the humerus or brachioradialis and inserting into the radius, pronator teres or supinator muscle has been considered as a variation of the brachioradialis muscle (Dawson, 1822; Meckel, 1823; Lauth, 1830; Halbertsma, 1864; Gruber, 1868b; Testut, 1884; LeDouble, 1897; Spinner & Spinner, 1996). However, a similar description was used to report additional heads of the brachialis or biceps brachii muscles (Gruber, 1848; Wood, 1864, 1868; Macalister, 1864–66, 1966–69, 1875; Gruber, 1868a; Wolff-Heidegger, 1937).

The innervation of these variant muscles would be a good tool to assign each variation to its associated muscle. Consequently, innervation by the radial nerve would indicate that it is a derivative of the humero–radialis group of muscles, while innervation by the musculocutaneous nerve would support it as a derivative of the anterior musculature of the arm (Rolleston, 1887; Lewis, 1989). However, no references to the innervation were found in the available literature.

Therefore this study set out to establish the phylogenetic origin of the brachioradialis accessorius muscle and, with the help of its innervation, to determine its incidence and unreported detailed morphology.

According to the usual description in most anatomy texts, the median nerve in the forearm passes between the 2 heads of pronator teres. It continues distally between flexor digitorum superficialis and profundus almost to the retinaculum. Muscular branches leave the nerve near the elbow and supply all superficial muscles of the anterior part of the forearm except flexor carpi ulnaris. Many variations of the median nerve in the forearm have been reported (Urban & Krosman, 1992). The palmaris profundus is also a rare anomaly of the forearm (Dyreby & Engber, 1982). It originates from the radial side of the common flexor tendon in the proximal forearm and inserts into the undersurface of the palmar aponeurosis. The origin of palmaris profundus may be close to the median nerve and its branches, and may be involved in compressive neuropathy of the anterior interosseous nerve. Its tendon crossing through the carpal canal has been implicated in the carpal tunnel syndrome (reviewed by Lahey & Aulicino, 1986). In some cases, palmaris profundus was found enclosed in a common fascial sheath with the median nerve (Stark, 1992; Sahinoglu et al. 1994). To indicate its close association with the median nerve, the palmaris profundus was also named ‘musculus comitans nervi mediani’ (Sahinoglu et al. 1994). This article reports an unusual loop of the median nerve encircling an anomalous palmaris profundus in the forearm, which, to the best of our knowledge, has not been previously described.

The abdominal surgeon is largely unappreciative of the peritoneal reflections of the ileocaecal region. This is not surprising since the literature on this subject is sparse. The available descriptions in standard texts (Hollinshead, 1971; McMinn, 1994; Williams et al. 1995) outline the appendicular mesentery as developing from the lowest aspect of the ileal mesentery. The inferior ileocaecal recess develops between the mesentery of the appendix and the ileocaecal fold; the superior ileocaecal recess develops between the ileal mesentery and the vascular fold of the caecum. The clinical significance of these recesses is minimal as they neither predispose to entrapment of bowel nor serve as useful landmarks. In this context, the peritoneal flange on the anti-mesenteric aspect of the terminal ileum warrants consideration (Fig. 1). This extension of the ileocaecal fold along a variable distance on the antimesenteric terminal ileum is largely underappreciated as it may hold value as a useful landmark to the abdominal surgeon.

Teeth as a feeding mechanism in an oral cavity (mouth) are functionally and locationally linked with jaws. In fossils, teeth found in the oral cavity are usually linked with jaws, although mineralised structures with the same histology as teeth are known in fossils before jaws appeared. Denticles in the skin occur in both fossil and extant fish. Pharyngeal denticles also occur in both extant and fossil gnathostomes but in only a few fossil agnathans (thelodonts). Complex structures with dentine and enamel have been described in the earliest jawless vertebrates, conodonts. Such fossils have been used to suggest that teeth and jaws have evolved and developed independently. Our understanding of the developmental biology of mammalian tooth development has increased greatly in the last few years to a point where we now understand some of the basic genetic interactions controlling tooth initiation, morphogenesis and patterning. The aim of this review is to see what this developmental information can reveal about evolution of the dentition.

The sternalis muscle (musculus sternalis) is the name usually given to this common anatomical variant, but the terms ‘episternalis’, ‘presternalis’, ‘sternalis brutorum’, ‘rectus thoracis’, ‘rectus sterni’, ‘superficial rectus abdominis’ and ‘japonicus’ have also been used in the literature (for reviews see Le Double, 1879; Calori, 1888; Pichler, 1911; Blees, 1968). According to Turner (1867), Cabrolius was the first, in 1604, to describe sternalis. Nevertheless this muscle is often unknown even in clinical practice (Bailey & Tzarnas, 1999; Vandeweyer, 1999).

Thus far, investigations on the incidence of sternalis have been made both in large populations such as the American (Barlow, 1935) and small populations, for example in Taiwan (Shen et al. 1992; Jeng & Su, 1998). In Europe, all studies on the frequency of this muscle have been made amongst subpopulations in Western (e.g. Cunningham, 1888; Le Double, 1890, 1897) and Northern Europe (Gruber, 1860) although the reported frequencies have been quite different. There is a lack of information about sternalis in Eastern European populations. We therefore present data from a study on the incidence of sternalis muscle in Bulgaria.

Although the 3 genera of living lungfish have different-shaped adult tooth plates, their larval stages have similar patterns of development. The sequence in the pattern of initiation of teeth and their modification through ontogeny in Neoceratodus hatchlings provides a developmental model for fossil hatchling tooth plates (smallest 1–2 mm) recovered as 3-dimensional dentitions from Andreyevichthys. This Late Devonian lungfish demonstrates that these also have a similar dentition pattern and suggests strongly conserved developmental processes. We postulate that a specific pattern of development, derived within lungfish, has been conserved in extant forms through evolution from the earliest known lungfish. The most basal early dipnoan, Diabolepis speratus, is also known from juveniles with tooth plates formed in this pattern.

The lungfish pattern is in marked contrast to the typical linear rows of teeth with lingual replacement for each tooth position, characteristic of most osteichthyan and chondrichthyan dentitions. Uniquely for lungfish, teeth are only added to the lateral ends of the radial rows in the palatal and lingual dentition and are consolidated into dental plates without loss through shedding. It is proposed that this tooth pattern is set up from primordial teeth at the patterning stage of the dentition, one in each dentate region of the larval jaws. Although in post-Devonian lungfish marginal dentate bones are absent in the adult, in both the fossil and extant hatchling, teeth are present and function on some of the marginal bones. This pattern of development and loss is described and we conclude that in both forms it is also based on a radial pattern of successive tooth initiation. We propose that this ontogenetic pattern constrained the phylogenetic pattern of adult form, through evolution of dipnoan dentitions from 360 MYBP until the present. The universality amongst dipnoans and the implications for such a conserved constraint in the developmental module for the dentition is discussed.

Vertebrate somitogenesis has been shown to be associated with a molecular oscillator, the segmentation clock, whose periodicity matches that of the process of somitogenesis. The existence of such a clock in presomitic mesoderm (PSM) cells was originally proposed in theoretical models such as the ‘clock and wavefront’. Molecular evidence for the existence of this clock in vertebrates has been obtained on the basis of the periodic expression of several genes, most of which are related to the Notch signalling pathway. These genes are expressed in a dynamic sequence which appears as a wave sweeping caudo-rostrally along the whole PSM once during each somite formation. Notch-pathway mouse and fish mutants lose the dynamic expression of the cycling genes, indicating that Notch signalling is required for their periodic expression, or is required to coordinate the oscillations between PSM cells. Therefore Notch signalling is either part of the mechanism of the oscillator itself or acts as a cofactor required for cycling gene expression. A further potentially important role for the segmentation clock is to periodically activate Notch signalling in the rostral presomitic mesoderm, thereby generating the periodic formation of somite boundaries.

Aodán Breathnach, Emeritus Professor of Anatomy at the University of London at St Mary's Hospital Medical School, and Honorary Senior Research Fellow Division of Physiology and Institute of Dermatology, UMDS, St Thomas' Hospital, died in London after a prolonged illness on 17 April 2000, at the age of 77. He had been a member of the Anatomical Society of Great Britain and Ireland since 1949, of the Societas Dermatologica Italica from 1978, of La Société Française de Dermatologie Syphiligraphie since 1979, and of the Society for Investigative Dermatology since 1984. He is survived by his wife Babs, his two sons Stephen and Richard, and their families, to whom our deepest sympathy is extended.

The backbone or vertebral column is the defining feature of vertebrates and is clearly metameric. Given that vertebrae arise from segmented paraxial mesoderm in the embryo, this metamerism is not surprising. Fate mapping studies in a variety of species have shown that ventromedial sclerotome cells of the differentiated somite contribute to the developing vertebrae and ribs. Nevertheless, extensive studies in amniote embryos have produced conflicting data on exactly how embryonic segments relate to those of the adult. To date, much attention has focused on the derivatives of the somites, while relatively little is known about the contribution of other tissues to the formation of the vertebral column. In particular, while it is clear that signals from the notochord induce and maintain proliferation of the sclerotome, and later promote chondrogenesis, the role of the notochord in vertebral segmentation has been largely overlooked. Here, we review the established role of the notochord in vertebral development, and suggest an additional role for the notochord in the segmental patterning of the vertebral column.