Circumventing the polydactyly 'constraint': the mole's 'thumb'.

Talpid moles across all northern continents exhibit a remarkably large, sickle-like radial sesamoid bone anterior to their five digits, always coupled with a smaller tibial sesamoid bone. A possible developmental mechanism behind this phenomenon was revealed using molecular markers during limb development in the Iberian mole (Talpa occidentalis) and a shrew (Cryptotis parva), as shrews represent the closest relatives of moles but do not show these conspicuous elements. The mole's radial sesamoid develops later than true digits, as shown by Sox9, and extends into the digit area, developing in relation to an Msx2-domain at the anterior border of the digital plate. Fgf8 expression, marking the apical ectodermal ridge, is comparable in both species. Developmental peculiarities facilitated the inclusion of the mole's radial sesamoid into the digit series; talpid moles circumvent the almost universal pentadactyly constraint by recruiting wrist sesamoids into their digital region using a novel developmental pathway and timing.

The spatio-temporal pattern of testis organogenesis in mammals - insights from the mole.

Some cellular events are crucial in testis organogenesis, including Sertoli and Leydig cell differentiation, mesonephric cell migration and testis cord formation. These processes are controlled by transcription factors, paracrine signalling and hormones. Using the mole species Talpa occidentalis as an alternative animal model, we report the expression patterns of nine genes during testis differentiation and analyse their implications in the above-mentioned cellular processes. We show that: 1) Sertoli cell differentiation occurs very early and precedes mesonephric cell migration, indicating that the latter is not needed for the endocrine cytodifferentiation of Sertoli cells; 2) the time of Leydig cell differentiation is consistent with the participation of PDGFR-alpha in promoting the migration and/or proliferation of Leydig cell precursors, and with that of WNT4 signalling in inhibiting Leydig cell differentiation and 3) the formation of the tunica albuginea involves intragonadal cell migration/movement. These results demonstrate that testicular organogenesis in the mole differs from that in the mouse in some particular aspects, thus providing evidence that the spatio-temporal pattern of testis development is not highly conserved during mammalian evolution.

Meiosis onset is postponed to postnatal stages during ovotestis development in female moles.

In mammals, germ cells are important both during development and for the function of female gonads, whereas male gonads may develop in the absence of germ cells. The gonads of female moles (genus Talpa) develop according to a testis-like pattern which results in the formation of ovotestes. In this paper, we studied the expression pattern of several pre-meiotic and meiotic germ cell markers, in order to establish the precise time of meiosis onset in the mole species T. occidentalis, and to investigate the location and possible role of germ cells in ovotestis organogenesis. Our results evidenced that: (1) the asymmetrical distribution of primordial germ cells, which concentrate in the cortex of the XX gonad, is brought about by germ cell depletion from the medulla between the s5a and s5b stages, (2) XX germ cells enter meiosis postnatally, which is quite exceptional among eutherian mammals, and (3) XX but not XY germ cells of moles express DMRT1 during premeiotic stages of development, an expression pattern not described previously in vertebrates.

Early skeletal development in Talpa europaea, the common European mole.

An ontogenetic series of 22 cleared and double-stained prenatal specimens was used to study the sequence of ossification of selected postcranial skeletal elements of Talpa europaea. Results were compared with nine other therian mammals, with Alligator, Chelydra, and Lacerta as outgroups. Using the event-pairing method, shifts in the onset of ossification in T. europaea, Sus, and Homo were identified. In T. europaea, the ossification of the cervical vertebrae starts before the metatarsals. In Homo and Sus, the tarsals ossify before the pubic bone. These shifts in the sequence of ossification are unique among the mammals examined, whereas many other changes, characterising monophyletic groups and/or evolving convergently, were also identified. Particular attention was given to some peculiar calcified elements of the hand in T. europaea, which were identified as accessory ;sesamoid bones', and do not display a chondrified precursor. They start to calcify before all others of the hand and later fuse. They appear in all fingers and function as reinforcement for the distal phalanges, most likely as an adaptation for burrowing. The development of the sesamoid bones was examined using histological sections and macerated adults.

The mole's thumb -- evolution of the hand skeleton in talpids (Mammalia).

Osteological specimens representing 15 out of the 16 currently recognized talpid genera were examined and scored for seven discrete morphological characters of the hand. The phylogenetic distribution of these characters was studied in the context of alternative hypotheses of talpid relationships, using three species of shrews and a hedgehog as outgroups. All talpids show a similar number and arrangement of carpal bones. The most obvious differences concern the presence of additional sesamoid bones, the relative size of the os falciforme when present, and the degree of fusion of the scaphoid and lunate in the proximal carpal row. Marked differences in the relative length and proportions of the metacarpals also exist. The development of the carpals in Talpa europaea was studied through examination of histological sections of the hand of an embryo and a neonate. Whereas carpal anatomy in the neonate mirrors the arrangement and proportions of the adult, in the embryo the scaphoid and lunate are still separate, there are no signs of the os falciforme, and the size proportions of metacarpals to carpals are obviously different to those of the adult. A prehallux or tibial sesamoid, serial homologue to the os falciforme or prepollex (a radial sesamoid), does not have an obvious functional role, and its presence might be the result of a common epigenetic control in the hand and the foot resulting in a non-adaptive structure in the latter.

Testis-like development of gonads in female moles. New insights on mammalian gonad organogenesis.

Moles are unique among mammals because all females of several species of genus Talpa have bilateral ovotestes (gonads with both ovarian and testicular tissue). Based on the analysis of a large sample of embryos, foetuses and infants over a 13-year period, we have studied the development of the gonads in male and female moles of the species Talpa occidentalis. Several new field and laboratory procedures were developed specifically to obtain and manage this singular material. Our results reveal that gonads of female moles develop according to a testis-like pattern, which includes cord formation and mesonephric cell migration, and begins at the same time as testis differentiation in males. The first signs of sex differentiation do not appear in males but in females. Female (but not male) gonads are regionalised with a cortex (precursor of the ovarian tissue) and a medulla (precursor of the testicular tissue). Germ cells concentrate only in the cortex, so that the medulla soon becomes sterile. Testicular tissue development is transiently retarded in females for about a week before birth, and resumes afterwards. Development of the ovarian tissue in females is considerably delayed with respect to that of testicular tissue in both males and females. The molecular characterisation of peritubular myoid cells, which are exclusive of testes, evidences the presence of testicular tissue in the gonads of female moles, which also contain Leydig cells. However, the absence of fully differentiated Sertoli cells indicates that these cells are not responsible for triggering the differentiation of such a testicular tissue. Our results are also discussed regarding the definition of Sertoli cell morphology and function, and the possible role of germ cells in the sex-reversal process. Differences observed between XX and XY gonad development in moles suggest that the mammalian testis-determining gene, SRY, has an "anti-regionalisation" role during gonadal development, at least in those mammalian species in which regionalisation of the female gonad occurs.

Abnormal sex-duct development in female moles: the role of anti-Müllerian hormone and testosterone.

We have performed a morphological, hormonal and molecular study of the development of the sex ducts in the mole Talpa occidentalis. Females develop bilateral ovotestes with a functional ovarian portion and disgenic testicular tissue. The Müllerian ducts develop normally in females and their regression is very fast in males, suggesting a powerful action of the anti-Müllerian hormone in the mole. RT-PCR demonstrated that the gene governing this hormone begins to be expressed in males coinciding with testis differentiation, and expression continues until shortly after birth. Immunohistochemical studies showed that expression occurs in the Sertoli cells of testes. No expression was detected in females. Wolffian duct development was normal in males and degenerate in prenatal females, but developmental recovery after birth gave rise to the formation of rudimentary epididymides. This event coincides in time with increasing serum testosterone levels and Leydig cell differentiation in the female gonad, thus suggesting that testosterone produced by the ovotestes is responsible for masculinisation of female moles. During postnatal development, serum testosterone concentrations decreased in males but increased in females, thus approaching the levels that adult males and females have during the non-breeding season.

Early development of a somatosensory fovea: a head start in the cortical space race?

Star-nosed moles have 11 mechanosensory appendages surrounding each nostril, and primary afferents from a single appendage-the tactile fovea-are greatly over-represented in somatosensory cortex. It was found that the foveal appendage led development in the periphery, had the greatest innervated surface area in embryos, and developed mature nerve terminals and epidermal sensory organs first; also, in developing cortex, markers for metabolic activity (cytochrome oxidase) appeared first in the fovea representation. This developmental sequence may provide the fovea with an advantage in a competition for cortical space, and account for the much larger areas of cortex devoted to foveal afferents.

The development of a biological novelty: a different way to make appendages as revealed in the snout of the star-nosed mole Condylura cristata.

The nose of the star-nosed mole Condylura cristata is a complex biological novelty consisting of 22 epidermal appendages. How did this new set of facial appendages arise? Recent studies find remarkable conservation of the genes expressed during appendage formation across phyla, suggesting that the basic mechanisms for appendage development are ancient. In the nose of these moles, however, we find a unique pattern of appendage morphogenesis, showing that evolution is capable of constructing appendages in different ways. During development, the nasal appendages of the mole begin as a series of waves in the epidermis. A second deep layer of epidermis then grows under these superficial epidermal waves to produce 22 separate, elongated epidermal cylinders embedded in the side of the mole's face. The caudal end of each cylinder later erupts from the face and rotates forward to project rostrally, remaining attached only at the tip of the snout. As a result of this unique 'unfolding' formation, the rostral end of each adult appendage is derived from caudal embryonic facial tissue, while the caudal end of each appendage is derived from rostral facial tissue. This developmental process has essentially no outgrowth phase and results in the reversal of the original embryonic orientation of each appendage. This differs from the development of other known appendages, which originate either as outgrowths of the body wall or from subdivisions of outgrowths (e.g. tetrapod digits). Adults of a different mole species (Scapanus townsendii) exhibit a star-like pattern that resembles an embryonic stage of the star-nosed mole, suggesting that the development of the star recapitulates stages of its evolution.

[The intermediate nerve system]. / Sistema promezhutochnogo nerva.

A comparative-embryological study (223 fetuses of man, cat, dog, albino rat and mole) has established a similarity in the development and structure of the intermediate nerve in mammalia. The nerve under study represents a complex structural-functional system. Its afferent component includes a geniculate ganglion, a nucleus of the solitary tract, root, peripheral branches. A superior salivary nucleus, a root, pterygopalatine, submandibular, sublingual and auricular (in man) parasympathetic ganglia of the head, peripheral branches. The intermediate nerve fibers are included as additional sources in branches of other cranial nerves, vegetative ganglia and plexuses being spread on a considerable territory of the head. The intermediate nerve is an isolated anatomical structure and deserves being detached into a separate pair of cranial nerves.

[Embryonal development and the structure of the intermesenteric plexus in humans and various mammals]. / Embrional'noe razvitie i stroenie mezhbryzheechnogo spleteniia u cheloveka i nekotorykh mlekopitaiushchikh.

The intermesenteric plexus is an independent formation, it is connected with other plexuses in the abdominal and pelvic cavities and participates in innervation of organs. In the species investigated (mole, rat, cat, dog, man) connections between the intermesenteric and other vegetative plexuses in the abdominal and pelvic cavities vary according to their amount and complexity, they are most abundant and complex in man. In the latter the plexus is also mostly rich in neuro-fibrillar and neuro-cellular elements. In the intermesenteric plexus of the man and the animals studied there is a rather big part of vegetative ganglia, which can be considered as peripheral centers of the internal abdominal organs innervation.

Ontogeny of relationship of middle ear and temporomandibular (squamomandibular) joint in mammals. II. Morphology and ontogeny in insectivores.

The ontogeny of the mandibular joint and the middle ear region was studied in Erinaceus europaeus, Sorex araneus, Talpa europaea and Elephantulus rozeti. During development, a passage connection was found between the mandibular condyle and Meckel's cartilage that is produced by the primordium of the lateral pterygoid muscle. The articular disk is formed apart and it appears later in development.

The pineal gland of the mole (Talpa europaea L.).

The ultrastructure of the pinealocytes in the fetal mole (Talpa europaea) was examined and compared with that in the adult mole. The parenchyma of the fetal pineal gland consists primarily of pinealocytes with very few dispersed "glial" cells. Three different morphological types of pinealocytes (I, II and III) were observed. Pinealocytes of types I and II, homologous to the rudimentary photoreceptor cells of lacertilians and birds, were commonly found, especially around the pineal lumen, which is still present in the fetus. These results support the concept of the phylogenetic origin of mammalian pinealocytes from the pineal photoreceptor cells of the nonmammalian vertebrates. Considering their synthetic/secretory activity, the pinealocytes of the mole fetus are characterized by the presence of accumulations of proteinaceous material (APM) in the cisternae of the granular endoplasmic reticulum (GER). Two types of APM were also found. One type shows a paracrystalline organization as described in the adult mole. In the fetus, however, the paracrystalline-organized APM occur only infrequently. A progressive transformation, via an increase in size or fusion of vacuoles containing material originating from the cisternae of the GER in APM, free of a paracrystalline structure, has been observed. Granular vesicles (GV) originating from the Golgi saccules were rarely observed. Sometimes GV and APM were found to be present in the same cell.

Morphogenesis of the fetal membranes of an American mole, Scalopus aquaticus.

Scalopus membranes are characterized by: Superficial nidation; antimesometrial orientation of the embryonic disc; amniogenesis by folding; an extensive but transitory choriovitelline placenta; a large yolk sac with late and incomplete inversion; large persistent allantoic vesicle; a very broad, thin, villous, epitheliochorial chorioallantoic placenta of annular shape interrupted mesometrially, dotted with numerous areolae, and bordered by a nonvillous sparsely vascular chorioallantoic membrane connected with the persistent bilaminar omphalopleure by a very narrow rim of chorion. There is no decidua. Electron microscopy shows that at 8 mm, CR, (limb bud embryo) the uterine epithelium of the interhemal membrane may be 0.5 micron or less in thickness, but that it shows no signs of degeneration. Trophoblastic microvilli often penetrate the epithelium to within 0.2 micron of its base. At this time there is active secretion by the uterine glands, and cellular hypertrophy and cytolysis of the epithelium at the gland mouths, with active phagocytosis by the areolar cytotrophoblast. The occurrence of absorptive areolae in an insectivore emphasizes the probable primitiveness of this widely distributed placental mechanism. In spite of similarities of the yolk sac to that of rabbits and rodents, the bilaminar omphalopleure produces no invasive trophoblastic giant cells. The definitive membranes of Parascalops breweri and Scapanus latimanus are like those of Scalopus. The placentae of Talpa europaea, Condylura cristata, and Neurotrichus gibbsii are discoid and relatively much smaller, thicker and more complex in internal structure. There is some reason to believe that the fetal membrane systems of moles and shrews (Soricoidea) are more like those of the ancestral mammalian stock than are those of any other recent eutherians.