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A B S T R A C TThe sexual differentiation in t h e colonial ascidian Botryllus schlosseri was considered with respect to colony sexualization and sexualization of t h e single zooids of t h e same blastogenic generation.The colony sexualization was investigated histologically in a series of young colonies reared in t h e laboratory and fixed periodically throughout a sequence of blastogenic generations. The newly set colonies lack gonad blastemata; next, germ cells appear and several generations are involved in various phases of hyposexuality prior to the appearance of generations in which mature both eggs and sperm.The sexual contribution of t h e zooids of the same generation was found to vary according to a n intrinsic bilateral asymmetry (the gonadogenic potential differs both contralaterally in each zooid and between zooids of a different, dextral or sinistral, parental origin) and in proportion to t h e quantity of germ cells that have been transferred from preceding generations.The transfer of germ cells from one to t h e following generations was investigated by fusing colonies of opposite pigment genotypes, then crossing t h e separated parabionts to a common genotype, and scoring t h e offspring pigment phenotypes. This experiment has shown t h a t not only immature oocytes but also undifferentiated germ cells were exchanged between t h e parabionts and then gradually matured as eggs or sperm in a prolonged series of generations.The primordial germ cells differ in size from hemoblasts, t h e source of hemocytes. However, further experiments a r e needed in order to decide between t h e two contrasting hypotheses of germ cell origin: either segregation from t h e atrial epithelium of buds or development from hemoblasts.The sexual differentiation in t h e colonies of Botryllus has been recently re-considered by Mukai and Watanabe ('76a,b). Nevertheless, many points still remain controversial.The hermaphroditic colonies reach sexual maturity only after having gone through several blastogenic generations. Male sexual maturity is attained first, though immature eggs are also present. Next, maturing gonads of either sex are produced in sequential generations. Seasonal sexual cycles in the wild, and succeeding cycles in t h e laboratory under constant temperature conditions have been described for B. schlosseri (Sabbadin, '55a, '60). Whether germ cells are present in the newly set colonies or during t h e periods or sexual inactivity and transferred to and gradually matured in t h e following generations

The late differentiation of the ectodermal layer is analysed in the ascidians Ciona intestinalis and Botryllus schlosseri, by means of light and electron microscopy, in order to verify the possible presence of placodal structures. Cranial placodes, ectodermal regions giving rise to nonepidermal cell types, are classically found exclusively in vertebrates; however, data are accumulating to demonstrate that the nonvertebrate chordates possess both the genetic machinery involved in placode differentiation, and ectodermal structures that are possible homologues of vertebrate placodes. Here, the term "placode" is used in a broad sense and defines thickenings of the ectodermal layer that can exhibit an interruption of the basal lamina where cells delaminate, and so are able to acquire a nonepidermal fate. A number of neurogenic placodes, ones capable of producing neurons, have been recognised; their derivatives have been analysed and their possible homologies with vertebrate placodes are discussed. In particular, the stomodeal placode may be considered a multiple placode, being composed of different sorts of placodes: part of it, which differentiates hair cells, is discussed as homologous to the octavo-lateralis placodes, while the remaining portion, giving rise to the ciliated duct of the neural gland, is considered homologous to the adenohypophyseal placode. The neurohypophyseal placode may include the homologues of the hypothalamus and vertebrate olfactory placode; the rostral placode, producing the sensorial papillae, may possibly be homologous to the placodes of the adhesive gland of vertebrates.

A new mechanoreceptor organ, the "coronal organ," located in the oral siphon, is described by light and electron microscopy in the colonial ascidians Botryllus schlosseri and Botrylloides violaceus. It is composed of a line of sensory cells (hair cells), accompanied by supporting cells, that runs continuously along the margin of the velum and tentacles of the siphon. These hair cells resemble those of the vertebrate lateral line or, in general, the acoustico-lateralis system, because they bear a single cilium, located centrally or eccentrically to a hair bundle of numerous stereovilli. In contrast to other sensory cells of ascidians, the coronal hair cells are secondary sensory cells, since they lack axonal processes directed towards the cerebral ganglion. Moreover, at their base they form synapses with nerve fibers, most of which exhibit acetylcholinesterase activity. The absence of axonal extensions was confirmed by experiments with lipophilic dyes. Different kinds of synapses were recognized: usually, each hair cell forms a few afferent synapses with dendrites of neurons located in the ganglion; efferent synapses, both axo-somatic (between an axon coming from the ganglion and the hair cell) and axo-dendritic (between an axon coming from the ganglion and an afferent fiber) were occasionally found. The presence of secondary sensory cells in ascidians is discussed in relation to the evolution of sensory cells and placodes in vertebrates. It is proposed that the coronal organ in urochordates is homologous to the vertebrate acoustico-lateralis system.

In this work we set out to determine if the murine macrophage J774 cell line can be used to produce myogenic growth factors. Activated J774 macrophages were grown in serum-free conditions. The macrophage-conditioned medium (MCM) was then used to treat cultures of primary myoblasts and regenerating muscle tissue, in vitro and in vivo respectively. MCM activity in vitro was tested by analyzing the expression of muscle-specific transcription factors, in parallel with the proliferation and differentiation rates of the cells. The macrophage-secreted factors greatly enhanced the proliferative potential of both rat and human primary myoblasts and were found to be highly muscle-specific. In vivo, MCM administration markedly enhanced the regenerative processes in damaged muscles. The ability to produce large amounts of macrophage-secreted myogenic factor(s) in the absence of serum holds great promise for its biochemical characterization and successive application in therapeutic protocols, both for ex vivo gene therapy and for muscle repair.