The insertion of mesenchyme cells into the ectoderm during differentiation in Sea urchin embryos

Summary During the course of sea urchin development, from early blastula to pluteus larva, there are two major visible processes toward which all activities seem to be focused. They are the differentiation of the larval skeleton by the primary mesenchyme cells and the differentiation of the primitive gut by the secondary mesenchyme cells. These activities take place within the shell-like layer of epithelial cells, or ectodermal wall. The interactive role of the ectodermal wall with the mesenchyme cells is not yet clearly understood. A number of earlier studies have proposed that the ectoderm may have an inductive influence on the mesenchyme cells and that its inner surface forms a molecular template for guiding the mesenchyme cells. In this report, we suggest an additional role for the ectodermal wall. We show that some primary mesenchyme cells and secondary mesenchyme cells insert between the cells of the ectodermal wall in order to firmly anchor the anlage of the larval skeleton and primitive gut during differentiation. This mechanism may provide a physical basis for maintaining the stable positional relationship of the anlage during development.     

Reproduction and the central project of evolutionary theory

In much of the discourse of evolutionary theory, reproduction is treated as an autonomous function of the individual organism — even in discussions of sexually reproducing organisms. In this paper, I examine some of the functions and consequences of such manifestly peculiar language. In particular, I suggest that it provides crucial support for the central project of evolutionary theory — namely that of locating causal efficacy in intrinsic properties of the individual organism. Furthermore, I argue that the language of individual reproduction is maintained by certain methodological conventions that both obscure many of the problems it generates and serve to actively impede attempts to redress those difficulties that can be identified. Finally, I suggest that inclusion of the complexities introduced by sexual reproduction — in both language and methodology — may radically undermine the individualist focus of evolutionary theory.I am indepted to the Rockefeller Foundation for a Humanities Fellowship that supported this research during the spring of 1986. I am also grateful to Richard Lewontin, Diane Paul, and Lisa Lloyd for many extremely helpful conversations.     

Unilateral cleft lip repair

The marking of the medial lip segment of the Millard rotation advancement procedure for repair of the unilateral cleft lip has been altered in the uppermost portion by utilizing tissue from the columellar base. Once adequate length has been obtained, cutback is utilized at approximately 90 degrees. With adequate full-thickness release of this medial lip segment and subsequent rotation into the proper position, the C flap is advanced into the donor defect of the columellar base and is also used to lengthen the shortened columella on the cleft side. This results in placement of a scar that will closely simulate the "mirror image" of the noninvolved philtral column. Fifty-seven patients with unilateral cleft lip have been repaired utilizing this technique during the past 14 years. Several of these children have required secondary surgeries because of mucosal irregularities or residual nasal deformities, but none has required additional surgery because of inadequate rotation of the medial lip segment or for correction of any donor-site defect at the base of the columella.     

Permeabilization ofNeurospora crassa hyphae with toluene-ethanol and filipin

Young hyphae ofNeurospora crassa were made permeable to UDP-glucose and trypan blue by treatment with toluene-ethanol and filipin. Less than 2% of treated cells survived treatment with 8% and 16% toluene-ethanol, while 25% survived treatment with 4% toluene-ethanol. Similarly, 98% of treated cells were killed by treatment with 16 g/ml filipin. Electron microscopy revealed that toluene-ethanol-treated cells lost pieces of plasma membrane and contained a number of vacuole-like structures; filipin-treated cells were less affected. Both filipin- and toluene-ethanol-treated cells were able to incorporate UDP-glucose into insoluble material (likely glycogen and glucan).     

Ultraviolet photoproducts at the ochre suppressor mutation site in the glnU gene of Escherichia coli: relevance to "mutation frequency decline"

Summary Ochre suppressor mutations induced by UV in the Escherichia coli glnU tRNA gene are CG to TA transitions at the first letter of the anticodon-encoding triplet, CAA. Premutational UV photoproducts at this site have long been known to exhibit an excision repair anomaly (mutation frequency decline or MFD), whereby post-irradiation inhibition of protein synthesis enhances their excision and reduces suppressor mutation yields ten-fold. We sought to clarify the basis of this unique repair response by determining the spectrum of UV photoproducts on both strands of a 36 by region of glnU which includes the anticodon-encoding triplet. We found that four different photolesions are produced within the 3 by sequence corresponding to the tRNA anticodon: (i) on the transcribed strand, TC (6–4) photoproducts and TC cyclobutane dimers are formed in equal numbers at the site of the C to T transition, indicating that this site is a hotspot for the usually less frequent (6–4) photoproduct; (ii) on the nontranscribed strand, TT dimers are found opposite the second and third letters of the anticodon-encoding triplet, adjacent to the mutation site; and (iii) on the nontranscribed strand, an alkali-sensitive lesion other than a (6–4) photoproduct is formed, apparently at the G in the mutation site. We suggest that mutation frequency decline may reflect excision repair activity at closely spaced UV lesions on opposite strands, resulting in double-strand breaks and the death of potential mutants.     

Cytogenetic analysis of 400 sperm from three translocation heterozygotes

Summary Sperm chromosome complements were studied in three men who carried reciprocal translocations. A total of 400 sperm were karyotyped after in vitro penetration of hamster eggs: 217 sperm from t(2;9) (q21;p22), 164 from t(4;6) (q28;p23) and 19 from t(7;14) (q21;q13). All possible 22 and 31 meiotic segregations were observed for t(2;9) and t(4;6); for t(7;14) only 22 segregations were observed. For alternate segregations, the number of normal sperm was not significantly different from the number of sperm carrying a balanced form of the translocation in any of the translocations, as theoretically expected. The percentage of sperm with an unbalanced form of the translocation was 57% for t(2;9), 54% for t(4;6) and 47% for t(7;14). There was no evidence for an interchromosomal effect in any of the translocations since the frequencies of numerical abnormalities (unrelated to the translocation) were within the normal range of control donors. The frequencies of X- and Y-bearing sperm did not differ significantly from 50%. Results from a total of 17 reciprocal translocations studied by sperm chromosomal analysis were reviewed.     

GABA A -receptors: Structural requirements and sites of gene expression in mammalian brain

GABA_A-receptors, the major synaptic targets for the neutotransmitter GABA, are gated chloride channels. By their allosteric drug-induced modulation they serve as molecular control elements through which the levels of anxiety, vigilance, muscle tension and epileptiform activity can be regulated. Despite their functional prominence, the structural requirements of fully functional GABA_A-receptors are still elusive. Expression of cDNAs coding for the ₁- and ₁-subunits of rat brain yielded GABA-gated chloride channels which were modulated by barbiturates but displayed only agonistic responses to ligands of the benzodiazepine receptor. GABA_A-receptors with fully functional benzodiazepine receptor sites were formed when the ₁- and ₁-subunits were coexpressed with the ₂-subunit of rat brain. These receptors, however, failed to show cooperativity of GABA in gating the channel. In order to determine the subunit repertoire available for receptor assembly in different neuronal populations in vivo, the sites of subunit gene expression were (₁, ₂, ₃, ₅, ₆, ₁, ₂, ₃, ₂) mapped by in situ hybridization histochemistry in brain sections. The mRNAs of the ₁-, ₁- and ₂-subunits were co-localized e.g. in mitral cells of olfactory bulb, pyramidal cells of hippocampus as well as granule cells of dentate gyrus and cerebellum. The lack of colocalization in various other brain areas points to an extensive receptor heterogeneity. The presence of multiple GABA_A-receptors in brain may contribute to synaptic plasticity, differential responsiveness of neurons to GABA and to variations in drug profiles.Special issue dedicated to Dr. Erminio Costa