Ursula K. Abbott

Mechanisms of genetic sex determination, gonadal sex differentiation, and germ-cell development in animals.

Publisher Summary This chapter focuses on the basic events of genetic sex determination and primary sex differentiation. Several models are discussed to explain the genetics of sex determination. The most recent and best developed theory of sex determination based on genetic inheritance involves expression of the gene for the H-Y cell-surface antigen in male cells. Observations in several mammalian species have revealed a consistent correlation between the presence of H-Y antigen and testicular development. Sexual differentiation of the gonad results from the development of only one gonadal component, either the cortex or the medulla. Differentiation of the testis is marked by continued development of the medullary cords, into which the cortical germ cells migrate. There are certain important distinctions between the mammalian and avian gonadal development. The most striking difference is that in the avian female only the left gonad differentiates into a functional ovary, whereas in mammals female gonad development is bilaterally symmetrical.

Heart Rate of the Developing Chick Embryo.

Summary A new technique was used to measure the heart rate of developing chick embryos. Ballistocardiography should afford ideal conditions for observing embryonic heartbeats with a minimum of disturbance to the embryo. The same embryos can be measured on each day of incubation. The heart rate increased from day 4 through day 10 and then decreased until hatching time. The average frequency at 10 days was 280 bpm and 263 bpm at 19 days of incubation. Male embryos consistently had slower average heart rates than did females. No significant difference was observed in the average daily heart rate of embryos from 4 different genotypes, even though differences are apparent between adults of these genotypes. Mechanical disturbances or alterations in temperature had a marked effect on the heart rate of the embryo. Brief exposure to room temperature or slight injuries, such as those following insertion of electrodes, usually result in a decreased heart rate.

Upper beak truncation in chicken embryos with the cleft primary palate mutation is due to an epithelial defect in the frontonasal mass

In this study, we used the chicken mutant strain known as cleft primary palate (cpp) to study the mechanisms of beak outgrowth. cpp mutants have complete truncation of the upper beak with normal development of the lower beak. Based on structural analysis and grafts of facial prominences, we localized the defect to the frontonasal mass and its derivatives. Several explanations that would account for the outgrowth defect were investigated, including abnormal expression of genes in the frontonasal epithelium, intrinsic defects in epithelium and/or mesenchyme defects in epithelial–mesenchymal signalling, a localized decrease in cell proliferation or a localized increase in programmed cell death. One of the genes expressed in the frontonasal epithelial growth zone, Fgf8, failed to down‐regulate and was maintained for at least 48 hr beyond the time when down‐regulation normally occurs. Recombination experiments further illustrated that the frontonasal mass epithelium was abnormal in the cpp mutants, whereas mutant mesenchyme was capable of normal outgrowth when combined with wild‐type epithelium. Cell proliferation was not decreased in mutant embryos nor was cell death initially increased. Later, at stages 31–32, when the prenasal cartilage begins directed outgrowth, there was an increase in cell death within the mutant upper but not lower beak cartilage. The cpp beak truncation, therefore, is due to an epithelial defect in the frontonasal mass that is coincident with a failure to down‐regulate expression of Fgf8. Developmental Dynamics 230:335–349, 2004.

Scoliosis in chickens.

Scoliosis developed in 55 per cent of sexually mature birds (68 per cent of male and 46 per cent of female birds) in a highly inbred line of chickens originally produced from white Leghorns. The curve could first be detected at five to six weeks of age and progressed until spontaneous fusion of the thoracic vertebrae occurred. Studies of these chickens indicated that abnormalities of growth and development of the spine are not the primary cause of the scoliosis. Preliminary studies of the paravertebral musculature also indicated that simple muscle imbalance is not responsible for the curve. Initial studies of collagen extracted from the scoliotic line of chickens showed it to be more soluble than similar collagen extracted from white Leghorn controls.

Shh, HoxD, Bmp‐2, and Fgf‐4 gene expression during development of the polydactylous talpid2, diplopodia1, and diplopodia4 mutant chick limb buds

Several polydactylous mutants affect the pattern of asymmetry along the anteroposterior axis of the vertebrate limb. In talpid2, diplopodia1, and diplopodia4 chick limb mutants, there is a preaxial extension that results in broader limb buds. Talpid2 shows reduction of the long bones and 9-10 syndactylous digits, none of which are specifically recognizable as members of the normal digit complement. In diplopodia1 and diplopodia4 extra digits are present preaxially in addition to the normal digits. This phenotype resembles the duplications obtained by grafting a polarizing region to the anterior margin of the limb bud. The abnormal skeletal pattern along the anteroposterior limb axis in both mutants suggests alterations in the signaling pathways that mediate growth and patterning of the limb. In situ hybridization studies reveal that whereas shh transcripts are restricted to the posterior limb margins, HoxD, Bmp-2, and Fgf-4 genes are ectopically expressed in the anterior region of the talpid2, diplopodia1, and diplopodia4 limb buds. The results obtained give insights into the molecular basis of talpid2 and diplopodia mutations and also into the possible roles of shh, Bmp-2, HoxD, and Fgf-4 genes in vertebrate limb morphogenesis.

Inductive Interactions between Human Dermis and Chick Chorionic Epithelium

In a study of specificity in mesenchymal-epithelial interactions, human embryonic dermis has been recombined with chick chorionic epithelium and cultured for 7 days on a host chick chorioallantoic membrane. Dermis from the sole of the foot or palm of the hand induces chick chorionic epithelium to form an epidermis that resembles chick rather than human epidermis. Chick epithelium, though it has the capacity to respond to a human dermal stimulus, is limited to forming chick-type tissue. The human dermis was modified in its turn by culture in combination with chick epithelium.

Chick gonad differentiation following excision of primordial germ cells

Abstract The differentiation of embryonic chick gonads lacking germ cells was compared to that of normal chick gonads to determine whether the somatic elements of sterile avian gonads will undergo normal sexual differentiation. Primordial germ cells were removed by surgical excision of anterior germinal crescent from early embryos, Hamburger and Hamilton stages 6–11. Surgically treated and control embryos were sacrificed at 6, 15, and 20 days of incubation, and their gonads were studied histologically. Analysis of differentiation was based on morphological criteria at the cellular, tissue, and organ levels. In both male and female embryos, the somatic elements of the gonads differentiated normally in the absence of germ cells. The significance of these results for understanding the controls of differentiation of both the somatic gonad and the germ cells in birds is discussed and correlated with similar results in mammals.

Neuromorphometric changes in the ventral spinal roots in a scoliotic animal.

The morphologic changes were investigated In the ventral and dorsal spinal nerve roots in a strain of white leghorn chickens that genetically was predisposed to have Idiopathic scoliosis. In the ventral spinal roots in chickens with scollotic curves, small mylinated fibers were increased in number and appeared in clusters. The number of small fibers did not correlate with the degree of the duration of the deformity. In 8 of the 15 chickens of the genetic scoliotic strain that did not have spinal curves, changes in the mylinated fibers, similar to those in chickens with scoliotic curves, were seen. These findings suggest that abnormalities in the myelinated fibers of the ventral spinal nerve roots may be the primary genetic lesion that, with variable penetrance, influences the development of scollosis.

Ontogeny of hemoglobins: Evidence for hemoglobin M

Abstract A new autosomal codominant hemoglobin mutation alters hemoglobin M of the primitive red cell line and hemoglobin D found in definitive cells. That Hb M and Hb D are altered by the same gene mutation supports the idea that Hb M shares a polypeptide chain with Hb D. It is concluded that in the switch from primitive hemoglobins to those of the definitive type, there are at least two α chains conserved; α A of Hb E in Hb A and α D of Hb M in Hb D.

Collagen crosslinking and cartilage glycosaminoglycan composition in normal and scoliotic chickens

The amounts of lysine-derived crosslinks in collagens from tendon, cartilage, intervertebral disc, and bone and changes in the composition of sternal cartilage glycosaminoglycans were estimated in two lines of chickens, a control-isogenic line and a line that develops scoliosis. In the scoliotic line, scoliosis first appears at 3-4 weeks and progressively increases in severity and incidence so that 90% of the birds express the lesion by week 10. We have reported previously that cartilage, tendon, and bone collagens from scoliotic birds are more soluble than corresponding collagens from normal birds. Herein, collagen crosslinking and altered proteoglycan metabolism are examined as possible mechanisms for the differences in collagen solubility. At 1 week of age there were fewer reducible crosslinking amino acids (hydroxylysinonorleucine, dihydroxylysinonorleucine, and lysinonorleucine) in collagens from sternal cartilage and tendon in the scoliotic line than in the isogenic line. However, by week 3 and at weeks 5 or 7 values were similar in both groups. The amounts of hydroxypyridinium in vertebral bone and intervertebral disc collagen were also similar in both groups of birds. Consequently, differences in collagen crosslinking do not appear to be a persistent developmental defect underlying the expression of scoliosis in the model. However, differences were observed in cartilage proteoglycans and glycosaminoglycans from the scoliotic line that were not present in cartilage from the isogenic line. The average molecular weight of the uronide-containing glycosaminoglycans was 30% less in the scoliotic line than in the isogenic line, i.e., 12,000 compared to 18,000. The size distribution of cartilage proteoglycans from the scoliotic line also differed from that of proteoglycans from the isogenic line.(ABSTRACT TRUNCATED AT 250 WORDS)