• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 77
  • 37
  • 4
  • 4
  • 3
  • 3
  • 1
  • 1
  • Tagged with
  • 162
  • 64
  • 63
  • 29
  • 29
  • 27
  • 20
  • 18
  • 18
  • 15
  • 14
  • 14
  • 14
  • 13
  • 12
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

A Developmental Synapomorphy of Squamate Reptiles

Stewart, James R., Blackburn, Daniel G. 01 November 2019 (has links)
The reptilian clade Squamata is defined primarily by osteological synapomorphies, few of which are entirely unambiguous. Studies of developing squamate eggs have revealed a uniquely specialized feature not known to occur in any other amniotes. This feature—the yolk cleft/isolated yolk mass complex—lines the ventral hemisphere of the egg. During its formation, extraembryonic mesoderm penetrates the yolk and an exocoelom (the yolk cleft [YC]) forms in association with it, cutting off a thin segment of yolk (the “isolated yolk mass” [IYM]) from the main body of the yolk. The YC–IYM complex has been observed and described in more than 65 squamate species in 12 families. In viviparous species, it contributes to the “omphaloplacenta,” a type of yolk sac placenta unique to squamates. The only squamates known to lack the IYM are a few highly placentotrophic skinks with minuscule eggs, viviparous species in which it clearly has been lost. Given its absence in mammals, chelonians, crocodylians, and birds, the YC–IYM complex warrants recognition as a developmental synapomorphy of the squamate clade. As in extant viviparous lizards and snakes, the YC–IYM complex presumably contributed to the placenta of extinct viviparous squamates.
2

Characterisation and cryopreservation of Bapedi ram semen in tris egg yolk extender supplemented with phosphatidylcholine

Mafolo, Kgaogelo Stimela January 2018 (has links)
Thesis (M.Sc. (Animal Production)) -- University of Limpopo, 2018 / The study was conducted to determine the macroscopic and microscopic raw semen characteristics of Bapedi rams, to evaluate the effect of different egg yolk (EY) concentrations in Tris-based extenders on cryopreservation of Bapedi ram semen and to determine the effect of supplementing different phosphatidylcholine (PC) concentrations in Tris-based extenders with or without egg yolk on cryopreservation of Bapedi ram semen. Semen ejaculates were collected from four matured Bapedi rams aged 2-4 years using artificial vagina (AV) and pooled to eliminate individual differences. The first experiment was performed to characterise Bapedi ram semen parameters immediately after semen collection. The macroscopic semen parameters such as volume, pH and concentration and microscopic semen parameters such as motility, viability and morphology, membrane integrity and acrosome integrity were evaluated. The experiment was replicated 8 times and the data was subjected to descriptive statistics. The second experiment evaluated the effect of Tris-based extenders with five different EY concentrations (0, 5, 10, 15 and 20 %) on the microscopic quality of cryopreserved Bapedi ram semen. The treatments were subjected to a Completely Randomized Design (CRD) and replicated 4 times. The third experiment evaluated the effects of different PC concentrations supplemented to Tris-based extenders with or without 10% EY and the PC was added as liposomes. The experiment was a 2 x 4 factorial design in a CRD with two concentrations of EY: 0 and 10 %, and four concentrations of PC: 0, 0.25, 0.50, 0.75 mg/ml in Tris-based extenders. Pooled semen samples were divided into 5 and 8 aliquots to comply with objective 2 and objective 3, respectively. The semen aliquots were diluted with Tris-based extenders and equilibrated in a refrigerator at 5°C for another 4 hours. The semen was frozen using a programmable freezer and plunged into liquid nitrogen tank (-196°C).The volume, sperm concentration and pH of Bapedi ram semen ranged between 0.4-1.5 ml, 0.52-8.84 × 109 sperm/ml, and 5-7, respectively. The average total motility (TM), progressive motility (PM) and rapid motility (RM) characteristics were 85.95±2.58 %, 29.33±2.11 % and 39.47±4.99 %, respectively. The results for the mean percentage live spermatozoa, abnormalities, intact membrane and intact acrosome were 70.19±2.29 %, 2.50±1.34 %, 72.39±1.71 % and 75.37±5.39 %, respectively. There was a general decrease trends in frozen-thawed motility characteristics such as TM, PM and RM as compared to raw semen (p<0.05). The frozen-thawed semen in Tris-based extenders with 10, 15 and 20% EY concentrations resulted in significantly (p<0.05) higher TM, PM and RM motility characteristics compared to 0 and 5%. The percentage of live spermatozoa, membrane and acrosome integrities were found higher in raw semen than in frozen–thawed semen of respective extenders (p<0.05). The supplementation of PC in extenders either with or without EY did not improve the TM, PM and RM parameters (p>0.05). The membrane integrity in extenders either with or without EY were not influenced by the supplementation PC after freezing and thawing (p>0.05). The supplementation of PC in treatments with EY did not improve the acrosome integrity (p>0.05). Interestingly, the supplementation 0.75 mg/ml PC resulted in acrosome integrity that was not significantly different (P>0.05) to treatments with EY. In conclusion, the macroscopic and microscopic semen parameters of raw Bapedi ram semen were characterized. The use of 10% EY concentration resulted in higher motility parameters and membrane integrity of frozen-thawed Bapedi ram semen. However, 20% EY resulted in higher acrosome integrity of frozen-thawed Bapedi ram semen. The supplementation of PC in extenders in extenders with or without EY did not improve the motility parameters, percentage live spermatozoa and membrane integrity. However, the acrosome integrity was improved in extenders without EY supplemented with 0.75 mg/ml PC / Agricultural research council professional development programme (ARC-PDP)
3

Studies on egg yolk

Chang, Charlotte Mary Nowak, January 1969 (has links)
Thesis (Ph. D.)--University of Wisconsin--Madison, 1969. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.
4

Amniote Yolk Sacs: Diversity in Reptiles and a Hypothesis on Their Origin

Elinson, Richard P., Stewart, James R., Bonneau, Laurie J., Blackburn, Daniel G. 08 July 2014 (has links)
Oviparous amniotes produce a large yolky egg that gives rise to a free-living hatchling. Structural characteristics and functional attributes of the egg are best known for birds, which have a large mass of fluid yolk surrounded by an extraembryonic yolk sac. Yolk nutrients are delivered to the embryo via the vascular yolk sac. This developmental pattern and nutrient transport mechanism is thought to be representative of all other lineages of amniotes. Recent discovery of a snake with cellularized yolk organized around a meshwork of blood vessels reveals an additional pattern for yolk mobilization, which may also occur in other squamate reptiles (lizards and snakes). This complex yolk sac raises interesting questions about developmental mechanisms and suggests a possible model for the transition between the egg of anamniotes and that of amniotes.
5

Interactions between hydrophobically modified starch and egg yolk proteins in solution and at oil/water interfaces

Magnusson, Emma January 2009 (has links)
<p>A common modification of starch is esterfication with anhydrous octenyl succinic acid (OSA).  The modification makes the polymer surface active and it also incorporates a carboxyl group to the starch, which can be negatively charged. The characteristics of OSA starch make it interesting for usage in combination with egg yolk proteins in food emulsions. It is not only the individual ingredients that affect the product; interactions between ingredients and ingredient-dispersion medium have a great impact on factors such as structure and stability. Knowledge about how the interactions affect emulsion properties would make it possible to predict the behavior of an emulsion, which would be a great advantage in the formulation of food emulsions. Therefore, this is a subject of interest.</p><p>The purpose of this master thesis was to further investigate the interactions between OSA starch and α – β-livetin in solutions and in emulsions. First, the charges of the macromolecules were studied by titration. Interactions in solution were then analyzed through turbidity and solubility measurements. The adsorption of OSA starch onto livetin and the interfacial rheology were also studied. Finally, an emulsion stability experiment was made.</p><p>Strong interactions between the two macromolecules were observed in solutions at pH 4.0. This was probably due to hydrophobic interaction; however it could also be explained by electrostatic interaction. In the emulsions the adsorption of starch onto livetin was highest at pH 4.5, and then decreased with increasing pH values. The absence of OSA starch adsorption at pH 4.0, despite the strong interaction in solution, could be explained by complex formation immediately in solution. Less starch would then be able to reach the interface and adsorb. In the interfacial rheology experiments, an indication of decreased complex dilational modulus of the interfacial layer, caused by OSA starch addition was seen at low pH values. This could be due to aggregation of the proteins and formation of an uneven interfacial layer. OSA starch would then be able to adsorb and disturb the elasticity. Some differences in the stability of an emulsion only containing livetin, and an emulsion with both livetin and OSA starch could be observed. However, more investigations are needed to be made to understand the underlying mechanisms.</p>
6

Interactions between hydrophobically modified starch and egg yolk proteins in solution and at oil/water interfaces

Magnusson, Emma January 2009 (has links)
A common modification of starch is esterfication with anhydrous octenyl succinic acid (OSA).  The modification makes the polymer surface active and it also incorporates a carboxyl group to the starch, which can be negatively charged. The characteristics of OSA starch make it interesting for usage in combination with egg yolk proteins in food emulsions. It is not only the individual ingredients that affect the product; interactions between ingredients and ingredient-dispersion medium have a great impact on factors such as structure and stability. Knowledge about how the interactions affect emulsion properties would make it possible to predict the behavior of an emulsion, which would be a great advantage in the formulation of food emulsions. Therefore, this is a subject of interest. The purpose of this master thesis was to further investigate the interactions between OSA starch and α – β-livetin in solutions and in emulsions. First, the charges of the macromolecules were studied by titration. Interactions in solution were then analyzed through turbidity and solubility measurements. The adsorption of OSA starch onto livetin and the interfacial rheology were also studied. Finally, an emulsion stability experiment was made. Strong interactions between the two macromolecules were observed in solutions at pH 4.0. This was probably due to hydrophobic interaction; however it could also be explained by electrostatic interaction. In the emulsions the adsorption of starch onto livetin was highest at pH 4.5, and then decreased with increasing pH values. The absence of OSA starch adsorption at pH 4.0, despite the strong interaction in solution, could be explained by complex formation immediately in solution. Less starch would then be able to reach the interface and adsorb. In the interfacial rheology experiments, an indication of decreased complex dilational modulus of the interfacial layer, caused by OSA starch addition was seen at low pH values. This could be due to aggregation of the proteins and formation of an uneven interfacial layer. OSA starch would then be able to adsorb and disturb the elasticity. Some differences in the stability of an emulsion only containing livetin, and an emulsion with both livetin and OSA starch could be observed. However, more investigations are needed to be made to understand the underlying mechanisms.
7

The effects of alcohols on pinocytosis and proteolysis in the rat yolk sac in vitro

Steventon, Glyn B. January 1987 (has links)
No description available.
8

Placental Transfer of Nutrients During Gestation in the Viviparous Lizard, Pseudemoia spenceri

Thompson, M. B., Stewart, J. R., Speake, B. K., Russell, K. J., McCartney, R. J. 01 July 1999 (has links)
Energy, ionic, protein and lipid contents and fatty acid profiles for the major lipid classes of freshly ovulated eggs and neonates of the viviparous lizard, Pseudemoia spenceri, were measured. Litter size is 1.7 ± 0.1, with larger females producing larger neonates. Placentotrophy results in approximately 23% more dry matter in the neonates than in the fresh egg. The increase in the quantity of protein and lipid during development is not significant and is reflected in the similarity of energy densities of eggs and neonates. As a percentage of dry matter, neonates have slightly lower proportions of lipid and protein than eggs because of significant uptake of ash, calcium, potassium and sodium, but not of magnesium, across the placenta. The amounts of triacylglycerol and phospholipid are not significantly different between the egg and the neonate, but neonates contain significantly more cholesterol and cholesteryl ester. The amounts of the major fatty acids, palmitic and oleic acids, recovered from the total lipids of the neonate do not differ significantly from the amounts present in the egg lipids, but the neonates contain significantly less linoleic and α-linolenic acids and more palmitoleic, stearic and arachidonic acids than the eggs. The amount of docosahexaenoic acid recovered from the lipids of the neonate is 2.6-times greater than the amount initially present in the egg. P. spenceri has a relatively larger egg and a smaller reliance on placentotrophy than other species in the same genus, all of which have a similar placental morphology. Nevertheless, the pattern of embryonic nutrition includes both obligative and facultative placentotrophy. All the major components of yolk of oviparous species are present in eggs of P. spenceri, but most are augmented during development by placental transfer.
9

Placental Nutrition in a Viviparous Lizard (Pseudemoia pagenstecheri) With a Complex Placenta

Thompson, Michael B., Stewart, James R., Speake, Brian K., Russell, Kylie J., McCartney, Ruth J., Surai, Peter F. 01 July 1999 (has links)
The composition of egg yolks and neonates of the viviparous lizard, Pseudemoia pagenstecheri, one of the most placentotrophic reptiles studied to date, are described. Neonates (43.3 ±5.2 mg) have twice the dry mass of the initial eggs (22.0 ±1.9 mg). The protein content of neonates (29.1 ±1.1 mg) is more than twice that of eggs (12.2 ±1.1 mg), while the energy content (908.1 ±107.4 J) is 1.6 times higher than that of the egg (565.0 ±42.9 J). The energy densities of eggs (27.5 kJ g-1) and neonates (23.1 ±0.3 kJ g-1) are similar to the energy densities of eggs and neonates of oviparous species. The total ash per neonate (4.1 ±0.4 mg) is three times greater than that of the egg contents (1.4 ±0.2). Neonates contain significantly more calcium, sodium and potassium, but not magnesium, than do eggs. Thus, the placenta has a quantitatively important role in supplying nutrients for the embryo. The proportions of triacylglycerol (66%), phospholipid (19%), and free cholesterol (5%) in the eggs are similar to those in eggs of birds and crocodilians, but the proportion of cholesteryl esters (7%) is much higher in eggs of P. pagenstecheri. The proportion of docosahexaenoic acid in the egg phospholipid is relatively low (1.4%) but rises to 5.4% in the neonate. The eggs contain vitamin E (mainly in the form of α-tocopherol) and vitamin A, but no detectable carotenoids. The overall composition of the eggs is not substantially different from that of oviparous species, suggesting that the small egg size relative to neonate size is a result of a reduction in egg size rather than modification by omission of some nutrients from the yolk. The pattern of placental nutrient provision of P. pagenstecheri contains both an obligate and a facultative component suggesting that enhancement of offspring quality through facultative placentotrophy is a general characteristic of placental reptiles independent of pattern of embryonic nutrient provision.
10

The 'giant' yolk sac : an in vitro model for studying early placental transport

Dunton, Anne January 1988 (has links)
In the rat, before the establishment of the chorioallantoic placenta, the nutritional requirements of the post-implantation embryo, are met solely by the visceral yolk sac and therefore a study of its structure and functions is essential to a full understanding of early embryonic nutrition. A method has been developed for maintaining the rat visceral yolk sac in organ culture over a prolonged period, having first removed the embryo by microsurgery at 9.5 days or alternatively allowing it to die within its own amnion. The yolk sac continues to grow as a closed vesicle, and can reach a diameter of 2cm. The system has been called the 'giant' yolk sac. The 'giant' yolk sac and in vivo yolk sac have been compared using various criteria. A detailed morphological study was made, including a quantitative analysis of the vacuolar compartment. The endocytic capacity of both systems was studied using three different substrates; those used were 125I-polyvinylpyrrolidone (PVP), a non-degradable macromolecule, taken up in the fluid phase and accumulated within the yolk sac tissue, 125I-bovine serum albumin (BSA) taken up by adsorptive pinocytosis and digested within the lysosomes and 125I-IgG (and colloidal gold-IgG) taken up with great efficiency by specific receptor mediated endocytosis. Also a preliminary study of 14C-amino acid uptake was made. In many instances the 'giant' yolk sac functioned very similarly to the in vivo yolk sac and therefore seems an ideal model for studying transport across an epithelial sheet. It is particularly useful as its continuous epithelium separates the exocoelom from the external culture medium. The fluid maintained within the exocoelom of the 'giant' yolk sac should be an excellent source of processed histiotroph essential for embryonic nutrition during organogenesis. Experiments carried out indicate that some of the trophic factors necessary for growth are present in this fluid.

Page generated in 0.0483 seconds