Histogenesis of the yolk sac in the chick

Morphological research on amniote eggs and embryos: An introduction and historical retrospective

Evolution of the terrestrial egg of amniotes (reptiles, birds, and mammals) is often considered to be one of the most significant events in vertebrate history. Presence of an eggshell, fetal membranes, and a sizeable yolk allowed this egg to develop on land and hatch out well-developed, terrestrial offspring. For centuries, morphologically-based studies have provided valuable information about the eggs of amniotes and the embryos that develop from them. This review explores the history of such investigations, as a contribution to this special issue of Journal of Morphology, titled Developmental Morphology and Evolution of Amniote Eggs and Embryos. Anatomically-based investigations are surveyed from the ancient Greeks through the Scientific Revolution, followed by the 19th and early 20th centuries, with a focus on major findings of historical figures who have contributed significantly to our knowledge. Recent research on various aspects of amniote eggs is summarized, including gastrulation, egg shape and eggshell morphology, eggs of Mesozoic dinosaurs, sauropsid yolk sacs, squamate placentation, embryogenesis, and the phylotypic phase of embryonic development. As documented in this review, studies on amniote eggs and embryos have relied heavily on morphological approaches in order to answer functional and evolutionary questions.

Functional morphology, diversity, and evolution of yolk processing specializations in embryonic reptiles and birds

Evolution of the terrestrial, amniotic egg of vertebrates required new mechanisms by which yolk material could be processed for embryonic use. Recent studies on each of the major extant reptile groups have revealed elaborate morphological specializations for yolk processing, features that differ dramatically from those of birds. In the avian pattern, liquid yolk is housed in a yolk sac whose endodermal lining absorbs and digests yolk material and sends resultant nutrients into the blood circulation. In snakes, lizards, turtles, and crocodilians, as documented herein, the yolk sac becomes invaded by endodermal cells that proliferate and phagocytose yolk material. Blood vessels then invade, and the endodermal cells become arranged around them, forming elongated "spaghetti-like" strands that fill the yolk sac cavity. This pattern provides an effective means by which yolk material is cellularized, digested, and transported by vitelline vessels to the developing embryo. Phylogenetically, the (non-avian) "reptilian" pattern was ancestral for sauropsids and was modified or replaced in ancestors to birds. This review postulates that evolution of the "avian" pattern involved increased reliance on extracellular digestion of yolk, allowing embryonic development to occur more rapidly than in typical reptiles. Comparative studies of yolk processing that draw on morphological, biochemical, molecular approaches are needed to explain how and why the "reptilian" pattern was replaced in birds or their archosaurian ancestors.

Histological analyses demonstrate the temporary contribution of yolk sac, liver, and bone marrow to hematopoiesis during chicken development

The use of avian animal models has contributed to the understanding of many aspects of the ontogeny of the hematopoietic system in vertebrates. However, specific events that occur in the model itself are still unclear. There is a lack of consensus, among previous studies, about which is the intermediate site responsible for expansion and differentiation of hematopoietic cells, and the liver's contribution to the development of this system. Here we aimed to evaluate the presence of hematopoiesis in the yolk sac and liver in chickens, from the stages of intra-aortic clusters in the aorta-genital ridges-mesonephros (AGM) region until hatching, and how it relates to the establishment of the bone marrow. Gallus gallus domesticus L. embryos and their respective yolk sacs at embryonic day 3 (E3) and up to E21 were collected and processed according to standard histological techniques for paraffin embedding. The slides were stained with hematoxylin-eosin, Lennert's Giemsa, and Sirius Red at pH 10.2, and investigated by light microscopy. This study demonstrated that the yolk sac was a unique hematopoietic site between E4 and E12. Hematopoiesis occurred in the yolk sac and bone marrow between E13 and E20. The liver showed granulocytic differentiation in the connective tissue of portal spaces at E15 and onwards. The yolk sac showed expansion of erythrocytic and granulocytic lineages from E6 to E19, and E7 to E20, respectively. The results suggest that the yolk sac is the major intermediate erythropoietic and granulopoietic site where expansion and differentiation occur during chicken development. The hepatic hematopoiesis is restricted to the portal spaces and represented by the granulocytic lineage.

Characterization of calcium carbonate crystals in pigeon yolk sacs with different incubation times

Calcium carbonate crystals are known to form in the yolk sacs of fertile pigeon eggs at late stages of incubation. The composition and structure of these crystals were investigated, the crystallization environment was inspected, and the physical chemistry constants of the yolk fluid were determined through the incubation period. Polarized light microscopy was used to observe the generation and distribution of calcium carbonate crystals in the yolk sac. In addition, X-ray diffraction was employed to analyze the composition and crystal phase of the yolk sac. A decalcification and deproteination method was established to analyze the ultrastructure and composition of the crystals, as well as the internal relationship between inorganic and organic phases of the crystals. Additionally, scanning electron microscopy, transmission electron microscopy, X-ray energy dispersive spectroscopy, and Fourier transform infrared spectroscopy were used to evaluate the characteristics of the crystals. Our results demonstrated that the calcium carbonate crystals were mainly composed of vaterite and calcite, with vaterite being the major component. Vaterite, a type of biomaterial generated by an organic template control, presented as a concentric hierarchical spherical structure. The organic nature of the biomaterial prevented vaterite from transforming into calcite, which is more thermodynamically stable than vaterite. Additionally, the configuration, size, and aggregation of vaterite were also mediated by the organic template. This bio-vaterite was found during the incubation period and is valuable in calcium transport during embryonic development.

Two-step consumption of yolk granules during the development of quail embryos

The mechanism of yolk consumption was studied morphologically and biochemically in Japanese quail Coturnix japonica. The amount of yolk granules in the yolk (or 'yolk cell') decreased in two steps during embryonic development. In the first step, during days 0-4 of incubation, the yolk-granule weight decreased at a rate of 13 mg/day. This decrease was due to segregation by endodermal cells that were newly formed in the developing yolk sac. In the second step after day 6, the decrease was drastic at a rate of 29.8 mg/day during days 6-12 and very slow thereafter. The decrease at the second step was due to the enzymatic digestion of yolk granules by cathepsin D that coexisted in yolk spheres. This digesting reaction was triggered by the solubilization of the granules with high concentrations of salts that were supplied after disruption of the limiting membrane of yolk spheres. The 'yolk cell' seemed to die around day 5 of incubation. Thus the digestion products might be taken up together with yolk lipids by endocytosis into the endodermal cells and transported to blood vessels.

Proteolytic activity in the yolk sac membrane of quail eggs

Extraembryonal degradation of yolk protein is necessary to provide the avian embryo with required free amino acids during early embryogenesis. Screening of proteolytic activity in different compartments of quail eggs revealed an increasing activity in the yolk sac membrane during the first week of embryogenesis. In this tissue, the occurrence of cathepsin B, a lysosomal cysteine proteinase, and cathepsin D, a lysosomal aspartic proteinase, has been described recently (Gerhartz et al., Comp Biochem Physiol, 118B:159-166, 1997). Determination of cathepsin B-like and cathepsin D-like proteolytic activity in the yolk sac membrane indicated a significant correlation between growth of the yolk sac membrane and proteolytic activity, shown by an almost constant specific activity. Both proteinases could be localized in the endodermal cells, which are in direct contact to the yolk. The concentration of proteinases in the endodermal cells appears to be almost unaltered in the investigated early stage of quail development, whereas the amount of endodermal cells increases rapidly, seen by a complicated folding of the yolk sac membrane. In the same cells quail cystatin, a potent inhibitor of quail cathepsin B (Ki 0.6 nM), has been localized at day 8 of embryonic development. Approximately at this stage of development, the quail embryo stops metabolizing yolk. In conclusion, it is strongly indicated that the amount of available free amino acids, produced by proteolytic degradation and supporting embryonic growth, is regulated by the growth of the yolk sac membrane.

Effects of cytochalasin B on calcium transport by 1,25(OH)2D3- or PTH-treated chick embryonic yolk sac in vitro and in vivo

The present study was done to elucidate the mechanism(s) by which calcium is taken up or transported across the yolk sac membrane of the embryonic chick. We examined the effect of various inhibitors or experimental conditions on the uptake of calcium in vitro. Treatment with ouabain, verapamil, antimycin A and calcium ionophore A23187; substitution of choline for sodium or potassium in the buffer; or incubation of the tissue at 0 degree C had no significant effect on calcium uptake by the yolk sac membrane. Dinitrophenol (DNP) and lanthanum chloride (LaCl2) reduced 45Ca uptake from day 6 and 9 embryos by 15% and 30%, respectively. Cytochalasin B decreased the uptake of 45Ca in yolk sac membrane disks of day 6 embryos, but not in older embryos. The effects of cytochalasin B were explored further in embryos pretreated with either 1,25(OH)2D3 or PTH, both of which enhance calcium uptake. Cytochalasin B decreased calcium uptake in 9-day and 12-day vitamin D-treated embryos to about 60% of their hormone-enhanced level and also decreased PTH-stimulated 45Ca uptake into yolk sac disks by about 50% in embryos of all age groups tested. We also examined the effect of cytochalasin B on 45Ca transport across the yolk sac membrane in vivo. Cytochalasin B did not affect this transport in control (vehicle-treated) embryos. However, it significantly decreased the enhanced in vivo 45Ca transport in day-9 and -12 vitamin D-treated embryos approximately 30% and 45%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin D and chick embryonic yolk calcium mobilization: identification and regulation of expression of vitamin D-dependent Ca2(+)-binding protein, calbindin-D28K, in the yolk sac

The developing chick embryo acquires calcium from two sources. Until about Day 10 of incubation, the yolk is the only source; thereafter, calcium is also mobilized from the eggshell. We have previously shown that during normal chick embryonic development, vitamin D is involved in regulating yolk calcium mobilization, whereas vitamin K is required for eggshell calcium translocation by the chorioallantoic membrane. We have studied here the biochemical action of 1,25-dihydroxy vitamin D3 in the yolk sac by examining the expression and regulation of the cytosolic vitamin D-dependent calcium-binding protein, calbindin-D28K. Two types of embryos are used for this study, normal embryos developing in ovo and embryos maintained in long-term shell-less culture ex ovo, the latter being dependent solely on the yolk as their calcium source. Our findings are (1) calbindin-D28K is expressed in the embryonic yolk sac, detectable at incubation Days 9 and 14; (2) the embryonic yolk sac calbindin-D28K resembles that of the adult duodenum in both molecular weight (Mr 28,000) and isoelectric point, as well as the presence of E-F hand Ca2(+)-binding structural domains; (3) systemic calcium deficiency caused by shell-less culture of chick embryos results in enhanced expression of calbindin-D28K in the yolk sac during late development; (4) yolk sac calbindin-D28K expression is inducible by 1,25-dihydroxy vitamin D3 treatment in vivo and in vitro; and (5) immunohistochemistry revealed that yolk sac calbindin-D28K is localized exclusively to the cytoplasm of the yolk sac endoderm. These findings indicate that the chick embryonic yolk sac is a genuine target tissue of 1,25-dihydroxy vitamin D3.

Changes in proteins from yolk and in the activity of benzoyl-L-tyrosine ethyl ester hydrolase from the yolk sac membrane during embryonic development of the chicken

The wet weight and nitrogen content in the water soluble fraction and granule fraction of chicken egg yolk changed during embryonic development. Rapid decreases between Days 14 and 16 were the largest changes observed. When protein components in the soluble and granule fractions of yolk from developing eggs were analyzed by polyacrylamide gel electrophoresis under denaturing conditions, the intensities of bands of some high molecular-weight proteins decreased during incubation, while smaller proteins increased. Most of such changes were observed after Day 16. The activities of proteinases and hydrolases assayed with protein substrates or synthetic substrates at a neutral pH were higher in the yolk sac membrane than in yolk itself. A benzoyl-L-tyrosine ethyl ester hydrolase showed the highest specific activity among the enzymes examined in yolk sac membrane homogenates. Its activity increased after Day 5 of embryogenesis, and became maximum on Days 14 to 20. The data suggest that some of the yolk proteins are degraded by yolk sac membrane enzymes during the latter half of the incubation period.

Host-parasite interaction and development of infraforms in chicken embryos infected with Coxiella burnetii via the yolk sac

Two phase I strains of Coxiella burnetii of different virulence were injected into the yolk sacs of chicken embryos, and the yolk sacs and livers were examined at intervals by light, fluorescent, and electron microscopy. The high absorptive and digestive capacities of the yolk endoderm contributed to he entrance of the organisms into endodermal epithelial cells where C. burnetii multiplied. Organisms multiplied not only inside specific vacuoles originating from phagolysosomes but also in the cytoplasm itself. Lysis of the limiting membrane of some phagolysosomes, a normal function of endodermal cells, as well as rupture of vacuoles, provided the release of C. burnetii into the cytoplasm. The C. burnetii strain of greater virulence infected 100% of the endodermal cells, whereas the strain of lesser virulence infected only 60%. Budding of very small particles from the C. burnetii bodies was demonstrated. The particles were regarded as filterable forms of the organism. Despite the enormous multiplication of C. burnetii in the endodermal cells, organisms were only rarely detected in the vitelline blood vessels and liver sinusoids of the embryos. Peculiarities of the infectious process of C. burnetii in chicken embryos and possible mechanisms of limitation of spread of the infection are discussed.

Structure of the endodermal epithelium of the chick yolk sac during early stages of development

The structure of the areas pellucida and vasculosa of the early chick embryo (stages 11-29) was examined by light, transmission and scanning electron microscopy. The most striking feature of the endodermal cells of these areas is the presence of large intracellular yolk drops which are characteristic of the regions in which they are found; lipid-like homogeneous drops in the area pellucida, heterogeneously composed pleomorphic drops in the mid-region of the area vasculosa and granular drops at the periphery of the area vasculosa in the region of the sinus terminalis. On morphological criteria it is postulated that granular drops may arise by direct engulfment of extracellular yolk, but this does not appear to be true for pleomorphic or homogeneous drops. Since the apical junctions between endodermal cells across the yolk sac are tight, they seal off the extraembryonic compartment from the vitelline circulation and presumably prevent intercellular passage of the yolk constituents. Thus the endodermal epithelium must mediate the transport of nutrients from the yolk mass to the developing embryo. Endodermal cells exhibit a variation across the yolk sac in the presence and number of structures associated with uptake of extracellular materials. The mid-region of the area vasculosa appears to be the most endocytotically active region with an abundance of microvilli, bristle-coated pits and vesicles and apical canaliculi and vacuoles. There is a close association between the endoderm and vitelline blood vessels and this association is maintained, as the yolk sac develops, by the formation of small vessels juxtaposed between the vascular surface of the endoderm and the walls of the large vitelline vessels.