Extraembryonic Membranes: The thin layers of tissue that surround the developing embryo. There are four extra-embryonic membranes commonly found in VERTEBRATES, such as REPTILES; BIRDS; and MAMMALS. They are the YOLK SAC, the ALLANTOIS, the AMNION, and the CHORION. These membranes provide protection and means to transport nutrients and wastes.Oviparity: The capability of producing eggs (OVA) from which young are hatched outside the body. While mostly referring to nonmammalian species, this does include MAMMALS of the order MONOTREMATA.Serous Membrane: A thin lining of closed cavities of the body, consisting of a single layer of squamous epithelial cells (MESOTHELIUM) resting on a thin layer of CONNECTIVE TISSUE, and covered with secreted clear fluid from blood and lymph vessels. Major serous membranes in the body include PERICARDIUM; PERITONEUM; and PLEURA.Viviparity, Nonmammalian: The capability of bearing live young (rather than eggs) in nonmammalian species. Some species of REPTILES and FISHES exhibit this.Allantois: An extra-embryonic membranous sac derived from the YOLK SAC of REPTILES; BIRDS; and MAMMALS. It lies between two other extra-embryonic membranes, the AMNION and the CHORION. The allantois serves to store urinary wastes and mediate exchange of gas and nutrients for the developing embryo.Chorioallantoic Membrane: A highly vascularized extra-embryonic membrane, formed by the fusion of the CHORION and the ALLANTOIS. It is mostly found in BIRDS and REPTILES. It serves as a model for studying tumor or cell biology, such as angiogenesis and TISSUE TRANSPLANTATION.Uterus: The hollow thick-walled muscular organ in the female PELVIS. It consists of the fundus (the body) which is the site of EMBRYO IMPLANTATION and FETAL DEVELOPMENT. Beyond the isthmus at the perineal end of fundus, is CERVIX UTERI (the neck) opening into VAGINA. Beyond the isthmi at the upper abdominal end of fundus, are the FALLOPIAN TUBES.Pregnancy: The status during which female mammals carry their developing young (EMBRYOS or FETUSES) in utero before birth, beginning from FERTILIZATION to BIRTH.Endoderm: The inner of the three germ layers of an embryo.Membranes: Thin layers of tissue which cover parts of the body, separate adjacent cavities, or connect adjacent structures.Membrane Lipids: Lipids, predominantly phospholipids, cholesterol and small amounts of glycolipids found in membranes including cellular and intracellular membranes. These lipids may be arranged in bilayers in the membranes with integral proteins between the layers and peripheral proteins attached to the outside. Membrane lipids are required for active transport, several enzymatic activities and membrane formation.Cell Membrane: The lipid- and protein-containing, selectively permeable membrane that surrounds the cytoplasm in prokaryotic and eukaryotic cells.Intracellular Membranes: Thin structures that encapsulate subcellular structures or ORGANELLES in EUKARYOTIC CELLS. They include a variety of membranes associated with the CELL NUCLEUS; the MITOCHONDRIA; the GOLGI APPARATUS; the ENDOPLASMIC RETICULUM; LYSOSOMES; PLASTIDS; and VACUOLES.Yolk Sac: The first of four extra-embryonic membranes to form during EMBRYOGENESIS. In REPTILES and BIRDS, it arises from endoderm and mesoderm to incorporate the EGG YOLK into the DIGESTIVE TRACT for nourishing the embryo. In placental MAMMALS, its nutritional function is vestigial; however, it is the source of INTESTINAL MUCOSA; BLOOD CELLS; and GERM CELLS. It is sometimes called the vitelline sac, which should not be confused with the VITELLINE MEMBRANE of the egg.Membrane Potentials: The voltage differences across a membrane. For cellular membranes they are computed by subtracting the voltage measured outside the membrane from the voltage measured inside the membrane. They result from differences of inside versus outside concentration of potassium, sodium, chloride, and other ions across cells' or ORGANELLES membranes. For excitable cells, the resting membrane potentials range between -30 and -100 millivolts. Physical, chemical, or electrical stimuli can make a membrane potential more negative (hyperpolarization), or less negative (depolarization).Membranes, Artificial: Artificially produced membranes, such as semipermeable membranes used in artificial kidney dialysis (RENAL DIALYSIS), monomolecular and bimolecular membranes used as models to simulate biological CELL MEMBRANES. These membranes are also used in the process of GUIDED TISSUE REGENERATION.Embryo, Mammalian: The entity of a developing mammal (MAMMALS), generally from the cleavage of a ZYGOTE to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the FETUS.Gastrula: The developmental stage that follows BLASTULA or BLASTOCYST. It is characterized by the morphogenetic cell movements including invagination, ingression, and involution. Gastrulation begins with the formation of the PRIMITIVE STREAK, and ends with the formation of three GERM LAYERS, the body plan of the mature organism.Gene Expression Regulation, Developmental: Any of the processes by which nuclear, cytoplasmic, or intercellular factors influence the differential control of gene action during the developmental stages of an organism.Trophoblasts: Cells lining the outside of the BLASTOCYST. After binding to the ENDOMETRIUM, trophoblasts develop into two distinct layers, an inner layer of mononuclear cytotrophoblasts and an outer layer of continuous multinuclear cytoplasm, the syncytiotrophoblasts, which form the early fetal-maternal interface (PLACENTA).Ectoderm: The outer of the three germ layers of an embryo.Erythrocyte Membrane: The semi-permeable outer structure of a red blood cell. It is known as a red cell 'ghost' after HEMOLYSIS.Embryonic and Fetal Development: Morphological and physiological development of EMBRYOS or FETUSES.Membrane Fluidity: The motion of phospholipid molecules within the lipid bilayer, dependent on the classes of phospholipids present, their fatty acid composition and degree of unsaturation of the acyl chains, the cholesterol concentration, and temperature.Embryonic Development: Morphological and physiological development of EMBRYOS.Blastocyst: A post-MORULA preimplantation mammalian embryo that develops from a 32-cell stage into a fluid-filled hollow ball of over a hundred cells. A blastocyst has two distinctive tissues. The outer layer of trophoblasts gives rise to extra-embryonic tissues. The inner cell mass gives rise to the embryonic disc and eventual embryo proper.Placenta: A highly vascularized mammalian fetal-maternal organ and major site of transport of oxygen, nutrients, and fetal waste products. It includes a fetal portion (CHORIONIC VILLI) derived from TROPHOBLASTS and a maternal portion (DECIDUA) derived from the uterine ENDOMETRIUM. The placenta produces an array of steroid, protein and peptide hormones (PLACENTAL HORMONES).Amnion: The innermost membranous sac that surrounds and protects the developing embryo which is bathed in the AMNIOTIC FLUID. Amnion cells are secretory EPITHELIAL CELLS and contribute to the amniotic fluid.Chorion: The outermost extra-embryonic membrane surrounding the developing embryo. In REPTILES and BIRDS, it adheres to the shell and allows exchange of gases between the egg and its environment. In MAMMALS, the chorion evolves into the fetal contribution of the PLACENTA.Cell Membrane Permeability: A quality of cell membranes which permits the passage of solvents and solutes into and out of cells.Chimera: An individual that contains cell populations derived from different zygotes.Basement Membrane: A darkly stained mat-like EXTRACELLULAR MATRIX (ECM) that separates cell layers, such as EPITHELIUM from ENDOTHELIUM or a layer of CONNECTIVE TISSUE. The ECM layer that supports an overlying EPITHELIUM or ENDOTHELIUM is called basal lamina. Basement membrane (BM) can be formed by the fusion of either two adjacent basal laminae or a basal lamina with an adjacent reticular lamina of connective tissue. BM, composed mainly of TYPE IV COLLAGEN; glycoprotein LAMININ; and PROTEOGLYCAN, provides barriers as well as channels between interacting cell layers.Germ Layers: The three primary germinal layers (ECTODERM; ENDODERM; and MESODERM) developed during GASTRULATION that provide tissues and body plan of a mature organism. They derive from two early layers, hypoblast and epiblast.Embryo Loss: Early pregnancy loss during the EMBRYO, MAMMALIAN stage of development. In the human, this period comprises the second through eighth week after fertilization.Mesoderm: The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube.Cell Lineage: The developmental history of specific differentiated cell types as traced back to the original STEM CELLS in the embryo.GATA6 Transcription Factor: A GATA transcription factor that is expressed predominately in SMOOTH MUSCLE CELLS and regulates vascular smooth muscle CELL DIFFERENTIATION.Embryonic Stem Cells: Cells derived from the BLASTOCYST INNER CELL MASS which forms before implantation in the uterine wall. They retain the ability to divide, proliferate and provide progenitor cells that can differentiate into specialized cells.Cell Differentiation: Progressive restriction of the developmental potential and increasing specialization of function that leads to the formation of specialized cells, tissues, and organs.In Situ Hybridization: A technique that localizes specific nucleic acid sequences within intact chromosomes, eukaryotic cells, or bacterial cells through the use of specific nucleic acid-labeled probes.Models, Biological: Theoretical representations that simulate the behavior or activity of biological processes or diseases. For disease models in living animals, DISEASE MODELS, ANIMAL is available. Biological models include the use of mathematical equations, computers, and other electronic equipment.Dosage Compensation, Genetic: Genetic mechanisms that allow GENES to be expressed at a similar level irrespective of their GENE DOSAGE. This term is usually used in discussing genes that lie on the SEX CHROMOSOMES. Because the sex chromosomes are only partially homologous, there is a different copy number, i.e., dosage, of these genes in males vs. females. In DROSOPHILA, dosage compensation is accomplished by hypertranscription of genes located on the X CHROMOSOME. In mammals, dosage compensation of X chromosome genes is accomplished by random X CHROMOSOME INACTIVATION of one of the two X chromosomes in the female.Membrane Transport Proteins: Membrane proteins whose primary function is to facilitate the transport of molecules across a biological membrane. Included in this broad category are proteins involved in active transport (BIOLOGICAL TRANSPORT, ACTIVE), facilitated transport and ION CHANNELS.Blastoderm: A layer of cells lining the fluid-filled cavity (blastocele) of a BLASTULA, usually developed from a fertilized insect, reptilian, or avian egg.Genomic Imprinting: The variable phenotypic expression of a GENE depending on whether it is of paternal or maternal origin, which is a function of the DNA METHYLATION pattern. Imprinted regions are observed to be more methylated and less transcriptionally active. (Segen, Dictionary of Modern Medicine, 1992)SOXF Transcription Factors: A subclass of closely-related SOX transcription factors. Members of this subclass are expressed in VASCULAR ENDOTHELIAL CELLS and may play a role in vasculogenesis.Cell Line: Established cell cultures that have the potential to propagate indefinitely.Molecular Sequence Data: Descriptions of specific amino acid, carbohydrate, or nucleotide sequences which have appeared in the published literature and/or are deposited in and maintained by databanks such as GENBANK, European Molecular Biology Laboratory (EMBL), National Biomedical Research Foundation (NBRF), or other sequence repositories.Membrane Glycoproteins: Glycoproteins found on the membrane or surface of cells.Lipid Bilayers: Layers of lipid molecules which are two molecules thick. Bilayer systems are frequently studied as models of biological membranes.Viscera: Any of the large interior organs in any one of the three great cavities of the body, especially in the abdomen.Embryonic Induction: The complex processes of initiating CELL DIFFERENTIATION in the embryo. The precise regulation by cell interactions leads to diversity of cell types and specific pattern of organization (EMBRYOGENESIS).Cells, Cultured: Cells propagated in vitro in special media conducive to their growth. Cultured cells are used to study developmental, morphologic, metabolic, physiologic, and genetic processes, among others.Gene Targeting: The integration of exogenous DNA into the genome of an organism at sites where its expression can be suitably controlled. This integration occurs as a result of homologous recombination.Mutation: Any detectable and heritable change in the genetic material that causes a change in the GENOTYPE and which is transmitted to daughter cells and to succeeding generations.Genes, Lethal: Genes whose loss of function or gain of function MUTATION leads to the death of the carrier prior to maturity. They may be essential genes (GENES, ESSENTIAL) required for viability, or genes which cause a block of function of an essential gene at a time when the essential gene function is required for viability.Vitelline Duct: The narrow tube connecting the YOLK SAC with the midgut of the EMBRYO; persistence of all or part of it in post-fetal life produces abnormalities, of which the commonest is MECKEL DIVERTICULUM.Chick Embryo: The developmental entity of a fertilized chicken egg (ZYGOTE). The developmental process begins about 24 h before the egg is laid at the BLASTODISC, a small whitish spot on the surface of the EGG YOLK. After 21 days of incubation, the embryo is fully developed before hatching.Base Sequence: The sequence of PURINES and PYRIMIDINES in nucleic acids and polynucleotides. It is also called nucleotide sequence.Mitochondrial Membranes: The two lipoprotein layers in the MITOCHONDRION. The outer membrane encloses the entire mitochondrion and contains channels with TRANSPORT PROTEINS to move molecules and ions in and out of the organelle. The inner membrane folds into cristae and contains many ENZYMES important to cell METABOLISM and energy production (MITOCHONDRIAL ATP SYNTHASE).Morphogenesis: The development of anatomical structures to create the form of a single- or multi-cell organism. Morphogenesis provides form changes of a part, parts, or the whole organism.Body Patterning: The processes occurring in early development that direct morphogenesis. They specify the body plan ensuring that cells will proceed to differentiate, grow, and diversify in size and shape at the correct relative positions. Included are axial patterning, segmentation, compartment specification, limb position, organ boundary patterning, blood vessel patterning, etc.Microscopy, Electron: Microscopy using an electron beam, instead of light, to visualize the sample, thereby allowing much greater magnification. The interactions of ELECTRONS with specimens are used to provide information about the fine structure of that specimen. In TRANSMISSION ELECTRON MICROSCOPY the reactions of the electrons that are transmitted through the specimen are imaged. In SCANNING ELECTRON MICROSCOPY an electron beam falls at a non-normal angle on the specimen and the image is derived from the reactions occurring above the plane of the specimen.Germ Cells: The reproductive cells in multicellular organisms at various stages during GAMETOGENESIS.Hyalin: A clear, homogenous, structureless, eosinophilic substance occurring in pathological degeneration of tissues.Protein Transport: The process of moving proteins from one cellular compartment (including extracellular) to another by various sorting and transport mechanisms such as gated transport, protein translocation, and vesicular transport.Embryo, Nonmammalian: The developmental entity of a fertilized egg (ZYGOTE) in animal species other than MAMMALS. For chickens, use CHICK EMBRYO.Synaptic Membranes: Cell membranes associated with synapses. Both presynaptic and postsynaptic membranes are included along with their integral or tightly associated specializations for the release or reception of transmitters.X Chromosome Inactivation: A dosage compensation process occurring at an early embryonic stage in mammalian development whereby, at random, one X CHROMOSOME of the pair is repressed in the somatic cells of females.Bone Morphogenetic Protein 4: A bone morphogenetic protein that is a potent inducer of bone formation. It also functions as a regulator of MESODERM formation during EMBRYONIC DEVELOPMENT.Signal Transduction: The intracellular transfer of information (biological activation/inhibition) through a signal pathway. In each signal transduction system, an activation/inhibition signal from a biologically active molecule (hormone, neurotransmitter) is mediated via the coupling of a receptor/enzyme to a second messenger system or to an ion channel. Signal transduction plays an important role in activating cellular functions, cell differentiation, and cell proliferation. Examples of signal transduction systems are the GAMMA-AMINOBUTYRIC ACID-postsynaptic receptor-calcium ion channel system, the receptor-mediated T-cell activation pathway, and the receptor-mediated activation of phospholipases. Those coupled to membrane depolarization or intracellular release of calcium include the receptor-mediated activation of cytotoxic functions in granulocytes and the synaptic potentiation of protein kinase activation. Some signal transduction pathways may be part of larger signal transduction pathways; for example, protein kinase activation is part of the platelet activation signal pathway.X Chromosome: The female sex chromosome, being the differential sex chromosome carried by half the male gametes and all female gametes in human and other male-heterogametic species.