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Placentation
Placentation refers to the formation, type and structure, or arrangement of the placenta. The function of placentation is to transfer nutrients, respiratory gases, and water from maternal tissue to a growing embryo, and in some instances to remove waste from the embryo. Placentation is best known in live-bearing mammals (theria), but also occurs in some fish, reptiles, amphibians, a diversity of invertebrates, and flowering plants. In vertebrates, placentas have evolved more than 100 times independently, with the majority of these instances occurring in squamate reptiles. The placenta can be defined as an organ formed by the sustained apposition or fusion of fetal membranes and parental tissue for physiological exchange. This definition is modified from the original Mossman (1937) definition, which constrained placentation in animals to only those instances where it occurred in the uterus. In mammals In live bearing mammals, the placenta forms after the embryo implants into th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Implantation (embryology)
Implantation (nidation) is the stage in the embryonic development of mammals in which the blastocyst hatches as the embryo, adheres, and invades into the wall of the female's uterus. Implantation is the first stage of gestation, and when successful the female is considered to be pregnant. In a woman, an implanted embryo is detected by the presence of increased levels of human chorionic gonadotropin (hCG) in a pregnancy test. The implanted embryo will receive oxygen and nutrients in order to grow. There is an extensive variation in the type of trophoblast cells, and structures of the placenta across the different species of mammals. Of the five recognised stages of implantation including two pre-implantation stages that precede placentation, the first four are similar across the species. The five stages are migration and hatching, pre-contact, attachment, adhesion, and invasion. The two pre-implantation stages are associated with the pre-implantation embryo. In humans followi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. Characteristics Therian mammals give birth (''see viviparity'') to live young without a shelled egg. This is possible thanks to key proteins called syncytins which allow exchanges between the mother and its offspring through a placenta, even rudimental ones such as in marsupials. Genetic studies have suggested a viral origin of syncytins through the endogenization process. The marsupials and the placental mammals evolved from a common therian ancestor that gave live birth by suppressing the mother's immune system. While the marsupials continued to give birth to an underdeveloped fetus after a short pregnancy, the ancestors of placental mammals gradually evolved a prolonged pregnancy. Therian mammals no longer have the coracoid bone ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Strepsirrhini
Strepsirrhini or Strepsirhini (; ) is a suborder of primates that includes the lemuriform primates, which consist of the lemurs of Madagascar, galagos ("bushbabies") and pottos from Africa, and the lorises from India and southeast Asia. Collectively they are referred to as strepsirrhines. Also belonging to the suborder are the extinct adapiform primates which thrived during the Eocene in Europe, North America, and Asia, but disappeared from most of the Northern Hemisphere as the climate cooled. Adapiforms are sometimes referred to as being "lemur-like", although the diversity of both lemurs and adapiforms does not support this comparison. Strepsirrhines are defined by their "wet" (moist) rhinarium (the tip of the snout) – hence the colloquial but inaccurate term "wet-nosed" – similar to the rhinaria of canines and felines. They also have a smaller brain than comparably sized simians, large olfactory lobes for smell, a vomeronasal organ to detect pheromones, and a bic ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cleavage (embryo)
In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a compact mass called the morula. Cleavage ends with the formation of the blastula, or of the blastocyst in mammals. Depending mostly on the concentration of yolk in the egg, the cleavage can be holoblastic (total or entire cleavage) or meroblastic (partial cleavage). The pole of the egg with the highest concentration of yolk is referred to as the vegetal pole while the opposite is referred to as the animal pole. Cleavage differs from other forms of cell division in that it increases the number of cells and nuclear mass without increasing the cytoplasmic mass. This means that with each successive subdivision, there is roughly half the c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chorion Frondosum
The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles ( amniotes). It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitelline membrane in other animals. In insects it is developed by the follicle cells while the egg is in the ovary.Chapman, R.F. (1998) "The insects: structure and function", Section ''The egg and embryology''. Previewed in Google Bookon 26 Sep 2009. Structure In humans and other mammals (excluding monotremes), the chorion is one of the fetal membranes that exist during pregnancy between the developing fetus and mother. The chorion and the amnion together form the amniotic sac. In humans it is formed by extraembryonic mesoderm and the two layers of trophoblast that surround the embryo and other membranes; the chorionic villi emerge from the chorion, invade the endometrium, and allow the transfer of nutrients from maternal blood to fet ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Blood Vessel
The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away from the tissues. Blood vessels are needed to sustain life, because all of the body's tissues rely on their functionality. There are five types of blood vessels: the arteries, which carry the blood away from the heart; the arterioles; the capillaries, where the exchange of water and chemicals between the blood and the tissues occurs; the venules; and the veins, which carry blood from the capillaries back towards the heart. The word ''vascular'', meaning relating to the blood vessels, is derived from the Latin ''vas'', meaning vessel. Some structures – such as cartilage, the epithelium, and the lens and cornea of the eye – do not contain blood vessels and are labeled ''avascular''. Etymology * artery: late Middle English; from L ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pregnancy
Pregnancy is the time during which one or more offspring develops (gestation, gestates) inside a woman, woman's uterus (womb). A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but can also occur through assisted reproductive technology procedures. A pregnancy may end in a Live birth (human), live birth, a miscarriage, an Abortion#Induced, induced abortion, or a stillbirth. Childbirth typically occurs around 40 weeks from the start of the Menstruation#Onset and frequency, last menstrual period (LMP), a span known as the Gestational age (obstetrics), gestational age. This is just over nine months. Counting by Human fertilization#Fertilization age, fertilization age, the length is about 38 weeks. Pregnancy is "the presence of an implanted human embryo or fetus in the uterus"; Implantation (embryology), implantation occurs on average 8–9 days after fertilization. An ''embryo'' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rats
Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' ( pack rats), ''Bandicota'' (bandicoot rats) and ''Dipodomys'' (kangaroo rats). Rats are typically distinguished from mice by their size. Usually the common name of a large muroid rodent will include the word "rat", while a smaller muroid's name will include "mouse". The common terms ''rat'' and ''mouse'' are not taxonomically specific. There are 56 known species of rats in the world. Species and description The best-known rat species are the black rat (''Rattus rattus'') and the brown rat (''Rattus norvegicus''). This group, generally known as the Old World rats or true rats, originated in Asia. Rats are bigger than most Old World mice, which are their relatives, but seldom weigh over in the wild. The term ''rat'' is also used in the names of other small mammals that are ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mice
A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus''). Mice are also popular as pets. In some places, certain kinds of field mice are locally common. They are known to invade homes for food and shelter. Mice are typically distinguished from rats by their size. Generally, when a muroid rodent is discovered, its common name includes the term ''mouse'' if it is smaller, or ''rat'' if it is larger. The common terms ''rat'' and ''mouse'' are not taxonomically specific. Typical mice are classified in the genus ''Mus'', but the term ''mouse'' is not confined to members of ''Mus'' and can also apply to species from other genera such as the deer mouse, ''Peromyscus''. Domestic mice sold as pets often differ substantially in size from the common house mouse. This is attributable to breeding an ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Guinea Pig
The guinea pig or domestic guinea pig (''Cavia porcellus''), also known as the cavy or domestic cavy (), is a species of rodent belonging to the genus '' Cavia'' in the family Caviidae. Breeders tend to use the word ''cavy'' to describe the animal, while in scientific and laboratory contexts, it is far more commonly referred to by the common name ''guinea pig''. Despite their common name, guinea pigs are not native to Guinea, nor are they closely related biologically to pigs, and the origin of the name is still unclear. They originated in the Andes of South America. Studies based on biochemistry and hybridization suggest they are domesticated animals that do not exist naturally in the wild, descendants of a closely related cavy species such as '' C. tschudii''. They were originally domesticated as livestock for a source of meat, and are still consumed in some parts of the world. In Western society, the guinea pig has enjoyed widespread popularity as a pet since its intr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rabbit
Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit species and its descendants, the world's 305 breeds of domestic rabbit. ''Sylvilagus'' includes 13 wild rabbit species, among them the seven types of cottontail. The European rabbit, which has been introduced on every continent except Antarctica, is familiar throughout the world as a wild prey animal and as a domesticated form of livestock and pet. With its widespread effect on ecologies and cultures, the rabbit is, in many areas of the world, a part of daily life—as food, clothing, a companion, and a source of artistic inspiration. Although once considered rodents, lagomorphs like rabbits have been discovered to have diverged separately and earlier than their rodent cousins and have a number of traits rodents lack, like two extra i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, and language. Humans are highly social and tend to live in complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. Social interactions between humans have established a wide variety of values, social norms, and rituals, which bolster human society. Its intelligence and its desire to understand and influence the environment and to explain and manipulate phenomena have motivated humanity's development of science, philosophy, mythology, religion, and other fields of study. Although some scientists equate the term ''humans'' with all members of the genus '' Homo'', in common usage, it generally refers to ''Homo sapiens'', the only extant member. Anato ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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