Ontogeny is the origination and development of an organism, usually from the time of fertilization of the egg to adult. The term can also be used to refer to the study of the entirety of an organism's lifespan.
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm cell. The resulting fusion of these two cells produces a single-celled zygote that undergoes many cell divisions that produce cells known as blastomeres. The blastomeres are arranged as a solid ball that when reaching a certain size, called a morula, takes in fluid to create a cavity called a blastocoel. The structure is then termed a blastula, or a blastocyst in mammals.
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.
Gastrulation is the stage in the early embryonic development of most animals, during which the blastula, or in mammals the blastocyst is reorganized into a two-layered or three-layered embryo known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body, and internalized one or more cell types including the prospective gut.
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm and endoderm. It emerges and originates from the outer layer of germ cells. The word ectoderm comes from the Greek ektos meaning "outside", and derma meaning "skin".
The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the embryoblast which subsequently forms the embryo, and an outer layer of trophoblast cells called the trophectoderm. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. In the late blastocyst the trophectoderm is known as the trophoblast. The trophoblast gives rise to the chorion and amnion, the two fetal membranes that surround the embryo. The placenta derives from the embryonic chorion and the underlying uterine tissue of the mother.
A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans produce two or three primary germ layers. Some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as bilaterians produce a third layer between these two layers, making them triploblastic. Germ layers eventually give rise to all of an animal's tissues and organs through the process of organogenesis.
Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation form the internal organs of the organism.
The serous membrane is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane that covers internal organs is called visceral, while the one that covers the cavity wall is called parietal. For instance the parietal peritoneum is attached to the abdominal wall and the pelvic walls. The visceral peritoneum is wrapped around the visceral organs. For the heart, the layers of the serous membrane are called parietal and visceral pericardium. For the lungs they are called parietal and visceral pleura. The visceral serosa of the uterus is called the perimetrium. The potential space between two opposing serosal surfaces is mostly empty except for the small amount of serous fluid.
In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo after passing through an organizational checkpoint during mid-embryogenesis. In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.
The primitive node is the organizer for gastrulation in most amniote embryos. In birds it is known as Hensen's node, and in amphibians it is known as the Spemann-Mangold organizer. It is induced by the Nieuwkoop center in amphibians, or by the posterior marginal zone in amniotes including birds.
The lateral plate mesoderm is the mesoderm that is found at the periphery of the embryo. It is to the side of the paraxial mesoderm, and further to the axial mesoderm. The lateral plate mesoderm is separated from the paraxial mesoderm by a narrow region of intermediate mesoderm. The mesoderm is the middle layer of the three germ layers, between the outer ectoderm and inner endoderm.
In amniote embryonic development, the epiblast is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other layer is the hypoblast. It drives the embryo proper through its differentiation into the three primary germ layers, ectoderm, mesoderm and endoderm, during gastrulation. The amniotic ectoderm and extraembryonic mesoderm also originate from the epiblast.
The bilaminar embryonic disc, bilaminar blastoderm or embryonic disc is the distinct two-layered structure of cells formed in an embryo. In the development of the human embryo this takes place by day eight. It is formed when the inner cell mass, also known as the embryoblast, forms a bilaminar disc of two layers, an upper layer called the epiblast and a lower layer called the hypoblast, which will eventually form into fetus. These two layers of cells are stretched between two fluid-filled cavities at either end: the primitive yolk sac and the amniotic sac.
Eye formation in the human embryo begins at approximately three weeks into embryonic development and continues through the tenth week. Cells from both the mesodermal and the ectodermal tissues contribute to the formation of the eye. Specifically, the eye is derived from the neuroepithelium, surface ectoderm, and the extracellular mesenchyme which consists of both the neural crest and mesoderm.
Enterocoelom describes both the process by which some animal embryos develop and the origin of the cells involved. In enterocoely, a mesoderm is formed in a developing embryo, in which the coelom appears from pouches growing and separating from the digestive tract. As the incipient coelomic epithelium originates from archenteral diverticula, the endoderm therefore gives rise to the mesodermal cells.
The development of fishes is unique in some specific aspects compared to the development of other animals.
Human embryonic development or human embryogenesis is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilization occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form the single cell zygote and the germinal stage of development commences. Embryonic development in the human, covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. The eight weeks has 23 stages.
In amniote embryology, the hypoblast is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac, which in turn gives rise to the chorion.
The development of the digestive system in the human embryo concerns the epithelium of the digestive system and the parenchyma of its derivatives, which originate from the endoderm. Connective tissue, muscular components, and peritoneal components originate in the mesoderm. Different regions of the gut tube such as the esophagus, stomach, duodenum, etc. are specified by a retinoic acid gradient that causes transcription factors unique to each region to be expressed. Differentiation of the gut and its derivatives depends upon reciprocal interactions between the gut endoderm and its surrounding mesoderm. Hox genes in the mesoderm are induced by a Hedgehog signaling pathway secreted by gut endoderm and regulate the craniocaudal organization of the gut and its derivatives. The gut system extends from the oropharyngeal membrane to the cloacal membrane and is divided into the foregut, midgut, and hindgut.
