Last updated Standard Event System character depiction
The "Standard Event System" (SES) to Study Vertebrate Embryos was developed in 2009 to establish a common language in comparative embryology.[1] Homologous developmental characters are defined therein and should be recognisable in all vertebrate embryos. The SES includes a protocol on how to describe and depict vertebrate embryonic characters. The SES was initially developed for external developmental characters of organogenesis, particularly for turtle embryos. However, it is expandable both taxonomically and in regard to anatomical or molecular characters. This article should act as an overview on the species staged with SES and document the expansions of this system. New entries need to be validated based on the citation of scientific publications. The guideline on how to establish new SES-characters and to describe species can be found in the original paper of Werneburg (2009).[1]
SES-characters are used to reconstruct ancestral developmental sequences in evolution such as that of the last common ancestor of placental mammals.[2] Also the plasticity of developmental characters can be documented and analysed.
SES-staged species
Overview on the vertebrate species staged with SES.
New SES-characters are continuously described in new publications. Currently, characters of organogenesis are described for Vertebrata (V), Gnathostomata (G), Tetrapoda (T), Amniota (A), Sauropsida (S), Squamata (SQ), Mammalia (M), and Monotremata (MO). In total, 166 SES-characters are currently defined.
Character complex
Character
Character description
Reference
Illustration
A. Egg (V01).
A1. Egg lay (V01a).
Most authors begin to create their staging tables shortly after or around egg lay time.
The dorsal borders of the neural folds come in touch and begin to form the neural tube that encloses the neural tube. The anterior and posterior regions of the primitive streak remain open.
Correlated to the internal differentiation and partly due to the carapace forming in turtles somite borders become blurred in specific regions until somites are completely inconspicuous.
The total number of somite pairs is count, filled in a formula for each specimen and afterwards grouped within the somite cluster of five somites each. Seldom the left and the right side show a different number of mesodermal segments. By definition the row with the maximum is to be count. Often a somite pair underlies a forming process. This one should also be counted.
The forming of a mesencephalic (or/and a diencephalic) projection in the posterodorsal region of the head is characterized by a continuing growth and can not be defined as a distinguishable event. But the disappearance of the structure resulting in a flat occipital head region can be well defined and is possibly associated to skull ossification.
An optic vesicle forms lateral of the prosencephalic region. It can be mistaken for the trigeminal ganglion which is proportional enlarged in Tachyglossus or Monodelphis at this early period of development. But the ganglion lies more caudally.
The optic or choroid fissure represents the blood vessel agglomeration, which supplies the developing lens. It forms a clear streak between the lens and the ventral most curvature of the optic cup. When the lens is completely formed the optic fissure disappears, which is a fluent process that can not be defined as a distinct event.
Although in many references a tail is arbitrarily described very early or in association to the development to the hind limb bud here the occurrence of the tail bud is defined as a distinct constriction of the caudal body region.
On the distal part of the forelimb bud an apical ectodermal ridge (AER) is formed in a horizontal longitude. Often it is only visible as a slightly eruption and in a particular angle of view.
In the distal region of the forelimb a round digital plate is formed by flattening of its paddle like end in a horizontal plane. The digital plate is clearly separated from the tube shaped leg by a surrounding step.
Scales on the dorsum of the head occur. The forming of scutes on throat and lower eyelid are encoded separately. Due feathers and scales are assumed to be homologous the scale characters are also applicable to bird development. Mammalian hears are formed differently and are not regarded here.
O. Maxillary process of the mandibular arch (G01).
O1. Maxillary bud (G01a).
The maxillary process of the mandibular arch occurs as a bud posterior to the eye. Often it is clearly seen as the anterodorsal process of the first pharyngeal arch.
O. Maxillary process of the mandibular arch (G01).
O2. Maxillary process posterior to eye (G01b).
The maxillary process lies posterior to the eye for a long period. First, when a clear rostrad development of the maxillary process is recognizable, its position at the level of the posterior margin of the optic cup should be noted. For identifying the level of the maxillary process in respect to the eye the ventral border of the telencephalic/diencephalic head region must be orientated horizontally.
O. Maxillary process of the mandibular arch (G01).
O4. Maxillary process anterior to lens (G01d).
The tip of the maxillary process is located beyond the optic fissure and is situated around the level of the anterior borders of pupil, iris, lens and scleral papillae.
The first pharyngeal (mandibular) arch is generally the first pharyngeal arch to occur as a bud. It forms later on a dorsal maxillary and a ventral mandibular process.
P2. Mandibular process of the mandibular arch posterior eye (G02b).
The mandibular process lays posterior to the eye for a long period. First, when a clear rostrad development of the mandibular process is recognizable, its position around the level of the posterior margin of the optic cup should be noted. For defining the level of the mandibular process in respect to the eye the ventral border of the telencephalic/diencephalic head region must be orientated horizontally.
The tip of the mandibular process is located beyond the optic fissure and is situated around the level of the anterior borders of pupil, iris, lens and scleral papillae.
During development of different species a 90° cervical flexure can occur and reverse several times. Only the first occurrence of a 90° cervical flexure is to be noted.
The caruncle (egg tooth) is first visible as a medial calcification of the skin that covers the symphysis of the maxillaries and lies between the nasal openings. Later on it enlarges and fuses with the maxillaries to get a mechanical support for slashing the egg while hatching.
The everted hemipenes are seen as paired structures sticking out on each side of the cloaca. Depending on the taxa, different forms are possible. The hemipenes are only found in males.
Embryo drawing is the illustration of embryos in their developmental sequence. In plants and animals, an embryo develops from a zygote, the single cell that results when an egg and sperm fuse during fertilization. In animals, the zygote divides repeatedly to form a ball of cells, which then forms a set of tissue layers that migrate and fold to form an early embryo. Images of embryos provide a means of comparing embryos of different ages, and species. To this day, embryo drawings are made in undergraduate developmental biology lessons.
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.
Embryology is the branch of animal biology that studies the prenatal development of gametes, fertilization, and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology.
Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula is a hollow sphere of cells known as blastomeres surrounding an inner fluid-filled cavity called the blastocoel. Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.
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 multilayered structure 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.
Amniotes belong to the clade Amniota, a clade of tetrapod vertebrates that comprises sauropsids and synapsids. They are distinguished from the other living tetrapod clade—the lissamphibians—by the development of three extraembryonic membranes, thicker and more keratinized skin, and costal respiration.
In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle, it is, by definition, a chordate. The notochord consists of inner, vacuolated cells covered by fibrous and elastic sheaths, lies along the anteroposterior axis, is usually closer to the dorsal than the ventral surface of the embryo, and is composed of cells derived from the mesoderm.
The long-beaked echidnas make up one of the two extant genera of echidnas, spiny monotremes that live in New Guinea; the other being the short-beaked echidna. There are three living species and one extinct species in this genus. The extinct species were present in Australia. Echidnas are one of the two types of mammals that lay eggs, the other being the platypus. The echidnas retain reptilian features such as egg-laying but display mammalian features such as fur and lactation.
A neurula is a vertebrate embryo at the early stage of development in which neurulation occurs. The neurula stage is preceded by the gastrula stage; consequentially, neurulation is preceded by gastrulation. Neurulation marks the beginning of the process of organogenesis.
The pharyngeal arches, also known as visceral arches, are structures seen in the embryonic development of vertebrates that are recognisable precursors for many structures. In fish, the arches are known as the branchial arches, or gill arches.
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.
In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.
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 red-bellied short-necked turtle, also known commonly as the pink-bellied side-necked turtle and the Jardine River turtle, is a species of turtle in the family Chelidae. The species is native to Australia and New Guinea. There are two recognized subspecies.
The premaxilla is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals has been usually termed as the incisive bone. Other terms used for this structure include premaxillary bone or os premaxillare, intermaxillary bone or os intermaxillare, and Goethe's bone.
Rosa Susan Penelope Beddington FRS was a British biologist whose career had a major impact on developmental biology.
In developmental biology, von Baer's laws of embryology are four rules proposed by Karl Ernst von Baer to explain the observed pattern of embryonic development in different species.
Postparietals are cranial bones present in fish and many tetrapods. Although initially a pair of bones, many lineages possess postparietals which were fused into a single bone. The postparietals were dermal bones situated along the midline of the skull, behind the parietal bones. They formed part of the rear edge of the skull roof, and the lateral edge of each postparietal often contacts the tabular and supratemporal bones. In fish, the postparietals are elongated, typically the largest components of the skull roof. Tetrapods possessed shorter postparietals, which were reduced further and shifted towards the braincase in amniotes. At several points in synapsid evolution, the postparietals fused to each other and the tabulars during embryological development. This fusion produces the interparietal bone, which is inherited by mammals. Postparietals are common in extinct amphibians and early reptiles. However, most living amphibians and living reptiles lack postparietal bones, with a few exceptions.
This glossary of developmental biology is a list of definitions of terms and concepts commonly used in the study of developmental biology and related disciplines in biology, including embryology and reproductive biology, primarily as they pertain to vertebrate animals and particularly to humans and other mammals. The developmental biology of invertebrates, plants, fungi, and other organisms is treated in other articles; e.g. terms relating to the reproduction and development of insects are listed in Glossary of entomology, and those relating to plants are listed in Glossary of botany.
↑ Werneburg and Spiekman (2016). Mammalian embryology and organogenesis. From gametes to weaning. In: Zachos F., Asher R. (eds.). Mammalia. Series: The Handbook of Zoology / Handbuch der Zoologie. De Gruyter, Berlin
↑ Werneburg I, Hugi J, Müller J, Sánchez-Villagra MS (2009). Embryogenesis and ossification of Emydura subglobosa (Testudines, Pleurodira, Chelidae) and patterns of turtle development. Developmental Dynamics, Volume 238, Issue 11 Pages 2770-2786 http://onlinelibrary.wiley.com/doi/10.1002/dvdy.22104/abstract
↑ Nunes Silva R and Sobral Sampaio F (2014). Immunoreactivity of Mel1a-like melatonin receptor and NRH: Quinone reductase enzyme (QR2) in testudine whole embryo and in developing whole retinas Trends in Developmental Biology 8:39-46.
1 2 3 4 Polachowski KM and Werneburg I (2013). Late embryos and bony skull development in Bothropoides jararaca (Serpentes, Viperidae). Zoology
↑ Roscito and Rodriges (2012). Embryonic development of the fossorial gymnophthalmid lizards Nothobachia ablephara and Calyptommatus sinebranchiatus. Zoology 115:302-318
↑ Werneburg et al. (2015). Bony skull development in the Argus monitor (Squamata, Varanidae, Varanus panoptes) with comments on developmental timing and adult anatomy. Zoology 118(4):255-280
↑ Ollonen, J., Da Silva, F.O., Mahlow, K. and Di-Poï, N., 2018. Skull development, ossification pattern, and adult shape in the emerging lizard model organism Pogona vitticeps: a comparative analysis with other squamates. Frontiers in physiology, 9, p.278.
↑ González B, Soria-Escobar AM, Rojas-Díaz V, Pustovrh MC, Monsalve LS, Rougier GW (2020). The embryo of the silky shrew opossum, Caenolestes fuliginosus (Tomes, 1863): First description of the embryo of Paucituberculata. Journal of Morphology 2020: 1-12
1 2 Werneburg I, Tzika AC, Hautier L, Asher RJ, Milinkovitch MC, Sánchez-Villagra MR (2013). Development and embryonic staging in non-model organisms: the case of an afrotherian mammal. The Journal of Anatomy 222:2-18
1 2 3 4 5 6 7 8 Taro Nojiri, Dai Fukui, Ingmar Werneburg, Takashi Saitoh, Hideki Endo, Daisuke Koyabu (2021). Embryonic staging of bats with special reference to Vespertilio sinensis and its cochlear development. Development Dynamics, DOI10.1002/dvdy.325
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