• Source: Carnegie stages

      • In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.
      The stages are delineated through the development of structures, not by size or the number of days of development, and so the chronology can vary between species, and to a certain extent between embryos. In the human being, only the first 60 days of development are covered; at that point, the term embryo is usually replaced with the term fetus.
      It was based on work by Streeter (1942) and O'Rahilly and Müller (1987). The name "Carnegie stages" comes from the Carnegie Institution of Washington.
      While the Carnegie stages provide a universal system for staging and comparing the embryonic development of most vertebrates, other systems are occasionally used for the common model organisms in developmental biology, such as the Hamburger–Hamilton stages in the chick.


      Stages


      Days are approximate and reflect the days since the last ovulation before pregnancy ("Postovulatory age").


      = Stage 1: 1 days

      =
      fertilization
      polar bodies
      Carnegie stage 1 is the unicellular embryo. This stage is divided into three substages.


      Stage 1 a


      Primordial embryo. All the genetic material necessary for a new individual, along with some redundant chromosomes, are present within a single plasmalemma. Penetration of the fertilising sperm allows the oocyte to resume meiosis and the polar body is extruded.


      Stage 1 b


      Pronuclear embryo. Two separate haploid components are present - the maternal and paternal pronuclei. The pronuclei move towards each other and eventually compress their envelopes where they lie adjacent near the centre of the wall.


      Stage 1 c


      Syngamic embryo. The last phase of fertilisation. The pronuclear envelopes disappear and the parental chromosomes come together in a process called syngamy.


      = Stage 2: 2-3 days

      =
      cleavage
      morula
      compaction
      Carnegie stage 2 begins when the zygote undergoes its first cell division, and ends when the blastocyst forms a cavity and comprises more than 16 cells. At this point, it is called a morula.
      The cleavage divisions of CS2 embryos do not occur synchronously. And the fate of the blastomeres is not yet determined.
      The two-cell embryo is spherical and surrounded by the transparent zona pellucida. Each of the blastomeres that form is also spherical.
      On approximately day 3, at the eight-cell stage, compaction usually begins.


      = Stage 3: 4-5 days

      =
      blastocyst and blastocoele
      trophoblast and embryoblast
      Carnegie stage 3 begins when a cavity first appears in the morula and ends after the hatching from the zona pellucida when the embryo makes contact with the endometrial lining of the uterus.
      There are only two stage 3 embryos in the Carnegie collection.
      There are four characteristic processes that CS3 embryos go through cavitation, collapse and expansion, hatching, and discarding of cells.


      Cavitation


      The initiation of cavitation indicates the start of CS3. This process leads to the differentiation of blastocysts into outer trophoblast cells and inner embryoblasts.


      Collapse and expansion


      This process is seen in vitro and it is not known whether this occurs in vivo. In vitro, the blastocyst rapidly collapses and slowly re-expands before hatching from the zona pellucida.


      Hatching


      During this process, the blastocyst breaks through and escapes from the zona pellucida. This process must occur prior to implantation into the endometrium.


      Discarding of cells


      TEM inspection of in vitro blastocysts has allowed us to identify two types of cells that the developing embryo apparently discards. These are sequestered cells and isolated cells. Sequestered cells are groups of cells that are located in between the zona pellucida and the trophoblast. Isolated cells are mainly found in the blastocystic cavity.


      = Stage 4: 6 days

      =
      syncytiotrophoblast
      cytotrophoblast
      amniotic ectoderm


      = Stage 5 (a-c): 7-12 days

      =
      implantation
      bilaminar embryonic disc
      primary yolk sac
      amniotic cavity


      = Stage 6: c. 17 days

      =
      primitive streak
      primitive groove
      chorionic villi
      secondary yolk sac
      early gastrulation


      = Stage 7: c. 19 days

      =
      gastrulation
      neural plate
      start of hematopoiesis
      notochord


      = Stage 8: c. 23 days

      =
      primitive pit


      = Stage 9: c. 25 days

      =
      neural groove
      neural folds
      septum transversum
      placode
      early heart


      = Stage 10: c. 28 days

      =
      pharyngeal arches #1 and #2
      cardiac loop
      intermediate mesoderm


      = Stage 11: c. 29 days

      =
      sinus venosus
      mesonephric duct


      = Stage 12: c. 30 days

      =
      upper limb buds


      = Stage 13: c. 32 days

      =
      septum primum
      foramen primum


      = Stage 14: c. 33 days

      =
      ureteric bud


      = Stage 15: c. 36 days

      =
      Development of the Olfactory nerve and the early stage foot and hand plates


      = Stage 16: c. 39 days

      =
      lower limb buds


      = Stage 17: c. 41 days

      =
      implementation embryo in posterior uterus wall


      = Stage 18: c. 44 days

      =
      septum secundum


      = Stage 19: c. 46 days

      =
      Ectoderm: sensory placodes, lens pit, otocyst, nasal pits moved ventrally, fourth ventricle of brain
      Mesoderm: heart prominence, ossification continues
      Head: forebrain, eye, external acoustic meatus
      Body: straightening of trunk, heart, liver, umbilical cord


      = Stage 20: c. 49 days

      =
      Ectoderm: sensory placodes, lens pit, otocyst, nasal pits moved ventrally, fourth ventricle of brain
      Mesoderm: heart prominence, ossification continues
      Head: forebrain, eye, external acoustic meatus
      hearing - otic capsule connected with the basal plate and with the future exoccipitals. Tip of the cochlea is elongated and curled.
      Tensor tympani and stapedius present.


      = Stage 21: c. 51days

      =
      Ectoderm: sensory placodes, nasal pits moved ventrally, fourth ventricle of brain
      Mesoderm: heart prominence, ossification continues
      Head: nose, eye, external acoustic meatus
      Body: straightening of trunk, heart, liver, umbilical cord
      Limb: upper limbs longer and bent at elbow, foot plate with digital rays begin to separate, wrist, hand plate with webbed digits


      = Stage 22: c. 53 days

      =
      Mesoderm: heart prominence, ossification continues
      Head: nose, eye, external acoustic meatus
      Body: straightening of trunk, heart, liver, umbilical cord
      Limb: upper limbs longer and bent at elbow, foot plate with webbed digits, wrist, hand plate with separated digits


      = Stage 23: c. 56 days

      =
      Final embryonic stage, after this development is described as "fetal" through the entire second and third trimester.
      Mesoderm: ossification continues
      Head: eyelids, external ears, rounded head
      Body: straightening of trunk, intestines herniated at umbilicus
      Limbs: hands and feet turned inward


      See also


      Mammalian embryogenesis


      References



      Hill, M.A. (2016) Embryology Carnegie Stages. Retrieved August 19, 2016, from https://embryology.med.unsw.edu.au/embryology/index.php/Carnegie_Stages


      External links


      Swiss embryology (from UL, UB, and UF) iperiodembry/carnegie02
      UNSW Embryology - Carnegie Stages
      Overview at umich.edu
      Details on stages at lsuhsc.edu
      List of structures by week, at mrc.ac.uk
      Diagrams at actual size at Visible Embryo
      Virtual human embryo detailing stages

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