embryology Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos ...

, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

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 A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...

. 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 The Carnegie Institution of Washington (the organization's legal name), known also for public purposes as the Carnegie Institution for Science (CIS), is an organization in the United States established to fund and perform scientific research. Th ...

. 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 developmental biology, the Hamburger–Hamilton stages (HH) are a series of 46 chronological stages in chicken, chick development, starting from laying of the egg and ending with a newly hatched chick. It is named for its creators, Viktor Hambu ...

in the chick.

Stages

Days are approximate and reflect the days since the last

ovulation Ovulation is the release of eggs from the ovaries. In women, this event occurs when the ovarian follicles rupture and release the secondary oocyte ovarian cells. After ovulation, during the luteal phase, the egg will be available to be fertilize ...

before

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 occur ...

("Postovulatory age").

Stage 1: 1 days

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

polar bodies A polar body is a small haploid cell that is formed at the same time as an egg cell during oogenesis, but generally does not have the ability to be fertilized. It is named from its polar position in the egg. When certain diploid cells in animals ...

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 A morula (Latin, ''morus'': mulberry) is an early-stage embryo consisting of a solid ball of cells called blastomeres, contained in mammals, and other animals within the zona pellucida shell. The blastomeres are the daughter cells of the zygot ...

* 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

. 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 The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a specialized extracellular matrix that surrounds the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first ap ...

. 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 The blastocoel (), also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mammal ...

trophoblast The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the p ...

and

embryoblast The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of th ...

Carnegie stage 3 begins when a cavity first appears in the

and ends after the hatching from the

when the embryo makes contact with the endometrial lining of the

uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...

. 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

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 The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is ...

amniotic cavity The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shor ...

Stage 6: c. 17 days

* primitive streak *

primitive groove The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians the equivalent structure is the blastopore. During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with ...

* chorionic villi * secondary

* early gastrulation

Stage 7: c. 19 days

* gastrulation *

neural plate The neural plate is a key developmental structure that serves as the basis for the nervous system. Cranial to the primitive node of the embryonic primitive streak, ectodermal tissue thickens and flattens to become the neural plate. The region ant ...

* start of

hematopoiesis Haematopoiesis (, from Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells ...

* notochord

Stage 8: c. 23 days

* primitive pit

Stage 9: c. 25 days

neural groove The neural groove is a shallow median groove of the neural plate between the neural folds of an embryo. The neural plate is a thick sheet of ectoderm surrounded on either side by the neural folds, two longitudinal ridges in front of the primit ...

neural folds The neural fold is a structure that arises during neurulation in the embryonic development of both birds and mammals among other organisms. This structure is associated with primary neurulation, meaning that it forms by the coming together of t ...

septum transversum The septum transversum is a thick mass of cranial mesenchyme, formed in the embryo, that gives rise to parts of the thoracic diaphragm and the ventral mesentery of the foregut in the developed human being and other mammals. Origins The septum ...

placode A neurogenic placode is an area of thickening of the epithelium in the embryonic head ectoderm layer that gives rise to neurons and other structures of the sensory nervous system. Placodes are embryonic structures that give rise to structures such ...

* early

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...

Stage 10: c. 28 days

pharyngeal arches 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 arche ...

#1 and #2 * cardiac loop *

intermediate mesoderm Intermediate mesoderm or intermediate mesenchyme is a narrow section of the mesoderm (one of the three primary germ layers) located between the paraxial mesoderm and the lateral plate of the developing embryo. The intermediate mesoderm develop ...

Stage 11: c. 29 days

sinus venosus The sinus venosus is a large quadrangular cavity which precedes the atrium on the venous side of the chordate heart. In mammals, it exists distinctly only in the embryonic heart, where it is found between the two venae cavae. However, the sinus v ...

mesonephric duct The mesonephric duct (also known as the Wolffian duct, archinephric duct, Leydig's duct or nephric duct) is a paired organ that forms during the embryonic development of humans and other mammals and gives rise to male reproductive organs. Stru ...

Stage 12: c. 30 days

* upper limb buds

Stage 13: c. 32 days

septum primum During heart development of a human embryo, the single primitive atrium becomes divided into right and left by a , the septum primum. The septum primum () grows downward into the single atrium. Development The gap below it is known as the ostiu ...

* foramen primum

Stage 14: c. 33 days

ureteric bud The ureteric bud, also known as the metanephric diverticulum, is a protrusion from the mesonephric duct during the development of the urinary and reproductive organs. It later develops into a conduit for urine drainage from the kidneys, which, in ...

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 The septum secundum is a muscular flap that is important in heart development. It is semilunar in shape, and grows downward from the upper wall of the atrium immediately to the right of the septum primum and ostium secundum. It is important in t ...

Stage 19: c. 46 days

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

External links

*
UNSW Embryology

Carnegie Stages

List of structures by week, at mrc.ac.uk

References

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