The Carnegie Stages of Development of an Embryo

Embryology is the discipline that studies formation and development of embryos. The Carnegie stages refer to a standard system of 23 stages of the development of human embryo from day 1 to day 56. At the end of this development phase, the embryo is called a fetus. The name "Carnegie Stages" is derived from the Carnegie Institution of Washington where two major breakthrough studies were carried out by Streeter in 1942 and by O'Rahilly and Muller in 1987 on this very subject.

Stages 1 to 3: Days 1 to 5

• The first day is post fertilization where the egg meets the sperm and the zygote is formed. The size of the zygote is about 0.1 to 0.15mm in this stage. In second stage, the embryo is about 2 to 3 days old. It divides rapidly and from its two-cell size, it grows to 16 cells. This is also called the morula stage where compaction takes place, which is the division of cells until they are 16 in number from the original two. By the third stage, the embryo is about 2mm. The blastocyst cavity is formed which consists of 16 to 32 cells. There is segmentation of the outer cell mass.

Stages 4 to 6: Days 6 to 15

• The embryo would have the anchoring of the blastocyst on the endometrium at the fourth stage and there is first interaction between the mother and embryo tissues. The blastocyst is implanted in the uterus mucus of the mother. In the fifth stage, the amniotic sac is formed and embryo is able to fully penetrate into the endometrium. The primary and secondary umbilical vesicles are formed. At the sixth stage, cloacal membrane and the primitive streak will appear.

Stages 7 to 10: Days 15 to 23

• The neural plate is visible in the seventh stage. This is when the genesis of the blood system starts and blood is formed. At the eighth stage, the neural grove is formed and the embryo becomes pear shaped. The ninth stage is when somites first appear. The cardiac primordiurm is also formed here. In the tenth stage, the neural folds start fusing together. The optic sulcus and 2 pharyngeal bars are formed.

Stages 11 to 13: Days 23 to 32

• The optic vesicle is formed and the rostral neuropore closes in the stage 11. There are about 13 to 20 somites formed and the nasal placode is visible. The embryo is now about 5mm in the stage 12. The formation of upper extremity buds is visible. The lens disc and the cervical sinus are formed as well. In stage 13, the buds of lower extremities are prominent, the auditory pit is visible and there are more than 30 somites.

Stages 14 to 18: Days 32 to 48

• The endolymphatic appendage is distinctly visible in the stage 14. You can also see the lens pit and optic cup. The nasal pit is formed at stage 15; the hands, and the cerebral vesicles become visible. The retinal pigment is visible in the eye and the foot plate is developed in the stage 16. The nasal pit faces ventrally. You can see finger rays in stage 17. The trunk is straight and head is growing larger. At stage 18 the elbow is distinguishable. You can also see toe rays appearing. The nose and nipples are easily discernible.

Stages 19 to 23: Days 48 to 60

• The trunk becomes more elongated and straight in stage 19. By stage 20, you can see the upper limbs clearly bent at the elbows. Fingers grow longer and in shape in stage 21. Hands and feet come closer to one another. The eyelids are formed and you can see the ears as well by stage 22. And by stage 23, the head is rounded, limbs are longer and clearly developed.