**2.3. Development of upper and lower extremities**

The embryonic development of the limbs [1] is illustrated here using computer graphics [2].

**3. Representative external anomalies of human embryos**

midface, and occasionally limbs. It can be recognized as early as CS12 or 13.

sented in **Figure 1** shows single eye in the center of the face, without any nose.

Holoprosencephaly (HPE) refers to an anomaly in which the differentiation of the prosencephalon from the neural tube is defective, thus leading to malformations of the forebrain,

Congenital Anomalies in Human Embryos http://dx.doi.org/10.5772/intechopen.72628 27

HPE can be classified into three categories, depending on the degree of defect in the development of prosencephalon: alobar holoprosencephaly, semi lobar holoprosencephaly, and lobar

Alobar holoprosencephaly is the most severe, which usually associates with cyclopia, ethmocephaly, and cebocephaly. Cyclopia (**Figure 2A**) is characterized by a single eye centered in the middle of the face, caused by the fusion of the optic vesicles due to the lack of midline tissue. The name of this malformation is derived from the cyclops (or cyclopes) in Greek mythology, first mentioned in Homer's epic poem, "Odyssey" in the seventh century B.C. There are cases of cyclopia with incomplete fusion of optic vesicles, but either with the nose absent or complicated further with proboscis located above the orbit [3]. The cyclopic embryo pre-

Ethmocephaly is morphologically similar to cyclopia, except that both eyes exist with distinct orbits, although marked by hypotelorism, with proboscis located between the eyes (**Figure 2B**) [4]. Cebocephaly is also an anomaly that exhibits hypotelorism in the two distinct orbits, characterized by a single nostril, occasionally complicated by cleft lip and/or palate [5]. HPE is one of the most common lethal congenital anomalies that occur at embryonic stages, and the prevalence rate is approximately 1/250. However, most of them cannot survive to

develop into a fetus, which makes it a rare anomaly in newborns (1/10,000–20,000) [6].

**Figure 2.** Congenital anomalies in the CNS. (A) Embryo presenting cyclopia, (B) embryo presenting ethmocephaly, with proboscis located between the eyes, (C) exencephaly presenting the opening in the neural tube, (D) spina bifida occulta

observed dorsally, and (E) spina bifida occulta observed laterally.

**3.1. CNS anomalies**

prosencephaly.

*3.1.1. Holoprosencephaly (HPE)*

