At approximately day 1

Craniofacial: Mandible begins to ossify.

Duration: 1 week

Reference

O’Rahilly R, Muller F. 2001. Human embryology & teratology. Third edition. New York: Wiley-Liss.

At approximately day 1

Skeletal: The sternum arises from mesenchyme into bilateral cartilaginous bars that fuse together by about 8 weeks.

Duration: 2 weeks

Reference

O’Rahilly R, Muller F. 2001. Human embryology & teratology. Third edition. New York: Wiley-Liss.

At approximately day 1

The definitive cortex develops as a second wave of epithelial cells penetrates the mesenchyme and surrounds the fetal cortex.

Duration: 1 week

Reference [PubMed Link]

Sadler TW. 2000. Susceptible periods during embryogenesis of the heart and endocrine glands. Environ Health Perspect 108 (suppl 3):555-561.

At approximately day 1

Gonadotropin-releasing hormone detected.

Duration: 2 weeks

Reference

Forest, MG, Ducharme JR. 1993. Gonadotropic and gonadal hormones. In: Bertrand J, Rappaport R, Sizonenko PC, eds. Pediatric Endocrinology. Baltimore, MD: Williams & Wilkins.

At approximately day 1

Inferior and superior parathyroid glands are derived from the 3rd and 4th pharyngeal pouches, respectively.

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 1

Skeletal: Joints begin to develop.

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 1

Limbs: Handplates and footplates begin to form; the first hyaline cartilage models are formed.

Reference

Sadler TW. 2004. Langman’s Medical Embryology. Ninth Edition. Maryland: Lippincott Williams & Wilkins.

At approximately day 1

Liver parenchyma produces “definitive,” or adult, erythroblasts.

Duration: 2 weeks

Reference [PubMed Link]

Dienhart MK, Sommer RJ, Peterson RE, Hirshfield AN, Silbergeld EK. 2000. Gestational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin induces developmental defects in the rat vagina. Toxicol Sci 56(1):141-149.