At approximately day 1

Thyroid hormone is from maternal sources from weeks 3-12.

Duration: 9 weeks

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

Thyroid hormone affects brain development by promoting neuronal and glial proliferation and differentiation, playing a role in migration, regulating nerve growth factors, stimulating development of dopaminergic and cholinergic systems, and promoting myelination.

Duration: 37 weeks

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

First generation of hemopoietic cells (mostly “primitive,” or fetal, erythroblasts) proliferates in yolk sac and extraembryonic mesenchyme.

Duration: 1 week

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.

At approximately day 1

Embryonic thyroid visible.

Reference [PubMed Link]

Howdeshell KL. 2002. A model of the development of the brain as a construct of the thyroid system. Environ Health Perspect 110 (suppl 3):337-348.

At approximately day 3

Blood islands (clusters of hematopoietic stem cells, HSCs) first detected in yolk sac.

Reference [PubMed Link]

Baron MH, Isern J, Fraser ST. 2012. The embryonic origins of erythropoiesis in mammals. Blood 119(21):4828-4837.

At approximately day 4

Skeletal: Notochordal plate forms and becomes tubular.

Duration: 1 week

Reference

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

At approximately day 5

Dorsal outgrowth of foregut endoderm is visible.

Reference [PubMed Link]

Piper K, Brickwood S, Turnpenny LW, Cameron IT, Ball SG, Wilson DI, Hanley NA. 2004 Apr. Beta cell differentiation during early human pancreas development. J Endocrinol 181(1):11-23.

At approximately day 6

Maternal-embryo transfer of thyroxine (T4) detected.

Duration: 6 weeks

Reference [PubMed Link]

Contempre B, Jauniaux E, Calvo R, Jurkovic D, Campbell S, Morreale de Escobar G. 1993. Detection of thyroid hormones in human embryonic cavities during the first trimester of pregnancy. Journal of Clinical Endocrinology & Metabolism 77(6):1719-1722.

At approximately day 7

Skeletal: Paraxial mesoderm surrounding the notochord begins to divide into somites, of which the ventromedial part (sclerotome) will form the vertebrae and ribs.

Reference

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