At approximately day 1

Thyrotropin-releasing hormone and luteinizing hormone-releasing hormone present.

Reference [PubMed Link]

Winters AJ, Eskay RL, Porter JC. 1974. Concentration and distribution of TRH and LRH in the human fetal brain. Journal of Clinical Endocrinology & Metabolism 39(5):960-963.

At approximately day 1

Thyroid-stimulating hormone (TSH) detected.

Duration: 2 weeks

Reference [PubMed Link]

Fisher DA, Dussault JH, Sack J, Chopra IJ. 1976. Ontogenesis of hypothalamic-pituitary-thyroid function and metabolism in man, sheep, and rat. Recent Prog Horm Res 33:59-116.

At approximately day 1

Differentiation of the thyroid gland occurs.

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

Skeletal: Ossification becomes evident in the vertebral arches and continues through 25 years of age

Reference

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

At approximately day 1

Definitive erythroblasts enter the bloodstream and gradually outnumber circulating primitive erythroblasts.

Duration: 30 weeks

Reference [PubMed Link]

Migliaccio G, Migliaccio AR, Petti S, Mavilio F, Russo G, Lazzaro D, Testa U, Marinucci M, Peschle C. 1986. Human embryonic hemopoiesis. Kinetics of progenitors and precursors underlying the yolk sac----liver transition. The Journal of Clinical Investigation 78(1):51-60

At approximately day 4

Craniofacial: Most membrane bones of skull are present.

Reference

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

At approximately day 4

Glucagon and somatostatin are expressed in isolated epithelial cells, and insulin expressing cells are clustered and more prevalent.

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.