At approximately day 1

Hypothalamus begins to develop from the forebrain.

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

Primordial adrenal gland appears.

Reference [PubMed Link]

Kaplan SL, Grumbach MM, Aubert ML. 1976. The ontogenesis of pituitary hormones and hypothalamic factors in the human fetus: maturation of central nervous system regulation of anterior pituitary function. Recent Prog Horm Res 32:161-243.

At approximately day 1

Hypophysial (Rathke’s) pouch appears.

Reference

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

At approximately day 1

Skeletal: Sclerotomic segments of the vertabral column become separated by less dense intersegmental tissue.

Reference

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

At approximately day 1

Epithelial buds enlarge and a ductal system develops by growth and branching.

Duration: 3 weeks

Reference [PubMed Link]

Montgomery RK, Mulberg AE, Grand RJ. 1999. Development of the human gastrointestinal tract: twenty years of progress. Gastroenterology 116(3):702-731.

At approximately day 1

Thyroglobulin present in follicle cells.

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

Endodermal thyroid develops as a flask-like vesicle with a narrow neck connected to the buccal cavity.

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 3

Thyroid vesicle enlarges and becomes bilobed; stalk ruptures and thyroid becomes a solid mass of laterally expanding tissue by week 6.3.

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 3

Pharyngeal arches begin to form from neural crest cells.

Reference

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

At approximately day 3

First hepatic colonization of hematopoietic stem cells .

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

Hypothalamic sulcus is visible.

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 7

Skeletal: Sclerotome cells form into mesenchyme (embryonic connective tissue).

Reference

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

At approximately day 7

Limbs: Limb buds become visible

Duration: 1 week

Reference

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

At approximately day 7

Arterial cluster formation occurs.

Reference [PubMed Link]

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