Mnemonic
- Week 1
- hCG secretion begins around the time of blastocyst implantation.
- Blastocyst “sticks” on day six.
- Week 2
- Formation of bilaminar embryonic disc; two layers = epiblast, hypoblast.
- Week 3
- Formation of trilaminar embryonic disc via gastrulation (epiblast cell invagination through primitive streak); three layers = endoderm, mesoderm, ectoderm.
- Notochord arises from midline mesoderm and induces overlying ectoderm (via SHH) to become neural plate, which gives rise to the neural tube via neurulation.
- Week 4
- Heart begins to beat (four chambers). Cardiac activity visible by transvaginal ultrasound.
- Upper and lower limb buds begin to form (four limbs).
- Week 8
- Genitalia have male/female characteristics (pronounce “geneightalia”).
Abnormalities of morphogenesis
- Agenesis: Absent organ due to absent primordial tissue.
- Aplasia: Absent organ despite presence of primordial tissue.
- Association: a collection of malformations that are often seen together and do not have a known, common cause
- Hypoplasia: Incomplete organ development; primordial tissue present.
- Disruption: 2° breakdown of tissue with normal developmental potential (eg, amniotic band syndrome).
- Deformation: Extrinsic mechanical distortion (eg, congenital torticollis); occurs during fetal period.
- Malformation: Intrinsic developmental defect (eg, cleft lip/palate); occurs during embryonic period.
- Arteriovenous malformation (an abnormal, high-flow connection between arteries and veins bypassing capillaries which develops due to disrupted angiogenesis)
- Sequence: Abnormalities result from a single 1° embryologic event (eg, oligohydramnios → Potter sequence).
- Field defect: Disturbance of tissues that develop in a contiguous physical space (eg, holoprosencephaly).
Aortic arches
Aortic Arch Derivative | Adult Derivative |
---|---|
First | Part of maxillary artery |
Second | Hyoid artery, Stapedial artery |
Third | Common carotid artery, Proximal internal carotid artery |
Fourth | On left → aortic arch, On right → proximal right subclavian artery |
Sixth | Proximal pulmonary arteries, On left → ductus arteriosus |