Early embryonic development

Why this matters

Everything you will treat for the rest of your career was assembled in about eight weeks, and most of the face was assembled by a cell population that has no name in the classical three-germ-layer story. Enamel comes from oral surface ectoderm; dentine, pulp, cementum, periodontal ligament and alveolar bone come from cranial neural crest ectomesenchyme. Clefts, hypodontia, Apert and Down syndrome, and the enamel and dentine dysplasias all arise in this window, and every one of them arrives in a dental chair.

The big picture

One cell becomes a two-layer disc inside an invading trophoblast, the disc becomes three layers through a streak, and a fourth population peels off the closing neural tube and migrates into the face to build almost every dental tissue except enamel. Dates in this story belong to clocks; numbers belong to denominators.

The 15 cards in this lesson

1. 01Name the clock and the endpoint, or two true numbers will look like a contradiction
2. 02The whole embryonic period is three weeks of assembly plus five of refinement
3. 03One oocyte and four spermatozoa: the asymmetry that predicts which parent's age does what
4. 04The most famous number in reproductive biology rests on two ovaries
5. 05Paternal age acts through mutation, maternal age through missegregation
6. 06Apert and achondroplasia are paternal, new, and sitting in your chair
7. 07The maternal age curve is sigmoidal, and the error may be set up before the mother was born
8. 08Fertilisation is a six-day window, a few hundred sperm and two blocks that mostly work
9. 09Week one: the embryo divides without growing and arrives in the uterus as a blastocyst
10. 10Implantation: an invading syncytium outside, a two-layered disc inside
11. 11All three germ layers come out of the epiblast
12. 12Enamel is ectoderm; every other dental tissue is neural crest
13. 13Clinical alert — in the lesson
14. 14Four mechanisms of congenital abnormality, and one that is nobody's fault at all
15. 15Clinical alert — in the lesson

You have got it when

You can (1) name the clock and endpoint behind a developmental date and convert post-fertilisation to LMP dating; (2) explain why paternal age drives point mutations while maternal age drives missegregation; (3) describe fertilisation, cleavage, implantation and their evidence limits; (4) say why all three germ layers come from the epiblast; (5) name the neural crest derivatives in a tooth and the one that is not;…