Embryology Lecture 1

✽ Development ☞ Growth (↑ in mass of tissues) ☞ Differentiation (↑ in complexity) ✽ Development does not stop at birth ☞ Development of teeth, breast… ☞ Brain triples in weigth between birth & 16 yrs ☞ Most development completed by the age of 25 ✽ Divided into: ☞ Prenatal period (embryonic period: 3rd – 8th week) ☞ Postnatal period

✹ Infancy (neotate – 1st yr) ✹ childhood (13 mo – 12 yrs) ✹ puberty (12 – 15 ♀ , 13 – 16 ♂ yrs) ✹ adolescence (12 – 17 yrs) ✹ adulthood (18 – 21 yrs)

PURPOSE OF LEARNING EMBRYOLOGY. BASIC FACTS OF EARLY HUMAN DEVELOPMENT.

☞ Literally means “study of embryos”, 3rd – 8th week ☞ Generally refers to “prenatal development”: both embryos and fetuses. ☞ Developmental anatomy: prenatal and postnatal periods. ☞ Teratology: study of abnormal development (Birth defects, congenital malformations) DIFFERENCES BETWEEN CONCEPTUS, EMBRYO, FETUS

☞ Normal development vs. birth defects ☞ Illuminates gross anatomy: normal / abnormal ☞ Obstetrics: applied embryology: mother vs embryo/fetus ☞ Pediatrics → congenital anomalies: spina bifida, congenital heart diseases ☞ Surgery → cleft palate, cardiac defects

1. Preformation: 18th century. ➝ Development is merely the growth of an already present diminutive being. 2. Epigenesis: (Caspar Friedrich Wolff, 1759) ➝Development results from growth & differentiation of specialized cells. 3. Recapitulation (Carl Ernst von Baer, 1928): Biogenetic Law ➝ Early stages in devpt are not like the adult stages. 4. Induction (Hans Spemann, 1869-1941): Nobel Prize 1935. ➝ Inducer, e.g. optic vesicle → lens (one tissue determines the fate of another) ➝ Organizer ➝ Homeoboxes (DNA sequences, encoding AA)

5. Germ layers ➝ Morphogenetic processes - cavitation - invagination - migration - proliferation ➝ Primordium (anlage) – earliest discernible indication of an organ or a part of it. 6. Epithelio - mesenchymal interactions ➝ skin, hair, nails: Mesenchyme induces & specifies an epithelium. 7. Morphogenetic fields: organizing factors ➝ determination: ectoderm → neural tissue 8. Morphogenetic processes ➝ rearrangement of cells - Relative cell movement - Cell adhesiveness - Invagination - Condensation - Fusion - Cell death - Proliferation - Differential growth rates

9. Cell death (necrosis): genetic control ➝ Interdigital cell death ➝ Establishment of definite number of neurons ✹ ↓ cell volume ✹ chromatin condensation ✹ cell dies ✹ apoptosis ✹ autophagy (with / without lysosomal participation) ✹ swelling of a cell & cellular membrane ruptures 10. Mechanical factors

Figure 1 : The dwarf embryo as imagined by Leonardo da Vinci from the 15th century (on the left)

Turner's syndrome

Figure 7 : Teratoma in the coccygeal region at the end of the spine

Trisomy 18: flecked, crossed-over fingers; deformed feet

Trisomy 13: Cleft palate, lip and jaw Trisomy 18: Dolichocephaly

Turner's syndrome