embryology

• the embryonic period starts from the 3rd week of the prenatal period, 2 weeks after the last menstrual period
• fertilisation occurs in the oviduct (fallopian tube)
• egg is surrounded by the zona pellucida (ZP). this is a glycoprotein layer that, when fertilised, starts a chemical reaction to ensure only one sperm can implant.
• the cellular layer of the egg is the corona radiata
• sperm binding to ZP triggers acrosome and cortical reactions to limit sperm to one
• cleavage occurs from day 2-3. the zygote undergoes progressive divisions, still encased in ZP. Mass of cells by day 4/5 is called a morula. individual cells are known as blastomeres. These are pleuripotent. Cell number increase, cell size stable
• blastulation at day 4. Morula undergoes compaction to become a blastocyst. Peripheral cells become trophoblasts, which form the placenta. Central cells become the inner cell mass, which forms the embryo proper. The trophoblast cells form a cavity that creates a basolateral Na+ gradient; water follows.
• on day 5 the zygote reaches the uterus and becomes a blastocyst
• Hatching occurs at day 5. The ZP is enzymatically digested to form a hole that the blastocyst pops out of in preparation for implantation. It absorbs nutrients from uterine secretions
• Implantation occurs from day 6-10. Apposition occurs (embryo makes contact with uterine wall). ICM aligns closest to the wall. Adhesion occurs (embryo penetrates uterine wall). Trophoblast cells and ICM undergo differentiation
• During implantation, the trophoblasts differentiate into syncytiotrophoblasts at the invading front to form the placenta precursor, and cytotrophoblasts that remain around the embryo.
• During implantation, the ICM differentiates into the epiblast (embryo proper) and the hypoblast (primitive endoderm and future extraembryonic structures)
• ectopic pregnancy is when the embryo implants outside of uterus - unilateral lower abdominal/pelvic pain, cramping, vaginal bleeding
• chemical pregnancy is when a fertilised egg fails to develop - late period, cramping, tissue loss
• a blighted ovum is when a fertilised egg has the supporting cells, but no or an improperly developed ICM - ultrasound diagnosis, induce miscarriage or wait for natural
• miscarriage occurs in up to 50% of fertilisation events, usually in the first 12 weeks - cramping, vaginal bleeding, tissue loss
• week 1 involves fertilisation and early cell division (cleavage)
• week 2 involves the development of nutrient support systems
• at the start of week 2, an embryo becomes bilaminar with the epiblast and hypoblast
• the amnion is derived from the epiblast, and the chorion is derived from the hypoblast
• in week 2, the synctiotrophoblast continues to invade, eventually making contact with uterine capillaries
• syncytiotrophoblast produces bHCG to maintain ovarian hormone function until the placenta can create it itself
• extraembryonic structures formed in week 2: amnion + cavity, umbilical connecting stalk (allantois), chorion + chorionic cavity
• the amnion is initially the 'dorsal' extension of epiblast with fluid cavity abutting the epiblast layer. It grows and folds with the embryo. The amniotic cavity expands to fill chorionic cavity, where the two membranes will eventually meet and fuse.
• The connecting stalk/allantois is the extension by which the embryo remains attached to the placenta, umbilical vessels form within
• the chorion forms from hypoblast-derived extraembryonic mesoderm. its cavity is also called the extraembryonic coelom. This disappears as the amniotic cavity grows.
• In the placenta, chorion develops into villi, which are bathed in maternal blood to facilitate nutrient transfer
• hypoblast cells > extraembryonic endoderm > yolk sac
• the mother's blood and foetal blood do not come into contact!!! nutrient transfer happens across the membrane
• chorionic villus sampling (CVS) has the same DNA as the embryo, but differentiates separately, so you can sample some to test for genetic abnormalities
• the cytrophoblastic shell separates the decidua basalis in the placenta from the area in which nutrient exchange occurs. This is between the main stem villus and endometrial veins/arteries that run around in a bloody fluid soup
• the yolk sac's role is in nutrient transfer in early implanted embryo via blood vessels that form in mesoderm walls
• the yolk sac is responsible for blood cell development until the liver takes over in week 6
• blood vessels will develop into umbilical blood vessels
• the secondary (definitive) yolk sac can be seen by ultrasound
• the prechordal plate is the thickening of hypoblast cells at one site of embryonic disc, which is the future mouth site.
• in week 3, the germ layers are formed and neurulation occurs
• gastrulation is the process of the embryo becoming trilaminar
• in gastrulation, the epiblast forms three primary germ layers; the endoderm (stomach), mesoderm (middle), and ectoderm (spine)
• Gastrulation begins at the primitive streak, which coincides with the first missed menstrual period, and defines positional differences (cranial/caudal etc)