Embryology day 6-20

First, Embryology in context It will be covered again… ESA1 Gastro: development of foregut, midgut, hindgut ESA 2 Repro: fertilization, urogenital embryology, abnormal implantation Musculoskeletal: muscles and bones Cardiovascular: heart and circulation ESA 3 Urinary: renal developmental abnormalities

Early Development in Brief… Week 1: Egg travels down fallopian tube, dividing as it goes, until it reaches the uterus Week 2: ‘The week of 2s’ Blastocyst divides in 2: ‘Baby’, Placenta Week 3: Embryo now has 3 layers Ectoderm, Endoderm, Mesoderm

Early Development in pictures

WEEK ONE

Week one Day 5 Blastocyst reaches uterus Day 5-10 Implantation of blastocyst into uterine wall

Up to 50% of blastocysts are abnormal, and may be lost before the mother even knows she is pregnant.

Abnormalities in week one Abnormal implantation: Ectopic pregnancy Abnormal blastocyst: Abortion

WEEK TWO PLACENTA BABY BABY IN 2 LAYERS 2 CAVITIES

Abnormalities in week two Abnormal blastocyst: Abortion Possible molar pregnancy (see repro module)

WEEK 3 Baby develops 3 layers …and things start to get …complicated

Some words… Embryo = ? Fetus = ? ‘Embryo’ = ‘that which grows’ 0-8 weeks (not quite human-looking) Fetus = ‘offspring’ 8 weeks – term (looks human)

Week 2 is the ‘week of 2s’

Some words ‘Blast’ = immature Blastocyst = ? ‘Tropho’ = nourish Trophoblast = ? Embryoblast = ?

More words Trophoblast divides in two: Embryoblast divides in two: Cytotrophoblast syncytiotrophoblast Embryoblast divides in two: Epiblast Hypoblast

Week two: PLACENTA Blastocyst divided into placenta and baby Placenta = Trophoblast 2 layers: cytotrophoblast has proliferating cells that move into syncytiotrophoblast Syncytiotrophoblast invades maternal capillaries to form UTERO-PLACENTAL CIRCULATION

Week two: BABY Inner cell mass of blastocyst becomes embryoblast Divides in two: epiblast and hypoblast Epiblast from all 3 embryological layers Hypoblast forms lining of future yolk sac

Week two: ANOTHER LAYER (not baby, not placenta) A new layer of mesoderm forms outside between baby and placenta from the hypoblast ‘Extra-embryonic mesoderm’ A big space develops within it, so that mesoderm now lines a new cavity: chorionic cavity The connecting stalk becomes the….

This mesoderm invaginates the primary villi of the cytotrophoblast, and forms capillaries These now link up baby and mother via the connecting stalk to nourish the baby

More words Chorion = ‘skin’ = outer membrane Amnion = ‘lamb’ = inner membrane which surrounds baby Yolk sac = provides nourishment

Syncytiotrophoblast (‘invading layer’) Utero-placental circulation Trophoblast (future placenta) Blastocyst Cytotrophoblast (proliferating cell layer) Primary villi Epiblast Ectoderm, mesoderm, endoderm Embryoblast (future baby) Hypoblast Lining of secondary yolk sac, Extra-embryonic mesoderm

WEEK 3 Baby develops 3 layers …and things start to get …complicated

So: Gastrulation The process by which 3 layers are formed First, a streak appears on the surface of the epiblast Cells start to move toward it, then invaginate underneath First ones to invaginate become endoderm, next ones become mesoderm, ones on top are ectoderm

Week three: THE TRICKY BIT Gastrulation occurs Formation of 3 layers of Neurulation begins Formation of neural tube

Endoderm forms… Gut Lung Bladder

Mesoderm forms… Muscle Skeleton Connective tissue Mesentery (in gut) Heart One foregut structure (from dorsal mesentery) Joins extra-embryonic mesoderm

Ectoderm forms… Nervous tissue Skin Mouth and anus

Abnormalities week 3 onward Abnormal development of three layers: Neural tube defects Lower limb abnormalities Craniofacial abnormalities

Abnormalities associated with defective gastrulation Often cause death Toxins (alcohol) may kill off cells migrating along the midline: brain and facial deformities Insufficient mesoderm may form caudally: sirenomelia (legs fused ‘mermaid-like’)

Are these good ESA 1 questions? The gut is comprised of which embryological layer? In which week of development does this layer begin to develop? What is likely consequence of a significant chromosomal abnormality in the first weeks of development? What function does the yolk sac have in the developing embryo? How does a neural tube defect occur?

Note: Remember that exam questions must… …examine clinically important material …be clear and unambiguous …not test irrelevant details …ideally integrate with other subjects (therefore learn your GI embryology for ESA1)