Embryology day 6 20 First Embryology in context

Embryology day 6 -20

First, Embryology in context • It will be covered again… • ESA 1 – 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 2 s’ – 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 2 CAVITIES BABY IN 2 LAYERS

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 2 s’

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

More words • Trophoblast 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 UTEROPLACENTAL 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) Epiblast Embryoblast (future baby) Hypoblast Primary villi Ectoderm, mesoderm, endoderm 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 ESA 1)