Week 3 Flashcards
Lewis Wolpert Quote
It is not birth, marriage or death, but gastrulation, which is truly the mostimportant time in your life. Lewis Wolpert (1986) …
WEEK 3 OF DEVELOPMENT
RAPID DEVELOPMENT OF CONCEPTUS (embryo+placenta)
TRILAMINAR EMBRYONIC DISC FORMS (week 2=bilaminar, week three–>trilaminar)
FORMATION OF:
PRIMITIVE STREAK, EMBRYONIC MESODERM, NOTOCHORD
NEURAL TUBE
SOMITES
INTRAEMBRYONIC COELOM
BLOOD VESSELS AND CELLS
CHORIONIC VILLI
PRIMITIVE STREAK AND FORMATION OF THE TRILAMINAR EMBRYO
ORIENTATION
BILAMINAR EMBRYONIC DISK SITS ON TOP OF YOLK SAC
BIRD’S EYE VIEW OF EMBRYO SHOWING POSITION OF PS
migration of epiblast cells move towards line, pinch off groove, move away.
Oropharyngeal membrane-remains bilaminar
Cardiogenic area-heart formation
most of hypoblast replaced by migrating cells from epiderm–>endoderm
Rest of epiderm=mesoderm
Gastrulation continues through week 4
GASTRULATION
Formation of the three germ layers
GASTRULATION- an epithelial to mesenchymal transition
mesoderm goes everywhere except oropharangeal membrane and cloacal membrane.
the three layers formed in gastrulation become everything.
MIGRATION OF EPIBLAST CELLS
OROPHARYNGEAL MEMBRANE AND CLOACAL MEMBRANE – DISC REMAINS BILAMINAR
IF DEVELOPMENT PROCEEDS NORMALLY THE 3 GERM LAYERS GIVE RISE TO THE TISSUES/PORGANS INDICATED IN THIS SLIDE
CONGENITAL DEFECTS OF PRIMITIVE STREAK
Teratoma in female
TERATOMA = TUMOR OF PRIMITIVE STREAK ORIGIN
EXPLAIN SITES AND CONTENTS
MORE COMMON IN FEMALES THAN MALES
MAY CONTAIN TISSUES FROM ALL THREE GERM LAYERS
CAN BECOME MALIGNANT - REMOVED SURGICALLY
Sacrococygeal teratoma=tumor. Forms when primitive streak fails to recede. Pluripotent cells, so inside tumor there are all tissue types. Surgically removed after birth. Typically benign, but can develop to become malignant. happens in 1/35,000 births. Most common form of cancer in newborns.
Other Examples of Teratomas
CONGENITAL DEFECTS OF PRIMITIVE STREAK-CAUDAL DYSGENESIS
CAUDAL DYSGENESIS OR DISPLASIA
Sirenomelia
CAUDAL DYSGENESIS
NOT ENOUGH MESODERM IN CAUDAL REGION
EFFECTS DEVELOPMENT OF LIMBS (SHORT AND FUSED)
VERTEBRA, KIDNEYS AND GENITAL ORGANS ALSO AFFECTED - SMALL OR AGENESIS
Not enough mesoderm. Caudal area most affected. under-developed lower half of body. Might be affected by the gene Brachyury, which encodes a transcription factor that causes teh formation of mesoderm. Graded affects
NEURULATION - WEEK 3
NEURULATION = DIFFERENTIATION OF NEURAL ECTODERM - IMPT
ECTODERM OVER NOTOCHORD THICKENS = NEURAL PLATE
NOTOCHORD INDUCES FORMATION OF PLATE
NEURAL PLATE INVAGINATED AND SOME CELLS CAUGHT AT CREST OF INVAGINATION DIFFERENTIATE
NEURAL FOLDS MEET AND FUSE DORSALLY
CLOSURE BEGINS IN MIDDLE OF EMBRYO AND PROCEEDS ANT AND POSTERIORLY
CLOSURE FORMS:
NEURAL TUBE
NEURAL CREST CELLS
SURFACE ECTODERM RESEALS
NCC LATER MIGRATE TO MANY PLACES - HAVE MANY DERIVATIVES
Neural tube: plate of thick cell layers
cells on edge of fold pinch together. the neural tube becomes teh spinal cord and brain
Neural crest cells have many derivatives. It begins formation in the middle of the embryo and works its way out.
The openings left in the tail and head are neuropores. they eventually close
NEURULATION (images)
Mesoderm Development
During week three mesoderm differentiates into distinct regions
Paraxial mesoderm
Intermediate mesoderm
Lateral mesoderm - somatic and splanchnic
DEVELOPMENT OF NOTOCHORDAL MESODERM
MESODERM DIFFERENTIATE AS SHOWN ABOVE
MENTION DERIVATIVES OF EACH TYPE OF MESODERM
Notochord–>below neural tube. Notochord eventually becomes part of spinal cord
Paraxial mesoderm makes blocks of tissue (somites) form caudally ~44 pairs of somites after somatogenesis
DEVELOPMENT OF PARAXIAL MESODERM
DEVELOPMENT OF THE SOMITES
SOMITE FORMATION
SOMITES FORM IN PAIRS ON EITHER SIDE OF NOTOCHORD
APPEAR AS BUMPS EXTERNALLY
BEGINS ON DAY 20 AND EXTENDS INTO WEEK 4
42-44 PAIRS OF SOMITES FORM
EACH SOMITE DIFFERENTIATES AS ABOVE
DEVELOPMENT OF INTERMEDIATE MESODERM
DEVELOPMENT OF LATERAL PLATE MESODERM
Summary of Mesoderms and Their Fates
VASCULOGENESIS
VASCULOGENESIS:
MESODERM RESPONDS TO FGF2 – DIFFERENTIATES INTO HEMANGIOBLASTS – A PRECURSOR CELL THAT GIVES RISE TO BLOOD VESSELS AND BLOOD CELLS
SOME HEMANGIOBLASTS BECOME PLURIPOTENT STEM CELLS THAT EVENTAULLY GIVE RISE TO ALL THE TYPES OF BLOOD CELLS
SOME HEMAGIOBLASTS RESPOND TO VEGF AND DIFFERENTIATE INTO ANGIOBLASTS. VEGF IS SECRETED BY SURROUNDING MESENCHYME
ANGIOBLASTS THEN RESPOND TO VEGF BY DIFFERENTIATING INTO ENDOTHLIAL CELLS – CELLS THAT LINE OUR BLOOD VESSELS
HEMANGIOBLASTS -DEVELOP FROM MESODERM - AGGREGATE TO FORM BLOOD ISLANDS – EXPLAIN ISLAND
AS HEMANGHIOBLASTS DIFFERENTIATE, ENDOTHELIUM BECOMES LOCATED ON THE OUTSIDE AND CELLS CAUGHT IN CENTER DIFFERENTIATE INTO BLOOD CELLS.
ADJACENT BLOOD ISLANDS FUSE FORMING PRIMITIVE BLOOD VESSELS
EVENTUALLY BLOOD ISLANDS HOOK UP TO FORM CIRCULATORY SYSTEM
HEART BEGINS FORMING VIA THIS PROCESS IN CARDIOGENIC AREA- BECOMES FUNCTION BY END OF THIRD WEEK
FIRST ORGAN SYSTEM TO FUNCTION IN HUMAN
HISTOSECTION SHOWING A BLOOD ISALND
MOUSE HETEROZYGOUS FOR VEGF – ONE MUTATED ALLELE – BLOOD VESSELS DO NOT FORM IN YOLK SAC – IT DIES
ANGIOGENESIS
ONCE PRIMITIVE VASCULATURE HAS FORMED, VESSELS CAN SPROUT AND GROWTH OFF THE FIRST VESSELS
MANY FACTORS CAN REGULATE THE COMPLEX PROECESS OF ANGIOGENESIS
VEGF IS ONE FACTOR IMPORTANT IN THIS PORCESS
Primitive Circulation as It Appears in A Human Embryo on Day 21
Teratogenesis and Week 3
Embryo is very sensitive to teratogens during week 3
E.g., alcohol kills cells in anterior midline of embryonic disc during week3
Results in craniofacial deficiencies in midline= holoprosencephaly - craniofacial deficiencies in midline
Woman may not know she is pregnant at this time
