Drosophila Emrbyonic Stages Flashcards
Stage 4
- 50 minutes
- syncytial blastoderm stage
- blastoderm nuclei perform last 4 cleavage divisions (10-13)
- blastoderm nuclei to outside = clear rim with visible nuclei
- pole cells form, to posterior
First cells to form in the embryo
pole cells
Stage 4 key events
- syncytial divisions 10-13
* pole cell formation
Stage 5
- blastoderm nuclei elongate
- pole cells start to move to dorsal sides
- membrane furrows separate the individual blastoderm nuclei
- nuclei sperical to elongate
Stage 5 key event
cellularization
Stage 6
- gastrulation - invagination of mesoderm initiated
- endoderm - anterior and posterior midgut invaginate
- blastoderm cells at posterior form a dorsal plate under pole cells - onset of germ band elongation
- cephalic furrow visible at 65% (from post)
Stage 6 key events
- onset of gastrulation
- formation of ventral and cephalic furrow
- dorsal shift of pole cells
Stage 7
• gastrulation completed
• midgut and hindgut invaginatele cells tilts inward
• posterior dorsal plate that carries the pole cells tilts inward and forms a pocket
• whole pocket called proctodeal invagination (part of anal passage)
• 3 folds - cephalic, anterior, posterior furrows
(only cephalic lasts til stage 9)
Stage 7 key events
- completion of gastrulation
* pole cells in a pocket
Stage 8
- rapid germ band elongation
* posterior tip of the germ band to 60% egg length (from posterior)
Germ band elongation
the gradual extension of the germ band over the dorsal side of the embryo
Stage 8 key events
rapid phase of germ band extension to 60% egg length
Stage 9
- first neural stem cells (neuroblasts) segregate from the outer ectoderm
- by the end, the germ band exhibits a 3-layered architecture
Three layered architecture
mesoderm
neuroblasts
outer ectoderm
(stage 9)
Stage 9 key events
- germ band elongation to 70% egg length
* beginning of nervous system formation
Stage 10
- appearance of stodeum
- germ band continues to stretch to 75%
- yolk sac
Stodeum
gives rise to foregut and annexes
• formed by cells that are internalized at the anterior ventral side of the embryo
• site of internalization = stomodeal invagination
Stage 10 key events
- germ band elongates to 75% egg length
* stomodeum invaginatees
Stage 11
- first signs of germ band retraction
- segmental furrows that subdivide the germ band into metameric units
- thorax and abdomen boundaries = deep folds
- head’s gnathan segments visible ventral to cephalic furrow
- cephalic furrow becomes shallower and disappears
Stage 11 key events
segmentation becomes visible
Stage 12
• shortening of the germ band
- segmentation becomes prominent
- yolk sac pushed to dorsal side and is covered by amnioserosa
• anterior and posterior midgut meet at 60% egg length
• ventral cord separates completely from the epidermis= first axons in the ventral nerve cord appear
Stage 12 key events
- onset of germ band retraction
* fusion of anterior and posterior midgut
Stage 13
- initiated by completion of germ band shortening
- cells in primordial organs start to differentiate
- after germ band retraction = dorsal closure and head involution
- midgut primordium forms 2 longitudinal plates flanking the central yolk that fuse to form closed tubes
Dorsal closure
- starts in stage 13
- epidermal primordium from ventral to dorsal
- closes the gap in the dorsal part
• begins at both ends, towards middle
Stage 13 key events
• start of organ differentiation and dorsal closure
Stage 14
• begins with onset of head involution • 3 major morphogenetic events - progression of dorsal closure - closure of the midgut - head involution
- epidermis covers 80% of embryo (leaving small part on dorsal side)
- most head structures (inc gnathal segments) brought inside
- obv by involution of clypeolabrum - nose shaped at front
- midgut plates fuse ventrally and dorsally to form closed tubes
Gnathal segments
mandible, maxilla, and labium
Clypeolabrum
part of the mouth
Stage 14 key events
- progression of dorsal closure
* head involution
Stage 15
- gut forms a closed tube, encloses the yolk sac, develops 3 constrictions
- anterior and posterior, midgut lumen is continuous w/ the lumen of the foregut and hindgut
- dorsal closure is complete
- head involution continues
Stage 15 key events
dorsal closure of epidermis and midgut
Stage 16
- 3 circular midgut constrictions become visible that subdivide the midgut into 4 chambers
- head involution is completed
- shortening of the ventral nerve cord - posterior tip to about 60% egg length
Stage 16 key events
- subdivision of the midgut
* shortening of the ventral nerve cord
Stage 17
- until the hatching
- differentiation of the organs complete
- fully developed tubes of the digestive system are visible
- movements of the embryos w/i the egg shells can be observed
Stage 17 key events
- completion of organogenesis
* movements of the embryo w/i the vitelline envelope
