Gastrulation

1
Gastrulation

• establishes multilayered body plan
• layers formed ectoderm, mesoderm, endoderm
• patterns vary throughout animal kingdom
• cell movements involve entire embryo

2
Types of cell movements

• epiboly movement of epith. sheets as a unit to
  enclose deeper layers of embryo
• invagination infolding of a region of cells
• involution inward movement of an outer layer
  spreads over internal surface of external cells
• ingression migration of indiv. cells from
  surface to interior
• delamination splitting of one cellular sheet
  into 2 or more parallel sheets

3
Sea urchin gastrulationcell movements

• after hatching, veg. side thickens, flattens
• specialized cells (primary mesenchyme) appear in
  veg. plate
• extend filopodia dissociate from epith. sheet
  and enter blastocoel
• lineage pm cells are daughter cells of
  micromeres
• explore environment localize in ventrolateral
  regions of blastocoel
• form ventrolateral clusters and syncytial cables
• secrete calcium carbonate spicules (larval
  skeleton)

4
Cues for migration of pm cells

• initial contacts of pm cells
• other cells in veg. plate
• hyalin layer
• basement membrane lining blastocoel
• change in affinity directs migration
• decreased affinity for neighbors, hyalin layer
• 100 fold increased affinity for basal lamina,
  other ECM components of blastocoel

5
Cues for adhesion of pm cells

• exist in ventral region (evidence migration of
  donor cells)
• sensed by filopodia of pm cells

6
Archenteron invagination Stage I

• other cells in veg. plate fill in gaps as pm
  cells leave gt flattening
• veg. plate invaginates
• invag. archenteron (primitive gut endoderm)
• opening blastopore (anus)

7
Stage I Mechanisms

• caused by buckling of hyalin layer
• laminae in hyalin layer
• outer mostly hyalin
• inner mesh of fibropellin proteins
• veg. plate cells secrete chondroitin sulfate
  proteoglycan (CSPG) into inner laminae
• CSPG absorbs water gt swelling gt buckling
• veg. plate invaginates
• second force may come from movements of epith.
  cells adjacent to veg. plate

8
Stages II III

• Stage II archenteron extends dramatically
• due to convergent extension (cells intercalate to
  narrow tissue)
• Stage II formation of secondary mesenchyme at
  tip of archent.
• filopodia attach to wall, pull up archent.
• mouth forms where archent. meets wall

9
Avian gastrulation

• differs from sea urchins and amphibians similar
  to mammals
• cleavage gave rise to blastodisc atop yolk
• 2 regions
• area pellucida (clear center above subgerm.
  cavity)
• area opaca (contact with yolk)
• most cells remain at surface (epiblast)
• hypoblast forms from polyinvagination islands and
  posterior margin

10
Epiblast and hypoblast

• epi. and hypo. joined at area opaca space
  blastocoel
• epiblast gt embryo extraembryonic membranes
• hypoblast gt yolk sac, stalk to digestive tube
• next major event formation of primitive streak

11
Primitive Streak

• originates as thickening of epiblast near
  posterior region
• narrows and moves anteriorly
• cells converge depression forms (primitive
  groove)
• anterior end of streak gt Hensens node
• fate of cells migrating through streak / node
• node gt form foregut, head mesoderm, notochord
• streak gt endoderm, mesoderm

12
Formation of endoderm, mesoderm

• 1st cells to migrate form foregut (endoderm)
• cells move thru blastocoel displace hypoblast
  (Fig. 6.28)
• remaining hypoblast in anterior region forms
  germinal crescent
• next, epiblast cells ingress to form head
  mesoderm, notochord (head process appears)
• future meso., endo. ingress down length of embryo
• streak regresses

13
Endoderm, mesoderm contd.

• outcome
• epiblast is now ectoderm
• mesoderm fills blastocoel and notochord has
  formed
• endoderm has displaced hypoblast below area
  pellucida
• anterior-posterior gradient of maturity
  established
• ectoderm proliferates surrounds yolk by epiboly
  (takes 4 days!)

14
Mechanisms Posterior endoderm, mesoderm

• select cells in epiblast express HNK-1 on cell
  surface
• HNK-1sulfated glucuronic acid
• migrate to posterior margin to form meso., endo.
• attracted to chemical in posterior margin
  repelled by anterior marginal tissue
• destroy HNK-1 cells with antibodies gt no
  posterior endo. or meso.

15
Gastrulation in mammals

• gastrulation proceeds as though inner cell mass
  were on top of imaginary yolk
• modification trophoblast cells (supplemented by
  mesoderm)
• 1st step formation of hypoblast (primitive
  endoderm or yolk sac endoderm)
• result gt 2 layered embryo (epiblast, hypoblast)
• top layer of epiblast splits off to form amnion
  (and amniotic cavity)
• cavity fills with amniotic fluid - shock
  absorber

16
Gastrulation in mammals

• cells on surface of embryonic epiblast (future
  endoderm, mesoderm) begin to migrate thru
  primitive streak
• cells cease to express E-cadherin gt migrate
  individually
• those migrating thru Hensens node form notochord
  
• while rearrangement of inner cell mass takes
  place, trophoblast cells begin to form placenta

17
Extraembryonic membranes

• yolk sac (hypoblast), amnion (epiblast),
  allantois (outpocketing of yolk sac), chorion
  (trophoblasts)
• initial trophoblast cells cytotrophoblasts
• give rise to syncytiotrophoblast cells (nuclear
  division without cytokinesis)
• together, the 2 form the chorionic villi
• developmental pattern
• primary villi (cyto. encased in syncytio.)
• secondary villi (penetrated by mesoderm)
• tertiary villi (blood vessels, branching)

18
Placenta

• cytotrophoblasts adhere to uterine endometrium
  (e.g. P-cadherin)
• cytotrophoblasts also secrete proteases to break
  assoc. between endometrial cells, remodel blood
  vessels
• villi are surrounded by lacunae (cavities filled
  with blood)
• maternal blood never mixes with blood of embryo
• villi chorion (embryonic / fetal portion of
  placenta)
• modified endometrium maternal portion of
  placenta

19
The placenta as endocrine tissue

• besides facilitating exchange of gases
  nutrients, placenta secretes pregnancy hormones
• 3 major hormones produced
• human chorionic gonadotropin (hCG) gt stimulates
  progesterone production in corpus luteum
• progesterone (after 1st trimester primate
  experiments)
• chorionic somatomammotropin (placental lactogen)
  gt maternal breast development