Chapter 5: Formation of Germ Layers and Early Derivatives Flashcards
gastrulation
process by which germ layers are formed through cell movements
embryonic inductions
signals which are exchanged between the germ layers or other tissue precursors
inductors acts on another (responding tissue) so that the developmental course of the latter is different from what it would have been in the absence of the inductor
embryonic shield
cells of inner cell mass become rearranged into an epithelial configuration
epiblast
the main upper layer of cells of the inner cell mass
hypoblast
also known as primitive endoderm
lower layer of cells of inner cell mass
differentiation of cells into epiblast and hypoblast
some cells begin to express transcription factor nanog or Gata 6, which are arranged in a slat and pepper pattern within the inner cell mass
nanog cells become epiblast
Gata 6 cells become hypoblast; produce molecules that increase their adhesive properties, as well as their mobility; move towards lower surface of the ICM (inner cell mass)
anterior visceral endoderm and formation of anteroposterior polarity
lefty-1 and Cerberus-1 (Cer-1)
small group of hypoblast cells becomes translocated to the future anterior end of the embryo
cells first secrete lefty-1 and Cer-1 which inhibit the activity of nodal and Wnt in the overylying epiblast but allow nodal and Wnt-3 expression in he posterior epiblast
bilaminar disk
what the inner cell mass becomes when the hypoblast becomes a well-defined layer, and the epiblast has taken on an epithelial configuration
epiblast is on the dorsal surface and hypoblast on the ventral surface
cell and tissue lineages in the mammalian embryo
[check flashcards]
pg. 76; Fig. 5.1
amnion
layer of extraembryonic ectoderm that ultimately encloses the entire embryo in a fluid-filled chamber called the amniotic cavity
origins of the major extraembryonic tissues
[check textbook for possible test diagram]
pg. 77; Fig. 5.2
parietal endoderm
a continuous layer of extraembryonic endoderm that is lining the inner surface of the cytotrophoblast and comes from the hypoblast
primary yolk sac
complete when the spreading of the parietal endoderm is completed
extraembryonic mesoderm
first cells of extraembryonic mesoderm appear to arise as a transformation of parietal endodermal cells; later joined by extraembryonic mesoderm cells from the primitive streak
primitive streak
gastrulation begins here; a linear midline condensation of cells derived from the epiblast in the posterior region of the embryo through an induction by cells at the edge of the embryonic disk in that region
initially triangular, soon becomes linear and elongates. largely through a combination of proliferation and migration, as well as internal cellular rearrangements called convergent-extension movements
cell movement towards and through the primitive streak
[know Fig.5.5, pg. 79]
as cells of epiblast reach primitive streak, they change shape and pass through it on their way to forming new layers beneath (ventral to) the epiblast
endodermal precursor cells that pass through the anterior primitive streak largely displace the original hypoblast; displaced hypoblastic cells form extraembryonic endoderm
extraembryonic mesoderm
most posterior cells both to enter and leave the streak as it is beginning to elongate; lines the trophoblast and yolk sac, as well as forming blood islands
paraxial, lateral plate, and cardiac mesoderm
arise later and more anteriorly in the primitive streak than the extraembryonic mesoderm
primitive groove
a groove along the midline of the primitive streak that forms as a result of the movement of cells through the primitive streak
primitive node or Hensen’s node
found at the anterior end of the primitive streak; small but well-defined accumulation of cells
major posterior signaling center of the embryo; area through which cells migrate in a stream toward the anterior end of the embryo
body stalk
formed by much of the extraembryonic mesoderm; connects the caudal part of the embryo to the extraembryonic tissues that surround it; later becomes the umbilical cord
changes to cells as they move through primitive streak
bottle cells
mesenchymal cells
epiblast cells have properties typical of epithelial cells: apical and basal surfaces and connection to basal lamina
as they enter the primitive streak, these cells elongate, lose their basal lamina, take on characteristic morphology: now called Bottle cells
when they become free of epiblastic layer in the primitive groove, the bottle cells assume a morphology and characteristics of mesenchymal cells, which are able to migrate as individual cells if they are provided with the proper extracellular environment;
E-cadherin
as the cells convert from epithelial to mesenchymal configuration, they lose E-cadherin molecules
epithelial-mesenchymal transition:
N-cadherin
epiblast —> bottle cells —–> mesenchymal
lose E-cadherin molecules and begin to express the N-cadherin cell adhesion molecules (CAMs) which are necessary for their spreading out from the primitive streak in the newly forming mesodermal layer
hyaluronic acid
expressed by cells of the epiblast; enters the space between epiblast and hypoblast
associated with cell migration in developing systems; tremendous capacity to bind water; functions to keep mesenchymal cells from aggregating during cell migrations
ectoderm and endoderm
the mesoderm ultimately spreads laterally as a thin sheet of mesenchymal cells between the epiblast and hypoblast layers
by the time the mesoderm has formed a discrete layer, upper germ layer (remains of the former epiblast) is called the ectoderm
lower germ layer (displaced the original hypoblast) is called endoderm
regression of the primitive streak
primitive streak begins in the extreme caudal end of the embryo and expands cranially until about 18 days after fertilization
then, it regresses caudally and strings out the notochord in its wake
regression is accompanied by the establishment and patterning of the paraxial mesoderm (which gives rise to somites and ultimately the segmental axial structures of the trunk and caudal regions of the body)
tail bud
mass of mesenchymal cells;; the most caudal extent of the primitive streak; plays role in forming the most posterior portion of the neural tube
notochord
cellular rod running along he longitudinal axis of the embryo just ventral to the central nervous system
inductive signals from the notochord…
- stimulate the conversion of overlying surface ectoderm into neural tissue
- specify the identity of certain cells (floor plate) within the early nervous system
- transform certain mesodermal cells of the somites into vertebral bodies
- stimulate the earliest steps in development of the dorsal pancreas
notochord produces noggin, chording and shh; induces the neural plate within the overlying ectoderm; does not stimulate the formation of anterior parts of he brain or head structures
oropharyngeal membrane
small region where embryonic ectoderm and endoderm abut without any intervening mesoderm; marks the site of the future oral cavity
prechordal plate
located between the cranial tip of the notochord and the oropharyngeal membrane; small aggregation of mesodermal cells closely apposed to endoderm
emits molecular signaling that are instrumental in stimulating the formation of the forebrain; similar to the anterior visceral endoderm in mammals
first cells passing through the node form the prechordal plate
source of signals that are important for the survival of neural crest cells that emigrate from the early forebrain
notochordal process
rodlike aggregation of cells left behind as the primitive streak regresses and leaves the prechordal plate and notochordal precursor cells
molecular aspects of gastrulation
[see the diagram in notes]
[see Fig. 5-8, page 82]
establishment of anteroposterior axis
depends on the activation of Wnt antagonist Dkk 1 in the future anterior part of the embryo, thus confining Wnt activity to the future posterior part of the embryo, where it induces the expression of Nodal
later, the production of lefty-1 and Cer-1 (inhibitors of Nodal) confine Nodal expression in the posterior end of the embryo
Nodal induces the formation of the primitive streak, the definitive endoderm, and the mesoderm
chordin and Vg1 also induce the formation of a primitive streak
primitive node
cells found at the tip of the primitive streak; express chordin, goosecoid, and hepatic nuclear factor-3beta (now called Foxa-2)
Foxa-2 (HNF-3beta)
winged helix transcription factor; involved in the formation of the primitive node; vital for the establishment of midline structures cranial to the node;
required for initiation of notochord function
Goosecoid
homeodomain transcription factor; prominently expressed in organizer region; activates chordin, noggin
helps stimulate the formation of a secondary body axis
chordin and noggin
signaling molecules associated with the node; involved with neural induction
under the influence of goosecoid and Foxa-2, cells of the forming notochord produce noggin and chordin
nodal
expression on the left side of the embryo is key to forming left-right asymmetry
[see diagram of left/right axis formation in notes]
expressed throughout the posterior epiblast before gastrulation, but its activity is concentrated at the primitive node during gastrulation;
T gene
activated by products of Foxa-2 and goosecoid genes; necessary for normal movements of future mesodermal cells through the primitive streak during gastrulation
regulation of Hox around the primitive streak
three molecules: retinoic acid, Wnt and FGF act on transcription factor Cdx in the area of the regressing primitive streak just behind the last-forming somites
Cdx acts on the Hox genes
anterior visceral endoderm
anterior hypoblast; initiates head formation
subdivided into anterior and posterior portions; anterior part serves as a signaling center for early heart formation and posterior part becomes part of the prechordal plate complex and induces formation of head
head formation (Lim-1, cerebrus-like 1, Otx-2)
necessary to block BMP-4 signal using noggin and block Wnt signal using Dkk 1
Lim-1 mutant mice have no head; cerebrus-like 1 is also important to head formation
Otx-2 is another transcription factor present in the head signaling center, but is also a general marker of the induced anterior region of the central nervous system
BMP-4 and the induction of neural tissue
noggin and chordin, given off by the notochord, inhibit the inhibitor BMP-4 in the dorsal ectoderm; in the absence of BMP-4, dorsal ectoderm forms neural plate as a default state
neural plate
elongated patch of thickened epithelial cells; border of neural plate is specified by exposure of those cells to a certain [BMP-4]; this is the region from which the neural crest arises
restriction ==> determination ==> differentiation
a restriction even occurs at the point when cells become committed to a certain pathway and are no longer able to become another pathway
when the last restriction even occurs, the fate of the cells is fixed and they are said to be determined
restriction and determination signify the progressive limitation of the developmental capacities in the embryo
differentiation describes the actual morphological or functional expression of the portion of the genome that remains available to a group of cells;; phenotypic specialization of cells
CAMs
cell adhesion molecules; suspended cells of a similar type have a strong tendency to aggregate
molecular basis for left-right asymmetry
beating of cilia around the primitive node results in a directional current leading to the expression of nodal and lefty-1 along the left side of the primitive streak over a very restricted developmental time
lefty-1 helps prevent the diffusion of left-determining molecules to the right side of the embryo
nodal results in activation of Ptx-2 (TF on left side) which leads to later asymmetric development such as the rotation of the gut and stomach, position of spleen, asymmetric lobation of the lungs
shh, retinoic acid, and FGF-8 are swept to the left and lead to the expression of nodal and later Ptx-2
planar cell polarity
mechanism directing cells to orient themselves along an axis in the plane of a flat epithelial tissue;
in the node, Disheveled is concentrated in the posterior region of cells and Prickle is arranged along the anterior border
basal body of cells of the node is associated with Disheveled and the cilium that protrudes from the cells does so at an angle that produces leftward fluid current when the cilium beats
situs inversus
condition in which the left-right asymmetry of the body is totally reversed
Kartagener’s syndrome
situs inversus is associated with respiratory symptoms resulting from abnormalities of the dynein arms in cilia
cadherins
single transmembrane glycoproteins typically arranged as homodimers that protrude from the cell surface
in the presence of Ca2+, cadherin dimers from adjacent cells adhere to one another and cause the cells to become firmly attached to one another
Ig-CAMs and N-CAM
have immunoglobin-like extracellular domains; either bind to similar (hemophilic binding) or different (heterophilic binding) CAMs and do so without the mediation of calcium ions
N-CAM strongly expressed within the developing nervous system; do not bind to cells as tightly as cadherins and they provide for fine-tuning of intercellular connections
neural inductions and CAMs
before primary induction of CNS, ectoderm expresses N-CAM and E-cadherin (aka L-CAM);
after primary induction, cells within the newly formed neural tube continue to express N-CAM but they no longer express E-cadherin (also strongly express N-cadherin)
The ectoderm ceases to express N-CAM and continues to express E-cadherin
integrins
attaches cells to components of basal laminae and the ECM
