Chapter 5: Formation of Germ Layers and Early Derivatives

Question 1

gastrulation

Answer 1

process by which germ layers are formed through cell movements

Question 2

embryonic inductions

Answer 2

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

Question 3

embryonic shield

Answer 3

cells of inner cell mass become rearranged into an epithelial configuration

Question 4

epiblast

Answer 4

the main upper layer of cells of the inner cell mass

Question 5

hypoblast

Answer 5

also known as primitive endoderm

lower layer of cells of inner cell mass

Question 6

differentiation of cells into epiblast and hypoblast

Answer 6

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)

Question 7

anterior visceral endoderm and formation of anteroposterior polarity

lefty-1 and Cerberus-1 (Cer-1)

Answer 7

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

Question 8

bilaminar disk

Answer 8

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

Question 9

cell and tissue lineages in the mammalian embryo

Answer 9

[check flashcards]

pg. 76; Fig. 5.1

Question 10

amnion

Answer 10

layer of extraembryonic ectoderm that ultimately encloses the entire embryo in a fluid-filled chamber called the amniotic cavity

Question 11

origins of the major extraembryonic tissues

Answer 11

[check textbook for possible test diagram]

pg. 77; Fig. 5.2

Question 12

parietal endoderm

Answer 12

a continuous layer of extraembryonic endoderm that is lining the inner surface of the cytotrophoblast and comes from the hypoblast

Question 13

primary yolk sac

Answer 13

complete when the spreading of the parietal endoderm is completed

Question 14

extraembryonic mesoderm

Answer 14

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

Question 15

primitive streak

Answer 15

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

Question 16

cell movement towards and through the primitive streak

Answer 16

[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

Question 17

extraembryonic mesoderm

Answer 17

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

Question 18

paraxial, lateral plate, and cardiac mesoderm

Answer 18

arise later and more anteriorly in the primitive streak than the extraembryonic mesoderm

Question 19

primitive groove

Answer 19

a groove along the midline of the primitive streak that forms as a result of the movement of cells through the primitive streak

Question 20

primitive node or Hensen’s node

Answer 20

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

Question 21

body stalk

Answer 21

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

Question 22

changes to cells as they move through primitive streak

bottle cells

mesenchymal cells

Answer 22

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;

Question 23

E-cadherin

Answer 23

as the cells convert from epithelial to mesenchymal configuration, they lose E-cadherin molecules

Question 24

epithelial-mesenchymal transition:

N-cadherin

Answer 24

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

Question 25

hyaluronic acid

Answer 25

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

Question 26

ectoderm and endoderm

Answer 26

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

Question 27

regression of the primitive streak

Answer 27

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)

Question 28

tail bud

Answer 28

mass of mesenchymal cells;; the most caudal extent of the primitive streak; plays role in forming the most posterior portion of the neural tube

Question 29

notochord

Answer 29

cellular rod running along he longitudinal axis of the embryo just ventral to the central nervous system

inductive signals from the notochord…

1. stimulate the conversion of overlying surface ectoderm into neural tissue
2. specify the identity of certain cells (floor plate) within the early nervous system
3. transform certain mesodermal cells of the somites into vertebral bodies
4. 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

Question 30

oropharyngeal membrane

Answer 30

small region where embryonic ectoderm and endoderm abut without any intervening mesoderm; marks the site of the future oral cavity

Question 31

prechordal plate

Answer 31

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

Question 32

notochordal process

Answer 32

rodlike aggregation of cells left behind as the primitive streak regresses and leaves the prechordal plate and notochordal precursor cells

Question 33

molecular aspects of gastrulation

Answer 33

[see the diagram in notes]

[see Fig. 5-8, page 82]

Question 34

establishment of anteroposterior axis

Answer 34

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

Question 35

primitive node

Answer 35

cells found at the tip of the primitive streak; express chordin, goosecoid, and hepatic nuclear factor-3beta (now called Foxa-2)

Question 36

Foxa-2 (HNF-3beta)

Answer 36

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

Question 37

Goosecoid

Answer 37

homeodomain transcription factor; prominently expressed in organizer region; activates chordin, noggin

helps stimulate the formation of a secondary body axis

Question 38

chordin and noggin

Answer 38

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

Question 39

nodal

Answer 39

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;

Question 40

T gene

Answer 40

activated by products of Foxa-2 and goosecoid genes; necessary for normal movements of future mesodermal cells through the primitive streak during gastrulation

Question 41

regulation of Hox around the primitive streak

Answer 41

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

Question 42

anterior visceral endoderm

Answer 42

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

Question 43

head formation (Lim-1, cerebrus-like 1, Otx-2)

Answer 43

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

Question 44

BMP-4 and the induction of neural tissue

Answer 44

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

Question 45

neural plate

Answer 45

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

Question 46

restriction ==> determination ==> differentiation

Answer 46

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

Question 47

CAMs

Answer 47

cell adhesion molecules; suspended cells of a similar type have a strong tendency to aggregate

Question 48

molecular basis for left-right asymmetry

Answer 48

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

Question 49

planar cell polarity

Answer 49

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

Question 50

situs inversus

Answer 50

condition in which the left-right asymmetry of the body is totally reversed

Question 51

Kartagener’s syndrome

Answer 51

situs inversus is associated with respiratory symptoms resulting from abnormalities of the dynein arms in cilia

Question 52

cadherins

Answer 52

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

Question 53

Ig-CAMs and N-CAM

Answer 53

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

Question 54

neural inductions and CAMs

Answer 54

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

Question 55

integrins

Answer 55

attaches cells to components of basal laminae and the ECM