Download gastrulation of amphibia - Bejoy Narayan Mahavidyalaya

MILITA ROY
BEJOY NARAYAN MAHAVIDYALAYA
The six stages of vertebrate development
 Fertilization
 Cleavage
 Gastrulation
 Neurulation
 Neural crest formation
 Organogenesis
Cleavage in a frog embryo
Cross section of a frog blastula
Gastrulation - The point in embryogenesis where
the basic organization of the organism is
established.
A. Establishment of the three basic germ layers
Ectoderm, Mesoderm, Endoderm
B. Groups of cells that will give rise to specific organ
systems and tissues are moved into the right position
both externally and internally.
C. Groups of cells that will influence each others
differentiation are positioned so they can have an effect
on each other.
How is the characteristic body plan for any organism developed?
Gastrulation: the first step in the process of body formation. It transforms a complex
sphere into 3 basic germ layers from which all other tissues develop.
Ectoderm is the outer layer = forms epidermis and nervous system.
Mesoderm is in the middle and forms a many structures (i.e., heart, muscles).
Endoderm is the inner layer and forms the ‘gut’ and related organs.
The first change is to generate the rudiment of the digestive tract, hence the name
gastrulation (gastric = stomach).
1. How does gastrulation
occur ?
2. What are the
molecular and cellular
mechanisms that cause
movement and
rearrangement?
Gastrulation is the first step of morphogenesis
Morphogenesis is the process whereby individual cells undergo complex movements
that generate the organ rudiments. Gastrulation generates the three basic germ layers
from which organs arise.
How do sheets of cells (epithelia) move during gastrulation? 5 methods.
Invagination is the local inward movement of cells from a cavity
Involution is similar, but more dramatic. It is an inward expansion of epithelial cells
around an edge such as the blastpore.
Convergent extension is elongation of an epithelium in one direction while it shortens
in the other direction (stretching taffy). The cells can keep their relative positions and
elongate or they can interdigitate
interdigitate..
Epiboly is spreading movement of an epithelium to a deeper or thinner layer.
Delamination is the splitting of one layer into 2 different layers.
Different combinations of these basic movements yield a variety of changes that
characterize gastrulation
How do individual cells move during gastrulation?
Seven basic types of cell movement lead to the changes in epithelial sheets that
characterize gastrulation.
1. Migration is the movement of an individual cell over other cells or a substrate.
2. Intercalation is wedging of cells between their neighbors. Lateral intercalation
involves lateral movements of cells in the same layer between one another =
convergent extension. Radial intercalation involves wedging of 2 different layers. This
process often leads to epiboly, the surface area of the epithelium increases while the
thickness decreases.
3. Ingression is the movement of individual cells from an epithelium into an
embryonic cavity.
4. Shape changes are coordinated changes in cell shape that cause an epithelium to
invaginate, buckle or undergo convergent extension.
5. Cell division without growth increases the total number of ‘building blocks’
6. Changes in adhesiveness: cells adhere to one another and to extracellular material
via different types of specialized attachments. Loss of these attachments can facilitate
cell movement.
Gastrulation in Amphibians
Amphibian gastrulation is much more complex than sea urchins, due in part to the
large amount of yolk. There are also more layers in the blastula
Different areas of the gastrula show distinct behavior:
Animal cap: about 3 layers deep and derived from the animal hemisphere of the egg. It
expands by epiboly until it covers about half of the surface of the gastrula.
Non involuting marginal zone: a wide belt of cells 4 to 5 layers deep. It expands by
convergent extension to cover the lower part of the gastrula
Involuting marginal zone: involutes during gastrulation and comes to lie in the
interior. The limit of involution reaches the edge of the blastopore by the end of
gastrulation.
The superficial layer of the involuting zone forms the roof of the archenteron (the
lining of the future gut).
The deep layer of the involuting zone forms the future trunk mesoderm (muscles of
the body)
Deep zone: a ring of cells that migrate towards the animal pole. Deep zone cells
become head mesoderm and heart.
Vegetal base: formed by the large yolky cells in the vegetal hemisphere. During
gastrulation this base is tilted and displaced ventrally. It will form the floor of the
archenteron (gut).
Ectoderm = animal cap and non involuting marginal zone
Mesoderm = deep zone and deep layer of involuting marginal zone
Endoderm = vegetal cells and superficial layer of involuting marginal zone
Gastrulation in the frog