Download INSTITUTE OF ZOOLOGY AND FISHERY SCIENCE GASTRULATION

INSTITUTE OF ZOOLOGY AND FISHERY SCIENCE
INFRONT OF M S COLLEGE MAIN GATE
CHANDMARI, MOTIHARI, EC, BH-845401
Mobile- +91 78709 61145
[email protected]
www.izfs.net
GASTRULATION
INTRODUCTION
The ontogenic development requires six processes:
1. Gametogenesis
2. Fertilization
3. Cleavage 4. Gastrulation
5. Organogenesis
6. Growth and Differentiation
As the fertilized egg is formed by the fusion of female, ovum with the sperm of the same species the
development begins. The cleavage begins forming blastula. Now the embryo enters into the so called
Gastrulation, by which the origin of three germinal layers take place. It will later on enter into the
organogenesis and the various organs are formed.
DEFINITION
According to BALINSKY, 1970, it is one of the displacements of parts of the early embryo so that the
endodermal and mesodermal organ rudiments are removed from the surface of the embryo. The single
layer of cells, blastoderm gives rise to three germinal layers, the ectoderm, mesoderm and endoderm.
PROMINENT FEATURE
1. A rearrangement of cells of the embryo by means of morphogenetic movements.
2. The rhythm of cellular division is slowed down.
3. Growth is insignificant.
Following types of morphogenetic movements are involved in Gastrulation of vertebrates:
1. Epiboly 2. Emboly 3. Invagination 4. Involution 5. Convergence
6. Divergence 7. Infiltration 8. Delamination 9. Concrescence etc.
1.
2.
3.
4.
5.
6.
GASTRULATION IN AMPHIOXUS
The egg is microlecithal. Cleavage is holoblastic. First cleavage is meridional or vertical. Second is
also vertical but at right angle to the first. The third cleavage is horizontal but it occurs above the
equatorial plate giving rise to four smaller cells called micromeres and four larger cells called
macromeres.
4th, 5th and 6th divisions are latitudinal and a ball of cells, morula is formed.
A cavity called blastocoel is formed. The roof of this cavity is occupied by micromeres and the
vegetal poles by the macromeres.
During Gastrulation the main processes involved are Invagination, involution and Epiboly.
As Gastrulation begins, increase in mitotic activity occurs in the presumptive ectoderm (both neural
and epidermal), notochord and mesoderm.
The onset of Gastrulation is marked by flattening of prospective endoderm or macromeres of
vegetal pole. It gradually invaginates inwardly into the blastocoel. Thus, the blastocoel is reduced
1
and a new cavity, the archenteron is formed. Its opening is called blastopore and its rim is bounded
by four lips.
7. A crescent of cells, the chordal cells lying along the mid dorsal edge of the roof of archenteron.
8. The cells of the mesodermal crescent pass up dorsally to a position on either side of the chordal
cells and forming the part of the roof of archenteron.
9. While the cells of the lips are rolling into their position by involution the ectodermal cells of the
animal half are spreading out by Epiboly.
10. The blastopore is very broad in early stage, but its lips begin to contract and it reduces. It also shifts
from the antero-dorsal to the posterior.
11. The lateral horns of the mesodermal crescent converge towards dorsal side and come to lie on both
sides of presumptive notochord.
12. The remainder of the lateral, ventral and anterior part of the gastrula consists of endodermal cells.
The presumptive material of nervous system comes over the notochordal material. In this way, the
embryo becomes of three layers.
GASTRULATION IN AMPHIBIA
Eggs are telolecithal. Cleavage is holoblastic. By some divisions the morula is formed. At the animal pole
there is micromere and at the vegetal pole there are yolk laden megameres. The cells are arranged in a layer
surrounding a cavity called blastocoel. The layer of cells at the vegetal pole is very much thicker than the
animal pole, so the blastocoel becomes eccentric.
The first trace of Gastrulation is the formation of a depression on the dorsal side of the embryo
(invagination). So a cavity is soon formed. This cavity is lined on all sides by invaginated cells and
represents the archenteron. Its opening is called blastopore.
FATE MAP
According to VOGT, 1925, a fate map has been prepared:
1. Animal pole area will develop into ectoderm to form nervous system and skin.
2. Immediate grey crescent or marginal zone material for notochord and mesoderm.
3. Area on and around the vegetal pole endodermal lining of the alimentary canal.
A. ECTODERM: - By the further Invagination the archenteron increases and the blastocoel
decreases. The upper margin of blastopore is called dorsal lip and the lower edge is called ventral
lip. Inward moving cells form a border beneath the outer cell by a process called involution.
When the inward movement of the cells is in the progress through the dorsal
lip another type of movement occurs in outside called Epiboly. The pigmented cells of animal pole
start to enclose the micromere of vegetal pole. It covers the outer part of embryo. Small mass of
macromeres remain uncovered for a while and acts as a yolk plug. At this stage, the embryo
becomes two layered. Each layer having many layers of cells. The outer layer is ectoderm having
the material for the epidermis and nervous system.
B. MESODERM AND NOTOCHORD: - The notochord cells rolls over the dorsal lip of blastopore
in the interior. The notochordal material becomes concentrated on the dorsal side.
The prechordal plate which in the blastula lies just below the presumptive notochord is the first
mesoderm to invaginate and becomes a part of archenteron roof in front of the notochordal material.
Most of the mesoderm invaginates into the interior by the rolling over the lateral and ventral lips of
the blastopore and comes to lie between the ectoderm on the outside, on the endoderm on the
inside. The notochordal and mesodermal materials in this stage are in the form are one continuous
epithelial sheet called chorda mesodermal mantle.
2
C. ENDODERM: - Finally the two lips of blastopore become fused and two mesodermal sheets unite
to form a continuous mesoderm. The endoderm which until now was on the ventral side starts to
move upwards along the two borders. When the two ends meet dorsally it forms the third innermost
strata. The cavity is between the endoderm is known as gut cavity.
In this way, the embryo becomes three layered and layer formed are ectoderm,
mesoderm and endoderm. The special feature in amphibian development is the formation of
mesoderm first and then the endoderm.
GASTRULATION IN BIRDS
The egg is telolecithal. The cleavage is meroblastic. The segmentation is restricted only at the
blastodisc. By early divisions a disc is formed having smaller cells. A cleft appears which separates
the disc from the underlying yolk. The new cavity in between is called sub-germinal space. Disc is
then called as blastoderm, the cells of which continue to divide. It has two types of cells:
1. The peripheral part which lies in contact with yolk is called area opaca and
2. Inner layer is called area pellucid. Later on, the blastoderm becomes double layered. Upper
layer is called epiblast and the lower layer is called hypoblast. The cavity in between these two
is called blastocoel. The space between the hypoblast and yolk is called sub- germinal cavity. It
is the primordial archenteron. The epiblast contains presumptive ectodermal and ventral areas at
the anterior portion while the posterior half comprises of presumptive notochordal and
mesodermal cells. The hypoblast transforms into the endoderm and the epiblast is converted
into ectoderm and mesoderm.
ORIGIN OF ECTODERM, MESODERM AND ENDODERM: - The Gastrulation begins with the movement of cell called
immigration. The hypoblast cells from posterior end start migration towards the anterior end along the median line. The cells of
epiblast move downward the hypoblast. These involuted cells occupy a position in between the epiblast and hypoblast, and
migrate from there the lateral and anterior ends between the epiblast and hypoblast. This movement has been studied by
SPRATT, 1946. The stripe of the blastoderm becomes thicken at the median line called primitive streak. It has a narrow furrow
called primitive groove. At this anterior end there is a Hensen’s node. The entire primitive streak is a mass of moving cells.
As the cells of epiblast migrate into an anterior, whose areas of the blastoderm disappear from the surface.
They are replaced by the adjoining areas moving towards the mid-line and taking their place in the primitive streak.
Thus the primitive streak persists, although the cells do not stay in the same place but are constantly
replaced. The first areas to start Invagination are presumptive endoderm, the notochord and the presumptive head process. The
presumptive notochordal cells become concentrated in the primitive streak in the deeper parts in the Hensen’s node. It may be
called as head process or notochordal process. The narrow canal penetrates into the notochordal process and its cavity may be
recognized as a part of archenteron.
The endoderm starts invaginating at an early stage, the cells of presumptive
endoderm penetrates into the hypoblast. The endoderm lying in the posterior part of the primitive streak after Invagination
moves laterally replacing the original hypoblast.
As the notochordal and endodermal material disc appears from the surface the presumptive somites areas converge medially and
enter the primitive streak at its inner end. After passing into the interior the cells of the somites migrate outward and forward and
distributed in stripes, on each side of the notochordal process.
The elongation of the primitive streak is only temporary. As the cells destined to become notochord, endoderm and
mesoderm migrate into anterior. The primitive streak begins to shrink. The gut of embryo is derived from epiblast and not by the
hypoblast.
The neural plate appear in brain region while the Gastrulation in movement are in full swing as the Hensen’s node read the
parts of the neural plate becomes differentiated. The neural tube is formed. In this way the embryo becomes three layered.
------------------****------------------
3