The Cycle of Life: Gastrulation and Tubulation (Amphioxus and Amphibians)

What is gastrulation

A series of cell and tissue movements in which the blastula stage embryo fold inward, producing a 3 layered embryo, the gastrula

What are the three embryonic germ larders

1. Ectoderm

2. Endoderm

3. Mesoderm

Monoblastic

One layer - the ectoderm

Diploblastic

Two layers - endoderm and ectoderm

Triploblastic

Three layer - Endo, Ecto, and Mesoderm

1. Gastrulation of Amphioxus

1. Invagination of the prospective endoderm and formation of the blastopore

2. Elongation

3. Closing of the blastopore

A

Blastula of amphioxus

B

Invagination blastula

C

Early gastrula

D

Later gastrula

E

Section of late gastrula

F

Enterocoelic pouches

G

Somite formation

H

Neurulation

1

Blastocoel

2

Macromeres

Gastrocoel

4

Dorsal lip

5

Blastoporcre

6

Ventral lip

7

Gastrula in the amphioxus

8

Gastrococoel

9

Endoderm

10

Ectoderm

11

Mesoderm

12

Micromeres

Invagination of the prospective endoderm and formation of blastopore

1. blastula has micromeres in upper part and macromeres at the lower part. The slower dividing macromeres are pushed by faster dividing micromeres and then micromeres start moving downwards (Epiboly).

2. This process starts at vegetal pole. As invagination continues, the monoblastic blastula starts converting into diploblastic gastrula.

3. Initially the invaginating cavity is very small but gradually it deepens into the blastocoel

4. The new cavity is called Archenteron or Gastrocoel . As the archenteron advances, the old cavity (blastocoel) starts Obliterating.

5. The archenteron communicate to outside by a pore called blastopore.

6. Initially the blastopore is a small pore but very is soon it becomes a well developed structure and can be divided into three lips- one dorsal, two lateral and one ventral.

Invagination

A process in which the layer of cells itself goes into the interior of developing embryo

Elongation

1. The wide mouth of the archenteron gradually narrows into a triangular hole which is now situated at the posterior end of gastrula (Blastopore.)

2. When the inner wall (hypoblast) of the gastrula touches the outer wall (epiblast), the original blastocoel disappears.

3. As the gastrula elongates, the blastopore gradually becomes smaller.

4. At this stage the gastrula is elongated structure with an upper flat surface and a lower ventrally bulging surface and is slightly compressed.

Closing of the blastopore

After the completion of the invagination and involution the blastopore starts diminishing and finally leaves a narrow crevices.

Structure of a fully formed gastrula

1. contains well-defined three terminal layers, the ectoderm, mesoderm and endoderm.

2. The upper portion is covered by neural ectoderm and all sides by the epidermal ectoderm.

Gastrocoel

A spacious cavity that is present in the interior of the embryo which contains various cell types at various portions

A

Roof is formed of notochoradal cells

B

Sides contain mesoderm cells

C

Rest is endodermal

Tubulation

Is the formation of organ rudiments from three germinal layers, so that organs can develop from them

Tubulation steps

1. Neurulation

2. Notogenesis

3. Mesogenesis

4. Development of endoderm and gut

Neurulation - formation of neural tube (Amphioxus)

1. The upper surface of the embryo, which is the neural ectoderm starts thickening due to the change in cell shape, called neural plate.

2. The neural plate invaginates to form a neural groove flanked by neural folds.

3. Soon the groove is closed into a tube from anterior to posterior end of the embryo.

4. Anteriorly the neural tube opens outside by a neuropore.

5. Posteriorly also there is a canal covered by upcoming epidermal ectoderm called the neurocentric canal.

6. Later on the anterior portion of the tube makes brain and the posterior makes the spinal cord.

A

The upper surface of the embryo, which is the neural ectoderm starts thickening due to the change in cell shape, called neural plate.

B

The neural plate invaginates to form a neural groove flanked by neural folds.

C

Soon the groove is closed into a tube from anterior to posterior end of the embryo.

D

Anteriorly the neural tube opens outside by a neuropore.

E

Posteriorly also there is a canal covered by upcoming epidermal ectoderm called the neurocentric canal.

Notogensis - formation of notocord (Amphioxus)

1. The roof of the Archenteron is invaginated to form a closed structure called notochordal cord.

2. Later on it develops into a notochord just below the neural tube and above the alimentary canal.

F

Notogenesis

Mesogenesis - formation of mesoderm (Amphioxus)

1. The lateral portions on both sides are evaginated to form a cord of cells from anterior to posterior end containing a space derived from the archenteron called as Coelom.

2. Later on the cord is cut off in different segments to form somites.

Differentiation of mesoderm and coelom

1. Mesoderm segment continues to grow.

2. It grows downward on either side of the gut until the bottom meets the gut, then the two sides join together.

3. Mesoderm is divided into two parts: A- epimere -top piece-.(dorsal mesoderm) B- hypomere- bottom piece- (lateral mesoderm).

The bottom piece of the mesoderm will have the following

1. Visceral wall of the mesoderm is attached to the intestinal wall, forming a double wall

2. Somatic mesoderm adheres to the stratum corneum, forming a double wall

3- They are separated by a cavity

Top piece of the mesoderm is divided into three parts

1. Myotome

2. Dermatome

3. sclerotome

Myotome

Middle portion between and ventral to sclerotome and dermatome (muscle segment)

Dermatome

Outermost portion near skin ectoderm and is the internal content of the skin

Sclerotome

Surrounding portion notocord and neural tube . It is upper adjacent to the dorsal cord that extends upward around the neural tube forming a fibrous envelope of connective tissue that protects the notochord and the nerve tube of the young animal

Splanchopleure

Visceral wall of the mesoderm is attached to the intestinal wall, forming a double wall. (Internal mesoderm + endoderm)

Somatopleure

External mesoderm + ectoderm. adheres to the stratum corneum forming a double wall

Coelom cavity

They are separated by a cavity

1

Somatic mesoderm

2

Neural tube

3

Splanchnic mesoderm

5

Myotome

4

Dermatome

6

Sclerotome

7

Coelom cavity

8

Somatopleure

9

Splanchopleura

10

Gut cavity

11

Differentiation of the mesoderm

Development of gut(Amphioxus)

1. After separation of notochord and mesoderm from the endoderm, the edges of these endoderm cells grow towards each other and fuse below the notochord to form a tubular gut or mesenteron.

2. Later the cells lining the gut become ciliated, and the splanchnic layer of mesoderm surrounding them gives rise to a layer of smooth muscles of the gut.

3. After separation of anterior gut diverticulum, cells of the floor of anterior part of gut become columnarand form an elongated groove, the hypopharyngeal groove or endostyle.

4. Much later a midgutdiverticulum or liver will grow out from the gut on the right side. The anterior part of gut also enlarges to form the pharynx.

1

Left anterior gut diverticulum

2

Neuropore

3

Somites

6

Gut or enteron

Hatching of larva

Soon after gastrulation, when only two pairs of somites are formed, the ciliated embryo hatches from the vitelline membrane. The embryo is now called a larva. It swims actively on the surface of sea with the help of cilia. It does not feed since it has no mouth and anus.

Gastrulation of Amphibians

larger amount of yolk contained within the egg - the cells at the animal pole are retained for the formation of the embryo while the yolk-filled cells at the vegetal pole are used more by the embryo as an energy source.

- gastrulation is initiated by invagination of cells from animal pole undergo epiboly, when reach blastopore the cells (Bottle cells) will travel inward. That form the dorsal lip of the blastopore.

- Bottle cells continue to migrate, form leading edge of archenteron.

- cell movements cause the pushing out of the ectoderm and inward movement of the endoderm and yolk-filled cells

- The archenteron is formed and the blastocoel is slowly filled with cells and lost.

- the yolk-filled cells of the vegetal pole remain for a short time to fill the space between the dorsal and ventral lip of the blastopore, thus forming the yolk plug.

Bottle cells

1

Gastrulation begins when cells in the region of the gray crescent move inward, forming the dorsal lip of the future blastopore.

2

Cells of the animal pole spread out, pushing surface cells below them toward and across the dorsal lip. These cells move into the interior of the embryo, where they form the endoderm and mesoderm.

3

The archenteron expands, destroying the blastocoel. The blastopore lip forms a circle, with cells moving to the interior all around the blastopore; the yolk plug is visible through the blastopore.

The anterior-posterior axis of the embryo

The first mesodermal cells to migrate over the dorsal blastopore lip will induce the ectoderm above them to produce anterior structures such as the forebrain; mesoderm that involutes later will signal the ectoderm to form more posterior structures, such as the hindbrain and spinal cord.

Primary embryonic induction

The central nervous system forms through interactions with the underlying mesoderm

Neurulation- formation of neural tube ( Amphibians)

Transforming the Neural plate into a tube: The birth of the CNS

Formation of a dorsal, hollow neural tube by

1. Ectodermal cells flatten into neural plate

2. the centre of the plate sinks forming neural groove.

3. edge of plate is elevated to form neural folds.

4. neural folds fuse and form neural tube: anterior end develops into brain and posterior end develops into spinal cord

   
   

A

Dorsal surface

B

Transverse section

Notogenesis in Amphibians

The meso-endodermal cell lying in mid dorsal region of roof of archenteron separates from mesoderm layer forming the Notochord.

Neurulation- formation of neural tube in Amphibians

Notogenesis- formation of notocord in Amphibians

Development of Mesoderm

1. begins by splitting and separating from endoderm and appears as two cell masses on either side of the notochord.

2. A small incision appears inside each of two mesoderms and then gradually extends back and forth on both sides until each mesodermal plate separates into two layers between which there is a cavity known as the coelom cavity.

3. Mesoderm separates into two layers: External and called somatopleurem and Interior is known as the splanchnopleure

differentiation of mesoderm and coelom

1. The dorsal region or the muscular region

2. The intermediate or renal segment

3. The ventral or lateral region

The dorsal region or the muscular region

1. myotome -muscles are formed

2. dermatome - dermis of the skin

3. sclerotome - creating the skeleton

The intermediate or renal segment

gives future urogenital system

the ventral or lateral region

located on either side of the intestine

layers of the ventral or lateral region

1. outer layer (somatopleure) and it adheres to the ectoderm layer forming outside where they form a double membrane (somatic crystal)

2. Inner layer (splanchnopleure) and it adheres to the endoderm layer, forming a double membrane (visceral crystal)

3. between them there is a coelm cavity

A-B

Formation of the mesoderm begins by splitting and separating from endoderm and appears as two cell masses known as mesoderms on either side of the

notochord.

C-D

A small incision appears inside each of two mesoderms and then gradually extends back and forth on both sides until each mesodermal plate separates into two layers between which there is a cavity known as the coelom cavity

1

Somite

2

Intermediate mesoderm

3

Lateral plate mesoderm

4

Three divisions of mesoderm