Cleavage in Amphioxus

Cephalochordates are marine lancelets: slender, laterally compressed, translucent animals about 5 to 7 cm long, which inhabit sandy bottoms of coastal waters worldwide. Lancelets originally bore the generic Amphioxus (Gr. amphi, both ends, + oxys, sharp). Sexes are separate in Amphioxus. Sex cells are set free in the atrium and then pass out the atriopore to the outside, where fertilization occurs. Larvae hatch soon after fertilizing eggs and gradually assume the shape and size of adults.

1.0Systematic Position

• Phylum: Chordata
• Subphylum: Cephalochordata
• Class: Leptocardii
• Family: Branchiostomatidae
• Genus: Amphioxus

2.0General Characters of Amphioxus

• Marine, found in shallow water on the sandy coasts in tropical and temperate regions 
• A burrowing form where the anterior end protrudes out, nocturnal  
• It has a transparent spindle-like body, compressed, looks like a lance, pointed at both ends, hence called a lancelet 
• Body-anterior trunk having an oral hood, mouth at the anterior end above the mouth is the rostrum,11 pairs of oral tentacles/oral cirri surrounding the mouth and posterior tail – 4.5to five cms 
• The anterior end protrudes in the front – rostrum, having a dorsal fin, ventral fin and caudal fin
• Anus at the posterior end .presence of metapleural folds, atriopore at the junction of metapleural folds, presence of" V" shaped blocks –Myotomes. 
• The sexes are separate, and sexual dimorphism is not distinct. There are 26 pairs of gonads arranged in two rows, one on each side of the body. Gonoducts are absent, and the gametes are released through the atriopore.

3.0Reproduction in Amphioxus

• Amphioxus is a Cephalochordate animal. 
• The sexes are separate, but apart from the reproductive organs, there is no difference between the male and female. 
• The gonads occur in 26 pairs of pouches arranged metamerically along the body wall. They project into the atrium and largely fill its cavity.
• The inner or mesial face of each pouch is covered by atrial epithelium, which is pushed inwards by the growth of the gonads. 
• Thus, the gonads are present in a single layer of epithelium, which is shown by the development, which is coelomic. Hence, each gonad is surrounded by a closed coelomic sac.

4.0Amphioxus -Organisation of the Gametes 

a) Spermatozoon in Amphioxus:

The mature male reproductive cell in Amphioxus is called a spermatozoon.

b) Structure:

It measures approximately 15 to 20 micrometers in length and is divided into three main regions:

1. Head
2. Middle piece
3. Tai

• On the head, the acrosome is present. 
• The head shows a big nucleus. 
• Around the nucleus, a thin sheet of Cytoplasm is present. 
• It is called Manchetty. 
• The middle piece is small and has a long tail.

5.0OVA

• Amphioxus's unfertilized ovum or secondary oocyte is 0.10 mm to 0.12 mm in diameter. 
• It is microlecithal and isolecithal 
• It is surrounded by a plasma membrane, which is further surrounded by a thin vitelline membrane of mucopolysaccharides. 
• Its nucleus is eccentric, lying at the animal pole 
• The egg undergoes its first maturation division before it leaves the ovary 
• The animal pole is free from the yolk, and the Cytoplasm towards the vegetal pole contains some yolk distributed evenly.

6.0Fertilization

• Fertilization happens externally; the sperm entering the ovum near the vegetal pole activates the egg to complete its second meiosis division; both the second polar body and fertilization membrane are produced. 
• Fertilization is completed by the union of two pronuclei, male and female. 
• The lancelet egg can be considered a telolecithal due to its polarity and isolecithal due to the distribution of yolk and isolecithal because of its little county of yolk. 

Early cleavage in amphioxus

• During the earliest cleavages, growth does not occur. 
• Cleavage planes pass entirely through the egg; they are holoblastic. 
• The first two are meridional, giving the four-cell stage. 
• The third is equatorial, with 4 micromeres and 4 macromeres. 
• The third cleavage is meridional at two levels, giving rise to 16 blastomeres. The next two levels are equatorial (32) blastomeres arranged in two four-tiered cleavages. The last regular cleavage is meridional from 64 blastomeres.

7.0Gastrulation in Amphioxus

Cleavage and Gastrulation in Amphioxus

At the start of cleavage in the Amphioxus egg, three distinct cytoplasmic regions can be identified:

1. Vegetal Region:
2. Located near the vegetal pole, this area contains a dense mass of cytoplasm rich in yolk.
3. Animal Hemisphere:
4. This region contains clearer cytoplasm with less yolk, making it more transparent.
5. Basophilic Crescent:
6. Positioned roughly in the marginal zone, this area has cytoplasm that stains deeply with basic dyes. Although it contains little yolk, it is distinct from the animal cytoplasm and has a crescent shape.

Cleavage and Blastomere Formation

As cleavage progresses, these regions divide into blastomeres:

• The vegetal pole material forms larger cells at and around the vegetal pole of the developing blastula.
• The animal hemisphere consists of smaller, clear-cytoplasm cells that are columnar and tightly packed, forming a columnar epithelium.
• The crescent-shaped basophilic region gives rise to cells that are loosely packed and show bulging outer surfaces, especially in early cleavage stages.

Fate of the Cytoplasmic Regions

• Clear Cytoplasm (animal region) → Forms the epidermis (skin)
• Granular Cytoplasm (vegetal pole) → Develops into the lining of the alimentary canal (gut)
• Basophilic Crescent → Forms muscles and body cavity lining, representing the mesoderm

8.0Blastula Formation

Cleavage results in a blastula with:

• A large central cavity called the blastocoele
• A blastoderm made of a single layer of columnar cells
• Larger cells at the vegetal pole and a thicker blastoderm in that region

Gastrulation Process

Gastrulation begins when:

• The blastoderm at the vegetal pole flattens and bends inward, transforming the spherical embryo into a cup-shaped gastrula.
• The gastrula has:
• A double-layered wall: outer (ectoderm) and inner (endoderm + mesoderm)
• A cavity between the two layers — the remnants of the blastocoele
• A new cavity inside called the archenteron (primitive gut)
• An opening to the outside known as the blastopore

Blastopore and Tissue Movement

• The blastopore is initially wide, but soon contracts as the mesodermal crescent and presumptive notochord move inward.
• These inward movements, called invagination, help form the endoderm, mesoderm, and notochord.
• As development continues, the blastopore lips shrink, and the embryo begins to elongate, shaping the future body axis.

9.0Formation of the primary organ rudiments in the Amphioxus

• The epithelial sheets representing various parts of the developing animal break up into discrete cell masses, forming the primary organ rudiments.
• Within the embryo, presumptive tissues for the notochord, mesoderm, and gut are separated by gaps along their boundaries.
• As the mesoderm material splits from both the notochord and endoderm, it forms cuboidal cell masses arranged in segments along the animal's body, called mesodermal segments. These segments eventually develop cavities inside them, which will later become the coelom or secondary body cavities of the adult.
• When the notochord and mesodermal segments detach from the endoderm, the endoderm's free edges approximate and fuse along the dorsal midline, forming a closed sac. The cavity of this sac becomes the lumen of the alimentary canal.
• In the ectoderm, the tissue destined to form the nervous system separates from the surrounding epidermis and forms an elongated neural plate. The plate sinks beneath the surrounding ectoderm and is covered by the epidermal epithelium. The neural plate then rolls into a tube, with the lateral edges folding upward and fusing along the midline, forming the neural tube, which will develop into the spinal cord.
• The neural tube does not close fully at the anterior end, leaving an opening called the neuropore, which remains open until later in development.
• As the epidermis moves forward over the neural plate, it covers the blastopore, which is eventually sealed off from the exterior. The blastopore opens into a space within the neural tube, forming the neurenteric canal, which briefly connects the archenteron (future gut) with the neural tube.
• After the separation of the neural tube and the archenteron, the neurenteric canal disappears, and the alimentary canal forms its own cavity.
• By this stage, the early Amphioxus embryo has developed most of the key features of a chordate animal.