Module 4 – Cleavage and Blastulation

Cleavage

Rapid series of mitotic divisions that occur in the early embryo after fertilization.

Zona pellucida

Cleavage divisions occur within the __

Blastomeres

These divisions occur within the zona pellucida and result in smaller daughter cells called __

• Holoblastic cleavage (Complete)

• Meroblastic cleavage (Incomplete)

Types of Cleavage

Holoblastic cleavage (Complete)

• The entire egg is subdivided into blastomeres.

• Occurs in eggs with less yolk.

• Examples: Mammals, amphibians, molluscs

Meroblastic cleavage (Incomplete)

• Only a portion of the egg is divided into blastomeres.

• Occurs in yolk-rich eggs.

• Examples: Birds, reptiles, fishes, insects

• Microlecithal (Oligolecithal)

• Mesolecithal (Mediolecithal)

• Macrolecithal (Polylecithal)

Types of Egg Based on Amount of Yolk

Isolecithal, Telolecithal, Centrolecithal

Types of Egg Based on Distribution of Yolk

Microlecithal (Oligolecithal)

• Very small amount of yolk

• Mammals, echinoderms

Mesolecithal (Mediolecithal)

• Moderate amount of yolk

• Amphibians, some fishes

Macrolecithal (Polylecithal)

• Enormous amount of yolk

• Birds, reptiles

Isolecithal

• Yolk is evenly distributed throughout the cytoplasm

• Mammals, molluscs

Telolecithal

• Yolk is concentrated at the vegetal pole, displacing cytoplasm to the animal pole

• Birds, reptiles

Centrolecithal

• Yolk concentrated at the center; cytoplasm surrounds yolk as a thin layer

• Insects

Morula

Commonly referred to as a “ball of cells,” it is a solid spherical mass of blastomeres formed after several rounds of cleavage.

Morum

Morula originates from the latin word __, meaning mulberry

• Outer blastomeres

• Inner blastomeres

Morula is divided into

Outer blastomeres

Form tight junctions; Destined to become trophoblast or trophoectoderm

Inner blastomeres

Form gap junctions; Destined to become inner cell mass

Compaction

• During this stage, blastomeres undergo structural reorganization, forming tight junctions among outer cells and gap junctions among inner cells. This results in a compact, cohesive mass of cells, marking the transition from the morula to early blastocyst

• The embryo now occupies approximately 60–70% of the perivitelline space, creating a larger perivitelline space compared to the morula stage

Blastulation

• Process that transforms the morula into a blastocyst (in mammals) or blastula in general embryological terms.

Blastocoele

Blastulation is marked by the formation of a fluid-filled cavity known as the __

Inner cell mass (ICM) / embryoblast

Collection of cells localized inside one pole (end) of the blastocyst; will give rise to the embryo proper

Trophoblast / trophoectoderm

Surface cells of the blastocyst; will contribute to placenta and supporting structures

Polar trophoectoderm

The __ overlies the inner cell mass.

Mural trophoectoderm

The __ refers to the trophoblast cells not in contact with the ICM.

Blastocoele

Fluid-filled cavity

Blastocyst

Embryo transfer is best done on the __ stage.

Horses, dogs, cats, rabbits

Marked round to oval expansion

Cattle, sheep, pigs

Thread-like elongation

Primates, rodents, guinea pigs

Minimal expansion, blastocyst invades endometrium

Embryonic disc

Due to cell proliferation of the inner cell mass cells, a thickened __ becomes evident at the blastocyst surface.

Delamination

A key event that follows is __, where a group of cells break loose and migrate to form a new cell layer inside the trophoblast layer

Epiblast

• Multilayered sheet of cells positioned adjacent to the trophoblast.

• Gives rise to the entire embryo proper by forming the three primary germ layers during the process of gastrulation.

• Also contributes to some extraembryonic structures such as the amnion

Hypoblast

• Epithelial layer facing the blastocoele

• While it does not contribute to the embryo proper, it helps form extraembryonic tissues, including the yolk sac endoderm.

Coelom

Because of the separation of the hypoblast layer from the epiblast, a space called __ becomes established

Blastodisc

A 3 mm‐diameter structure termed the __, from which the chick embryo develops, occupies a position at the animal pole in direct contact with the yolk mass.

Subgerminal cavity

The region clear of yolk is termed the __

• Area opaca

• Area pellucida

The blastodisc, which is now referred to as the blastoderm, is composed of two regions

Area opaca

• Peripheral region

• Consists of a layer of large cells.

• Overlies the unaltered yolk.

• The cells of the area opaca digest the underlying yolk which nourishes the embryo

Area pellucida

• ▪Central region

• Consists of a thin layer of cells.

• Translucent and overlies the subgerminal cavity.

• The embryo develops within the area pellucida.