Blastoderm

A blastoderm (germinal disc, blastodisc) is a single layer of embryonic epithelial tissue that makes up the blastula.^[1] It encloses the fluid-filled blastocoel. Gastrulation follows blastoderm formation, where the tips of the blastoderm begins the formation of the ectoderm, mesoderm, and endoderm.^[2]

A chick blastoderm before primitive streak formation shows a light circular region in the center. This spot marks where the embryo is starting to take shape

The blastoderm is a thin sheet of cells that forms on the surface of the yolk soon after fertilization in many animals, including birds, fish, amphibians, and even insects.^[1] It markss one of the earliest organized stages in embryonic growth, laying down a foundation that future tissues and organs grow from.^[3] In birds, the blastoderm separates into zones that will develop into both the embryo and the membranes that help protect and feed it.^[4]

This stage connects fertilization and gastrulation when the body plan begins to form and the first tissues start developing.^[1]

Formation and Early Structure

A bird's blastoderm starts as a flat layer of dividing cells that rests on the surface of the yolk. As development continues, these cells spread and separate into the area pellucida and area opaca, which form the foundation for embryonic and supporting membranes.

According to Developmental Biology by Scott F. Gilbert (2000)^[1] and The Atlas of Chick Development by Bellairs & Osmond (2014),^[4] this stage involves a complex, coordinated pattern of cell movement that prepares the embryo for gastrulation. The University College London (2009)^[4] tutorial gives a much simpler breakdown, explaining how the blastoderm thickens and reorganizes before any distinct, visible body structures appear.

Structure and Function

The blastoderm forms a thin layer of cells that lies on top of the yolk or along the inner surface of the egg membrane, depending on the species. In birds, this sheet separates into 2 regions: the area pellucida and the area opaca surrounding it. The inner section forms the body of the embryo, while the outer ring contributes to the membranes that provide nutrition and protection during early growth.^[1][4]

It plays an important role by anchoring the embryo and setting up the framework that later defines its body axis.^[5]

This is a comparison of a fertilized and unfertilized chicken egg. (a) shows the blastoderm, a small disc of dividing cells forming on the yolk surface, while (b) shows the blastodisc, which lacks cell development.

The blastoderm is a very active tissue. Its cells constantly divide and coordinate with one another, keeping the layer stable while preparing for gastrulation. Research on chicken embryos shows that these cells produce high levels of DNA repair genes and limit those that trigger apoptosis, helping preserve genetic stability during fast cell growth.^[6] Signaling systems such as Wnt and BMP guide how cells begin to move and specialize into the germ layers that later build the body's major tissues.^[7]

Blastoderm Development Across Species

With the chick embryo being one of the most well-known examples, the blastoderm stage is very common to many animal groups. Work by Evanthia Nikolopoulo et al. (2017)^[3] and Wang & Steinbeisser (2009)^[7] shows that similar cellular changes occur in vertebrates as the blastoderm transitions into gastrulation.

Within birds (like chickens), it appears as a flat disc of cells sitting on the yolk surface, separating into the area pellucida and area opaca, which both form the embryo and supporting membranes.^[5] In species like fish and amphibians, the blastoderm spreads over the yolk through a process called epiboly, enclosing it as growth continues, while in mammals it forms part of the epiblast, which later produces all the tissues of the embryo. Insects will develop a syncytial blastoderm first before the membrane forms around each nucleus to create individual cells.^[8]

Species	Blastoderm Formation	Unique Features	Source
Birds	Blastoderm appears as a disc of cells on the yolk that divides into the area pellucida and area opaca.	This is the classic model for embryology studies.	Bellairs & Osmond (2014)[4]
Fish (teleost)	The blastoderm spreads over the yolk surface through epiboly movements.	This early work was described by Jane M. Oppenheimer (1947).	Oppenheimer (1947)[9]
Amphibians	Blastoderm forms as a thin cap of cells that folds inward during gastrulation.	Amphibian blastoderms share similar cell movements with other vertebrates.	Nikolopoulou et al. (2017)[3]
Mammals	The blastoderm develops into the epiblast, the layer that produces all embryonic tissues.	They connect directly to later gastrulation stages.	Wang & Steinbeisser (2009)[7]
Insects(Drosophila)	It starts as a syncytial blastoderm, where nuclei share a common cytoplasm, later forming cell membranes.	Shows early patterning before cellularization.	Jaeger (2012)[8]

In oviparous animals

In chicken eggs, the blastoderm represents a flat disc after embryonic fertilization.^[5] At the edge of the blastoderm is the site of active migration by most cells.^[10]

DNA repair genes are highly expressed in chicken blastoderms.^[6]

Cellular and Genetic Activity

When Wnt signaling is active, it prevents the breakdown of β-catenin, allowing it to enter the nucleus and regulate genes involved in cell differentiation during early embryonic development.

An overview of BMP signaling and its modulators. This pathway controls early cell fate in the blastoderm, guiding the formation of neural and non-neural ectoderm.

The research from Rengaraj et al. (2022), Chicken blastoderms and primordial germ cells possess a higher expression of DNA repair genes and lower expression of apoptosis genes to preserve their genome stability^[6] found that chicken blastoderm cells show high activity of genes involved in DNA repair and low activity of genes that trigger apoptosis. These patterns help the early embryo maintain a stable genome during intense periods of growth.

Wang & Steinbeisser (2009)^[7] also described how signaling systems like Wnt and BMP guide the organization of blastoderm cells as they move into their proper layers during gastrulation.

Historical Perspective

Jane M. Oppenheimer (1947)^[9] described how the blastoderm forms and spreads in teleost fishes, highlighting the physical and cellular mechanisms involved. Although written decades before molecular biology became central to developmental science, this work remains valuable for its careful observation.

See also

• Blastodisc
• Embryology
• Cleavage
• Gastrulation