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A 'germ layer' is a collection of cells, formed during animal embryogenesis. Germ layers are only really pronounced in the vertebrates. However, all animals more complex than sponges (eumetazoans and ) produce two or three 'primary tissue layers' (sometimes called primary germ layers). Animals with radial symmetry, like cnidarians, produce two called ectoderm and endoderm, making them diploblastic. Animals with bilateral symmetry produce a third layer in-between called mesoderm, making them triploblastic. Germ layers will eventually give rise to all of an animal’s tissues and organs through a process called organogenesis.

Contents

Germ layers

Development

Endoderm

Mesoderm

Ectoderm

Neural crest

References

See also

Germ layers

Caspar Friedrich Wolff observed organization of the early embryo in leaf-like layers. Later, Heinz Christian Pander discovered germ layers while studying chick embryos.
Among animals, sponges show the simplest organization, having a single germ layer. Although they have differentiated cells (e.g. collar cells), they lack true tissue coordination. Diploblastic animals, Cnidaria and ctenophores, show an increase in complexity, having two germ layers, the endoderm and ectoderm. Diploblastic animals are organized into recognisable tissues. All higher animals (from flatworms to man) are triploblastic, having in addition a mesoderm. Triploblastic animals develop recognisable organs.

Development

Fertilization leads to the formation of a zygote. During the next stage, cleavage, mitotic cell divisions transform the zygote into a tiny ball of cells called a blastula. This early embryonic form undergoes a massive reorganization called gastrulation forming a gastrula with either two or three layers (the germ layers). In all vertebrates, these are the forerunners of all adult tissues and organs.
The appearance of the archenteron marks the onset of gastrulation.

Endoderm

Main articles: Endoderm

The 'endoderm' is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm.
The endoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tube excepting part of the mouth and pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm). The lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas, the epithelium of the auditory tube and tympanic cavity, of the trachea, bronchi, and air cells of the lungs, of the urinary bladder and part of the urethra, and that which lines the follicles of the thyroid gland and thymus are also formed by this layer.
The endoderm forms: stomach, colon, liver, pancreas, urinary bladder; lining of urethra, epithelial parts of trachea, lungs, pharynx, thyroid, parathyroid, intestine.

Mesoderm

Main articles: Mesoderm

The 'mesoderm' germ layer forms in the embryos of animals more complex than cnidarians, making them triploblastic. During gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm.
This key innovation evolved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals. The formation of a mesoderm led to the formation of a coelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions protect them from shocks.
The mesoderm forms: skeletal muscle, skeleton, dermis of skin, connective tissue, urogenital system, heart, blood (lymph cells), and spleen.

Ectoderm

Main articles: Ectoderm

The 'ectoderm' is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.
The ectoderm forms: Central nervous system, retina and lens, cranial and sensory, ganglia and nerves, pigment cells, head connective tissue, epidermis, hair, mammary glands.

Neural crest

Due to the great importance of the neural crest it has been referred to as the fourth germ layer.

References

★ Evers, Christine A., Lisa Starr. ''Biology:Concepts and Applications.'' 6th ed. United States:Thomson, 2006. ISBN 0-534-46224-3.

See also

★ Cellular differentiation

★ Embryogenesis

★ Histogenesis

★ Neurulation

★ Organogenesis