Germ layer

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Germ layers
)

A germ layer is a primary layer of

organs through the process of organogenesis
.

History

Cleavage and division of the cell of an egg of a vertebrate (Remak, 1855).

Huxley in 1871, and "ectoderm" and "endoderm" by Lankester
in 1873.

Evolution

Gastrulation of a diploblast: The formation of germ layers from a (1) blastula to a (2) gastrula. Some of the ectoderm cells (orange) move inward forming the endoderm (red).

Among

triploblastic, possessing a mesoderm
in addition to the germ layers found in Diploblasts. Triploblastic animals develop recognizable organs.

Development

gastrula with either two or three layers (the germ layers). In all vertebrates, these progenitor cells differentiate into all adult tissues and organs.[5]

In the

implantation. The inner cell mass initially has two layers: the hypoblast and epiblast. At the end of the second week, a primitive streak appears. The epiblast in this region moves towards the primitive streak, dives down into it, and forms a new layer, called the endoderm, pushing the hypoblast out of the way (this goes on to form the amnion.) The epiblast keeps moving and forms a second layer, the mesoderm. The top layer is now called the ectoderm.[6]

Gastrulation occurs in reference to the primary body axis. Germ layer formation is linked to the primary body axis as well, however it is less reliant on it than gastrulation is. Hydractinia shows that germ layer formation that transpires as a mixed delamination.[7]

In mice, germ layer differentiation is controlled by two

pluripotent mouse embryonic stem cells to select a germ layer fate. Sox2 promotes ectodermal differentiation, while Oct4 promotes mesendodermal differentiation. Each gene inhibits what the other promotes. Amounts of each protein are different throughout the genome, causing the embryonic stem cells to select their fate.[8]

The germ layers

Endoderm

The endoderm produces tissue within the lungs, thyroid, and pancreas.
Main article: Endoderm

The endoderm is one of the germ layers formed during animal

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 tract except part of the mouth and pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm). It also forms the lining cells of all the glands which open into the digestive tract, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and alveoli of the lungs; the bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus.

The endoderm forms: the

parathyroid
.

Mesoderm

The mesoderm aids in the production of cardiac muscle, skeletal muscle, smooth muscle, tissues within the kidneys, and red blood cells.
Main article: Mesoderm

The mesoderm germ layer forms in the

triploblastic animals. During gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm.[9] The formation of a mesoderm leads to the development of a coelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions protects them from shocks.[10]

The mesoderm has several components which develop into tissues: intermediate mesoderm, paraxial mesoderm, lateral plate mesoderm, and chorda-mesoderm. The chorda-mesoderm develops into the notochord. The intermediate mesoderm develops into kidneys and gonads. The paraxial mesoderm develops into cartilage, skeletal muscle, and dermis. The lateral plate mesoderm develops into the circulatory system (including the heart and spleen), the wall of the gut, and wall of the human body.[11]

Through cell signaling cascades and interactions with the ectodermal and endodermal cells, the mesodermal cells begin the process of differentiation.[12]

The mesoderm forms: muscle (

Dentine of teeth, genitourinary system, serous membranes, spleen and notochord
.

Ectoderm

melanocytes
.
Main article: Ectoderm

The ectoderm generates the outer layer of the embryo, and it forms from the embryo's epiblast.[13] The ectoderm develops into the surface ectoderm, neural crest, and the neural tube.[14]

The surface ectoderm develops into:

sebaceous glands, cornea, tooth enamel, the epithelium of the mouth and nose
.

The neural crest of the ectoderm develops into:

melanocytes
, facial cartilage.

The neural tube of the ectoderm develops into:

motor neurons, retina
.

Note: The anterior pituitary develops from the ectodermal tissue of Rathke's pouch.

Neural crest

Because of its great importance, the neural crest is sometimes considered a fourth germ layer.[15] It is, however, derived from the ectoderm.

See also

Micrograph of a teratoma, a tumour that characteristically has tissue from all three germ layers. The image shows tissue derived from the mesoderm (immature cartilage - left-upper corner of image), endoderm (gastrointestinal glands - center-bottom of image) and ectoderm (epidermis - right of image). H&E stain.

References

  1. ^ Gilbert, Scott F (2003). "The Epidermis and the Origin of Cutaneous Structures". Developmental Biology. Sinauer Associates.
  2. ^ Remak, R. (1855). Untersuchungen über die Entwickelung der Wirbelthiere. Berlin: G. Reimer. link.
  3. S2CID 23450709
    .
  4. ^ Weyers, Wolfgang (2002). 150 Years of cell division. Dermatopathology: Practical & Conceptual, Vol. 8, No. 2. link Archived 2019-04-02 at the Wayback Machine.
  5. ^ Gilbert, Scott F (2000). "Comparative Embryology". Developmental Biology. Sinauer Associates.
  6. ^ Gilbert, Scott F (2000). "Early Mammalian Development". Developmental Biology. Sinauer Associates.
  7. S2CID 220121520
    .
  8. PMID 21663792
    .
  9. PMID 32119281
    , retrieved 2022-02-27
  10. ^ "Coelom". Biology Dictionary. 2017-06-07. Retrieved 2022-02-23.
  11. ^ Gilbert, Scott F (2003). "Paraxial and Intermediate Mesoderm". Developmental Biology. Sinauer Associates.
  12. PMID 12781678
    .
  13. ^ Gilbert, Scott F (2003). "Early Mammalian Development". Developmental Biology. Sinauer Associates.
  14. ^ Gilbert, Scott F (2003). "The Central Nervous System and The Epidermis". Developmental Biology. Sinauer Associates.
  15. S2CID 27150120
    .
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