| Lens placode | |
|---|---|
 Transverse section of head of chick embryo of fifty-two hours’ incubation. | |
| Details | |
| Precursor | surface ectoderm |
| Gives rise to | lens |
| Identifiers | |
| Latin | placoda lentis |
| TE | placode_by_E5.16.3.1.0.0.13 E5.16.3.1.0.0.13 |
| Anatomical terminology | |
The lens placode is a thickened portion of ectoderm that serves as the precursor to the lens.
Invagination is the process of folding in cells. The lens placode invaginates to later develop the lens or lens pit. The development of the lens placode is typically seen between 44 and 50 hours; invagination occurs shortly after at around the 50–55-hour mark. [1]
Both the formation of the lens placode, and the invagination of this to the lens pit are both morphogenetic events. [2]
Chick embryos studies show the lens placode contains a cuboidal to columnar cell shape and that it is not multilayered. Furthermore, the density of the cell appears to double in size once the placode has developed. And while the cell density increases, the surface area is not impacted. [2]
The restricted expansion hypothesis” states that the adhesion of the ectoderm cells to the matrix is a key factor in the lens placode formation. This adhesion is accompanied by cell proliferation, which also impacts crowding and cell elongation. [2]
Pax6 is a transcription factor that is essential to the development of the lens placode. More specifically, it is needed for the surface ectoderm to fully develop. Pax6 has been identified as a necessary transcription factor for the thickness of the lens placode. [3]
SOX2 is a transcription factor that works alongside Pax6 to develop the lens placode. They maintain the same protein levels in the ectoderm. Therefore, SOX2 and Pou2f1 are involved in the development of the lens placode. [4]
In the developing chordate, the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural fold become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into the closed neural tube. In humans, neural tube closure usually occurs by the fourth week of pregnancy.
The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures 5 cm2 (0.78 sq in) and lies on the roof of the nasal cavity about 7 cm (2.8 in) above and behind the nostrils. The olfactory epithelium is the part of the olfactory system directly responsible for detecting odors.
A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans produce two or three primary germ layers. Some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as bilaterians produce a third layer between these two layers, making them triploblastic. Germ layers eventually give rise to all of an animal's tissues and organs through the process of organogenesis.
Oct-4, also known as POU5F1, is a protein that in humans is encoded by the POU5F1 gene. Oct-4 is a homeodomain transcription factor of the POU family. It is critically involved in the self-renewal of undifferentiated embryonic stem cells. As such, it is frequently used as a marker for undifferentiated cells. Oct-4 expression must be closely regulated; too much or too little will cause differentiation of the cells.
The neural plate is a key developmental structure that serves as the basis for the nervous system. Cranial to the primitive node of the embryonic primitive streak, ectodermal tissue thickens and flattens to become the neural plate. The region anterior to the primitive node can be generally referred to as the neural plate. Cells take on a columnar appearance in the process as they continue to lengthen and narrow. The ends of the neural plate, known as the neural folds, push the ends of the plate up and together, folding into the neural tube, a structure critical to brain and spinal cord development. This process as a whole is termed primary neurulation.
In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.
Paired box protein Pax-6, also known as aniridia type II protein (AN2) or oculorhombin, is a protein that in humans is encoded by the PAX6 gene.
Bone morphogenetic protein 4 is a protein that in humans is encoded by BMP4 gene. BMP4 is found on chromosome 14q22-q23.
Eye formation in the human embryo begins at approximately three weeks into embryonic development and continues through the tenth week. Cells from both the mesodermal and the ectodermal tissues contribute to the formation of the eye. Specifically, the eye is derived from the neuroepithelium, surface ectoderm, and the extracellular mesenchyme which consists of both the neural crest and mesoderm.
Otic vesicle, or auditory vesicle, consists of either of the two sac-like invaginations formed and subsequently closed off during embryonic development. It is part of the neural ectoderm, which will develop into the membranous labyrinth of the inner ear. This labyrinth is a continuous epithelium, giving rise to the vestibular system and auditory components of the inner ear. During the earlier stages of embryogenesis, the otic placode invaginates to produce the otic cup. Thereafter, the otic cup closes off, creating the otic vesicle. Once formed, the otic vesicle will reside next to the neural tube medially, and on the lateral side will be paraxial mesoderm. Neural crest cells will migrate rostral and caudal to the placode.
The frontonasal process, or frontonasal prominence is one of the five swellings that develop to form the face. The frontonasal process is unpaired, and the others are the paired maxillary prominences, and the paired mandibular prominences. During the fourth week of embryonic development, an area of thickened ectoderm develops, on each side of the frontonasal process called the nasal placodes or olfactory placodes, and appear immediately under the forebrain.
The development of fishes is unique in some specific aspects compared to the development of other animals.
The nasal placode gives rise to the olfactory epithelium of the nose. Two nasal placodes arise as thickened ectoderm from the frontonasal process. They give rise to the nose, the philtrum of the upper lip, and the primary palate.
SRY -box 2, also known as SOX2, is a transcription factor that is essential for maintaining self-renewal, or pluripotency, of undifferentiated embryonic stem cells. Sox2 has a critical role in maintenance of embryonic and neural stem cells.
Transcription factor SOX-3 is a protein that in humans is encoded by the SOX3 gene. This gene encodes a member of the SOX family of transcription factors involved in the regulation of embryonic brain development and in determination of cell fate. The encoded protein acts as a transcriptional activator.
Homeobox protein SIX3 is a protein that in humans is encoded by the SIX3 gene.
SOX1 is a gene that encodes a transcription factor with a HMG-box DNA-binding domain and functions primarily in neurogenesis. SOX1, SOX2 and SOX3, members of the SOX gene family, contain transcription factors related to SRY, the testis-determining factor.
Forkhead box protein E3 (FOXE3) also known as forkhead-related transcription factor 8 (FREAC-8) is a protein that in humans is encoded by the FOXE3 gene located on the short arm of chromosome 1.
Pax-6, member of the Pax gene class, is responsible for carrying the genetic information that will encode Pax-6 (protein) which dictates the development of the olfactory epithelium, eyes and central nervous system in vertebrates. Pax-6 is expressed as a transcription factor when neural ectoderm receives a combination of weak sonic hedgehog and a strong TGF-Beta signaling gradients. Expression is first seen in the forebrain, hindbrain, head ectoderm and spinal cord followed by later expression in midbrain. Expression in the head ectoderm will give rise to the nasal placodes and to the eye placodes. The nasal placodes will give rise to the olfactory epithelium while eye placodes give rise to the lens and cornea. Expression in the different brain regions is orchestrated with the combinatorial expression of other transcription factors to give rise to the central nervous system. Experiments in mice demonstrate that a deficiency in Pax-6 leads to decrease in brain size, brain structure abnormality leading to Autism, lack of iris formation or a thin cornea. Knockout experiments produced eyeless phenotypes reinforcing the gene’s role in eye development. Advancing research in this area may lead us to better understanding of the complexity seen in neural development and maybe one day be able to grow eye tissue in vitro.
The evo-devo gene toolkit is the small subset of genes in an organism's genome whose products control the organism's embryonic development. Toolkit genes are central to the synthesis of molecular genetics, palaeontology, evolution and developmental biology in the science of evolutionary developmental biology (evo-devo). Many of them are ancient and highly conserved among animal phyla.
This article incorporates text in the public domain from the 20th edition of Gray's Anatomy (1918)
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