Flashcards in Chapter 4: Ocular Development Deck (39):
How many germ layers are present during gastrulation?
What germ layers do the eyes and orbit develop from?
The eyes and orbit develop from all three germ layers, with neural crest cells making a substantial contribution
What is the earliest sign of ocular development?
Formation of the Lens Placode is the first sign of ocular development
What is the Lens Placode?
small surface ectoderm thickenings on both sides of the developing head
What are optic vesicles? When during embryogenesis do they develop?
optic pits that fill up to form pouches that are derived from ectoderm. They connect to the forebrain via a narrow neck. They develop around the same time as the lens placode.
What is the optic cup?
bi-layered invagination of the optic vesicles that occurs after they come into contact with the surface ectoderm
What are the two layers of the optic cup?
the inner layer, which becomes the neurosensory retina
the outer layer, which becomes the RPE
How does the Lens begin to form?
As the optic cup forms, the surface ectoderm begins to invaginate
How does lens formation contribute to development of the anterior segment?
As the surface ectoderm is invaginating, the area b.w the optic cup and surface ectoderm is invaded by migrating mesodermal and neural crest cells
What are the steps in Lens Formation?
Lens Plate= proliferation of surface ectoderm
Lens Pit= inward invagination of lens plate
Lens Vesicle= Pit deepens, closes, and detaches from surface
What is the Lens Vesicle?
a single layered structure composed of cuboidal cells surrounding a large lumen
sits in the optic cup
Describe the formation of the primary lens fibers
Posterior Lens epithelium proliferates, elongates, and fills in the optic vesicle
Called the embryonal nucleus
How does the lens capsule form?
the remaining epithelial cells form the lens capsule, which is composed of type IV collagen
What do the zonular fibers develop from?
they develop as a part of the vitreous and ciliary body (via mainly ectomesenchymal cells)
What does the cornea develop from?
Corneal epithelium develops from surface ectoderm, and the stroma/endothelium develop from neural crest derived ectomesenchymal cells
What cell layers does the Uvea develop from?
Develops from a combination of neural ectoderm, mesoderm, and neural crest cells
Neural Crest cells--pigmented epithelial components
Mesoderm--spincter and dilator muscles, non-pigmented parts of the uvea (i.e. iris stroma and ciliary muscle)
What are the initial two layers involved in retinal development?
Neural retina forms from the inner layer of the optic cup
RPE forms from the outer layer of the optic cup
What germ layers are involved in formation of the vitreous?
Neural Ectoderm-- contribute to primary vitreous CT fibers
Describe the formation of the primary vitreous?
Forms a central conical structure that contains the hyaloid vasculature and is surrounded by secondary vitreous
What is the significant of the Neural Retina Leucine Zipper (Nrl)?
is a transcription factor of the Maf subfamily, is an intrinsic regulator of Rod photoreceptor development
What are the first retinal cells to differentiate?
When do the ILM/ELM of the retina form?
They form when cells cease proliferating and begin to differentiate
What is the only pigmented tissue in the body that does not form from neural crest cells?
RPE, which forms from proliferating pseudo stratified columnar epithelial cells
What structures does the optic nerve develop from?
the optic stalk, which is the narrow neck that connects the optic cup to the forebrain
is initially formed from neuroectodermal cells surrounded by neural crest cells
What germ layer/s form the Sclera?
formed from ectomesenchymal elements
When do the scleral spur and Tenons Capsule form?
They begin to form at the time of EOM insertion
Orbital development involves key contributions from which germ cell layers?
all of them--ectoderm, mesoderm, endoderm, and neural crest cells
What are the EOMs formed from?
They form from paraxial and prechordal mesoderm
Signals from what primitive ocular structure are necessary for proper migration of neural crest cells into the eye surrounding tissues?
The optic cup--interactions b.w optic cup, mesoderm, and neural crest cells are crucial for proper development and organization
What are some consequences of delayed innervation of EOM during development?
Innervation by the wrong nerve (occurs in Marcus Gunn Jaw Winking syndrome and Duane Syndrome)
Can also cause premature differentiation into CT (i.e. fibrosis)
When do bone/fat/cartilage/CT of the orbit develop? What do they develop from?
They develop by the 4th WGA from frontonasal and maxillary processes of neural crest cells that occupy the space around the optic cup
What is the only bone that isn't membranous during development? And what does it consist of?
The sphenoid bone is the only bone that isn't membranous, and its made of cartilage instead.
When does ossification of orbital bones occur during development?
Ossification occurs around the 3rd month of gestation
What does the upper eyelid develop from?
it develops from a proliferation of surface ectoderm in the region of the future outer can thus at 4-5 WGA
What type of cells eventually infiltrate the eyelids and form the palpebral muscles?
When does the lacrimal gland begin to develop?
When does tear production by the lacrimal gland begin?
occurs >20 days after birth
What are homeobox genes?
they are the blueprint for embryonic program--serve as the main regulators of downstream development