sensory placodes and epidermal appendages

Question 1

placodes

Answer 1

thickened regions of non-neuronal ectoderm which make organs of site, hearing, smell, and hairs
- places are specified by interactions with surrounding neural plate, mesoderm, and endoderm; different interactions specify different placed types

Question 2

sensory placodes

Answer 2

in the head, form the olfactory epithelium of the nose, the inner ear, the eye lens, and portions of cranial sensory ganglia

Question 3

non-sensory placodes

Answer 3

in the head, produce oral epithelium for tooth development and cutaneous structures throughout the body including hair, feathers, mammary, and sweat glands

Question 4

anterior placodes

Answer 4

• adenohypophyseal (anterior pituitary)
• lens (eye)
• olfactory (nerve and some brain neurons)

Question 5

intermediate placodes

Answer 5

ophthalmic and maxillomandibular ( distal neurons of the trigeminal ganglion; together, these placodes comprise the trigeminal placed; the trigeminal nerve sense touch, temp, pain)

Question 6

posterior placodes

Answer 6

-otic (inner ear sensory epithelium, neurons of cochlear-vestibular ganglion)
- lateral line
- epibranchial nerves
- geniculate (nerves that innervate taste buds, tonsils, ear lobes)
- petrosal (nerves that innervate tongue, carotid sinus, and body)
- nodose (makes vagus nerve that innervates body organs such as heart, lungs, GI tract)

Question 7

induction of the otic-epibranchial placed (OEP) in the posterior pre-placodal region

Answer 7

1. Fgf from mesoderm promotes Wnt/Fgf expression in neural ectoderm
2. mesodermal Fgf induces the posterior placodal area (PPA) in proximal non-neuronal ectoderm; pharyngeal endoderm begins Fgf signaling
3. Wnt/Fgf signaling promotes otic placode, but Wnt inhibits epibranchial placode; nearby pharyngeal Fgf overrides Wnt to promote epibranchial placode specification, and BMPs promote subsequent epibranchial neurogenesis

Question 8

formation of the inner ear structure and brain connection

Answer 8

• the otic placode begins invagination by first expanding the basal cell surface, forming the otic pit
• apical constriction further drives invagination, forming the otic cup
• closure of the otic cup forms the otic vesicle or otocyst
• note the similarity with neural tube closure
• otic cup cells near the neural tube become specified as neural progenitors
• these cells leave the epithelium via delamination, becoming neuroblasts
• these neuroblasts differentiate into the cochleovestibular ganglion, sending axonal connections to the neural tube
• likewise, neuroblasts delaminate from the epibranchial placode, migrate dorsally, and mix with neural crest to form the epibranchial ganglia and nerves

Question 9

vertebrate eye formation

Answer 9

• the forebrain neural tube (diencephalon) bulges outward laterally and folds upon itself to make the optic vesicle
• optic vesicle contact with the overlying ectoderm induces the lens placode, starting with lens cell elongation/thickening
• the optic vesicle becomes a bilayered optic cup; outer cell become retina pigmented epithelium and inner cells become neural retina
• invagination of the lens placode forms the lens vesicle
• closure of the lens vesicle forms the lens and overlying cornea

Question 10

reciprocity

Answer 10

the optic vesicle induces lens placode, which induces neural retina
- when the optic vesicle (neural tube) contacts the overlying ectoderm, the optic placode is induced to form
- this induction is mediated by Fgf8, BMP4, and Delta signaling to the ectoderm, and causes these cells to elongate and taken on a thicker placode morphology
- presumptive lens cells of the placode send Fgf signals back to the optic vesicle, which Vsx2 expression and differentiation of neural retina

Question 11

development of the epidermis

Answer 11

• the epidermis is derived from ectoderm covering the embryo after neurulation
• BMP signaling promotes epidermis (and blocks neural genes)
• the epidermis begins as one cell layer, but quickly becomes two with the outer becoming the periderm, a temporary outer layer that is shed while the inner layer becomes true epidermis
• the inner basal layer sects a basal lamina, and attached cells behave as stem cells
• asymmetric cell divisions produce a stem cell retained at the basal lamina and a sister cell that moves upward and differentiates into cells of the upper layers
• notch signaling promotes epidermal cell division, expression of keratin proteins, and directs their assembly into structural filaments
• differentiation produces keratinocytes that bind tightly to each other to produce a water impermeable seal composed of protein and lipid
• keratinocytes at the surface are dead and comprise the stratum corner layer