#developmental stages

Developmental biology Basics

Which is NOT a primary developmental process?

Classical developmental biology defines four overlapping processes: growth via cell proliferation and size increase, pattern formation establishing body axes and positional information through morphogen gradients like Sonic hedgehog and BMP, morphogenesis shaping tissues through cell movement and adhesion, and differentiation producing specialized cell types. Migration in mammalian development often considered component of morphogenesis rather than separate primary category in Gilbert classification. Some textbooks include migration as part of morphogenesis alongside growth and differentiation. Therefore migration alone generally not listed as independent primary process compared with other three core pillars plus differentiation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: Primary developmental processes.

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Eye Lens Induction -l

Which structure forms first in eye development?

Eye morphogenesis begins shortly after neurulation when optic field is specified in ventral forebrain. FGF and Wnt antagonists cause lateral walls of diencephalon to bulge outward forming optic vesicles around embryonic day 8 in mouse, stage 10 in chick. These vesicles grow toward surface ectoderm to induce lens placode. Lens vesicle, cornea and pigmented retina differentiate only after vesicle contact and folding into optic cup. Therefore optic vesicle evagination precedes all other eye structures, establishing appositional relationship necessary for subsequent reciprocal induction events and patterning of anterior visual system components.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 16: Sequence of vertebrate eye development - optic vesicle forms first.

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amphibian Development-l

Specification of germ layers in amphibians begins:

Axis and germ layer specifications in amphibians rely on maternally deposited RNAs and proteins localized during oogenesis rather than zygotic transcription initially. VegT, Vg1, Wnt11 mRNAs accumulate vegetally months before fertilization through localization machinery involving cytoskeleton. These determinants pre-pattern embryo, ensuring vegetal cells are biased toward endoderm even before sperm entry occurs. Post-fertilization events like cortical rotation merely redistribute dorsalizing factors dorsally, but initial animal-vegetal polarity exists prior to fertilization. This explains why isolated vegetal explants differentiate into gut without any additional external signals or zygotic input.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 2: Maternal mRNA localization before fertilization specifies germ layers.

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Teratogens

Maximum fetal susceptibility to teratogens occurs between:

Maximum susceptibility window corresponds to active organogenesis when organ anlagen originate and differentiate. Before week three, totipotency allows compensation or embryonic loss without specific defects. After week eight, organ templates established and growth predominates. Between weeks three and eight neural tube closes, pharyngeal arches form face, limb buds outgrow, heart septates, all highly dependent on morphogen gradients. Interruption causes severe structural anomalies like spina bifida, cleft lip, phocomelia. Hence greatest risk for major anatomical birth defects occurs during weeks three to eight, defining critical period for most classic teratogens affecting structural development.

Ref: Sadler, Langman's Medical Embryology, 14th ed., Chapter 8: Critical period weeks 3-8.

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