Embryogenesis: Definition, Stages, and Development [MCAT]

Sonic Hedgehog Gene

In embryology and developmental biology, embryogenesis is the development of an animal or plant embryo, starting with fertilization of an egg cell by a sperm cell. After fertilization, the ovum becomes a diploid zygote.

CNS development and Limb development

Forms forebrain, separates right and left brain, and establishes midline

If there’s a mutation, then we have a whole brain with no separation of right and left, which is bad. This is called holoprosencephaly. Left and right hemispheres fail to separate. There are also facial abnormalities, cleft lip/palate and even severe cyclopia (single eye like a cyclops).

Sonic Hedgehog gene is also important in limb development. Limbs develop along 3 planes: 1) Proximal to distal 2) dorsal-ventral axis, and 3) Anterior-Posterior Axis

Anterior-Posterior: Anterior towards head, Posterior towards toes

Dorsal: Extensors and Ventral: Flexors

  1. Proximal to Distal Development: Apical ectodermal ridge is made up of ectoderm overlying mesoderm. APical means tip, AER is at the tip –> important for limb growth. The ridge influences the mesoderm to grow

Fibroblast Growth Factor from the fgf gene is expressed, eventually when the ridge APICAL ECTODERMAL RIDGE is removed, the FGF replaces it and induces normal growth.

FGF = helps with limb proximal-distal axis development

2. Dorsal to Ventral Development: Remember dorsal-ventral involves flexors and extensor development. The most important gene is Wnt-7 gene for dorsal development. WNT-71 activates LMX-1 gene and dorsalizes mesoderm. If LMX-1 is deleted, then you would have 2 ventral sides (2 soles on both surface of foot)

Wnt-7= makes it dorsal

LMX-1 = helps Wnt-7 to induce dorsal development

Absence of LMX-1 = 2 ventral sides (there’s no back of the limb)

Absence of Wnt-7 = makes it ventral (Engrailed1 gene represses Wnt-7)

Wnt genes is originally derived from drosophila

Early embryo: Wnt gene regulates dorsal-ventral axis

and also in later embryogenesis, they’re responsible foro anteroposterior axis.

3. Anterior-Posterior Development

Depends on zone of polarizing activity (SHH in the zone of polarizing activity)

Homeobox genes or HOX genes code for transcription factors. Homeosis means transforming one structure into another, leading to body segment formation. 180 bases = for each homeobox gene

HOX gene mutation = abnormal limb formation

Example: hox mutation for fruit flies –> legs on head

Humans hox mutation –> polydactyly (extra fingers/toes) or syndactylyl (fused fingers/toes)


Embryogenesis Terms and Stages

Blastocyst: outer = trophoblast

Inner = inner cell mass = derivative of all the rest of the body

Blastocyst implants in uterus around 6-10 days after fertilization. b-hcg secretion begins

Morula –> blastula –> blastocyst –> Gastrula (3 layers structure) with 3 germ cell layers Ectoderm/Mesoderm/Endoderm

Inner cell mass –> forms a bilaminar disc in which 2 cell layers are separated by a basement membrane. This forms hypoblast (more inner) and epiblast (next to the amniotic cavity)

Epiblast = invaginates (creates primitive streak) == this means start of gastrulation – where germ layers are starting to form

notochord = mesoderm (becomes nucleus pulposus of spine)

induces overlying ectoderm to become neural plate, which then invaginates to become a neural crest

Mesenchyme = mostly mesoderm, surrounded by proteins and fluid and gives rise to most connective tissue

Embryonic Stages by Weeks

Embryonic period is 8 first weeks after fertilization (during the first 8 weeks) = organogenesis where organs form = most vulnerable period to teratogens or chemicals that are bad for development

Is embryonic period the same thing as Fetal Period? No, embryonic period refers to the first 8 weeks after fertilization; this involves formation of organs. However, in fetal period, this period refers to the organs growing bigger, or adult structures forming.

Week 4 = HEART 4 your heart (heart beats); Limbs form

Week 6 = ultrasound can detect fetal heart

Week 8 = limbs move

Week 10 = SRY gene from Y chromosome activates and leads to penis development. If there is no SRY gene –> then the clitoris of the baby girl develops.

Week 15 -20 = ultrasound detects/identifies sex of baby

“In three words I can sum up everything I’ve learned about life: it goes on.”

by Robert Frost

Special Structures to Remember in Embryogenesis

Special Structures to Remember:

Anterior Pituitary = Rathke’s pouch of ectoderm

Posterior Pituitary = Neural tube (this is why posterior pituitary is also called neurohypophysis, while anterior pituitary is called adenohypophysis)

Neural Tube = CNS

Neural crest = PNS

Adrenal Gland

Cortex = Mesoderm

Medulla = neural crest (epinephrine/norepinephrine)

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