Topic 2c: embryogenesis

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Fertilization

Cleavage

Blastula formation

Gastrulation/Organogenesis

Nerulation

the first 8 weeks of development are considered embryonic development; complete in the first trimester of gestation

fetal development occurs after those 8 weeks

gestation (development) period of humans is ~266 days (9 months), divided into 3 trimesters

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Fertilization:

the first step of embryonic development

haploid gametes fuse to form diploid zygote

should occur in the fallopian tube

limited by time/survival

sperm are able to survive up to 48 hours in the female reproductive system

although millions of sperm are released during ejaculation, only ~200 make it to the oocyte

oocytes last up to 24 hours

Sperm must penetrate the oocyte

secretions from the female reproductive system change the membrane composition of the sperm near its acrosome--> acrosomal reaction

results in release of acrosomal contents

allows sperm to penetrate the corona radiata (outer layer) of the oocyte

sperm must then pass through the zona pellucida (the next layer)

the first sperm to pass through shares it nucleus with the oocyte by fusing its membrane with the oocyte plasma membrane

this causes a depolarization of the oocyte membrane, resulting in it being impenetrable for fertilization by another sperm

the haploid nuclei fuse, forming the diploid zygote

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Cleavage:

zygote begins growing (mitosis)--> produces the embryo (ball of identical cells)

the zygote begins to divide ~1 day after fertilization, initiating cleavage (rapid cell division characteristic of early embryonic development).

during early cleavage, the embryo may split into 2 to form identical twins

division without increase in mass, just increase in the number of cells

Morula stage: after dividing for ~4 days, the zygote is a solid ball of cell

Blastula/blastocyst: reaches this stage after ~6 days of division; the zygote is a hollow ball of cells

blastocyst produces a key hormone: HCG (Human chorionic gonadotropin)

inner cell layers will continue to develop as the embryo and are the source of embryonic stem cells

trophoblast: the outer layer of cells of the blastocyst that aid in implantation and formation of extraembryonic membranes and the placenta

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Implantation:

implantation of the embryo: occurs ~1-2 weeks after fertilization; the developing embryo travels from the fallopian tube to the endometrium of the uterus

EXTRAEMBRYONIC MEMBRANES: form during implantation

amnion: fluid filled sac surrounding the embryo for protein and eventual cushioning of the fetus

allantois: membrane that eventually forms the umbilical cord (connects embryo and placenta)

yolk sac: the first blood cells form here

chorion: the embryo's side of the placenta

The placenta: develops from the chorion and grows in size during development. It provides nutrients and oxygen to the embryo and removes wastes. Produces hormones (relaxin, estrogen, progesterone, and HCG to maintain the pregnancy)

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Gastrulation:

takes place after blastula formation; occurs ~3 weeks after fertilization, the formation of 3 primary germ layers.

Once a cell enters a germ layer, its ability to differentiate into specific cell types is limited.

ectoderm: the outer layer of the gastrula; cells in this layer will become skin and nervous system cells

growth factors signal to the cells to differentiate into either cell type; if this signaling is disrupted, all of the ectoderm would become neural cells

mesoderm: the middle layer of the gastrula; cells in this layer will become bone, muscle, and internal organ cells

differing gene expression occurring in spatial pattern controls what tissue type the cells differentiate into

endoderm: the inner layer of the gastrula; cells in this layer will become linings of various tracts and organs

Organogenesis:

formation of the organs, begins during embryonic development and continues even after birth

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Neurulation:

development of the nervous system--> from neural plate to neural tube/crest.

neural crest: 2-strip tissue above the neural tube that develops into the peripheral nervous system

development begins with the neural plate--> a thin strip of cells

inner portion gives rise to the CNS

outer portion gives rise to the PNS

Then the inner neural plate becomes the cylindrical neural tube and the neural crest running above it forms the PNS

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Gene expression in embryos:

regulated by many factors

range of reaction: genes set the range for our phenotype and the environment determines where the phenotype falls

epigenetic markers: inherited by the embryo from gametes

reprogramming may erase some of the markers, but allows reprogrammed cells to have pluripotency--> embryonic stem cells

epigenetic markers have considerable influence on further gene expression and differentiation

Communication between cells is used to influence differentiation of cells via use of chemical messengers

also used to establish positional information

homeobox genes: produce proteins (transcription factors) that are essential for guiding the development of the shape of the embryo

induction and apoptosis also helps ensure that the right structures occur in the right places