Bio lecture 11 - early development 2 - amniotes

Amnion

fluid filled sac

- forms very early

- embryo floats within

- preventing desiccation

yolk sac

allows for nutrient uptake

allantois

early waste collector

chorion

contains blood vessels and allows gas exchange

placenta

unique to mammals

- fetal (chorion) and maternal (decidua) derived

- gas, nutrient, waste exchange

Cleavage - birds

- cleavage in bird eggs is meroblastic. birds form from a blastodisc. because of the yolky eggs of birds, the egg undergoes discoidal meroblastic cleavage

- the first cleavage occurs centrally in the blastodisc; other cleavages following forming a single-layer blasoderm

epiblast

embryo

hypoblast

contribute to extra embryonic membranes

cleavage - mammals

- mammalian cleavage is strikingly different from most patterns of embryonic cell division

- undergo unique holoblastic rotational cleavage (not always on the same axis)

- first cleavage is a normal meridional division; however, in the second cleavage, one of the two blastomeres divides meridionally and the other divides equatorially

cleavage - mammals continued

- mammalian cleavage occurs asynchronously

- mammalian embryos do not increase exponentially from 2- to 4-to-cell stages and frequently contain odd numbers of cells

- the switch from maternal to zygotic control occurs early during cleavage (2 cell mouse; 4/8 cell human)

- compaction of blastomeres is a phenomenon specific to mammalian cleavage

- after 3rd cleavage, blastomeres huddle together to form a compact ball of cells mediated by cadherin proteins

compaction to blastocyst

the 16-cell morula consists of a small group of internal cells surrounded by a larger group of external cells

compaction to blastocyst continued

- the mouse embryo proper is derived from inner cell mass (ICM) of the morula

- ICM will form embryo, and its associated yolk sac, allantois, and amnion

- outer cells form trophoblast

- by 64-cell stage layers - ICM and trophoblast - are separate and contribute solely to specific layer

- ICM becomes positioned one side of trophoblast cells

- through a process known as cavitation (driven by Na pumps) blastula with blastocoel develops.

escape from the zona pellucida

- as the embryo is moving through the oviduct to the uterus, blastocysts expand within the zona pellucida

- prevents adherence to the oviduct wall

- in the uterus, the embryo hatches using trypsin-like proteases secreted by trophoblast

- hatching allows contact with and invasion of the endometrium

formation of the bilaminar germ disc

the first segregation of cell within the ICM forms two layers - the hypoblast (primitive endoderm) and epiblast - collectively known as the BGD

- position in the ICM does not predict epiblast/hyoblast differentiation. TF expression 1 day before segregation predicts - Nanog (blue) forms epiblast and Gata6 (red) forms hypoblast

Bilaminar Germ disc

- hyoblast cells delaminate forming extra embryonic endoderm of the primitive yolk sac

- epiblast forms the embryonic epiblast and amnionic ectoderm and cavity which fills with amnion fluid

- trophoblast divides into cytotrophoblast (digest uterine lining and recruit maternal blood supply) and the syncytiotrophoblast (ingresses into uterine tissue)

Mammalian Gastrulation

- gastrulation begins at the posterior end of the embryon where cells of the node arise

- these cells migrate anteriorly forming the primitive streak

- a depression forms called the primitive groove and serves as an opening through which migration occurs

- thickening of primitive streak anteriorly forms "hensen's" node

mammalian gastrulation continued

- the node contains a funnel-shaped depression called the primitive pit

- epiblast cells migrate through the streak forming the mesoderm and endoderm

- cells that migrate through the pit will form the notochord

- migration is marked by down regulation of E-cadherin

- remaining epiblast cells form ectoderm

why is migration is marked by down regulation of E-cadherin?

binding adhesion protein and they need to be able to release

formation of extra embryonic membranes

- mammals have a distinct set of tissues, the placenta, that enable the fetus to survive within the maternal uterus

- the placenta is comprised of fetal and maternal tissues

- the cytotrophoblast secretes paracrine factors that attract maternal blood vessels and become lined by the trophoblast

- extra embryonic mesoderm joins the trophoblast to forma connecting stalk that will form the umbilical cord

formation of extra embryonic membranes continued

- the chorion is the trophoblast tissue and blood vessel containing mesoderm

- the placenta is the name given to the fusion of the chorion and uterine endometrium (decidua)

- fetal and maternal circulatory systems do not merge

mammalian axis formation - A-P axis

The mammalian embryo has two signaling centers:

1. the node

- the node is responsible for neural induction and setting the A-P axis

2. anterior visceral endoderm (AVE) (anterior hypoblast)

- the AVE sets primitive streak position and the head region

the 2 centers work together to form the anterior region of the embryo

Mammalian axis formation: the node

the node and derived tissues secrete BMP antagonists - chordin and noggin

- mice lacking chordin and noggin lack a forebrain, nose, and other facial structures

mammalian axis formation: the anterior visceral endoderm

- the AVE inhibits nodal by secreting 2 antagonists - lefty-1 (binds nodal receptor) and cerberus (binds nodal)

- blocking nodal creates an anterior region. nodal activity activated posterior genes

- the AVE functions to block nodal, cooperating with the node-produced mesoendoderm to promote head forming genes

- retinoic acid disrupts A-P axis

A=P patterning

- head region is devoid of nodal signaling and Bmps, FGFs, and Wnts are inhibited

- posterior region is characterized by nodal, BMPs, Wnts, FGFs, and RA

- RA, Wnts, and FGF work through the Cdx family of caudal related genes --> which in turn activate Hox genes

A-P patterning: The Hox code hypothesis

- A-P polarity in all vertebrates is specififed by hox gene expression

- hox genes are homologous to homeotic selector genes (Hom-C) of the fruit fly

- mammals have 4 copies of the complex per haploid set

- order on each chromosome is similar as is the expression pattern along A-P axis

- Hox/HOX are numbered 1-13 and equivalent genes in each mouse complex is called a paralogue (duplication)

expression of Hox genes along the dorsal axis

- Hox gene expression can be seen along the dorsal axis from anterior boundary of the hindbrain through the tail

- expression patterns suggest a code whereby certain combinations of Hox genes specify particular regions along the A-P axis

Left-Right axis in mammals

- mammalian body is not symmetrical

- heart on left

- spleen on left

- left lung vs right lung

- Iv gene (situs inversus viscerum) mutation results in random left-right axis

- Inv gene (inversion of embryonic turning) results in all asymmetrical organs to be on the wrong side

Left-Right axis in mammals continued

- ciliary cells of the node play a distinct role in L-R axis formation

- cilia cause fluid to flow from left to right

- remove dynein and asymmetrical organs were randomized

- the Iv gene codes for ciliary dynein

- crown cells (marked with immobile cilia) of the node are thought to sense flow and set left-right axis