embryology Flashcards
embryology
study of embryonic development
fertilization
fusion of sperm and egg –> zygote
cleavage
cell divisions
–> blastula/ blastocyst
gastrulation
blastula rearranges layers of cells –> gastrula
organogenesis
rudimentary organs form
4 stages
fertilization, cleavage, gastrulation and organogenesis
occur in all animals
as embryo develops, specific patterns of _________ direct cells to a specific fate
gene expression
haploid sperm and egg give rise to a
diploid zygote
3 key events of fertilization
- contact; sperm makes contact w protective layer (jelly coat ie zona pellucida) surrounding egg
- acrosomal reaction; enzymes dissolve jelly coat, molecules on sperm bind to receptors on egg
- cortical reaction; changes at surface of egg prevent further sperm entering; blocks polyspermy
how does acrosomal reaction work
- molecules in jelly coat trigger acrosomal reaction in the sperm, which release hydrolytic enzymes from acrosome (in the head of sperm) which digests a hole in jelly coat
- acrosomal process (actin filaments) pierce the jelly coat
- proteins on the tip of acrosomal process bind to receptors on egg plasma membrane; lock and key
acrosomal reaction: bind of proteins on acrosomal process to receptors on egg causes
fusion of plasma membranes of sperm and egg
fusion of plasma membranes of sperm and egg triggers
opening on Na+ channels, and Na+ rapidly diffuses into egg, causes depolarization and blocks other sperm from fusing
this is still a part of acrosomal reaction
this is short lived, fast block, not the same as cortical reaction which is longer lasting block
cortical reaction
vesicles called cortical granules in head of sperm go beneath plasma membrane and fuse with plasma membrane of egg
contents (enzymes) released into space between plasma membrane and outer vitelline layer (perivitelline space)
vitelline layer lifts away
hardens to form fertilization envelope
receptors clipped off so no further binding
cleavage
- succession of rapid cell division
in cleavage the large fertilized egg (zygote) becomes many smaller cells called
blastomeres
first 5-7 divisions in cleavage produce a hollow ball of cells called
blastula (blastocyst in mammals)
like a hollow ring of blastomeres
more divisions after blastula becomes a ball of cells surrounding a fluid filled space called
blastocoel
cleavage steps
zygote;
divides into blastomeres
more and more until become blastula
and then even more and fluid space in middle becomes blastocoel
(look at pic on slide 10)
2 types of cleavage patterns
holoblastic: therians, amphibians, echinoderms
meroblastic; monotremes, birds, reptiles, fish, insects
holoblastic or meroblastic is a function of
presence of yolk which impedes cleavage
holoblastic; hole aka minimal yolk
meroblastic; partial aka large yolk
holoblastic cleavage
yolk is concentrated towards one end; vegetal pole
less yolk at opposite end; animal pole
generates vegetal and animal hemispheres
impedes cleavage
in holoblastic cleavage an indentation forms that divides cells in half called a
cleavage furrow
first cleavage furrow parallel to lines connecting the poles
second is right angle to first
generates 4 blastomeres of equal size
3rd division is equatorial to prodice 8 cells
in holoblastic cleavage the presence of yolk in the vegetal hemispheres
displaces the cleavage furrow TOWARDS the animal hemisphere
this means SMALLER blastomeres in animal hemispheres
meroblastic cleavage
the volume of yolk in vegetal hemisphere so big that cleavage furrows cannot pass through it
CLEAVAGE OCCURS ONLY IN ANIMAL HEMISPHERE
cleavage occurs in a small disc at the animal pole; blastodisc
cleavage generates multicellular blastoderm
end result of holoblastic cleavage is
ball of cells (blastula or blastocyst) with fluid filled space called blastocoel
the blastocoel is on the animal hemisphere side
larger cells on the vegetal hemisphere side
in mice and humans the blastocyst contains a group of cells called
the outer layer of cells called the
the inner cell mass (embryoblast): forms the embryo and source of embryonic stem cells
trophoblast; does not contirbute to embryo; forms chorion –> placenta
gastrulation
at end of cleavage the embryo consists of a ball of cells with a fluid fileld space
gastrulation involves reorganization of cells to form 3 layer embryo
3 layer embryo is called
what are the 3 layers from in to out
gastrula
endoderm, mesoderm and ectoderm
each of embryonic germ layers contributes to dinstict set of tissues in animal
ectoderm will form epidermal layer of skin
mesoderm will form muscle, bone, kidneys, blood, gonads, tissue
endoderm will from lining of gut, the liver and the lungs
frog gastrulation
- group of cells on blastula form a small indented crease called blastopore
- sheet of cells at animal hemisphere migrate, move inside blastopore
- once inside, move back to animal pole
- organized into endoderm and mesoderm
- remaining cells at animal hemisphere become ectoderm
- cells at animal pole continue to spread over outer surface (ectoderm)
- as more cells enter through enlarging blastopore, new space forms called the archenteron
- blastocoel becomes smaller and smaller as archenteron becomes bigger
- eventually endoderm lined archenteron replaces blastocoel
- blastopore forms the anus
- archenteron becomes cavity of digestive tract
bird gastrulation
- unlike hollow ball of cells like the frog, the bird blastula is a DISC of cell on top of yolk mass called blastoderm
- blastoderm divides and forms 2 layers; epiblast and hypoblast
- blastocoel sits in between
- embryo will develop from epiblast
- cell from epiblast move to centre of blastoderm and inwards towards yolk
- produces a thickening: primitive streak
- some cells that have migrated through the primitive streak move downwards and form endoderm
- other move laterally and form mesoderm
- cells left behind on surface form ectoderm
mammal gastrulation
- inner cell mass (embryoblast) forms the embryo
- trophoblast form chorion
- trophoblast cell secrete enzymes that facilitate implantation into endometrium of uterus
- inner cell mass divides into epiblast and hypoblast
- epiblast forms embryo
- cells of epiblast move inwards via primitive streak to form mesoderm and endoderm
totipotent
can form ALL cell types plus the extraembryonic (placental) tissues
first few divisions after fertilization
pluripotent
- can form ALL cell cell types of the body but NOT the extraembryonic tissues
- cells from the inner cell mass
embryonic stem cells
ESCs isolated form embryos
pluripotent
induced pluripotent stem cells
IPSCs; produced from adult cells via reprogramming
multipotent
- can form MULTIPLE cell cell types of the body
- adult stem cells
- found in bone marrow and adipose tissue
- limited differentaition ability
- mesenchymal stem cells (MSCs)
part of organogenesis that is development of CNS is called
neurulation
neurulation: cells from dorsal mesoderm come together to form the
the notochord
in neurulation, the ectoderm above the notochord becomes the
neural plate
in neurulation, infolding of the neural folds adjacent to the neural plate generates the
neural tube; becomes brain and spinal cord
in vertebrates 2 types of cells develop near the neural tube
- neural crest cells; from neural folds, migrate to many parts of embryo; neurons, bone, pigment cells etc
- somites; from mesoderm, play major role in organizing the segmented structure of body; vertebrae and associated muscles
