DAT Embryology Continued Flashcards
Animal embryos follow four stages in growth
and development:
gametogenesis (sperm/egg
formation), embryonic development (fertilization
of egg until birth), reproductive maturity
(puberty), and aging process to death.
Stages of embryonic development
(sea urchin, echinoderm)
Fertilization - sperm penetrates
plasma membrane of secondary
oocyte
a. Recognition - before
penetrating, the sperm secretes
proteins that bind with
receptors that reside on a
glycoprotein layer surrounding
the plasma membrane of the
oocyte. In non-mammals, this
layer is called the vitelline
membrane. In mammals, this
layer is the zona pellucida. In
both organisms, the layer
ensures same species
fertilization.
Zona pellucida
external glycoprotein
membrane surrounding the plasma
membrane (jelly coat) of an oocyte. This
first appears in unilaminar oocytes, and
is secreted by both the oocyte and
follicular cells. At puberty, FSH
stimulates growth of granulosa cells
around the primary oocyte that secrete
the viscous zona pellucida.
Fertilization cannot occur until
capacitation and
acrosomal reaction have taken place.
capacitation,
secretions from the uterus’ wall and
uterine tube destabilize the plasma membrane
surrounding the head of the sperm (acrosome),
making the head more fluid, which helps prepare
it for fertilization and makes the sperm
hyperactive (faster and wiggle more).
Penetration
plasma membrane of
sperm and oocyte fuse, and the sperm
nucleus enters the oocyte
Formation of fertilization membrane
the vitelline layer forms a fertilization
membrane that blocks additional sperm
Completion of meiosis II in secondary
oocyte
sperm penetration triggers
meiosis II to complete.
Fusion of nuclei and replication of DNA
sperm and ovum nuclei fuse → diploid
zygote forms.
cleavage stage involves
rapid cell division of
the zygote without cell growth.
Embryo polarity
an egg has
an upper, animal pole and lower, vegetal
pole. Depending on the species, the
vegetal pole can contain more yolk
material, which is denser than the
cytoplasm and settles at the bottom. In
general, the vegetal pole differentiates
into extra-embryonic membranes that
protect and nourish the embryo. Polarity
is critical in setting up body axes.
Polar and equatorial cleavages
early
cleavages are polar, and divide the egg
into segments that stretch from pole to
pole, like the segments of an orange.
Others are parallel with the equator.
Note that in frogs, the horizontal
cleavage is closer to the animal pole.
Radial and spiral cleavages
occurs in deuterostomes. Radial
cleavage forms indeterminate cells at
animal and vegetal poles that are aligned
together, with top cells directly above
bottom cells. In protostomes, spiral
cleavage occurs, and determinate cells
are formed on top and are shifted
relative to those below.
Indeterminate and determinate
cleavages
in indeterminate cleavage,
blastomeres can individually complete
normal development if separated. In
determinate cleavage, blastomeres
cannot develop into a complete embryo
if separated; each is differentiated into
part of the embryo.
Morula
successive cleavage results in a
solid ball of ~8 cells, where the first 8
cells are totipotent, meaning the cells are
capable of giving rise to any cell type or
embryo
Blastula
cell division continues, and
liquid fills the morula and pushes cells
outward to form a circular cavity
surrounded by a single layer of cells. The
blastocoel is the fluid filled cavity.
Gastrula
formation of
the gastrula occurs with the invagination
of a group of cells into the blastula,
forming a two-layered embryo with an
opening from the outside into a center
cavity. forms 14 days post fertilization
Three germ layers
ectoderm,
mesoderm, and endoderm.
Ectoderm -
skin cells of epidermis, neuron on brain, pigment cells
Mesoderm
cardiac muscle cells, skeletal muscle cells, tubule cells of kidney, red blood cells, smooth muscle cells ( in the gut)
endoderm
lung cells, thyroid cells, digestive cells
Archenteron
the center cavity
formed by gastrulation that is
completely surrounded by
endoderm cells and gives rise to the
gut
Blastopore
opening into the
archenteron, becomes the mouth in
protostomes or the anus in
deuterostomes
Extra-embryonic membrane
development
in birds, reptiles, and
humans (collectively, amniotes),
membranes develop outside of the
embryo proper.
Chorion
outer membrane
Chorion birds and reptiles
functions
as a membrane for gas
exchange
Mammals chorion
chorion implants
into endometrium, and later,
the chorion and maternal tissue
(which is modified endometrial
tissue called deciduas basalis)
form the placenta.
Allantois
sac that buds off from
archenteron that eventually
encircles the embryo, forming
below the chorion.