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.
Allantois birds and reptiles
initially
stores waste products as uric
acid, and later fuses with the
chorion to form a membrane for
gas exchange with blood vessels
beneath it
Allantois mammels
allantois functions to
transport waste products to
placenta, and eventually forms
the umbilical cord between the
embryo and placenta.
Amnion
encloses the
amniotic cavity, and is a fluid-filled
cavity that cushions the developing
embryo, much like the coelom that
cushions internal organs in
coelomates. Amphibians do not
have an amnion.
Yolk sac birds and reptiles
digests
enclosed yolk, and blood vessels
transfer nutrients to embryo
yolk sac mammals
is empty and
contains no yolk as the umbilical
cord and placenta deliver
nutrients instead.
Organogenesis
cells
continue to divide after gastrulation
→ differentiate into specific tissues
and organs.
Notochord
cells along the
dorsal surface of the mesoderm
layer form the notochord, a stiff
cartilaginous rod that provides
support in lower chordates.
Neural tube
develops into the CNS, and
additional cells roll off the top of
the neural tube and form neural
crest cells, which form teeth,
bones, muscles of skull, pigment
cells in skin, and nerve tissue.
Gray crescent (frog)
sperm penetrates
frog egg → reorganization of
cytoplasm → pigmented cap of
animal pole rotates towards the point
of penetration while a gray crescent
shape region forms opposite of the
point of penetration.
Gastrulation (frog)
blastopore forms at
the border between the gray crescent
and the vegetal pole. During
gastrulation, cells migrate over the
top edge of, and into, the blastopore
through a process called involution,
forming the dorsal lip in the same
region previously occupied by the
gray crescent.
Yolk (frog)
cells from the vegetal pole rich in yolk
material form a yolk plug near the
dorsal lip
Blastodisc (bird)
flattened, disc shaped
region that sits on top of the yolk;
cleavage occurs here.
Blastocyst (humans and most mammals
- the blastula stage
consists of two parts: an outer ring of
cells (trophoblast) and inner cell mass
(embryonic disc)
Trophoblast
accomplishes
implantation by embedding into the
endometrium.
Embryonic disc
within the cavity
created by the trophoblast, the inner
cell mass clusters at one pole and
flattens into the embryonic disc,
analogous to the blastodisc of birds
and reptiles.
Influence of egg cytoplasm
cytoplasmic material is distributed
unequally in the egg (think gray crescent
in frogs and yolk in bird eggs), which
results in embryonic axes, such as animal
and vegetal poles. When cleavages
divide the egg, daughter cells have
different quality of cytoplasmic
substances, or cytoplasmic determinants.
These determinants are unique
substances that influence subsequent
development of each daughter cell.
Embryonic induction
influence of one
cell/group of cells over neighboring cells.
Organizers (controller cells) secrete
chemicals that diffuse among
neighboring cells, and influence their
development. The dorsal lip of the
blastopore, functioning as a primary
organizer, induces notochord
development in nearby cells.
Homeotic (Hox) genes
control
development by turning on/off other
genes that code for substances that
directly affect development of body
segments. An experiment in fruit flies
found that mutant homeotic genes
resulted in wrong body parts in wrong
places.
Homeobox (unique DNA segments
of 180 nucleotides)
identifies a
particular class of genes that control
development (encodes
homeodomain of protein that can
bind DNA). The homeobox
sequence is highly preserved across
species.
Embryonic lethals
mutations that
affect a process as fundamental as
segmentation, and cause death at
embryo/larval stage
Apoptosis
programmed cell death that
is part of normal cell development.
Apoptosis is essential for development
of the nervous system, operation of
immune system, and destruction of
tissue (webbing) between fingers and
toes.
Labor (three stages)
a series of strong
uterine contractions… i. Cervix thins out and dilates,
amniotic sac ruptures and releases
fluids
ii. Rapid contractions followed by
birth
iii. Uterus contracts and expels
umbilical cord and placenta
Fraternal/dizygotic twins
two
separate eggs are fertilized by two
different sperm, so the eggs are
not genetically identical, and are no
more related than ordinary siblings
Identical/monozygotic twins
result from indeterminate cleavage;
a single fertilized egg splits into
two, forming two genetically
identical offspring
notochord is derived from
mesoderm
Totipotent stem cells
can give rise
to any and all human cells, and even
an entire functional organism
Pluripotent
can give rise to all tissue
types, but not an entire organism
Multipotent
can give rise to limited
range of cells within a tissue type
Unipotent
just one single cell type
External development
fish and
amphibians have external fertilization
(ovuliparity) in water, to prevent
gametes from drying out, and to allow
sperm to swim to the egg. This requires
cooperative mating behaviors to ensure
simultaneous egg and sperm release.
Ovuliparity
external fertilization
Internal development -
reptiles, birds,
and some mammals (monotremes) have
internal fertilization, which requires
cooperative behavior leading to
copulation.
Viviparity
birth to live young that
was given nutrients during
development
Oviparity
egg is laid and hatches
later
Ovoviviparous
eggs are internal
and birthed as live young, but the
egg is not nourished in any way by
the parent
Non-placental internal development
certain animals like marsupials and
tropical fish spend a short time in the
uterus as embryos, then crawl out and
complete development attached to a
mammary gland in the mother’s pouch!
Placental internal development
major
components of this development in
humans include the umbilical cord and
placenta system. The oxygen is received
directly from the mother (as fetal lungs
are not functional until birth), as well as
nutrients. CO2 and metabolic wastes are
removed.
Placenta formation
begins with
chorion, and blood vessels of allantois
wall enlarge and become umbilical
vessels that connect the fetus with
the developing placenta.
Amniotes
group of tetrapods, four-
limbed animals with backbones or spinal
columns that have terrestrially adapted
eggs that are supported by several extra
embryonic membranes.
Monotremes
mammals that lay leathery
eggs, lack nipples, and are endothermic
(but have an unusually low body
temperature and metabolic rate
compared to other mammals)
In vitro
looks at cells and biological
molecules outside their normal biological
context, such as in a lab.
In vivo
normal biological environment
first trimester,
Organs of the fetus develop (critical development period)
second trimester
fetus is
very active during the second trimester and the
uterus grows enough for the pregnancy to be
noticeable.
