Animal
Development
Chapter 44
Fertilization
The union of sperm and egg to form a zygote.
It requires three main events occur.
Stage 1- Attachment
Sperm reach the egg membrane by burrowing
through the corona radiata and bind to the zona
pellucida. The acrosome of the sperm releases
enzymes to digest a portion of zona pellucida.
Stage 2- Prevention of
Polyspermy
The second a sperm touches the plasma
membrane of an egg, the membrane
depolarizes to temporarily repel sperm.
Vesicles in the egg cell release enzymes that
turn the zona pellucida impenetrable.
Stage 3- Becoming Diploid
The sperm releases its chromatin into the cell.
The chromatin reorganizes into chromosomes
in the sperm pronucleus. While this happens,
the secondary oocyte completes ovulation. A
nuclear envelope surrounds both the sperm
and egg pronuclei. The basal body of the
sperm forms a spindle apparatus of the zygote.
Embryonic Development
An embryo goes through 3 development stages
● Cellular
● Tissue
● Organ
Cellular Stages of Development
Part 1- Cleavage
Cleavage is division without growth. It involves
DNA replication and mitosis. The volume of
cytoplasm does not change, only the number of
cells. The cells get smaller and smaller.
16 uniform cells form a morula. The morula
continues dividing until it forms a blastula.
Cellular Stages of Development
Part 2- Blastula Formation
A blastula is hollow ball of cells with a blastocoel(fluid filled
center).
Blastula has two main parts
•Inner cell mass- becomes the embryo
•Trophoblast- becomes the placenta and aids in enodmetrium attachment
In amphibian, reptile, and bird embryos cleavage is not
uniform because yolk is present.
Tissue Stages of Development
Part 1- Early Gastrula
When the embryo attaches to the uterine wall, the inner cell
mass divides into: the epiblast (forms germ layers) and
hypoblast (forms yolk-produces first blood cells)
An early gastrula has 2 layers of cells.
● Ectoderm (outer layer)- nervous system, epidermis and
derivatives, epithelial linings of oral cavity and rectum
● Endoderm (inner layer)- epithelial linings of digestive
and respiratory tracts and bladder, thyroid glands
Tissue Stages of Development
Part 2- Late Gastrula
● Gastrulation is complete when the mesoderm is
developed. The mesoderm is the middle layer and
forms musculoskeletal, cardiovascular, urinary,
reproductive systems
The three layers are called the embryonic germ layers.
Organ Stages of
Development
•Mesoderm forms the notochord which is replaced
by the vertebral column in chordates.
•The ectoderm cells above the notochord form the
neural plateneural groove neural tube CNS.
•The nervous system forms from midline ectoderm
•Midline mesoderm form axial muscles.
•The endoderm forms a gut tube by folding.
•Somite- gives rise to muscles and vertebrae in
Extraembryonic Structures
1.
2.
3.
4.
Yolk- derived from the hypoblast; site of early blood cells creation in
humans. Source of nutrients for bird and reptile embryos.
Chorion- formed from the trophoblast; outer membrane of the embryo. Site
of implantation onto the endometrium. Contributes to formation of placenta
in mammals
Allantois- mammalian waste transporter. Later it becomes the umbilical
cord which carries oxygen, food, and wastes (including CO2) back and
forth from placenta to embryo
Amnion- formed from epiblast. Surrounds fluid-filled cushion that protects
the developing embryo.
Developmental Processes
1. Growth
2. Cellular Differentiation- cells become
specialized in structure and function
3. Morphogenesis- produces shape and form of
body
a.
b.
Pattern formation- how tissues and organs are
arranged
Apoptosis- programmed cell death
Cellular Differentiation
Cells are totipotent- each contain instructions
needed for every specialized cell type in the
body. Scientists believe differential gene
expression causes specialization.
Cytoplasmic Segregation
•The egg cell contains maternal determinants
which help influence the path of development.
•Cytoplasmic segregation helps determine how
the morula cells will develop.
Induction
•Induction is the ability of an embryonic cell to
influence the development of another tissue by
touch or chemical signaling
•This works with maternal determinants to
influence organ development.
Morphogenesis
•Pattern formation determines the placement of
axes, organs, and appendages.
•Homeoboxes are conserved genes that act as
main switches. Homeotic genes code for
transcription factors and determine how the
segments of an organism will develop.
•Apoptosis determines the shape of hands and
feet in human development.
Human Embryonic and Fetal
Development
Conception to parturition is 266 days in
humans.
Human development can be be divided by
embryonic development (months 1 and 2 when
major organs are developed) and fetal
development (months 3-9 when structures are
refined).
Trimesters
First trimester- embryonic and early fetal
development occur
Second trimester- development of organs and
organ systems
Third trimester- fetus grows rapidly and major
organ systems become functional
Embryonic Development
Week One- fertilization occurs in the upper third
of the oviduct. Cleavage begins 30 hours after
fertilization while the cell moves to the uterus.
On day three, the embryo is a morula. Day 5 is
the transformation into a blastocyst.
Embryonic Development
Week Two- Embryo begins implantation into
uterus wall. The trophoblast (single layer of cells
on blastocyst) secretes human chorionic
gonadotropin (HCG) to maintain the
endometrium.The inner cell mass detaches from
the trophoblast and flattens into a the embryonic
disk (endoderm and ectoderm). Gastrulation
occurs.
Embryonic Development
Week Three- Nervous system and heart
development occur
Weeks Four and Five- Umbilical cord forms and
limb buds appear. Sense organs become more
prominent
Embryonic Development
Weeks Six through Eight- Head achieves its
normal relationship with the body, nervous
system allows for reflex actions, and all organs
are established.
Embryo is the size of an aspirin tablet.
Placenta Structure and Function
Placenta functions in gas, nutrient, and waste
exchange between embryonic and maternal
cardiovascular systems.
Chemicals can cross the placenta. Each organ
has a “sensitive period” when normal
development can be altered.
Fetal Development
● Weight increases 600 times
● Genitalia appears in month 3
● Initially flexes limbs and nods its head, but
eventually moves to avoid discomfort
● Heartbeat can be heard at 16 weeks with a
stethoscope
● Can survive at 24 weeks outside the womb
Stages of Birth
Stage One- Cervix dialation
Stage Two- Baby is born and umbilical cord cut
Stage Three- Placenta is delivered.