Exercise 12
Reproduction and Development
INTRODUCTION
The fertilization of an egg
gg and the subsequent
q
development
p
of
an organism is a most remarkable process.
The fusing of the sperm and egg creates a cell, the zygote
zygote, which
may undergo a series of cell divisions to create the adult.
Although the specifics of development vary among organisms,
there
h are striking
iki similarities
i il i i found
f d among all
ll developing
d l i embryos.
b
In this lab, we will examine patterns of development among
animal forms and look at a plant embryo as well as the structures
that ensure its survival.
Exercise 12
Reproduction and Development
The development of an animal embryo can be divided into five
major processes:
1) gametogenesis
gametogenesis: the process of gamete production.
2) fertilization
fertilization: the fusion of male and female gametes
to form a single-celled zygote capable of
undergoing
g g development.
p
3) cleavage
cleavage: the mitotic divisions that divide the cytoplasm
into increasingly smaller cells, without an
increase in the total size of the cell mass.
mass
Exercise 12
Reproduction and Development
4) gastrulation:
gastrulation
t l ti
a stage
t
off cell
ll movementt andd rearrangementt
resulting in three different germ layers of cells. The
three germ layers have different potentials for
tissue specialization and development. As you
will observed in lab, the amount of yolk greatly
influences gastrulation and the development of
these three germ layers: ectoderm
ectoderm, endoderm
endoderm, and
mesoderm.
mesoderm
5) organogenesis
organogenesis:
i the
th process whereby
h b organs develop
d l from
f
the
th
three germ layers.
Exercise 12
Reproduction and Development
As a partt off this
A
thi laboratory
l b t
you will
ill observe
b
slides
lid off the
th
following:
mammalian ovary and testis
developmental stages of the starfish embryo
developmental stages of the chick embryo (33 and 72 hour)
In addition, you will observe the embryonic plant and the
structures that surround it.
Exercise 12
Reproduction and Development
REPRODUCTIVE ANATOMY
IIn this
hi section
i you will
ill examine
i the
h anatomy off the
h mammalian
li
ovary and testis.
Follicles within the ovary mature each month to form a
graafian follicle.
follicle After ovulation, the remnant of the follicle
becomes the corpus luteum which is responsible for the secretion
of progesterone and estrogen.
The ovaries, the uterus and the brain are linked in a complex
and integrated system that uses hormones as chemical messengers,
messengers
communicating the status of the systems and initiating changes
which must occur.
Exercise 12
Reproduction and Development
Refer to the following picture to review ovarian structure.
Note the graafian follicle and the corpus luteum.
graafian follicle
corpus luteum
Refer to Figure 12.2
in your lab manual
for comparison.
Exercise 12
Reproduction and Development
The structure of the male testis is composed of hundreds of tiny
seminiferous tubules,
tubules where sperm cells form from special cells
termed spermatogonia
spermatogonia.
The spermatogonia cells divide to form the primary spermatocyte
which is still diploid.
p
The primary spermatocytes divide by meiosis to form two new
haploid cells termed secondary spermatocytes which undergo a
second meiotic division to form spermatids
spermatids.
Once formed, the spermatids develop tails and mature to become
sperm cells
cells.
ll
Exercise 12
Reproduction and Development
The picture below represents a cross section of a seminiferous
tubule.
Refer to Figure 12.1
in your lab manual
for comparison.
comparison
Exercise 12
Reproduction and Development
STARFISH DEVELOPMENT
The starfish is an animal which practices external fertilization and
offers no support or protection for the young. Since the young must
provide for their own survival, they mature quickly into embryonic
forms
o sw
which
c ca
can feed
eed tthemselves
e se ves aand
d aaree mobile
ob e eenough
oug to avoid
avo d
certain predators.
The followingg stages
g of starfish development
p
occur in the water
column and lead to the production of a larval form responsible for
feeding before maturation into an adult starfish.
Exercise 12
Reproduction and Development
The following pictures represent various stages of starfish development.
UNFERTILIZED EGG
-The
h unfertilized
f ili d egg may or may not contain
i a clearly
l l
visible nucleus. Immature eggs, like this one, will
contain a large nucleus and nucleolus.
FERTILIZED EGG
-The
The fertilized egg (zygote) will have a dense
cytoplasm, without an obvious nucleus. A
fertilization membrane may be seen wrapped around
the zygote
yg in some instances.
Exercise 12
Reproduction and Development
MORULA
-The zygote undergoes rapid cell division without
cell growth (termed cleavage
cleavage) until a solid ball of
cells is produced termed the morula
morula. The cells
that make up the morula are termed blastomeres
blastomeres.
BLASTULA
-The center of the morula “hollows out” and creates
blastula The space inside the blastula is termed
blastula.
the blastocoel
blastocoel.
Exercise 12
Reproduction and Development
GASTRULA
-Cell migration results in the formation
of the gastrula
gastrula. The gastrula contains
three layers of cells termed germ layers.
The three germ layers include the ectoderm, endoderm and mesoderm.
Ectoderm - Cells of the ectoderm will form the outer skin and nervous system
y
of the adult starfish.
Endoderm - Cells of the endoderm form the digestive tract and associated
organs.
organs
Mesoderm - Cells of the mesoderm give rise to muscle, connective tissue
and reproductive tissues.
Exercise 12
Reproduction and Development
Compare the picture on the left with the diagram on
the right to familiarize yourself with these three
germ layers.
ectoderm
mesoderm
endoderm
Exercise 12
Reproduction and Development
As mesoderm is developed a primitive gut
is created termed the archenteron
archenteron. The opening
to the archenteron is known as the blastopore
and will become the anus in the adult animal.
archenteron
blastopore
Exercise 12
Reproduction and Development
Following the gastrula stage, two larval stages develop. As the
starfish egg does not contain enough yolk to support development, the
embryo uses the strategy of feeding as a free
free-living
living larval form to
obtain sufficient nutrition to complete development.
NOTE: You are not responsible for starfish larval
identification.
Metamorphosis occurs after
f sufficient
ffi i larval
l
l development
d l
for
f change
h
into the adult form.
Exercise 12
Reproduction and Development
A NOTE ON GASTRULATION
-Compare the models of the chick and starfish gatrula.
Notice that both models have the same colors representing endoderm
ectoderm and mesoderm, but the overall appearance of the gastrula differs.
Why is this the case?
Think about the difference in developmental environment
and the role of yolk in the chick egg.
egg
Exercise 12
Reproduction and Development
CHICK DEVELOPMENT
The chicken has certain features common with all other vertebrate
animals. Since all vertebrates are classified within the same large
taxonomic group (Phylum
Phylum Chordata),
Chordata they all demonstrate shared
features.
eatu es.
These features need not be present during the entire life of the animal
but must be found during at least some stage of the life cycle.
Features common to all chordates include a dorsal hollow nerve
cord, a cartilagenous rod known as a notochord
cord
notochord, gill slits found in
the pharyngeal area,
area and a post
post--anal tail.
tail
Exercise 12
Reproduction and Development
33 HOUR CHICK
-Use the diagram on the left to review
location of the following:
1) optic vesicles
2) heart
3) notochord
4) ddorsall hollow
h ll nerve cordd
5) somites
6) neural ridges
7) remnant of the primitive
streak
Exercise 12
Reproduction and Development
72 HOUR CHICK
-On the right is a
picture of a 72 hour
chick
hi k embryo.
b
Refer to the magnified photos
on the
h next slide
lid to review
i
features of the 72 hour chick.
Use the diagram on the right and
review the location of the following:
1)
2)
3)
4)
5)
6)
7)
8)
9)
forebrain
midbrain
idb i
hindbrain
eye
otic opening
somites
heart
pharyngeal gill slits
allantois
Refer to Figure
g
12.3 in the lab
manual for reference.
Exercise 12
Reproduction and Development
PLANT DEVELOPMENT
Just as animals undergo embryonic development, plants also
exhibit an embryonic condition.
condition In animals the embryo is
typically enclosed within a complex of supportive and
protective structures. The same can be said for the embryonic
plant.
A seed is much more than just a young plant. For example, the ordinary
pinto bean seed is covered by a protective coat that acts as a buffer
between the seed’s contents and the environment.
Exercise 12
Reproduction and Development
With the seed coat stripped away, a line can be seen which
divides the seed into two equal halves along the longitudinal axis.
Th function
The
f ti for
f these
th
structures
t t
(the
(th cotyledons
cotyledons)
t l d ) becomes
b
more
apparent when the two are separated, exposing the tiny embryonic
plant which was sandwiched in between the two cotyledons.
Filled with starch, the cotyledons nourish the developing
embryo until, or even after, it has produced leaves in
preparation for photosynthesis
photosynthesis.
Exercise 12
Reproduction and Development
PROCEDURE
1)) Obtain a soaked ppinto bean seed and remove the seed coat.
2) Carefully separate the two cotyledons and examine with the
dissecting microscope.
3) Draw and label the cotyledons and the embryonic plant
Exercise 12
Reproduction and Development
When the two cotyledons have been separated the seed should
appear similar to the picture depicted below:
Compare this specimen to the
illustration in Figure 12.4 of your
l b manual.
lab
l
Exercise 12
Reproduction and Development
Plants, like the pinto bean, containing two cotyledons are termed
dicots. However, not all plants contain two cotyledons. For example,
dicots
the corn seed depicted below only contains one cotyledon and is
termed a monocot
monocot.
Dicot
Monocot
Refer to Figure 12.4
off your lab
l b manuall for
f
a comparative diagram.
Exercise 12
Reproduction and Development
CONCLUSIONS
1) Specialized cells are formed in the process of developing
the sperm and egg cells that unite to form new individuals.
individuals
2) Plants and animals employ similar strategies during their
development.
3) Each organism has a set of specific developmental stages
that they must pass through in order to develop.
develop
4) Embryos that provide nourishment for their young develop
in a manner that is different from that of embryos resulting
from external fertilization.