Development
DEVELOPMENT LABORATORY
All living things must be able to reproduce
and develop. When we say that an organism
“develops” we are really looking at changes in
size and shape and at the emergence of different cellular characteristics.
The undifferentiated cells of an early embryo have
a “fate” or destiny – they will become certain types of
cells in the adult organism. Early embryonic cells will
eventually give rise to various tissues, organs and organ
systems. Early embryologists were able to construct
“fate maps” for the embryos of many different types of
organisms by lightly staining the cells of the early
embryo using vital dyes and observing where the different colors were found at a later stage. Thus, the fate of
individual cells can be determined.
Throughout your study you should remember that the entire process of development
begins with a single cell, the fertilized egg or
zygote, and the processes of cell division, cell
movement and cell differentiation result in the
development of a complex multicellular
organism.
GAMETES
I. Sperm
male gamete
produced in seminiferous tubules
formed by process of spermatogenesis
meiosis, cytokinesis, spermiogenesis
differentiation occurs in the epididymis
acquireing ability to move
composed of three parts
A. Head
1. nucleus
haploid
2. acrosome
found in front of nucleus
derived from Golgi complex
contains enzymes for protein and
sugar digestion and are used
to lyse outer covering of egg
B. Midpiece
contains mitochondria
C. Tail
a flagellum
used to propel the sperm forward
Examine the model of human sperm cell and
the slides of mammalian testes.
II. Ovum or Egg
female gamete
contains material needed for growth and
development of a new individual
known as oocyte before meiosis is
completed
actively accumulates cytoplasm
has prominent nucleus in various stages of
becoming haploid, depending on
species
surrounded by vitelline membrane outside
plasma membrane; essential for
species-species sperm binding; known
as zona pellucidum in mammals
ova classification
A. according to yolk content
1. microlecithal
small amount of yolk
2. mesolecithal
medium amount of yolk
3. macrolecithal
large amount of yolk
B. according to distribution of yolk
1. isolecithal
evenly distributed yolk
characteristic of isolecithal eggs
2. mildly telolecithal
yolk is found at one pole (vegetal
pole)
cytoplasm located at other pole
(animal pole)
characteristic of mesolecithal egg
3. extremely telolecithal
cytoplasm is contained in
blastodisc
found on surface of ovum with
yolk beneath
characteristic of macrolecithal
eggs
1
Development
III. Follicles
part of ovary
house ova
mammalian follicles have several parts
A. Granulose
layer of cells immediately around
oocyte
primarily source of estradiol
border antrum in larger follicles
B. Theca
layer cells outside of granulose
source of androgens, precursors of
estrogens
As follicles develop, they increase in size,
develop antrum, a fluid filled cavity, and
produce increasing amounts of hormones;
developing follicles are classified as
primordial, primary, secondary, and
tertiary or Graafian follicles; these latter
undergo ovulation
Examine the model of the frog egg, and the
chicken egg on display. Examine slides of
mammalian ovaries and be able to identify the
parts of follicles and the oocyte.
FERTILIZATION
Fertilization is two events:
I. Cytoplasmic Contact
between egg and sperm
activates egg
usually prevents other sperm from
entering egg
II. Pronuclear Union,
pronuclei from male and female open
chromosomes mix
these occur prior to mitotic cleavage
fertilized egg is known as zygote.
in frog egg gray crescent appears opposite point of
sperm entry
Study models of the fertilized frog egg and
identify the gray crescent.
The BLASTULA
I. Cleavage
series of mitotic divisions, often rapid
transforms zygotes into multicellular a
structure
no gain in volume or mass
each daughter cell is a blastomere.
results in
A. Blastula
a hollow ball
cavity is blastocoel
OR
B. Blastodisc
a disc of cellular tissue
formed in very yolky eggs
overlies large volume of undivided
yolky cytoplasm.
The type of cleavage that a zygote
undergoes is dependent upon (1) the
amount of yolk and (2) the distribution of
that yolk within the zygote:
C. Holoblastic Cleavage
cleavage plane cuts through the entire
(= holo-) zygote
divides the zygote into two large cells,
first cleavage is vertical, cutting
through both animal and vegetal
poles
second cleavage plane is at right
angles to first, oriented vertically.
third plane is horizontal or perpendicular to first two
1. holoblastic and equal cleavage
occurs if eight nearly equal cells
are formed
seen microlecithal eggs
blastomeres are of approximately
equal size
2. holoblastic and unequal cleavage
occurs when third cleavage plane
is located nearer to animal
pole
results in top group of four blastomeres being smaller than
bottom four
usually seen in mesolecithal eggs
blastomeres are unequal in size
due to unequal distribution of
yolk; animal pole cells have
less yolk and are smaller;
vegetal pole cells have more
yolk and are larger
results in a smaller blastocoel
2
Development
GASTRULATION
Interestingly, in the frog egg, the location of the
gray crescent will determine the placement of the first
cleavage division in the frog zygote. This plane will
always bisect the gray crescent. The second cleavage
division occurs at right angles and parallel to the first.
These events are genetically programmed within the
embryo and unfold following fertilization. The
importance of these first two cleavage divisions which
determine, first, the left and right halves of the organism
and, second, the front and rear cannot be underestimated.
II. Meroblastic Cleavage
in macrolecithal eggs
shows a spectrum of cleavage patterns
because egg cytoplasm divides
incomepletely (cytokinesis lags behind
mitoses)
some cells remain incompletely divided
(incomplete cleavage) or much of the
yolky mass of egg remaining
undivided (discoidal cleavage).
Cleavage ultimately results in an increase
in the number of cells and an increase in the
nuclear/cytoplasmic ratio, but does not generally lead to a change in size or shape of the
developing embryo. Cleavage also divides the
fertilized egg into smaller units that will be
more able to regulate their own metabolism.
Most importantly, however, cleavage localizes
different kinds of cytoplasm in discrete cellular
packages. Cleavage nuclei are thus segregated
into unitary cytoplasmic environments where
the interaction between nucleus and cytoplasm
and between adjacent cells may result in different populations of cells becoming differenttiated. This process leads to the formation of
different types of tissues with increasingly
specialized functions. As these tissues and the
organs they comprise develop, the embryo will
also change in shape – a process called
morphogenesis.
Cells must move and change their positions relative to other cells. This “migration” is
called gastrulation and the embryonic stage
formed as a result of the process is called a
gastrula. During gastrulation cells move into
definitive locations and as development
proceeds they become progressively more
differentiated, that is, their fate becomes determined. Cells of the blastula can generally be
transplanted to other locations and subsequently will assume a normal role for cells located in
the new site. However, as development and
differentiation continue, cellular potency is
reduced and, at some point, the transplantation
of eye tissue to the belly will cause an eye to
develop at this site, the transplantation of a
limb bud to the head will cause a limb to
develop at this point, etc. Thus, cells interact
with one another and development is really
underway.
Gastrulation is an early developmental
stage in which a hollow ball or disc of cells is
transformed into a tube (endoderm) within a
tube (ectoderm) containing mesoderm between
the inner and outer tubes. Somehow the migrating cells of the blastula must assume this
arrangement.
I. Amphioxus Gastrulation
simple invagination of cells
occurs at blastopore (opening into
primitive gut or archenteron)
outer cells become ectoderm
inner cells become endoderm
mesoderm forms from cells between
ecto- and endoderm and within
blastocoel; may involve some
independent cell migration
Study gastrulation in amphioxus using the
models.
Examine the cleavage stages of amphioxus and
frog embryo models on display. Consider what
happens to the size of the individual cells and
what happens to the size of the embryo itself.
Study the models of cleavage leading to the
blastula stage in amphioxus and the frog.
3
Development
II. Frog Gastrulation
in future anterior end of blastula (above the
gray crescent) cells begin to move
downward from animal pole and,
more slowly, from sides toward this
region; results in piling up of cells and
their movement inward to interior and
back under dorsal surface of embryo
forms crease in blastoderm known as
dorsal lip of blastopore
as development continues, crease moves
ventrally to form a complete
blastopore
internal movements slow
blastopore is filled with yolk-laden
endoderm cells forming a yolk plug.
blastodisc surface
forms mid-line thickening called
primitive fold and groove
cells move down into blastocoel
streaming anteriorly and laterally
anterior end of primitive groove,
primitive pit, is an active area of
movement
functionally equivalent to dorsal lip of
blastopore in frog.
B. Hypoblast
blastocoel separates these layers
as development continues, and with the
differentiation and beginnings of
organogenesis in more anterior tissues,
the primitive streak moves posteriorly
and is gradually obliterated.
Study gastrulation in the frog models.
III. Chicken Gastrulation
blastula stage consists of a blastodisc
which lies on top of an undivided
mass of yolky cytoplasm separated
from cytoplasm by subterminal space
cells of disc migrate to and delaminate
from upper layer to form two separate
tissues
A. Epiblast
cells move posteriorly and medially on
Examine a whole mount of the chicken embryo
at 18 hours. DO NOT USE HIGH POWER
ON WHOLE MOUNT SLIDES! Find the
notochord; head fold, anteriorly; neural tube;
primitive folds; primitive groove; primitive pit;
the area pellucida (where the blastodisc is
separated from the underlying yolk by the
subgerminal space); and area opaca (where
the blastodisc is in contact with the yolky
cytoplasm).
4
Development
AM
AO
AP
HN
HP
NO
anterior margin of mesoderm
area opaca
area pellucida
Hensen’s node, dorsal lip
head process, chordamesoderm
notochord
PA
PF
PG
PP
PS
proamnion
primitive fold, lateral lip
primitive groove, blastopore
primitive pit, anterior blastopore
primitive streak
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