Download Animal Development, Organogenesis, and Animal Tissues

Animal Development, Organogenesis, and Animal Tissues
Early development: Fertilization and early cleavage
Development in a multiceullar organism begins with the fusion of male and female
gametes. When egg and sperm come into contact, their haploid nuclei fuse, to form one
diploid cell called a zygote. The zygote rapidly begins the process of mitosis which
converts the zygote into a multicellular ball or disc, called the blastula. The cells of the
blastula are called blastomeres, and surround a cavity, the blastocoel.
Because early events in development are affected by the amount of yolk present in the
egg, we need to back up and think about the makeup of an egg in terms of its yolk
distribution. Eggs with small amounts of evenly distributed yolk are called isolecithal
eggs. Eggs containing large amounts of yolk concentrated at one end are telolecithal.
The yolk is concentrated in one region of the egg, called the vegetal hemisphere, or the
vegetal pole. The part of the egg that is devoid of yolk is called the animal hemisphere or
animal pole.
The end of early development is similar in all organisms, but the pattern of early mitotic
divisions can differ. One factor that influences the pattern of mitosis is the amount of
yolk present. In isolecithal eggs, which have minimal yolk, cleavage is holoblastic,
which means that cell divisions pass through the entire fertilized egg. In telolecithal eggs
the yolk retards cytoplasmic divisions and in some cases, only the animal pole shows cell
divisions. This process is called meroblastic cleavage, and it produces a cap of cells, a
blastoderm, during cell division that will ultimately give rise to the embryo. The
blastocoel forms between two layers of cells within the blastoderm.
Observation of early sea star development
Obtain a whole mount slide of sear star embryos and examine them under low and high
power. Use extreme care since this slide is much thicker than most. Examine the slide
and identify each characteristic stage.
Unfertilized egg: Cell is spherical with a prominent nucleus and nucleolus.
Zygote: The nuclear membrane disappears and the nucleous becomes indistinct.
The cell is round and homogeneous.
Two, four, and eight cell stages: Notice the patterns of cleavage. How does the
division producing the two cell stage compare with that producing the four cell
stage?
________________________________________________________________
Is the embryo increasing in size with subsequent cell divisions? _____________
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Notice the size of the cells in each of these stages of division. How do they
compare? ________________________________________________
Is cleavage in this animal holoblastic or meroblastic? _______________________
Morula: A morula is a ball of 16-32 cells without a central cavity. Compare the
size of the morula stage with the unfertilized egg. How do they compare?
__________________________________________________________________
Blastula: As cleavage continues, the cells become continually smaller and
become organized into a hollow ball, with a fluid filled blastocoel in the center.
A
B
C
D
E
Figure 1. Early sea star development. A. Unfertilized ovum. B. Zygote. C. Twocelled embryo. D. Four-celled embryo. E. Blastula
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Gastrulation
Gastrulation transforms the blastula into a gastrula made of three germ layers:
endoderm, ectoderm, and mesoderm. Early development is characterized by cell
division, but the focus of gastrulation is cell movement. Surface cells move into the
interior of the embryo (involution), forming a new hollow cavity, the archenteron. The
archenteron is lined by endoderm, the germ layer that will ultimately form the digestive
tract. The opening of the archenteron to the outside is the blastopore, which ultimately
becomes either the anus (in deuterostomes) or the mouth (in protostomes). The cells
that remain on the surface of the embryo become the ectoderm. A third layer of cells,
the mesoderm, develops between ectoderm and endoderm.
Observation of gastrulation it the sea star
In the sea star slide, locate embryos showing the initial steps of gastrulation and
subsequent stages of invagination. The early gastrula can be recognized by a small
outpocketing of cells producing into the blastocoel. These cells push into the blastocoel
through a region on the embryo surface called the blastopore. In this animal, the
blastopore becomes the anus of the organism. As cells continue to invaginate, or move
inward, a tube called the archenteron forms. Which embryonic germ layer lines the
archenteron? _________________________________________
In a late gastrula, the mesoderm begins as an outpocket of the endoderm and eventually
fills most of the blastocoel. As the tip of the archenteron meets the opposite wall of the
embryo, it fuses with surface cells and eventually becomes the mouth of the embryo.
A
B
Figure 2. Gastrulation in the sea star. A. Mid-gastrula. B. Late Gastrula
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Neurulation
Late in gastrulation ectodermal changes begin to occur which causes the formation of a
dorsal neural tube. This process, called neurulation, occurs only in chordates.
Ectodermal cells flatten into a neural plate, which extends the entire length of the
embryo. The center of the plate sinks, giving rise to a neural groove, and the edges of
the plate elevate to form neural folds. These folds move towards each other until they
eventually fuse, producing the hollow neural tube. The anterior end of the tube develops
into the brain, while the posterior end develops into the spinal cord.
Observation of neurulation in the chick embryo
Because the process of neurulation occurs only in chordates, we will switch over to the
chick embryo for its observation. The instructor has set up microscopes in which you
will observe this process. In early stages of neurulation, the central neural plate is
surrounded laterally by two prominent neural folds. As these folds move towards each
other, a distinct neural groove is produced. When the folds meet and fuse, notice that
some of the tissue in the neural folds does not become part of the closed neural tube, but
breaks off of the surface of the ectoderm as the neural crest. The cells of the neural
crest migrate throughout the body, and develop into a wide variety of neural and nonneural structures. Some of the structures produced from neural crest include sensory and
autonomic ganglia, glial cells, pigment cells, the adrenal medulla, and parts of the skull.
Neural plate
Neural folds
A
B
Neural crest
Neural groove
Figure 3. Sections through 24-hour
chick. A. Neural plate
B. Neural folds
C. Formation of neural tube nearly
complete
Neural tube
C
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Organogenesis
After the germ layers and nervous system have been established, organogenesis, or the
formation of organs and organ systems takes place. Ectoderm, forms both the neural
tube and skin and its associated glands. The mesoderm gives rise to muscles, the
skeleton, gonads, the excretory and circulatory systems. The endoderm develops in to
the lining of the digestive tract and the respiratory tract.
Observation of the 48 and 96-hour chick
Using the slides on demo locate the following structures:
48-hour chick- In the living embryo, the heart is already beating and pumping
blood through the vitelline vessels, which carry food from the yolk to the
embryo. There is a distinct atrium and ventricle at this stage. The anterior
neural tube has given rise to the brain and eyes are partially formed. Somites, or
blocks of tissue, lie on either side of the spinal cord.
96-hour chick- The brain has continued to develop, and there are distinct eyes
and ears by this stage. The heart is very conspicuous and additional blood
vessels are forming. Anterior and posterior limb buds are fairly well
developed, and will give rise to the wings and legs of the animal.
Based on your observation of the developing chick, what type of egg is the chicken egg?
_____________________________________________
This egg would undergo what type of cleavage? _______________________________
Describe the major differences between the 48 and 96-hour chick.
Animal Tissues
All of the tissues of the body are produced from one of the three germ layers. The four
types of tissue found in animals are epithelium, connective tissue, muscle tissue, and
nervous tissue. We will observe characteristics of some of these types of tissues as we
think about the germ layer from which they arise.
Observation of Ectodermal derivatives
As stated earlier, the ectoderm of the embryo gives rise to the skin and nervous system.
Examine a slide of skin. The outer layer, the epidermis is composed of stratified
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squamous epithelium. On the surface of this epithelium are many layers of dead cells.
These cells provide a thick waterproof barrier on the surface of the body. Because of the
presence of this layer, the epidermis is said to be a keratinized stratified squamous
epithelium. Below the epidermis is the dermis, a layer of dense connective tissue
containing elastic and collagen fibers, and fibroblasts. Only the dermis of the face is
ectodermally derived (it comes from neural crest). Most of the dermis of the body is
produced from mesoderm.
Human Skin
Epidermis
Dermis
The ectoderm also gives rise to the nervous tissues of the body. Observe a slide of
nervous tissue. Note the large prominent neurons with their prominent nuclei. Glial cell
nuclei are scattered over the tissue as dark spots.
Nervous Tissue
Glial cell nuclei
Neuron cell body
Observation of Mesodermal derivatives
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Many epithelial tissues, the connective tissues and muscle tissues of the body are
produced by the development of the mesoderm. There are numerous examples of each,
but here you will observe just a few.
Epithelial tissues are primarily categorized based on cell shapes and numbers of layers.
Observe the difference between simple cuboidal epithelium (from the tubules of the
kidney) and the stratified squamous epithelium you saw in the epidermis of the skin.
The kidney tubules are lined by a single layer of cube shaped cells, while the skin is
covered by many layers of cells, most of which are flattened with flat nuclei. Also
observe the lining of a blood vessel (also called the endothelium). It is made of a single
layer of cells. Which cells does the endothelium more closely resemble, the cells of the
kidney tubules
(cuboidal) or those in the many layers of the skin (squamous)? ____________________
Simple Cuboidal Epithelium
Simple Squamous
Epithelium
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The mesoderm also produces the connective tissues of the body. Observe one type of
connective tissue called adipose tissue. It is composed of rounded cells that store fat in
cytoplasmic vacuoles. The nuclei of these cells is flattened and pushed to the edges of
the cell.
Adipose tissue
Another type of connective tissue is cartilage. It is made of cells called chondrocytes,
which sit in spaces (lacunae) in the ground substance or matrix. Here you will observe
the most prevalent cartilage, hyaline cartilage. It is found in the nose, larynx, trachea,
and in the articular cartilages of joints.
Hyaline Cartilage
Chondrocyte in lacunae
Bone is also a mesodermally derived connective tissue. There are two types of bone
tissue; here you are observing compact bone. Compact bone is made of structural units
called osteons or Haversian Systems. At the center of each osteon is an opening, the
haversian canal, through which blood vessels and nerves travel. In the concentric rings
of the osteon are the osteocytes (bone cells) sitting in spaces in the bone matrix called
lacunae.
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Bone tissue
Osteon
Haversian Center
Muscle tissue comes in three varieties: skeletal, cardiac, and smooth. All three types
contain elongated cells, or fibers. Internally these cells contain the contractile proteins
actin and myosin, which allow them to contract. Observe a slide of skeletal muscle. Its
cells are extremely long and cylindrical. The striations on the surface of the cells are
produced by the orderly arrangement of actin and myosin filaments within the muscle
cell. A unique feature of skeletal muscle is the large number of nuclei found within each
cell.
Skeletal muscle
Cardiac muscle is also striated, but its cells are much shorter than skeletal muscle cells
and are often branched. Where skeletal muscle cells join together the membrane contains
large numbers of desmosomes and gap junctions. These regions are visible externally as
intercalated discs. A cardiac muscle cell has one nucleus (sometimes two) that is
centrally located and oval in shape.
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Cardiac Muscle
Intercalated Disc
Observation of endodermally derived tissues.
Endoderm produces the lining of the digestive tract and the respiratory system. Of
course, there are many organs associated with each of these systems, so again we will be
looking at a few select areas. First, observe a slide of the digestive tract, specifically the
intestine. Only the lining of this organ is produced from endoderm. The outer layers of
the organ, made of connective tissues and muscle, are produced from mesoderm. The
lining of this structure is made of simple columnar epithelium. Notice that its cells are
tall and narrow, and have oval shaped nuclei sitting near the basal edge of the cells.
Interspersed within this simple epithelium are goblet cells. Goblet cells are specialized
glandular cells that produce mucous. They are named for their clear staining apical
surface, which resembles the shape of a wine glass. Most of the digestive tract is lined
with this same kind of epithelium. However, at the ends of the digestive tract the tissue is
very different. Observe a slide of the esophagus. Again, only the lining of the esophagus
is made from endoderm. This lining is made of many layers of flattened cells and is
called stratified squamous epithelium. However, compare this stratified squamous
epithelium to that seen on the surface of the skin. Notice that the epithelium lining the
esophagus lacks the keratinized layer that is found on the surface of the skin. Therefore, it
is referred to as a non-keratinized epithelium. This same non-keratinized stratified
squamous epithelium is found lining the mouth and the anal regions of the digestive
tract.
Simple columnar epithelium of intestine
Goblet cells
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Stratified squamous epithelium lining
mouth
The endoderm also produces the respiratory system. Observe a slide of the lung. Most of
the lung is filled with air sacs, or alveoli. Observe the lining of these alveoli under high
power. What is the shape of the cells lining each alveolus?
_________________________________
Lung
Alveolus
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