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. Two-celled 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 non-neural 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 folds Neural plate

A B Neural groove

Figure 3. Sections through 24-hour chick. A. Neural plate

Neural crest

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

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. Glinuclei are scattered over the tissue as dark spots.

Neuron cell body

Glial cell nu

Nervous Tissue

Observation of Mesodermal derivatives

Dermis

al cell

clei

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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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