Lab Guide 11: Survey of Embryonic Development
Objectives:
Upon completing this lab the student should be able to:
1. Define gamete, ovum, spermatazoan, fertilization and zygote
2. Define and discuss the function of cleavage and gastrulation
3. Name the three primary germ layers and describe some of the tissue types to which they give rise
4. Identify the various structures of the developing embryo and placenta
6. Observe the early stages of embryological development in the sea urchin and by association the human embryo
Introduction
Congratulations! You have almost completed a full year of Anatomy and Physiology Laboratory. Your instructors have enjoyed teaching you and you can be assured that your hard work and perseverance are appreciated. We hope this course will prepare you well for the academic challenges of your upper division science courses and your chosen career. You have just one more exercise to complete.
Unlike previous lab exercises where your report was completed at home and turned in the following week, your laboratory instructor may ask you to complete this lab report in class and turn it in before leaving today.
Exercise 44. Survey of Embryonic Development, pp. 445-449.
Fertilization and Early Embryonic Development
We will not be doing this part of the lab as described in the lab manual. However, you should review this chapter and the background information provided here for a description of the terminology. We will try to observe fertilization and early development in the sea urchin as a "hands on" activity for this lab. The following terms that are important in describing human embryonic development do not apply to sea urchin development: inner cell mass, trophoblast, chorion, decidua basalis, decidua capsularis, fetus, chorionic villi, placenta, amnion, yolk sac, allantois, umbilical cord. See Figure 44.1, p. 446, in your lab manual for illustration of these structures. While there are numerous differences in the specific patterns and terminology describing sea urchin development, sea urchins provide a convenient model for observing fertilization, cleavage, and early embryonic structures such as the blastula and gastrula.
Each sperm cell (= spermatozoan) consists of a head, midpiece, and tail (= flagellum). In the head of the sperm is its nucleus containing its chromosomes made of the genetic material, DNA, and a vesicle, the acrosome, filled with digestive enzymes, such as acrosin, needed to penetrate the coverings of the egg. At ovulation, the egg consists of the egg cell proper, the zona pellucida, and corona radiata. The egg contains yolk and the nucleus with its genetic material, DNA. Recall that gametes, eggs and sperm, are haploid, i.e., they have half the usual number of chromosomes. The zona pellucida is a glycoprotein covering surrounding the egg. The corona radiata is a few layers of follicular cells that adhere to the egg when it is ovulated. For fertilization to occur the sperm must use the digestive enzymes contained in its acrosome to digest through the corona radiata and zona pellucida. It takes the combined activity of many sperm to remove the protective coverings of the egg so that one sperm finally makes contact with the egg cell membrane. When a sperm reaches the egg cell membrane, the sperm cell membrane will fuse with the egg cell membrane, and the sperm nucleus will be endocytosed into the egg, (= penetration). This event is sometimes called activation because it causes the egg cell to resume its metabolic activities, complete meiosis, and then rearrange the genetic material of the sperm and egg into one, diploid, nucleus.
At the instant a sperm penetrates the egg (See Fig. 28.2, p. 1112, in your text) a structure called the fertilization membrane forms. Penetration of the egg by the sperm triggers the egg cell membrane to depolarize, which in turn causes calcium ions to be released from vesicles inside the egg, which in turn causes granules to be released by exocytosis, which in turn change the chemical properties of the zona pellucida and render it impervious to the acrosin of other sperm. This prevents multiple fertilization or polyspermy from occurring.
Activity: Fertilization and Development in Sea Urchin
The formation of the fertilization membrane can be observed in sea urchin eggs. Place a drop of sea water containing sea urchin eggs on a depression slide and cover with a cover slip. Add a drop of sea water containing sea urchin sperm to the slide at the edge of the cover glass. Fertilizations may begin within 30 seconds, so start observing immediately.
After the sperm and egg nuclei fuse, the resulting diploid cell, a fertilized egg, is called a zygote. The zygote begins mitotic cell divisions, a process called cleavage in embryonic development. Each cleavage doubles the number of cells until a solid ball of cells called a morula is produced. Then a hollow space forms in the cell mass and the resulting hollow ball of cells is called a blastula. The hollow space is called the blastocoel cavity. In the human embryo a cluster of cells, the inner cell mass, remains on one side of the blastula. It is these cells of the inner cell mass that will develop into the human embryo. From this stage on, development of humans is different than that of sea urchins, although both progress through the stages described below. (See Fig. 28.4, p.1114, in your text.)
In human development the other cells of the blastocoel surrounding the inner cell mass are called the trophoblast . The trophoblast and part of the inner cell mass will give rise to the fetal portions of the placenta. This is the stage of development at which implantation into the uterine wall occurs in the human embryo. (See Fig. 28.4, p. 1114, in your text)
Sea urchin embryos proceed to the next stage of development called gastrulation as free living embryos. In gastrulation, the cells of the embryo begin to rearrange themselves to form three distinct tissue layers. These first three distinct tissue layers are known as primary germ layers and they are: an outer ectoderm, an inner endoderm, and a middle mesoderm. Ectoderm gives rise to epidermis and nervous system tissues, mesoderm to muscle and connective tissues, and endoderm gives rise to the epithelial surfaces of the digestives system and those organs, such as the respiratory tract and digestive glands that develop as outgrowths from the alimentary canal. Sea urchin eggs have been fertilized in advance at different times so that you can observe the various embryonic stages in the sea urchin embryos. Prepared slides of sea urchin development as well as wall posters are available for reference. Place a drop of water from each petri plate on the depression slide and try to observe all the embryonic stages.
You may choose to use much of this last lab to complete and turn in your lab report for this week so that your instructor may grade it and return it before the practical. You are advised also to review the previous weeks' labs to prepare for the final lab practical with any spare time. See Dr. Thompson's Review for the final practical exam at: http://www.apsu.edu/thompsonj/Anatomy%20&%20Physiology/2021%20Labs/2021%20final%20practical%20topic%20outline.htm The topics covered include: Respiratory Anatomy and Physiology, Digestive system gross anatomy and histology, Physiology of digestion, Digestive Enzyme function; Urinary system gross anatomy and histology; Dialysis; Urinalysis; and Reproductive System gross anatomy and histology, reproductive physiology, and embryonic development.
Assignment
Review Sheet Exercise 44, pp. 677-680.
Developmental Stages of the Human: #1, #4, 12