Lecture #3: Fertilization
& Development in Animals
The minute quantity of DNA in the zygote is the organic "blueprint" that contains the metabolic information needed to perpetuate the flux of energy we call life. During the long course of embryonic development to eventual adulthood, each and every cell in the body acquires its momentum for life from the zygote that forms upon fusion of the egg and sperm. This fusion is called fertilization.
I. FERTILIZATION IN A SEA URCHIN (deuterostome)
1. Certain primordial cells (immortal cells) called gonocytes are set aside early.
2. In the ovaries, the gonocytes grow unusually large to form oocytes with much cytoplasm.
3. In contrast, in the testes, the sperm cell has very little cytoplasm. (Fig 47.2) (sea urchin)
4. Acrosomes (acro = tip, top) of sperm contain hydrolytic enzymes to digest mucus around egg.
5. Acrosomal process has bindin (protein) that reacts with species specific receptors for bindin on the vitelline layer of the oocyte.
6. When membrane of sperm fuses with membrane of ovum, it depolarizes. (Fig 47.2)
a. Na+ flows in to discharge electric potential and cause fast block to polyspermy.
b. Ca++ is released from endoplasmic reticulum, or other cytosolic stores via IP3 signaling.
c. cortical granules inside membrane of ovum undergo exocytosis (Ca++ dependent) (Fig 47.3)
d. enzymes cause vitelline membrane to transform to hard fertilization membrane.
e. this is a long-term mechanism for preventing polyspermy, i.e., a slow block to polyspermy.
7. Such disturbance of the membrane induces embryonic development. (i.e., parthenogenesis)
a. injecting Ca++ can also induce embryonic development by exciting metabolic changes.
b. temperature shock can induce parthenogenesis of some eggs.
c. simply pricking the egg with a fine needle works on frog and rabbit ova. (pricking either depolarizes and/or causes Ca++ release)
8. True fertilization is when the sperm pronucleus and egg pronucleus unite (1 min).
9. Zygote contains the "blueprint", the "energy", and the "momentum" for life.
10. The ratio of cytoplasm to nuclear mass is high. (nutrient, RNA, DNA polymerase)
11. Within minutes after fertilization, respiration and protein synthesis increase.
12. The Ca++ leads to a change from pH 6.8 to 7.3, and this causes metabolic changes.
13. Summary of timetable of events in sea urchin fertilization (see Fig. 47.4)
II. FERTILIZATION IN A MAMMAL (e.g., a human) (Fig. 47.5)
1. mammalian eggs contain extracellular matrix called the zona pellucida.
2. the zona pellucida contains ZP3, which serves as a sperm receptor molecule.
3. sperm contact ZP3, causing an acrosomal reaction involving release of hydrolytic enzymes.
4. subsequent binding of the sperm membrane to the egg membrane induces depolarization.
5. depolarization is associated with signal transduction that leads to a cortical reaction.
6. this results in changes in the zona pellucida, and slowly blocks polyspermy.
III.
EMBRYONIC DEVELOPMENT
(Epigenesis = progressive development of form) (see the first half of page 999 in the text)
A. CLEAVAGE (=cutting)
1. Cleavage is a series of relatively rapid mitotic divisions of the zygote. (S® M® S® M® S)
2. The first cell division in a sea urchin zygote is in about 90 minutes, but humans take 36 hours.
3. Cleavage is not accompanied by protoplasmic growth in most animals. (Fig. 47.7)
4. Most animal eggs (and zygotes) have polarity (animal pole vs vegetal pole)
5. Some animal eggs (e.g., chicken) have enormous yolk to support cleavage.
6. As cleavage continues, Na+ accumulates in the interior, an osmotic gradient develops, and a fluid-filled blastocoel forms.
7. This period of rapid cell division ends at the blastula stage (@ 100-150 cells) of embryonic development.
B. GASTRULATION (= the establishment of cell layers thru movement and rearrangements)
1. Gastrulation is sometime called morphogenesis since cell layers begin to form.
2. Examine gastrulation in the frog embryo: (Fig. 47.10)
a. it begins with invagination of the blastula at the blastopore.
b. archenteron will become the GI cavity, and the blastopore becomes anus.
3. Formation of the ectoderm, endoderm, and eventually the mesoderm.
4. Developments from these three principle embryonic layers: (see Table 47.1)
a. Ectoderm gives nervous system, inner ear, eye lens, epidermis, pituitary gland, mouth, nasal epithelium, anal canal, rectum, and adrenal medulla.
b. Endoderm gives GI tract, lungs, liver, pancreas, thyroid, and bladder, and gonocytes.
c. Mesoderm gives notochord, muscle, bone, connective tissue, kidneys, blood, circulatory system, adrenal cortex, and gonads.
C. ORGANOGENESIS (the beginning of formation of distinct organs)
1. Neurulation in the frog (formation of nervous system starts first) (Fig 47.11)
a. a long groove forms extending the length of the embryo.
b. the dorsal folds of this groove move toward each other and fuse.
c. this neural tube will differentiate into the spinal cord and brain (CNS).
d. the neural crest will differentiate into adrenal medulla and autonomic nervous system.
D. DIFFERENTIATION (process of embryonic cells taking on special structure and function)
1. Unequal distribution of cytoplasm, ribosomes, etc. (heterogeneous organization of cytoplasm)
2. Cell-to-cell interactions compound the differences.
E. A BRIEF LOOK AT DEVELOPMENT OF THE CHICK EMBRYO (Fig. 47.12)
F. A BRIEF LOOK AT DEVELOPMENT OF THE HUMAN EMBRYO (Fig. 47.15)