Module 4
1. Module 4
1.7. Page 4
Module 4—From Fertilization to Birth
Lesson Summary
This lesson addressed the following focusing questions:
- What are the sequences of events and hormone controls that occur throughout fertilization and embryonic development?
- What major tissues and organs arise from the ectoderm, mesoderm, and endoderm?
- How are twins formed?
Prenatal development is divided into two main periods. The first period takes place from fertilization to the end of the eighth week of development. This period is called embryonic development, and the developing offspring is referred to as the embryo during this time. The period that takes place from the beginning of the ninth week to birth is called the fetal development period, and the offspring is referred to as the fetus during this time.
Once the egg and sperm have been correctly formed and released, both must meet inside the female’s reproductive system for natural fertilization to occur. The process of the sperm entering the egg thus combining the 23 chromosomes from the father’s sperm and the 23 chromosomes from the mother’s egg, is known as fertilization. Fertilization occurs in the oviduct, or Fallopian tube. Fertilization results in a single cell with 46 chromosomes, now called a zygote. While still in the oviduct, the zygote begins to divide through the process of cleavage, and at the 16 cell stage it is classified as a morula. The morula continues to divide and begins to fill with fluid. The resulting structure is then called a blastocyst.
The blastocyst forms two distinct layers of cells, an outer layer called the trophoblast, which will develop into the supportive membrane structure called the chorion. The mass of inner cells of the blastocyst will form the embryo. As the blastocyst reaches the uterus, the trophoblast produces and secretes an enzyme that digests some of the endometrial tissue, enabling the blastocyst to implant itself in the uterine lining.
As implantation is being completed, the amniotic cavity develops within the amnion,a type of sac. From the amniotic cavity, the embryonic disc forms. This disc will undergo the process of gastrulation—the formation of germ tissue layers (endoderm, mesoderm, and ectoderm). The event of gastrulation gives rise to a new term for the developing offspring, a gastrula. During gastrulation a series of changes occurs where cells begin differentiating.
Tremendous growth and organ development happen in the first trimester, with 90% of all organs being formed by the end of the embryonic period. At the same time as the embryonic growth is happening, supporting structures are also developing. The allantois, amnion, chorion, and yolk sac are formed to support the developing embryo and, later, the fetus. Some of these structures then give rise to the placenta and umbilical cord. The supporting structures help to nourish, protect, and remove wastes from the developing offspring during prenatal development. Once the baby is born, all of the supporting structures are expelled from the female’s body. These structures are then collectively referred to as the afterbirth.
Lesson Glossary
allantois: extra-embryonic membrane that forms the foundation for the umbilical cord, which connects the fetus with the placenta
amnion: extra-embryonic membrane that is a closed, fluid-filled sac surrounding the fetus; protects the fetus from impact and temperature change
blastocyst: embryonic stage that results when the morula hollows out; the outer layer is the trophoblast and becomes the chorion; the inner cell mass becomes the embryonic disc
chorion: outermost layer of the embryo; develops from the trophoblast layer of the blastocyst; will form the fetal blood pool of the placenta
cleavage: cell division in which the zygote divides into smaller, identical cells, called blastomeres, without increasing the total size of the cell mass
differentiation: the turning on and off of genes in a cell; results in specialized cell functions
dizygotic twins: twins that form when two eggs are fertilized by two sperms; also known as fraternal twins; may be of different sexes
ectoderm: germ layer that forms hair, nervous system, and skin
ectopic pregnancy: occurs when fertilization and implantation take place in the oviduct
embryo: the organism in the stages of development from the time the zygote begins cleavage until nine weeks of gestation, when the embryo becomes a fetus
endoderm: germ layer that forms gut lining, pancreas, and liver
extra-embryonic membranes: membranes surrounding the embryo, and later the fetus, that support its development in different ways; include the allantois, amnion, chorion, and yolk sac
fertilization: the fusing of a sperm nucleus with an egg nucleus to form a zygote; occurs in the oviduct
fetus: the organism in the stages of development from nine weeks until birth
gastrulation: the folding and differentiation of the embryonic disc into three germ layers, forming the embryonic stage called the gastrula
human chorionic gonadotropin (HCG): hormone released from the chorion after implantation; directs corpus luteum to continue making estrogen/progesterone, thereby maintaining the endometrium and preventing menstruation
implantation: burrowing of the blastocyst into the wall of the endometrium of the uterus
mesoderm: germ layer that forms muscle, bone, and muscular organs
monozygotic twins: identical twins formed when the embryonic mass produced by the union of one egg and one sperm splits into two separate, identical masses; are of the same sex
morphogenesis: the development of body organs and structures; begins with gastrulation
morula: embryonic stage of a solid ball of identical cells that results from cleavage of the zygote
neurulation: the process of forming the embryo's neural tube, which will develop into the brain and spinal cord
placenta: embedded in the endometrium; provides a location where nutrients, gases, and wastes can be exchanged between fetal and maternal blood pools
primary germ layers: three layers formed during gastrulation—endoderm, mesoderm, and ectoderm
umbilical cord: tube of blood vessels that connects the fetus to the placenta; for exchange of nutrients, gases, and wastes
yolk sac: extra-embryonic membrane that produces first blood cells
zygote: the one-celled product of egg and sperm fusion; also known as the fertilized egg