Module 9: Human Development and Anatomy Through the Lifespan
Module 9: Human Development and Anatomy Through the Lifespan
Learning Objectives:
By the end of this class, students will be able to:
- Identify key times and events in embryological development and describe the consequences of teratogen interference during those times.
- Understand critical periods of development and some of the consequences of disrupting typical development.
- Describe common changes that occur in the body with aging.
Terms to Know
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*Covered only in lecture, not in this text |
The topic of embryonic and fetal development could fill an entire course. This will only be a brief introduction to the topic.
Embryonic and fetal ages are expressed in terms of weeks from fertilization. The period of time required for full development of a fetus in utero is referred to as gestation. It can be subdivided into distinct periods:
- Embryonic period: from fertilization to week 8.
- Fetal period: from week nine until birth.
Embryonic Development
This information will be covered in the assignment and reviewed and built upon in lecture.
The first two weeks of prenatal development are sometimes referred to as the pre-embryonic stage. By the third week, the tissues that ultimately develop into all organ systems of the body have formed. By the end of the embryonic period, all of the organ systems are structured in rudimentary form, although the organs themselves are either nonfunctional or only semi-functional. The process of organ formation is called organogenesis.
Week 1 (Pre-Embryonic Period)
The terminology “embryo” is not typically used this early in gestation, but we will not ask you to know the other terms, so we will stick with “embryo” for simplicity.
Following fertilization, during the journey to the uterus, repeated rapid cell division occurs. Approximately 3 days after fertilization, a 16-cell embryo reaches the uterus. Once inside the uterus, the embryo floats freely for several more days, continuing to divide. At the end of the first week, the embryo embeds itself in the uterine lining, beginning the implantation process.
Week 2 (Pre-Embryonic Period)
At the beginning of the second week, the cells form into a two-layered disc of embryonic cells. Little embryonic growth occurs at this stage. Instead, the second week is a time when structures that support the embryo form. The amniotic fluid forms, which allows the embryo to float within it, protecting it from trauma and rapid temperature changes. The placenta begins to form in this stage as well. This is critically important because the embryo and fetus need oxygen, nutrients, and other factors to develop and grow. The placenta begins to form very early after implantation in order to provide these critical resources and support the development of all organ systems. Though it takes a few weeks before the placenta takes over this entire role, its development starts very early in gestation.
Week 3
As the third week of development begins, the embryo takes the form of a three-layered oval-s