Embryonic Development for AP Biology
Practice problems for these concepts can be found at: Human Reproduction Review Questions for AP Biology
Embryology, the study of embyronic development, is a detailed and complex field. Fortunately, you are not taking an AP exam in embryology. Stick to the basics here and do not let the complex details bog you down. Follow along with the pretty pictures, and the review questions at the end of this chapter will give you a good indication of the level of detail required for success on the embryology questions of the AP Biology exam.
Embryonic development begins as soon as the egg is fertilized to produce a diploid zygote (2n). This zygote then divides mitotically many times without increasing the embryo's overall size. During these "cleavage" divisions (Figure 16.5), cytoplasm is distributed unevenly to the daughter cells but genetic information is distributed equally. This disparity exists because different cells will later produce different final products and the uneven distribution of cytoplasm plays a role in that process.
These cleavage divisions take a while in humans. The first three divisions take 3 days to complete. After the fourth division, the one cell has become 16 cells and is now called a morula. As it undergoes its next round of cell divisions, fluid fills the center of the morula to create the hollow-looking structure known to embryologists as the blastula. The fluid-filled cavity in the blastula is known as the blastocoel. Up to this point, much of the dividing has occurred as the embryo moves toward the uterus through the fallopian tube. By the time the blastula has formed, it has reached the uterus and has implanted on the wall. The blastula contains two parts: an inner cell mass, which later becomes the embryo, and a trophoblast, which becomes the placenta for the developing fetus and aids in attachment to the endometrium. The trophoblast also produces human chorionic gonadotropin (HCG), which maintains the endometrium by ensuring the continued production of progesterone. The trophoblast later gives rise to the chorion, which we will discuss later.
Okay, here's where the discussion of embryology gets a little bit tricky. The next major stage of embryonic development after cleavage is gastrulation (also called morphogenesis). During gastrulation, cells separate into three primary layers called germ layers, which eventually give rise to the different tissues of an adult.
Let's look at this process in a bit more detail (See also Figure 16.6.) After the embryo attaches to the uterine wall, the inner cell mass divides into two major cell masses: the epiblast and the hypoblast. The hypoblast gives rise to the yolk sac, which produces the embryo's first blood cells. In birds and reptiles, the yolk sac provides nutrients to the embryo. In humans, the placenta fills this role.
The epiblast develops into the three germ layers of the embryo: the endoderm, the mesoderm, and the ectoderm.
Endoderm: inner germ layer; gives rise to the inner lining of the gut and the digestive system, liver, thyroid, lungs, and bladder.
Mesoderm: intermediate germ layer; gives rise to muscle, the circulatory system, reproductive system, excretory organs, bones, and connective tissues of the gut and exterior of the body.
Ectoderm: outer germ layer; gives rise to nervous system and skin, hair, and nails.
The separation of cells into the three primary germ layers sets the stage for cellular differentiation by which different cells develop into different structures with different functions. As far as this specific structural and functional differentiation is concerned, keep your focus to the basic development of the nervous system.
The human nervous system derives primarily from the ectoderm, but the mesoderm contributes a structure known as the notochord, which serves to support the body. In vertebrates, this is present only in the embryo. The cells of the ectoderm that lie above the notochord form the neural plate, which becomes the neural groove, which eventually becomes the neural tube. This neural tube later gives rise to the central nervous system. One other term you should be familiar with in the development of the mammalian embryo is the somite, which gives rise to the muscles and vertebrae in mammals.
There are four extraembryonic structures necessary to the healthy development of the embryo:
- Yolk sac: derived from the hypoblast; site of early blood cell creation in humans. Source of nutrients for bird and reptile embryos.
- Chorion: formed from the trophoblast; the outer membrane of the embryo. Site of implantation onto the endometrium. Contributes to formation of the placenta in mammals.
- Allantois: mammalian waste transporter. Later it becomes the umbilical cord, which carries oxygen, food, and waste (CO2) back and forth from placenta to embryo.
- Amnion: formed from epiblast. Surrounds fluid-filled cushion that protects the developing embryo. Present in birds, lizards, and humans, to name only a few.
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