MCAT Biology Review

Chapter 3: Embryogenesis and Development

Conclusion

In this chapter, we have seen a just-fertilized ovum (zygote) become an embryo. As organs develop and the body organizes into complex organ systems, that embryo will turn into a newborn baby. Development certainly does not stop there, however—humans nurture their young for years (sometimes decades!)—as they undergo physical, cognitive, and sexual development. Embryonic development is extremely important because it lays the foundation for further development to be able to proceed correctly. Most of the time, the process goes exactly as planned; however, this is not always the case. In medical school, you will learn all about the wide spectrum of teratology—the study of birth defects.

Adult structures that arise from embryonic germ layers are of special importance to us because they are commonly tested on the MCAT. For the remainder of embryology—from the first cleavage event to the last uterine contraction—focus on the main terminology and highlights of each stage, in addition to the differences between fetal and adult physiology. Now that we have seen from where the organ systems derive, we begin our survey of anatomy and physiology. For the next eight chapters (Chapters 4 to 11 of MCAT Biology Review), we will explore the cells, tissues, organs, and interactions of each of the major organ systems. Our discussion begins with the nervous system.

Concept Summary

Early Developmental Stages

· Fertilization is the joining of a sperm and an ovum.

o It usually occurs in the ampulla of the fallopian tube.

o The sperm uses acrosomal enzymes to penetrate the corona radiata and zona pellucida.

o Once it contacts the oocyte’s plasma membrane, the sperm establishes the acrosomal apparatus and injects its nucleus.

o When the first sperm penetrates, it causes a release of calcium ions, which prevents additional sperm from fertilizing the egg and increases the metabolic rate of the resulting diploid zygote. This is called the cortical reaction.

· Fraternal ( dizygotic ) twins result from the fertilization of two eggs by two different sperm. Identical ( monozygotic ) twins result from the splitting of a zygote in two. Monozygotic twins can be classified by the placental structures they share (mono- vs. diamniotic, mono- vs. dichorionic).

· Cleavage is defined as the early divisions of cells in the embryo. These mitotic divisions result in a larger number of smaller cells, as the overall volume does not change.

o The zygote becomes an embryo after the first cleavage because it is no longer unicellular.

o Indeterminate cleavage results in cells that are capable of becoming any cell in the organism, while determinate cleavage results in cells that are committed to differentiating into a specific cell type.

· The morula is a solid mass of cells seen in early development.

· The blastula (blastocyst) has a fluid-filled center called a blastocoel and has two different cell types, including trophoblasts (which become placental structures) and the inner cell mass (which becomes the developing organism).

o The blastula implants in the endometrial lining and forms the placenta.

o The chorion contains chorionic villi, which penetrate the endometrium and create the interface between maternal and fetal blood.

o Before the placenta is established, the embryo is supported by the yolk sac.

o The allantois is involved in early fluid exchange between the embryo and the yolk sac.

o The amnion lies just inside the chorion and produces amniotic fluid.

o The developing organism is connected to the placenta via the umbilical cord.

· During gastrulation, the archenteron is formed with a blastopore at the end. As the archenteron grows through the blastocoel, it contacts the opposite side, establishing three primary germ layers.

o The ectoderm becomes epidermis, hair, nails, and the epithelia of the nose, mouth, and anal canal, as well as the nervous system (including adrenal medulla) and lens of the eye.

o The mesoderm becomes much of the the musculoskeletal, circulatory, and excretory systems. Mesoderm also gives rise to the gonads and the muscular and connective tissue layers of the digestive and respiratory systems, as well as the adrenal cortex.

o The endoderm becomes much of the epithelial linings of the respiratory and digestive tracts, and parts of the pancreas, thyroid, bladder, and distal urinary tracts.

· Neurulation, or development of the nervous system, begins after the formation of the three germ layers.

o The notochord induces a group of overlying ectodermal cells to form neural folds surrounding a neural groove.

o The neural folds fuse to form the neural tube, which becomes the central nervous system.

o The tip of each neural fold contains neural crest cells, which become the peripheral nervous system (sensory ganglia, autonomic ganglia, adrenal medulla, and Schwann cells) as well as specific cell types in other tissues (calcitonin-producing cells of the thyroid, melanocytes in the skin, and others).

· Teratogens are substances that interfere with development, causing defects or even death of the developing embryo. Teratogens include alcohol, certain prescription drugs, viruses, bacteria, and environmental chemicals.

· Maternal health can affect development, including diabetes (increased fetal size and hypoglycemia after birth) and folic acid deficiency (neural tube defects).

Mechanisms of Development

· Cell specialization occurs as a result of determination and differentiation.

o Determination is the commitment to a specific cell lineage, which may be accomplished by uneven segregation of cellular material during mitosis or by morphogens, which promote development down a specific cell line. To respond to a specific morphogen, a cell must havecompetency.

o Differentiation refers to the changes a cell undergoes due to selective transcription to take on characteristics appropriate to its cell line.

· Stem cells are cells that are capable of developing into various cell types. They can be classified by potency.

o Totipotent cells are able to differentiate into all cell types, including the three germ layers and placental structures.

o Pluripotent cells are able to differentiate into all three of the germ layers and their derivatives.

o Multipotent cells are able to differentiate only into a specific subset of cell types.

· Cells communicate through a number of different signaling methods. An inducer releases factors to promote the differentiation of a competent responder.

o Autocrine signals act on the same cell that released the signal.

o Paracrine signals act on cells in the local area.

o Juxtacrine signals act through direct stimulation of the adjacent cells.

o Endocrine signals act on distant tissues after traveling through the bloodstream.

o These are often growth factors, which are peptides that promote differentiation and mitosis in certain tissues.

o If two tissues both induce further differentiation in each other, this is termed reciprocal induction.

o Signaling often occurs via gradients.

· Cells may need to migrate to arrive at their anatomically correct location.

· Apoptosis is programmed cell death via the formation of apoptotic blebs that can subsequently be absorbed and digested by other cells. Apoptosis can be used for sculpting certain anatomical structures, such as removing the webbing between digits.

· Regenerative capacity is the ability of an organism to regrow certain parts of the body. The liver has high regenerative capacity, while the heart has low regenerative capacity.

· Senescence is the result of multiple molecular and metabolic processes; most notably, the shortening of telomeres during cell division.

Fetal Circulation

· Nutrient, gas, and waste exchange occurs at the placenta.

· Oxygen and carbon dioxide are passively exchanged due to concentration gradients.

· Fetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin (primarily HbA), which also assists in the transfer (and retention) of oxygen into the fetal circulatory system.

· The placental barrier also serves as immune protection against many pathogens, and antibodies are transferred from mother to child.

· The placenta serves endocrine functions, secreting estrogen, progesterone, and human chorionic gonadotropin (hCG).

· The umbilical arteries carry deoxygenated blood from the fetus to the placenta; the umbilical vein carries oxygenated blood from the placenta back to the fetus.

· The fetal circulatory system differs from its adult version by having three shunts:

o The foramen ovale connects the right atrium to the left atrium, bypassing the lungs.

o The ductus arteriosus connects the pulmonary artery to the aorta, bypassing the lungs.

o The ductus venosus connects the umbilical vein to the inferior vena cava, bypassing the liver.

Gestation and Birth

· In the first trimester, organogenesis occurs (development of heart, eyes, gonads, limbs, liver, brain).

· In the second trimester, tremendous growth occurs, movement begins, the face becomes distinctly human, and the digits elongate.

· In the third trimester, rapid growth and brain development continue, and there is transfer of antibodies to the fetus.

· During birth, the cervix thins out and the amniotic sac ruptures. Then, uterine contractions, coordinated by prostaglandins and oxytocin, result in birth of the fetus. Finally, the placenta and umbilical cord are expelled.

Answers to Concept Checks

· 3.1

1. Determinate cleavage refers to cell division that results in cells having definitive lineages; that is, at least one daughter cell is already programmed to differentiate into a particular cell type. Indeterminate cleavage refers to cell division that results in cells that can differentiate into any cell type (or a whole organism).

2. Zygote → 2-, 4-, 8-, and 16-cell embryo → morula → blastula (blastocyst) → gastrula

3. Implantation occurs during the blastula (blastocyst) stage.

4. 

5. Induction is the process by which nearby cells influence the differentiation of adjacent cells. This ensures proper spatial location and orientation of cells that share function or have complementary functions.

6. Neural crest cells become the peripheral nervous system (including the sensory ganglia, autonomic ganglia, adrenal medulla, and Schwann cells) as well as specific cell types in other tissues (such as calcitonin-producing cells of the thyroid, melanocytes in the skin, and others).

· 3.2

1. Determination is the commitment of a cell to a particular lineage. Differentiation refers to the actual changes that occur in order for the cell to assume the structure and function of the determined cell type.

2. 

3. Autocrine (the signal acts on the same cell that secreted it), paracrine (the signal acts on local cells), juxtacrine (a cell triggers adjacent cells through direct receptor stimulation), endocrine (the signal travels via the bloodstream to act at distant sites)

4. Apoptosis is programmed cell death and results in contained blebs of the dead cell that can be picked up and digested by other cells. Necrosis is cell death due to injury and results in spilling of cytoplasmic contents.

· 3.3

1. The umbilical arteries carry deoxygenated blood. The umbilical vein carries oxygenated blood.

2. 

· 3.4

1. In the first trimester, organogenesis occurs (development of heart, eyes, gonads, limbs, liver, brain). In the second trimester tremendous growth occurs, movement begins, the face becomes distinctly human, and the digits elongate. In the third trimester, rapid growth and brain development continue, and there is transfer of antibodies to the fetus.

2. In the first phase of birth, the cervix thins out and the amniotic sac ruptures. In the second phase, uterine contractions, coordinated by prostaglandins and oxytocin, result in birth of the fetus. In the third phase, the placenta and umbilical cord are expelled.

Shared Concepts

· Behavioral Sciences Chapter 1

o Biology and Behavior

· Biochemistry Chapter 3

o Nonenzymatic Protein Function and Protein Analysis

· Biochemistry Chapter 6

o DNA and Biotechnology

· Biology Chapter 1

o The Cell

· Biology Chapter 2

o Reproduction

· Biology Chapter 5

o The Endocrine System