Ch. 3) Embryogenesis and Development: Concept Summary (BIO)


Fertilization is the joining of sperm and an ovum.

Where does fertilization usually occur?

Fertilization usually occurs in the ampulla of the fallopian tube.

What happens to the sperm at the site of fertilization?

The sperm uses acrosomal enzymes to penetrate the corona radiata and zona pellucida. Once it contacts the oocyte's plasma membrane, the sperm establishes the acrosomal apparatus and injects its pronucleus.

What is the cortical reaction?

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.

What do fraternal (dizygotic) twins result from?

Fraternal (dizygotic) twins result from the fertilization of two eggs by two different sperm.

What do identical (monozygotic) twins result from?

Identical (monozygotic) twins result from the splitting of a zygote in two.

Monozygotic twins

Monozygotic twins can be classified by the placental structures they share (mono- vs. diamniotic, mon- vs. dichronic).


Cleavage refers to 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.

What happens to the zygote after the first cleavage?

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

Indeterminate cleavage
Determinate cleavage

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 structures: the trophoblast (which becomes placental structures) and the inner cell mass (which becomes the developing organism).

What forms the placenta?

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


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

Before the placenta is established,what is the embryo is supported by?

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

What is the allantois involved in?

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

Where does the amnion lie?

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

The developing organism is connected to the placenta via what?

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

What occurs during gastrulation?

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.


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


The mesoderm becomes much of 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.


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.


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

Neural tube

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

Neural crest cells

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 th

What are teratogens?

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.

What kind of maternal conditions can affect development?

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


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

What must a cell have to respond to a specific morphogen?

To respond to a specific morphogen, a cell must have competency.


Differentation refers to the changes a cell undergoes due to selective transcription to take on charcteristics appropriate to its cell line.

What are stem cells?

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

Totipotent cells

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

Pluripotent cells

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

Multipotent cells

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

How do cells communicate?

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

Autocrine signals

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

Paracrine signals

Paracrine signals act on local cells.

Juxtacrine signals

Juxtacrine signals act through direct stimulation of adjacent cells.

Endocrine signals

Endocrine signals act on distant tissues after traveling through the blood stream.

What are growth factors?

Growth factors are peptides that promote differentiation and mitosis in certain tissues.

What is it called when two tissues both induce further differentiation in each other?

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

Signaling often occurs via __________.

Signaling often occurs via gradients.


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

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.

What occurs at the placenta?

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

Oxygen and carbon dioxide are passively exchanged due what?

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

Fetal hemoglobin (HbF)

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

The placental barrier also serves as what?

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

How does the placenta serve endocrine functions?

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

What do the umbilical arteries and veins do?

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

How does the fetal circulatory system differ from its adult version?

The fetal circulatory system differs from its adult version by having three shunts:
1) foramen ovale
2) ductus arteriosus
3) ductus venosus

What occurs during the first trimester?

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

What occurs during the second trimester?

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

What occurs during the third trimester?

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

What occurs during birth?

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.