Biology
CHAPTER 3: EMBRYOGENESIS AND DEVELOPMENT
Early Developmental Stages
➢ Fertilization is the joining of a sperm and an ovum.
○ It usually occurs in the ampulla of the fallopian tube.
○ 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. 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 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.
○ The zygote becomes an embryo aer the first cleavage because it is no longer unicellular.
○ 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).
○ 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, the embryo is supported by the yolk sac.
○ The allantois is involved in early fluid exchange between the embryo and the yolk sac.
○ The amnion lies just inside the chorion and produces amniotic fluid. 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.
○ 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.
○ 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. 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 aer the formation of the three germ
layers.
○ The notochord induces a group of overlying ectodermal cells to form neural folds
surrounding a neural groove.
○ The neural folds fuse to form the neural tube, which becomes the central nervous system.
○ 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 conditions can affect development, including diabetes (increased fetal size and
hypoglycemia aer birth) and folic acid deficiency (neural tube defects).
Mechanisms of Development
➢ Cell specialization occurs as a result of determination and differentiation.
○ 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. To respond to a specific morphogen, a cell
must have competency.
○ 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.
○ Totipotent cells are able to differentiate into all cell types, including the three germ layers
and placental structures.
○ Pluripotent cells are able to differentiate into all three of the germ layers and their
derivatives.
○ 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.
○ Autocrine signals act on the same cell that released the signal.
○ Paracrine signals act on local cells.
○ Juxtacrine signals act through direct stimulation of adjacent cells.
○ Endocrine signals act on distant tissues after traveling through the bloodstream.
○ These are oFTen growth factors, which are peptides that promote differentiation and
mitosis in certain tissues.
○ If two tissues both induce further differentiation in each other, this is reciprocal induction.
○ Signaling oen occurs via gradients.
➢ Cells may need to migrate to arrive at their 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);
this affinity 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).
CHAPTER 3: EMBRYOGENESIS AND DEVELOPMENT
Early Developmental Stages
➢ Fertilization is the joining of a sperm and an ovum.
○ It usually occurs in the ampulla of the fallopian tube.
○ 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. 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 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.
○ The zygote becomes an embryo aer the first cleavage because it is no longer unicellular.
○ 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).
○ 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, the embryo is supported by the yolk sac.
○ The allantois is involved in early fluid exchange between the embryo and the yolk sac.
○ The amnion lies just inside the chorion and produces amniotic fluid. 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.
○ 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.
○ 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. 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 aer the formation of the three germ
layers.
○ The notochord induces a group of overlying ectodermal cells to form neural folds
surrounding a neural groove.
○ The neural folds fuse to form the neural tube, which becomes the central nervous system.
○ 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 conditions can affect development, including diabetes (increased fetal size and
hypoglycemia aer birth) and folic acid deficiency (neural tube defects).
Mechanisms of Development
➢ Cell specialization occurs as a result of determination and differentiation.
○ 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. To respond to a specific morphogen, a cell
must have competency.
○ 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.
○ Totipotent cells are able to differentiate into all cell types, including the three germ layers
and placental structures.
○ Pluripotent cells are able to differentiate into all three of the germ layers and their
derivatives.
○ 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.
○ Autocrine signals act on the same cell that released the signal.
○ Paracrine signals act on local cells.
○ Juxtacrine signals act through direct stimulation of adjacent cells.
○ Endocrine signals act on distant tissues after traveling through the bloodstream.
○ These are oFTen growth factors, which are peptides that promote differentiation and
mitosis in certain tissues.
○ If two tissues both induce further differentiation in each other, this is reciprocal induction.
○ Signaling oen occurs via gradients.
➢ Cells may need to migrate to arrive at their 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);
this affinity 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).
