Ch. 3: Embryogenesis and Development Flashcards
Fertilization
Joining of a sperm and an ovum. After fertilization, zygote must travel to uterus for implantation. If it arrives too late, there will no longer be an endometrium capable of supporting the embryo.
Ampulla
Location in the fallopian tube where fertilization occurs, widest part of fallopian tube
Zygote
Diploid formed after sperm fertilizes egg
How does the Sperm penetrate into the egg?
The sperm uses acrosomal enzymes to penetrate the corona radiata and zona pellucida
Acrosomal apparatus
Tubelike structure formed when first sperm comes into direct contact with the secondary oocyte’s cell membrane. This structure extends to and penetrates the cell membrane. Its pronucleus may then freely enter the oocyte once meiosis II has come to completion
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
Fraternal (dizygotic) twins
Result from the fertilization of two eggs by two diff sperm. Both zygotes implant in the uterine wall and each develop own placenta, chorion, and amnion. Possible for placenta to overgrow onto each other
Identical (monozygotic) twins
Result from the splitting of a zygote in two. Conjoined twins occur when division is incomplete: the following depend on when separation occurred
- Monochorionic /monoamniotic: share same amnion and chorion
- Monochorionic/diamniotic: have own amnion but same chorion
- Dichorionic/diamniotic: have own amnion and chorion
Cleavage
Defined as the early divisions of cells in the embryo that happen as the zygote travels to uterus. First cleavage officially creates an embryo These mitotic divisions result in a larger number of smaller cells, as the overall volume does not change. Cells increase 2 ratios: nuclear to cytoplasmic and surface area to volume- thus increased area for gas and nutrient exchange relative to overall volume
When does a zygote become an embryo?
After the first cleavage bc it is no longer unicellular
Indeterminate cleavage
Results in cells that are capable of becoming any cell in the organism
Determinate cleavage
Results in cells that are committed to differentiating into a specific cell type
Morula
Several divisions later, solid mass of cells seen in early development
Blastula
Once morula is formed, it undergoes blastulation, which forms blastula, mammalian blastula is known as blastocyst; hollow ball of cells which as a has a fluid-filled center called a blastocoel which has 2 diff cell types: 1. Trophoblasts 2. Inner cell mass
Trophoblasts
Become placental structures– surround blastocoel and give rise to the chorion and later the placenta
Inner cell mass
Becomes the developing organism, protudes into the blastocoel and gives rise to the organism itself
How does the placenta form?
The blastula move through the fallopian tube to the uterus where it burros into the endometrial lining . trophoblast cells ae specialized to create an interface between the maternal blood supply and the developing embryo
Chorion
Arise from trophoblastic cells, Contains chorionic villi, which penetrate the endometrium and create the interface between maternal and fetal blood. Develops into the placenta. Also forms an outer membrane around the amnion adding an additional level of protection
Yolk Sac
Before the placenta is established, the embryo is supported by the yolk sac. Also the site of early blood cell development
Allantois
Involved in early fluid exchange between the embryo and yolk sac
Amnion
Surrounds allantois, thin tough membrane filled w amniotic fluid, Lies just inside the chorion and produces amniotic fluid which serves as a shock absorber during pregnancy
Umbilical cord
ultimately formed from remnants of yolk sac and allantois, Connects the developing organism to the placenta; consists of 2 arteries and one vein encased in a gelatinous substance
Gastrulation
Archenteron is formed w a blastopore at the end. As the archenteron grows through the blastocoel it contacts the opposite side, establishing 3 primary germ layers: ectoderm, endoderm, mesoderm (first invagination– imagnine pushing fist through a balloon)
Ectoderm
Becomes epidermis, hair, nails, and the epithelia of the nose, mouth and anal canal, as well as the nervous sys (including adrenal medulla) and lens of the eye
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
Endoderm
Becomes much of the epithelial linings of the respiratory and digestive tracts and parts of the pancreas, thyroid, bladder, and distal urinary tracts
Differentiation
Cells with the same genes develop into distinctly diff cell types w highly specialized functions
Selective transcription
Process by which differentiation occurs, only the genes needed for a particular cell type are transcribed. Related to the concept of induction
Induction
Ability of one group of cells to influence the fate of other nearby cells. Process is mediated by chemical substances called inducers which diffuse from the organizing cells to the responsive cells.
Neurulation:
Development of the nervous sys, begins after formation of the 3 germ layers (second invagination)
Notochord
Forms along the long axis of the organism like a primitive spine, Induces a group of overlying ectodermal cells to form neural folds surrounding a neural groove
Neural Tube
Neural folds fuse to form the neural tube which becomes the central nervous system
Neural crest cells
The tip of each neural fold contain 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
Substances that interfere w development, causing defects or even death of the developing embryo. They include alcohol, certain prescription drugs, viruses, bacteria, and environmental chemicals
Maternal Health and Development
Maternal health can affect development, including diabetes (increased fetal size and hypoglycemia after birth) and folic acid deficiency (neural tube defects)
Determination and differentiation
Cell specialization occurs as a result of these processes
Specialization
Initial stage of cell specialization where cell is reversibly designated to a specific cell type
Determination
Follows specialization, Commitment to a specific cell lineage, which may be accomplished by uneven segregation of cellular material during mitosis or by morphogens
Morphogens
Specific molecules secreted from nearby cells which promote development down a specific cell line
Competency
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 (structure, function, and biochemistry) appropriate to its cell line
Stem cells:
Cells that are capable of developing into various cell types, can be classified by potency
Totipotent Cells
Able to differentiate into all cell types including the 3 germ layers and placental structures
Pluripotent Cells
Able to differentiate into all 3 of the germ layers and their derivatives
Multipotent Cells
Able to differentiate only into a specific subset of cell types
Cell Communication:
Cells communicate through a number of different signaling methods
Inducer and Responder
An inducer releases factors to promote the differentiation of a competent responder
Autocrine Signaling
Act on the same cell that released the signal
Paracrine Signaling
Act on cells in the local area
Juxtacrine Signaling
Act through direct stimulation of the adjacent cells
Endocrine Signaling
Act on distant tissues after traveling through the bloodstream
Growth factors
Autocrine Signals, Paracrine Signals, Juxtacrine Signals, Endocrine Signals are often growth factors which are peptides that promote differentiation and mitosis in certain tissues
Reciprocal induction
Two tissues both induce further differentiation in each other
Signaling often occurs
Via gradients
Cells may need to
Migrate to arrive @ their anatomically correct location
Apoptosis
Programmed cell death via the formation of apoptotic blebs. Can be used for sculpting certain anatomical structures, such as removing the webbing between digits
Apoptotic Blebs
Can be absorbed and digested by other cells
Regenerative capacity
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
Result of multiple molecular and metabolic processes; most notably, the shortening of telomeres during cell division
What processes occur at the placenta?
- Nutrient, gas, and waste exchange occurs @ the placenta
- Oxygen and CO2 are passively exchanged due to concentration gradients
- serves endocrine functions: secreting estrogen, progesterone, and human chorionic gonadotropin (hCG)
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 sys
Placental Barrier
Among other functions, servers as immune protection against many pathogens, and antibodies are transferred from mother to child
Umbilical arteries
Carry deoxygenated blood from the fetus to the placenta
Umbilical vein
Carries oxygenated blood from the placenta back to the fetus
How does the fetal circulatory system differ from its adult version?
Fetal circulatory sys has 3 shunts: foramen ovale, ductus arteriosus, ductus venosus
Foramen Ovale
Connects the right atrium to the left atrium bypassing the lungs
Ductus arteriosus
Connects the pulmonary artery to the aorta, bypassing the lungs
Ductus venosus
Connects the umbilical vein to the inferior vena cava bypassing the liver
First trimester
Organogenesis occurs– development of heart, eyes, gonads, limbs, liver, brain
Second trimester
Tremendous growth occurs, movement begins, face becomes distinctly human, and the digits elongate
Third trimester
Rapid growth and brain development continue and there is transfer of antibodies to the fetus
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.
