Chapter 3: Embryogenesis and Development Flashcards
Approximately on what day of the menstrual cycle is a secondary oocyte ovulated from the follicle?
Day 14
A secondary oocyte can be fertilized in the fallopian tube for up to how many hours after ovulation?
24 hours
Fertilization usually occurs in the widest part of the fallopian tube, called the _________.
Ampulla
The first sperm to come into direct contact with the secondary oocyte’s cell membrane forms a tube-like structure known as what?
Acrosomal apparatus
Once the acrosomal apparatus penetrates the sperm cell membrane, what occurs?
The pronucleus may then freely enter the oocyte once meiosis II has come to completion
After sperm penetrates the cell membrane, what reaction occurs?
The cortical reaction = release of calcium ions
What 2 things does the cortical reaction do?
depolarizes the membrane of the ovum, which
1. prevents fertilization of the ovum by multiple sperm cells
2. increases the metabolic late of the newly formed diploid zygote
After the membrane is depolarized and rendered impenetrable, this is called what?
fertilization membrane
What are dizygotic twins?
2 different eggs are released by 2 different sperm
fraternal
What occurs in a dizygotic pregnancy?
Each zygote implants and each develops its own placenta, chorion, and amnion. If they implant close together, the placentas may grow onto each other.
What are monozygotic twins?
A single zygote splits into two
if incomplete, results in conjoined twins
What is used to classify monozygotic twins? What are the 3 types?
The number of structures they share
- Monochorionic/monoamniotic (share same chorion and amnion)
- Monochoroionic/diamniotic (share same chorion, but each have their own amnion)
- Dichorionic/diamniotic (each have their own chorion and amnion)
What determines the type of monozygotic twinning that occurs?
When the separation occurred
What occurs after fertilization in the fallopian tubes? What happens if this occurs too late?
The zygote travels to the uterus for implantation. If it occurs too late, there will no longer be an endometrium capable of supporting the embryo.
What is happening to the zyogte as it moves to the uterus for implantation?
It undergoes rapid mitotic cell divisions in a process called cleavage
What happens to the size of the embryo during the first few divisions? Why?
Stays unchanged; helps increase nuclear-to-cytoplasmic ratio and surface area-to-volume ratio
What are the 2 types of cleavage?
Indeterminate and determinate
What is indeterminate cleavage?
Results in cells that can still develop into complete organisms
e.g. monozygotic twins originate from indeterminately cleaved cells
What is determinate cleavage?
The cell fates are determined and the cells are committed to differentiating into a certain type of cell
After several divisions, the embryo becomes a solid mass of cells known as a what?
Morula
e.g. comes from latin for mulberry, which is what it kind of looks like
Once the morula forms, it undergoes what process?
Blastulation, which forms the blastula
What is a blastula?
A hollow ball of cells with a fluid-filled inner cavity
The fluid-filled inner cavity of a blastula is called a what?
blastocoel
The mammalian blastula is known as what? What are the 2 cell groups it is comprised of?
Blastocyst; trophoblast and inner cell mass (ICM)
What are the trophoblast cells?
They surround the blastocoel and give rise to the chorion and later the placenta
What is the inner cell mass?
Protrudes into the blastocoel and gives rise to the organism itself
The trophoblastic cells give rise to the ________, an extraembryonic membrane that develops into what?
chorion; placenta
What do the trophoblasts form and what are they used for?
chorionic villi, which are microscopic finger-like projections that penetrate the endometrium
These develop into the placenta and support maternal-fetal gas exchange
What connects the embryo to the placenta and what does it consist of?
Umbilical cord; 2 arteries and 1 vein encased in a gelatinous substance
What does the umbilical vein in the placenta carry? The umbilical arteries?
Vein = freshly oxygenated blood with nutrients (from placenta to embryo)
Arteries = deoxygenate blood and waste (from embryo to placenta)
Until the placenta is functional, what supports the embryo? What occurs here?
The yolk sac; the site of early blood cell development
What are 2 other extraembryonic membranes that support the embryo?
The allantois and amnion
What is the allantois?
Involved in early fluid exchange between the embryo and yolk sac; the allantois and the yolk sac remnants ultimately form the umbilical cord
What is the amnion?
A thin, tough membrane filled with amniotic fluid that surrounds the allantois. It also surrounds the embryo, serving as a shock absorber.
In addition to forming the placenta, the chorion also forms what around the amnion?
An outer membrane
After the cell mass implants, what begins?
Gastrulation = generation of 3 distinct cell layers
What marks the beginning of gastrulation?
A small invagination in the blastula
What occurs during gastrulation?
Cells move toward the invagination, resulting in elimination of the blastocoel (the inner cavity of the blastula), creating a tube
What is the result of gastrulation?
A gastrula
The membrane invagination into the blastocoel is called what? What does this eventually develop into?
Archenteron; the gut
What is the opening of the archenteron called? In humans, what does this eventually form?
Blastopore; the anus
Eventually, cells migrate into what remains of the blastocoel, which creates 3 layers called primary germ layers. What are these layers and how are they positioned?
Outer to inner:
1. Ectoderm
2. Mesoderm
3. Endoderm
What does each layer develop into?
1. Ectoderm
2. Mesoderm
3. Endoderm
- Ectoderm: Integument (epidermis, hair, nails; epithelia of nose, mouth, and lower anal canal), eye lens, nervous system, and inner ear
- Mesoderm: Musculoskeletal, circulatory, and most of excretory systems; gonads, muscular and connective tissue layers of digestive/respiratory systems and adrenal cortex
- Endoderm: Epithelial linings of digestive and respiratory tracts, including lungs. Pancreas, thyroid, bladder, and distal urinary tracts, as well as parts of liver.
How do difference cell types develop with highly specialized function?
Selective transcription (only necessary genes are transcribed)
What is induction?
Ability of one group of cells to influence fate of nearby cells
What mediates induction?
Inducers that are diffused from organizing cells to responsive cells
After the 3 germ layers are formed, what occurs?
Neurulation, or the development of the nervous system
In the first step of neurulation, a rod of mesodermal cells called the ________________ forms along the long axis of the organism like a primitive spine.
notochord
What does the notochord do to the overlying ectodermal cells?
Induces them to slide inward to form neural folds, which surround a neural groove
As the neural folds grow toward one another, they eventually fuse into what? What does this become?
Neural tube; central nervous system
What are located at the tip of each neural fold? What do these become once they migrate outward?
Neural crest cells; peripheral nervous system, among other cells (sensory/autonomic ganglia, adrenal medulla, Schwann cells, calcitonin-producing cells in the thryoid, melanocytes)
After the neural tube and crest are formed, what migrates over to cover them?
Ectodermal cells
What are the 3 stages of cell specialization?
- Specification
- Determination
- Differentiation
What occurs during cell specification?
The cell is irreversibly designated as a specific cell type
What occurs during cell determination?
The commitment of a cell to a particular function in the future
Can a cell still become any cell type even after undergoing specification? What about determination?
Yes to specification, no to determination. After determination, it is irreversibly committed to a specific lineage.
What are some ways in which cell determination can occur?
- Asymmetric distribution of mRNA and protein during cleavage
- Secretion of specific molecules from nearby cells (morphogens)
In order to produce what it needs to carry out the functions of its determined cell type, a cell must undergo what?
Differentiation
Cells with the greatest potency are described as what?
Totipotent; this includes embryonic stem cells (can differentiate into the placenta or embryo proper)
After the 16-cell stage, the cells of the morula begin to differentiate into what 2 groups?
The inner cell mass and trophoblast
After a few more cycles of cell division from the ICM and trophoblast stage, these totipotent cells start to differentiate into the 3 germ layers and are considered what?
How does this differ from totipotent?
Pluripotent; cannot develop into placental cells
As these pluripotent cells become more specialized, they are described as what? What does this mean?
Multipotent; can differentiate into multiple types of cells within a particular group
In order to be induced to differentiate, a responder cell must be what?
Competent
Cell-cell communication can occur via what 4 types of signals?
APEJ
- Autocrine
- Paracrine
- Endocrine
- Juxtacrine
____________ signals act on the same cell that secreted the signal in the first place.
Autocrine
____________ signals act on cells in the local area.
Paracrine
para-; beside, adjacent to
____________ signals involve secreted hormones that travel through the bloodstream to a distant target tissue.
Endocrine
____________ signals do not usually involve diffusion, but involve a cell directly stimulating receptors of an adjacent cell.
Juxtacrine
Next to, juxta
Inducers are often peptides that promote differentiation and mitosis in certain tissues. What are these referred to as?
Growth factors
What are the molecules that diffuse and cause cell determination called?
Morphogens
What are some common morphogens?
TGF-beta, sonic hedgehog (shh), epidermal growth factor (EGF)
Although induction/differentiation lead to the creation of different types of cells, these cells may not be in the correct location to carry out their function. They must undergo what process?
Cell migration
During apoptosis, the cell changes morphology and divides into self-contained protrustions called what?
Apoptotic blebs
Apoptotic blebs are broken apart into what? What happens to these?
Apoptotic bodies, which are digested by other cells
What is the benefit of other cells digesting apoptotic bodies?
Materials can be recycled
What is the benefit of apoptotic blebs forming?
prevents release of potentially harmful substances into extracellular environment
What is necrosis? Why can it be harmful?
Cell death by injury instead of pre-programmed death.
Necrosis can cause internal cellular substances to leak, causing tissue irritation / immune response.
What occurs during complete regeneration?
Extensive, stored clusters of stem cells migrate to the part of the body requiring regrowth, and the lost/damaged tissues are replaced with identical issues.
What occurs during incomplete regeneration?
In contrast to complete regeneration, the newly formed tissues are not identical in structure or function to the lost/injured tissue.
What occurs in cellular senescence and what causes it?
cells fail to divide; may be due to shortened telomeres, which are the ends of chromosomes.
Why do shortened telomeres cause biological aging?
Normal telomeres serve to prevent the loss of genetic info from chromosomal ends by preventing DNA from unraveling. But they are difficult to replicate, and shorten during each round of DNA synthesis. So eventually, they get too short, and the cell can no longer successfully replicate.
Certain cells, like germ, fetal, and tumor cells, express an enzyme called ________________. This enzyme is a reverse transcriptase that can synthesize the ends of chromosomes, preventing senescence.
telomerase
How does the body prevent maternal-fetal blood mixing while allowing nutrient, gas, and waste exchange to occur?
Nutrients and waste products are moved via diffusion.
Since diffiusion requires a gradient, there is a (lower/higher?) partial pressure of oxygen in maternal blood than in fetal blood.
Higher
To enhance the transfer of oxygen from maternal to fetal circulation, fetal blood cells contain what? What does this do?
fetal hemoglobin (HbF), has a greater affinity for oxygen than adult hemoglobin, which assists with the transfer and retention of oxygen into the fetal circulatory system
What other benefit does the placental barrier serve?
Immunity
But antibodies can cross through and start building immunity
What do the umbilical arteries do?
Carry blood away from the fetus toward the placenta
What does the umbilical vein do?
Carries blood toward the fetus from the placenta
Where does oxygenation occur? Thus, what type of blood do the umbilical arteries and umbilical vein carry? Oxygenated or deoxygenated?
Placenta, not the fetal lungs.
Umbilical arteries carry deoxygenated blood and the umbilical vein carries oxygenated blood.
What controls detoxification and emtabolism in the fetus?
Mother’s liver
Which of its own two body structures is the fetus not dependent on?
Lungs and liver
Which pathogens can cross the placental barrier?
TORCHES
Toxoplasma gondii
Rubella
Cytomegalovirus
HErpes or HIV
Syphilis
Since the lungs are underdeveloped and sensitive to the high blood pressures they will receive in postnatal life, what does the fetus construct in order to actively direct blood away from these organs as they develop?
how many of these?
3 shunts
What two shunts are used to reroute blood from the lungs in the fetus?
- Foramen ovale
- Ductus arteriosus
The foramen ovale is a one-way valve; what does it connect?
It connects the right atrium to the left atrium
What does the foramen ovale do?
Since it connects the left and right atriums, it allows blood entering the right atrium from the inferior vena cava to flow into the left atrium instead of the right ventricle.
This allows it to be pumped through the aorta into systemic circulation directly.
In the developing fetus, the right side of the heart is at a (lower/higher?) pressure than in the left side, which pushes blood through the opening.
Higher
What does the ductus arteriosus do?
Shunts leftover blood from the pulmonary artery to the aorta
The fetal liver is bypassed via the what? How?
Ductus venosus; it shunts blood returning from the placenta via the umbilical vein directly into the inferior vena cava
What occurs during the first trimester of pregnancy?
What is the fetus known as at the 8th week of gestation?
Major organs develop during first few weeks.
Heart beats at ~22 days, then eyes, gonads, limbs, and liver form.
At 5 weeks, the embryo is 10 mm in length.
At 6 weeks, it is 15 mm.
At 7 weeks, the cartilaginous skeleton starts to harden into bone.
At 8 weeks, most organs have formed, the brain is fairly developed. Known as a fetus now.
At the end of the first trimester, the fetus is 9 cm long.
What occurs during second trimester?
Grows a LOT. Begins to move in amniotic fluid. Looks more human. Lengthening fingers/toes.
At 6 months, it is 30-36 cm long.
What occurs during third trimester?
7th/8th months - rapid growth and brain development.
Antibodies are transported to the fetus, occurs most often in ninth month.
What is parturition?
Birth
What 2 things coordinate the rhythmic contractions of the uterine smooth muscle?
Prostaglandins and oxytocin (a peptide hormone)
What are the 3 basic phases of birth?
- Water breaking = cervix thins out and amniotic sac ruptures
- Contractions of uterus = leads to birth of fetus
- Afterbirth = placenta and umbilical cord are expelled
