Ch. 3: Embryogenesis and Development Flashcards
for how many hours after ovulation can a secondary oocyte be fertilized in the fallopian tube?
24 hrs
defn: ampulla
the wides part of the fallopian tube, where fertilization occurs
what happens when the sperm meets the secondary oocyte in the fallopian tube? (4)
- the sperm binds to the oocyte
- the sperm releases acrosomal enzymes that enable the head of the sperm to penetrate the corona radiata and zona pellucida
- the first sperm to come into direct contact with the oocyte’s cell membrane forms a tube-like structure known as the acrosomal apparatus (which extends to and penetrates the cell membrane)
- the sperm’s pronucleus may then freely enter the oocyte once meiosis II has come to completion
defn: cortical reaction
why does this occur?
what are the effects? (3)
a release of calcium ions
occurs after penetration of the sperm through the cell membrane
- calcium ions depolarize the membrane of the ovum which serves two purposes
- depolarization prevents fertilization of the ovum by multiple sperm cells
- the increased calcium concentration increases the metabolic rate of the newly formed diploid zygote
defn: fertilization membrane
the now depolarized and impenetrable membrane of the zygote
defn + process (3) + aka: dizygotic twins
FRATERNAL
forms from fertilization of two different eggs released during one ovulatory cycle by two different sperm
- each zygote will implant in the uterine wall
- each develops its own placenta, chorion, and amnion
- if the zygotes implant close together, the placentas may grown onto each other
are fraternal twins more genetically similar than other pairs of siblings?
no!
defn + aka: monozygotic twins
IDENTICAL
form when a single zygote splits into two (because the genetic material is identical, the genomes of the offspring will be also)
why do conjoined twins occur?
if division of a zygote (that would lead to identical twins if complete), is incomplete
how are monozygotic twins classified?
by the number of structures they share
what are the three types of monozygotic twins? define them.
MONOCHORIONIC/MONOAMNIOTIC: share the same amnion and chorion
MONOCHORIONIC/DIAMNIOTIC: each have their own amnion, share the same chorion
DICHORIONIC/DIAMNIOTIC: each have their own amnions and chorions
what happens as the result of more gestational structures being shared between twins?
there are more risks as the fetuses grow and develop
defn: cleavage
rapid mitotic cell divisions that the zygote undergoes as it moves to the uterus for implantation
what does the first cleavage officially create? why?
an embryo
because it nullifies one of the zygote’s defining characteristics: unicellularity
The total size of the embryo remains unchanged during the first few divisions. By dividing into progressively smaller cells, what two ratios are increased? what is the impact of this?
the nuclear to cytoplasmic (N:C) ratio
the surface area-to-volume ratio
impact: the cells achieve increased area for gas and nutrient exchange relative to overal volume
defn: indeterminate vs. determinate cleavage
INDETERMINATE = results in cells that can still develop into complete organisms
DETERMINATE = results in cells with fates that are already determined (these cells are committed to differentiating into a certain type of cell)
why do monozygotic twins have identical genomes?
they both originate from indeterminately cleaved cells of the same embryo
what does the embryo turn into several divisions later?
a solid mass of cells known as a morula
defn: blastulation/bastula
the morula undergoes blastulation once it is formed to form the blastula
blastula: a hollow ball of cells with a fluid-filled inner cavity (blastocoel)
defn + noteworthy cell groups: blastocyst
blastocyst = mammalian blastula
noteworthy cell groups: trophoblast and inner cell mass
func: trophoblast cells (3)
surround the blastocoel
give rise to the chorion and later the placenta
specialized to create an interface between the maternal blood supply and the developing embryo
func: inner cell mass (2)
protrudes into the blastocoel
gives rise to the organism itself
mnemonic: blastula
a BLASTula is an embryo with a BLASTed-out cavity
defn: implantation
the blastula moves through the fallopian tube to the uterus, where it burrows into the endometrium
defn + func: chorion (2)
an extraembryonic membrane that develops into the placenta
forms an outer membrane around the amnion (adds an additional layer of protection)
defn + func + origin: chorionic villi
defn: microscopic finger-like projections that penetrate the endometrium
func: support maternal-fetal gas exchange as they develop into the placenta
origin: form from trophoblasts
func + struct (2) + struct funcs (2): umbilical cord
connects the embryo to the placenta
consists of two arteries and one vein encased in a gelatinous substance
VEIN: carries freshly oxygenated blood rich with nutrients from the placenta into the embryo
UMBILICAL ARTERIES: carry deoxygenated blood and waste to the placenta for exchange
what is the embryo supported by until the placenta is functional?
the yolk sac (also the site of early blood cell development)
what group do the allantois and the amnion correspond to?
extraembryonic membranes
defn + func: allantois (2)
- involved in early fluid exchange between the embryo and the yolk sac
- the umbilical cord is ultimately formed from remnants of the yolk sac and the allantois
defn + func: amnion (1)
a thin, tough membrane filled with amniotic fluid
surrounds the allantois
what does amniotic fluid do?
it is the shock absorber of pregnancy
it lessens the impact of the maternal motion on the developing embryo
defn: gastrulation + when can this occur
can occur once the cell mass implants
defn: the generation of three distinct cell layers
what is the process of gastrulation? (4)
- begins with a small invagination in the blastula
- cells continue moving toward the invagination, resulting in elimination of the blastocoel
- if the two membranes could merge (as does in development) would create a tube through the middle of the balloon
- the result is a gastrula
defn + what does it develop into: archenteron
the membrane invagination in the blastocoel
later develops into the gut
defn + what does it develop into: blastopore
the opening of the archenteron
later develops into the anus in deuterostomes and the mouth in protostomes
origin: primary germ layers
some cells migrate into what remains of the blastocoel and establishes three layers of cells
what are the primary germ layers?
- ectoderm
- mesoderm
- endoderm
defn + gives rise to? (7): ectoderm
the outermost germ layer
- integument (1. epidermis, 2. hair, 3. nails, 4. epithelia of nose, mouth, and lower anal canal)
- lens of the eye
- nervous system
- inner ear
defn + gives rise to? (5): mesoderm
middle germ layer
- musculoskeletal system
- circulatory system
- most excretory systems
- gonads
- muscular and connective tissue layers of the digestive and respiratory systems and the adrenal cortex
defn + gives rise to? (6): endoderm
innermost germ layer
- epithelial linings of the digestive and respiratory tracts
- pancreas
- thyroid
- bladder
- distal urinary tracts
- parts of the liver
mnemonic: functions of the 3 primary germ layers
ECTODERM = “ATTRACTO”derm (things that attract us to others such as cometic features and smarts)
MESODERM = “MEANS”oderm (the means of getting around as an organism i.e. bones and muscles; the means of getting around in the body, such as the circulatory system; the means of getting around i.e. the gonads)
ENDODERM = linings of “ENDERNAL” (internal) organs (the digestive and respiratory tract and accessory organs attached to these systems)
defn + func: selective transcription
func: the reason for why cells with the same genes are able to develop into such distinctly different cell types with highly specialized functions
defn: only the genes needed for that particular cell type are transcribed
defn + func: induction (related to?)
related to: selective transcription
the ability of one group of cells to influence the fate of nearby cells
ensures the proximity of different cell types that work together within an organ
defn + loc + func (2): inducers
defn + func: chemical substances that mediate induction and are responsible for processes such as the guidance of neuronal axons
loc: diffuse from organizing cells to the responsive cells
when can neurulation occur and explain the process? (6) + defn
can occur once the 3 germ layers are formed
defn: development of the nervous system
- a rod of mesoderm cells (the NOTOCHORD) forms along the long axis of the organism (like a primitive spine)
- the notochord induces a group of overlying ectodermal cells to slide inward to form NEURAL FOLDS which surround a NEURAL GROOVE
- the neural folds grow toward one another until they fuse into a NEURAL TUBE which gives rise to the CNS
- at the tip of each neural fold are NEURAL CREST CELLS which migrate outward to form the peripheral nervous system and specific cell types in other tissues
- ectodermal cells will migrate over the neural tube and crest to cover the rudimentary nervous system
what is derived from neural crest cells? (6)
- peripheral nervous system (1. sensory ganglia, 2. autonomic ganglia, 3. adrenal medulla, 4. Schwann cells)
- calcitonin-producing cells of the thryoid
- melanocytes in the skin
defn: teratogens
substances that interfere with development, causing defects or even death of the developing embryo (early development is a highly sensitive time)
what factors influence the effects of the teratogen? (5)
- genetics of the embryo
- route of exposure
- length of exposure
- rate of placental transmission of the teratogen
- exact identity of the teratogen
what are common teratogens? (5)
- alcohol
- prescription drugs
- viruses
- bacteria
- environmental chemicals (inc. polycyclic aromatic hydrocarbons)
what can be the effect of overexposure to sugar in utero? (2)
- a fetus too large to be delivered
- a fetus that could become hypoglycemic soon after birth (due to synthesizing very high levels of insulin to compensate)
what effect does maternal folic acid deficiency have on the fetus?
may prevent complete closure of the neural tube (resulting in spina bifida) in which parts of the nervous system are exposed to the outside world or covered with a thin membrane, or anencephaly (the brain fails to develop)
what are the 3 main steps of cell specialization?
- specification
- determination
- differentiation
defn: specification
the cell is reversibly designated as a specific cell type
defn: determination
the cell is irreversibly committed to a specific lineage/cel type
what are the pathways by which determination may occur? (2)
- during cleavage, the existing mRNA and protein in the parent cell has been asymmetrically distributed between the daughter cells, and the presence of specific mRNA and protein molecules may result in determination
- may occur due to secretion of specific molecules (morphogens) from nearby cell that cause neighboring cells to follow a particular developmental pathway
defn: differentiation
a process that includes changing the structure, function, and biochemistry of the cell to match the cell type so that it can function as it is needed to function
defn: stem cells
cells that have not yet differentiated or that give rise to other cells that will differentiate
defn: potency
determines the tissues a particular stem cell can differentiate into
defn: totipotent (3) vs. pluripotent (2) vs. multipotent (2) (stem cells)
TOTIPOTENT
1. cells with the greatest potency
2. includes embryonic stem cells
3. can differentiate into any cell type (in the fetus or in placental structures)
PLURIPOTENT
1. includes cells after they have started differentiating into the three germ cell layers
2. these cells can differentiate into any cell type except in those found in the placental structures
MULTIPOTENT
1. as the cells continue to become more specialized
2. these can differentiate into multiple types of cells within a particular group
what are 3 concerns that come from transplanting stem cells?
- immunologic concerns (transplantation of stem cells of a different genetic makeup could evoke an immune response, resulting in rejection)
- once implanted, pluripotent cells may not necessarily differentiate into the desired tissue
- pluripotent cells may even become cancerous once implanted
what is 2 advantages and 2 disadvantages to using adult stem cells instead of embryonic stem cells for stem cell transplantation?
Disadvantage
1. these cells are multipotent at best, so they are only able to differentiate into only a few different cell types
2. challenging to induce differentiation into the correct cell type
Advantage
1. a stem cell can be taken from a patient, induced to become a different tissue type, and then implanted into the same patient
2. offers reduced risk of foreign tissue
what is the determination and differentiation of a cell dependent on? (2) + why
- location of the cell
- identity of the surrounding cells
why: the developing cell receives signals from organizing cells around it and may also secrete its own signaling molecules
defn: inducer vs. responder
INDUCER: cells in surrounding tissues of a developing cell induce the developing cell to become a particular cell type
inducer = the cell secreting the signal
RESPONDER = the cell that is induced (responsive cell)
defn: competent
able to respond to the inducing signal
in order to be induced, a responder must be competent
what are the 4 types of signals that cell-cell communication can occur?
- autocrine
- paracrine
- juxtacrine
- endocrine
defn: autocrine vs. paracrine vs. juxtacrine vs. endocrine signals
AUTOCRINE = act on the same cell that secreted the signal in the first place
PARACRINE = act on cells in the local area
JUXTACRINE = do not usually involve diffusion, but involve a cell directly stimulating receptors of an adjacent cell
ENDOCRINE = involve secreted hormones that travel through the bloodstream to a distant target tissue
defn: growth factors
inducers are often growth factors
peptides that promote differentiation and mitosis in certain tissues
defn + example: reciprocal development
induction is not always a one-way pathway
ex: differentiation of the lens then triggers the optic vesicle to form the optic cup which ultimately becomes the retina
what is one of the main methods of signaling that involves morphogens?
the use of gradients
locations closer to the origin of the morphogen will be exposed to higher concentrations, while areas farther away will have less exposure
what are 3 common morphogens?
- transforming growth factor beta (TGF-beta)
- sonic hedgehog (Shh)
- epidermal growth factor (EGF)
why is cell migration necessary in development?
cells are not always in the right location to carry out their function
defn + when does it occur + how does it occur (2): apoptosis
defn: programmed cell death
when: various times in development
how: 1. apoptotic signals 2. preprogramming
what are the two other structures that cells are broken down into during the process of apoptosis so that they can be digested by other cells?
cell –> apoptotic blebs –> apoptotic bodies
defn: necrosis
a process of cell death in which a cell dies as a result of injury
what is one factor that differentiates apoptosis from necrosis other than the fact that one is programmed and one is a result of injury?
APOPTOSIS = the blebs are contained by a membrane, so the release of potentially harmful substances being released into the extracellular environment
NECROSIS = internal substances can be leaked, causing irritation of nearby tissues or even an immune response
defn: regenerative capacity
the ability of an organism to regrow certain parts of the body
defn: complete regeneration vs. incomplete regeneration
COMPLETE = the lost or damaged tissues are replaced with identical tissues
INCOMPLETE = the newly formed tissue is not identical in structure or function to the tissue that has been injured or lost
why do species such as salamanders have an enhanced capacity to regenerate?
because they retain extensive clusters of stem cells within their bodies
defn: senescence
biological aging (can occur at the cellular or organismal level as changes accumulate over time) (also includes the accumulation of chemical and environmental damage(
cellularly: the failure of cells to divide normally after approx. 50 divisions in vitro
why do cells have a limit on how many times they can divide?
may be due to shortened telomeres (ends of chromosomes)
telomeres reduce loss of genetic info from chromosome ends and help prevent DNA from unraveling –> they are difficult to replicate so they shorten during each round of DNA synthesis
eventually the telomeres become too short and the cell is no longer able to replicate
defn + expressed in what kind of cells + func: telomerase
a reverse transcriptase enzyme that is able to synthesize the ends of chromosomes, preventing senescence
allows cells to divide indefinitely, may play a role in survival of cancer cells
expressed in germ cells, fetal cells, tumor cells
func (3): placenta
- the organ where nutrient, gas, and waste exchange occurs
- immunity (the fetus is immunologically naive, but accidental exposure can happen in utero –> the crossing over of antibodies across the placental membrane is protective)
- endocrine organ: it produces progesterone, estrogen, and hCG (essential for maintain pregnancy)
func (2) + where + how: fetal hemoglobin (HbF)
fetal blood cells contain this
- helps to further enhance the transfer of oxygen from maternal to fetal circulation
HOW: it has a greater affinity for oxygen than adult hemoglobin
- assists with transfer and retention of oxygen into the fetal circulatory system (waste material and carbon dioxide move in the opposite direction)
func: umbilical arteries, umbilical vein
umbilical arteries = carry blood away from the fetus toward the placenta (carry deoxygenated blood)
umbilical vein = carries blood toward the fetus from the placenta (carries oxygenated blood)
why does the fetus develop three shunts?
to actively direct blood away from the lungs and liver while they develop (the fetus does not depend on its own lungs and liver)
what are the three shunts that a fetus develops and what are their functions + defns + how?
- FORAMEN OVALE = a one-way valve that connects the right atrium to the left atrium
- allows blood entering the right atrium to flow into the left atrium instead of the right ventricle and thus be pumped through the aorta into systemic circulation directly
- accomplished by a pressure differential between the R and L side of the heart - DUCTUS ARTERIOSUS = shunts leftover blood from the pulmonary artery to the aorta and into systemic circulation (also accomplished by a pressure differential between the R and L side of the heart)
- DUCTUS VENOSUS = bypasses the liver
- shunts blood returning from the placenta via the umbilical vein into the inferior vena cava
why is the foramen ovale closed after birth? why was it open prior to birth?
the pressure differential that exists in the developing fetus reverses
the pressure differential: the right side of the heart is at a higher pressure than the left side which pushes blood through the foramen ovale in the developing fetus
how long is the gestation period of a human (days)? elephant (months)? mouse (days)?
HUMAN: 280 days
ELEPHANT: 22 months
MICE: 20 days
characteristics + process: first trimester (7)
- major organs begin to develop in the first few weeks
- heart begins to beat (22 days)
- eyes, gonad, limbs, and liver start to form
- embryo is 10mm long (week 5)
- cartilaginous skeleton begins to harden into bone (week 7)
- most of the organs have formed, the brain is fairly developed, and the embryo is a fetus (end of 8 wks)
- fetus is ~ 9 cm long (end of 3 months)
characteristics + process: second trimester (5)
- tremendous amount of growth
- begins to move within amniotic fluid
- face takes on a human appearance
- toes and fingers elongate
- fetus is 30 - 36 cm long (end of 6 months)
characteristics + process: third trimester (3)
- continued rapid growth and further brain development (month 7-8)
- antibodies are transported by highly selective active transport from the pregnant individual to the fetus for protection against foreign agents in preparation for life outside the womb (this begins earlier in pregnancy but is highest in the ninth month just before birth)
- growth rate slows, fetus becomes less active as it has less room to move about
defn: parturition
vaginal childbirth
how is vaginal childbirth accomplishe?
by rhythmic contractions of uterine smooth muscle coordinated by prostaglandins and the peptide hormone oxytocin
what are the three basic phases of birth? explain them.
- the cervix thins out and the amniotic sac ruptures (water breaking)
- strong uterine contractions result in the birth of the fetus
- the placenta and umbilical cord are expelled (afterbirth)
