MCAT Biology - Embryogenesis and Development

Ultrasonography

a radiographic technique performed by placing a probe that emits high-frequency sound waves near the tissue to be examined

fertilization

the formation of a diploid zygote from the union of a sperm and an ovum

ampulla

the widest part of the fallopian tube where fertilization occurs

acrosomal apparatus

tube-like structure when sperm comes into direct contact with the secondary oocyte’s cell membrane

cortical reaction

a release of calcium ions depolarize the membrane of the ovum to prevent fertilization of the ovum by multiple sperm cells and increases the metabolic rate of the newly formed diploid zygote

zygote

the diploid cell resulting after fertilisation

fertilization membrane

depolarized and impenetrable membrane of the secondary oocyte

Dizygotic (fraternal) twins

fertilization of two different eggs released during one ovulatory cycle by two different sperm; each placenta (may grow onto each other), chorion, amnion; as genetically distinct as regular siblings

Monozygotic (identical) twins

a single zygote splits into two; may share placenta, chorion, placenta; genetic material/genomes identical

conjoined twins

two offspring are physically attached due to poor division of zygote

Monochorionic/monoamniotic twins

share the same amnion and chorion

Monochorionic/diamniotic twins

each have their own amnion, but share the same chorion

Dichorionic/diamniotic twins

each have their own amnions and chorions

cleavage

zygote undergoes rapid mitotic cell divisions and becomes an embryo; increases two ratios: the nuclear-to-cytoplasmic (N:C) ratio and the surface area-to-volume ratio (more, smaller cells = same radius)

Indeterminate cleavage

results in cells that can still develop into complete organisms; totipotency

Determinate cleavage

results in cells with fates that are already determined; committed to differentiating into a certain type of cell

Differentiation

the process in which a stem cell changes from one type to a differentiated one, changing the structure, function, and biochemistry of the cell to match the cell type

morula

solid mass of cells causing by embryonic cleavage; Latin for ‘raspberry"‘

blastulation

morulas turns into blastula

blastula

a hollow ball of cells with a fluid-filled inner cavity

blastocoel

the fluid-filled inner cavity of a blastula/blastocyst

blastocyst

mammalian blastula; consists of trophoblast and inner cell mass

inner cell mass

gives rise to the organism; protrudes into the blastocoel

trophoblast

give rise to the chorion and placenta; surround the blastocoel

chorion

extraembryonic membrane that develops into the placenta

chorionic villi

microscopic finger-like projections that penetrate the endometrium; support maternal-fetal gas exchange, form from trophoblasts

umbilical cord

two arteries and one vein encased in a gelatinous substance that connects the embryo to the endometrium

ectopic pregnancy

blastula implants outside the uterus; >95% in the fallopian tube; often a surgical emergency

yolk sac

supports and nourishes embryo before placental site of early blood cell development

allantois

early fluid exchange between the embryo and the yolk sac; forms part of the umbilical cord

amnion

a thin, tough membrane filled with amniotic fluid that acts as a shock absorber and contains fetal cells that can be examined for chromosomal abnormalities as well as sex determination

Amniocentesis

process of aspirating amniotic fluid by inserting a thin needle into the amniotic sac; test for sex determination and chromosomal abnormalities

gastrulation

the generation of three distinct cell layers; occurs after implantation

gastrula

blastula after invagination and differentiation

archenteron

the membrane invagination into the blastocoel; develops into the gut

blastopore

the opening of the archenteron

deuterostomes

blastopore develops into the anus

ex. humans

protostomes

blastopore develops into the mouth

primary germ layers

three layers of cells in the gastrula

ectoderm

outermost layer of gastrula; gives rise to the integument

• epidermis

• hair

• nails

• epithelia of the nose, mouth, lower anal canal

• lens of the eye,

• nervous system (including adrenal medulla)

  • inner ear

mesoderm

middle layer of gastrula

• musculoskeletal

• circulatory

• excetory

• gonads

  • mucular and connective tissue layers of of the digestive and respiratory systems and the adrenal cortex

endoderm

innermost layer of gastrula

• the epithelial linings of the digestive and respiratory tracts

• lungs

• pancreas

• thyroid

• bladder

• distal urinary tracts

  • parts of the liver

selective transcription

only the genes needed for that particular cell type are transcribed

induction

the ability of one group of cells to influence the fate of nearby cells

inducers

by chemical substances which diffuse from the organizing cells to the responsive cells

organizing cells

cells that secrete inducers

responsive cells

cells that receive and respond to inducers

neurulation

development of the nervous system

notochord

a rod of mesodermal cells; forms along the long axis of the organism like a primitive spine; induces a group of overlying ectodermal cells to slide inward to form neural folds surrounding a neural groove

neural tube

gives rise to the central nervous system

neural crest cells

migrate outward to form the peripheral nervous system (including the sensory ganglia, autonomic ganglia, adrenal medulla, and Schwann cells) as well as specific cell types in other tissues (such as calcitonin-producing cells of the thyroid, melanocytes in the skin, and others); found at the tip of each neural fold

spina bifida

some or all of the spinal cord may be exposed to the outside world as result of a failure of the neural tube to close; range from no significant distress to death; prevented by folate supplements

anencephaly

the brain fails to develop and the skull is left open as result of a failure of the neural tube to close; universally fatal; prevented by folate supplements

Teratogens

substances that interfere with development, causing defects or even death of the developing embryo; will not have the same effect on every embryo or fetus

ex. alcohol, prescription drugs, viruses, bacteria, environmental chemicals including polycyclic aromatic hydrocarbons

pregnant individuals with diabetes and hyperglycemia

fetus that is too large to be delivered and that could become hypoglycemic soon after birth due to synthesising lots of insulin to compensate for high levels of sugar in utero

specification

initial stage of cell specialization in which the cell is reversibly designated as a specific cell type

determination

the iireversible commitment of a cell to a particular function in the future

morphogens

specific molecules secreted by neighbouring cells that cause neighboring cells to follow a particular developmental pathway

ex. transforming growth factor beta (TGF-β), sonic hedgehog (Shh), and epidermal growth factor (EGF)

stem cells

cells that have not yet differentiated or that give rise to other cells that will differentiate

potency.

determines the tissues a particular stem cell can differentiate into

totipotent

greatest potency; can differentiate into any cell type

ex. embryonic stem cells

pluripotent

differentiate into any cell type except for those found in the placental

structures; blastocyst/gastrula

multipotent

can differentiate into multiple types of cells within a particular group

ex. hematopoietic stem cells - capable of differentiating into all of the cells found in blood, but not skin, nerve, or muscle

responder

cell that is induced

competent

able to respond to the inducing signal

Autocrine

signals act on the same cell that secreted the signal

Paracrine

signals act on cells in the local area

Juxtacrine

signals do not usually involve diffusion, but involve a cell directly stimulating receptors of an adjacent cell

endocrine

signals involve secreted hormones that travel through the bloodstream to a distant target tissue

growth factors

highly specific peptides that promote differentiation and mitosis in certain tissues

ex. PAX6 is expressed in the ectoderm of the head → optic vesicle approaches the overlying ectoderm producing this factor → development of the lens of the eye is induced → triggers the optic vesicle → forms the optic cup → becomes the retina

reciprocal development

two-way induction

Cell Migration

Cells must be able to disconnect from adjacent structures and go to their correct location

ex. anterior pituitary gland originates from a segment of oral ectoderm and must migrate from the top of the mouth to its final location just below the hypothalamus

Apoptosis

programmed cell death; allows recycling of materials; prevents the release of

potentially harmful substances into the extracellular environment.

ex. fingers are originally webbed during development of the hand unril extra sells under go apoptosis

apoptotic blebs

the cell undergoes changes in morphology during apoptosis and divides into many self-contained protrusions

apoptotic bodies

blebs break apart into smaller pieces that are then digested by other cells

necrosis

a process of cell death in which a cell dies as a result of injury; internal substances can be leaked, causing irritation of nearby tissues or even an immune response

Regenerative capacity

the ability of an organism to regrow certain parts of the body

ex. liver - high; heart - low; kidney - moderate

complete regeneration

the lost or damaged tissues are replaced with identical tissues

ex. salamanders & newts (extensive clusters of stem cells)

incomplete regeneration

the newly formed tissue is not identical in structure or function to the tissue that has been injured or lost

ex. humans (scars)

Senescence

biological aging at the cellular and organismal level; results in the failure of cells to divide, normally after approximately 50 divisions in vitro

telomeres

ends of chromosomes; contains lots of guanine and cytosine to prevent DNA from unravelling; difficult to replicate so they shorten during each round of DNA synthesis

telomerase

reverse transcriptase that is able to synthesize the ends of chromosomes, preventing senescence; allows cells to divide indefinitely and may play a role in the survival of cancer cells

fetal hemoglobin (HbF)

greater affinity for oxygen than adult hemoglobin (primarily HbA); assists with the transfer (and retention) of oxygen into the fetal circulatory system

placenta

organ where nutrient, gas, antibodies and waste exchange between fetus and parent occurs; depends on the close proximity of the embryonic and maternal bloodstreams, facilitating diffusion between them without mixing; Fetal lungs do not function until birth

TORCHES infections

pathogens small enough to cross the placental barrier by diffusion

TOxoplasma gondii, Rubella, Cytomegalovirus, HErpes or HIV, and Syphilis

umbilical arteries

carry blood away from the fetus toward the placenta; deoxygenated

umbilical vein

carries blood toward the fetus from the placenta; oxygenated

fetal shunts

pattern of blood flow in the heart that deviates from the normal circuit of the circulatory system to actively directblood away from sensitive organs while they develop

foramen ovale

one-way valve that connects the right atrium to left atrium; allows blood entering the right atrium from the inferior vena cava to flow into the left atrium instead of the right ventricle, and thereby be pumped through the aorta into systemic circulation directly; right side higher pressure until birth - reverse

ductus arteriosus

shunts leftover blood from the pulmonary artery to the aorta

ductus venosus

shunts blood returning from the placenta via the umbilical vein directly into the inferior vena cava, bypassing the liver (smaller hepatic arteries exist)

Human gestation length

280 days, three trimesters

generally: larger animal, larger gestational period

first trimester

• organ development

• herat beat - 22 days

• eyes, gonads, limbs, liver

• 5 weeks - 10 mm

• 6 weeks - 15 mm

• 7 weeks - cartilaginous skeleton begins to harden into bone

• 8 weeks - most of the organs have formed, the brain is fairly developed, embryo → fetus

• 3 months - 9 cm long

Second Trimester

• moves in amniotic fluid

• face looks human

• toes an finegrs elongate

• 6 months - 30 - 36 cm

Third Trimester

• continued rapid growth and further brain development

• transport of antibodies to fetus

• growth rate slows

• less active

parturition

vaginal childbirth; accomplished by rhythmic contractions of uterine smooth muscle

prostaglandins

oxytocin

peptide hormone associate diwth childbirth, breatfeeding, and emotional attachment

birth process

1. the cervix thins out and the amniotic sac ruptures - water breaking.

2. strong uterine contractions result in the birth of the fetus

3. the placenta and umbilical cord are expelled

afterbirth

expulsion of the placenta and umbilical cord