MCAT Biology Pt. 1

Cell Theory

• all living things are composed of cells

• the cell is the basic functional unit of life

• cells arise only from preexisting cells

• cells carry genetic information in the form of deoxyribonucleic acid (DNA), This genetic material is passed on from parent to daughter cell.

Cell Theory vs Viruses

Viruses: not living bc violate third and fourth tenents of cell theory (unable to reproduce on own)

Eukaryotic cells

• Unicellular or multicellular

• Contain true nucleus enclosed in membrane

Prokaryotic cells

• Single celled

• Do not contain nucleus

Membrane Bound Organelles

• most cells have membrane enclosing semifluid cytosol where organelles are suspended

• membrane bound allow for compartmentalization of functions

• cytosol: allow for diffusion of molecules throughout the cell

Nucleus

• surrounded by nuclear membrane or envelope

• nuclear pores: allow for selective two-way exchange material between cytoplasm and nucleus

• Coding:

  • Genes: coding regions of DNA

  • Histones: organizing proteins where DNA wrapped around

  • Chromosomes: liner strands of DNA

• Nucleolus: subsection of DNA where rRNA is synthesized (25% of volume of nucleus, darker spot)

Mitochondria functions

• power plants of the cell

• semi-autonomous: own genes and replicated independently of nucleus via binary fission

• cytoplasmic / extranuclear inheritance: transmission of genetic material independent of nucleus (b/c engulfing of aerobic prokaryote by anaerobic prokaryote)

• in charge of kicking-off process called apoptosis via releasing enzymes from electron transport chain

Mitochondria parts

• Outer membrane: barrier between cytosol and inner environment of mitochondrion

• Inner membrane: arranged into infoldings called cristae, contain molecules and enzymes for electron transport chain

• Intermembrane space: between inner and outer membrane

• Mitochondrial matrix: space inside inner membrane

Lysosomes

• membrane bound structures

• hydrolytic enzymes for breaking down many different substrates (endocytosis and cellular waste products)

• endosomes: transport, package and sort cell material traveling to and from membrane → also HELP transport material to lysosomal pathway for degredation

• autolysis: destruction of cellular components and cell via hydrolytic enzymes released from lysosomes → apoptosis

Endoplasmic Reticulum (ER) structure

• interconnected membranes contiguous with nuclear envelope

• double membrane

Rough Endoplasmic Reticulum

• studded with ribosomes

• translation of proteins that will be secreted

Smooth Endoplasmic Reticulum

• lack ribosome

• use for lipid synthesis (like phospholipids in cell membrane)

• detoxification of certain drug and poison

Golgi Apparatus

• stacked membrane-bound sacks

• modify proteins made in ER

  • add groups like carbohydrates, phosphates and sulfates

  • introduce signal sequences which direct delivery of product to specific cellular location

  • repackaged in vesicles afterwards → exocytosis if exiting cell

Peroxisomes

• contain hydrogen peroxide

• break down long chain fatty acids via beta-oxidation

• synthesis of phospholipids

• also contain some enzymes invovled in pentose phosphate pathway

Cytoskeleton

• provide structure to cell and help to maintain its shape

Cytoskeleton - Microfilaments

• made up of solid polymerized rods of actin

• organized into bundles and networks and resistant to both compression and fracture → protection of cell

• also use ATP for movement by interacting with myosin

  • ex. muscle contraction

Cytoskeleton - Microfilaments: Cytokinesis

• cytokinesis: division of materials between daughter cells

  • cleavage furrow formed from microfilaments → ring at site of division between two new daughter cells

  • actin filaments here contract and ring becomes smaller to separate daughter cells

Cytoskeleton - Microtubules

• hollow polymers of tubulin protein

• radiate throughout cell

• primary pathways for carrying vesicles

  • use motor proteins like kinesin and dynein to carry vesicles

Cytoskeleton - Microtubules: Cilia and Flagella

• cilia: projection from cell involved in moving materials along surface of cell

• flagella: structures involved in moving of cell itself

• 9+2 structure (9 pair of microtubule forming in outer ring+ 2 pair in center)

  • structure for both cilia and flagella

Cytoskeleton - Microtubules: Centrosome

• found in region of cell called centrosome

  • nine triplets of microtubules with a hollow center

  • during mitosis:

    • centriole go to opposite poles of dividing cell and organize mitotic spindle

    • microtubule attaching to chromosomes via complexes called kinetochores

    • pulls sister chromatids apart

Cytoskeleton - Intermediate Filaments

• diverse group of filamentous proteins

• include keratin, desmine, vimentin, lamins

• involved in cell-cell adhesion and maintenance of overall integrity of cytoskeleton

• able to withstand tremendous amount of tension

• also helps anchor other organelles including nucleus

Epithelial Tissue

• cover body and line cavities, means of protection for pathogen invasion and desiccation

• underlying layer if basement membrane (connective tissue)

• in most organs, epithelial cells constitute parenchyma: functional parts of organ

• polarized: one side faces lumen (inside of organ/tube) or outside world, while other side interact with underlying blood vessel and structural cells

Types of Epithelial Cells

• Simple epithelia: one layer of cells

• stratified epithelial: multiple layers

• Pseudostratified epithelia: appear to have multiple layers due to differences in cell height but only one layer

• cuboidal cells: cube-shaepd

• columnar cells: llng adn thin

• Squamous cells: flat and scale-like

Connective tissue

• supports body and provide framework for epithelial cells to carry out functions

• main contributor to stroma: support structure

• bone, cartilage, tendons, ligaments, adipose tissue and blood

• most cells in connective tissues secrete materials like collagen and elastin to form extracellular matrix

Nucleoid region

• region where single circular molecule of DNA for prokaryote is concentrated

Prokaryote: Archaea

• single celled organisms that visually similar to bacteria but with genes and metabolic pathways more similar to eukaryotes than to bacteria

• extremophiles: most commonly isolated from harsh environment with hight temp, high salinity or no light but actually found in human body too

• use alternative sources of energy: photosynthetic, chemosynthetic

Archaea and Eukarya Common Origin

• translation with methionine

• contain similar RNA polymerases

• associate DNA with histones

• BUT archaea has single circular chromosome

Prokaryote: Bacteria

• cell membrane and cytoplasm

• some have flagella or fimbriae (like cilia)

Prokaryote: Bacteria - Targeting

• Can target via differences in flagella from eukarya vs bacteria

• differences in ribosomes (bacteria ribosomes smaller than eukarya)

Prokaryote: Bacteria and Eukarya Relationships

• mutualistic symbiotes: both humans and bacteria benefit from relationship

• parasites: provide no advantage or benefit to host, but rather cause disease

Shapes of Bacteria

• Cocci: spherical bacteria like strep pyogenes

• Bacilli: rod-shaped bacteria like e. coli

• Spirilli: spiral-shaped bacteria like treponema pallidum

Aerobes vs Anaerobes

• obligate aerobes: require oxygen for metabolism

• anaerobes: cellular metabolism that does not require oxygen

• obligate anaerobes: anaerobes tha cannot survive in oxygen-containing environment

• facultative anaerobes: can toggle between oxygen for aerobic metabolism or anaerobic metabolism if no oxygen present

• aerotolerant anaerobes: unable to use oxygen for metabolism but not harmed by presence in environment

Prokaryotic Cell structure: Cell Wall

• outer barrier of cell

• next layer is cell membrane (plasma membrane)

• envelope: cell wall + cell membrane

• provide structure and control movement of solute in and out of bacterium

Prokaryotic Cell structure: Cell Wall - Gram Positive and Gram Negative

• Gram positive: thick layer of peptidoglycan (polymeric substance made from amino acids and sugars)

  • provide protection from host organism’s immune system

  • contain lipoteichoic acid too

• Gram negative: thin layer of peptidoglycan

  • periplasmic space: between peptidoglycan walls and cell membrane

  • also contain outer membranes with phospholipids and polysaccharides

Prokaryotic Cell structure: Flagella

Chemotaxis: ability of cell to detect chemical stimuli and move away from or toward them

composed of:

• filament: hollow, helical struucture with flagellin

• basal body: complex structure that anchors flagellum to cytoplasmic membrane and motor of flagellum

• hook: connects filament and basal body

Prokaryotic Cell structure: Other Organelles

• plasmids: DNA acquired fomr external sources carried on these smaller circular structures

• cell membrane site of electron transport chain and generation of ATP

• primitive cytoskeleton

Binary Fission

• form of asexual reproduction in prokaryotes

• circular chromosome attach to cell wall and replicate while cell continues to grow in size and begin to grow inward to make two identical daughter cells

• faster than mitosis

Genetic Recombination

• Plasmids: extrachromosomal (extragenomic) material

  • often carry genes impart some benefit to bacterium, like antibiotic reistance or virulence factors: increase pathogenicity

  • episomes: subset of plasmids capable of integrating into genome of bacterium

Bacterial Transformation

• results from integration of foreign genetic material into host genome

• comes from other bacteria that upon lysing spill contents into vicinity of bacterium capable of transformation

• most gram-negative rods able to do this

Bacterial Conjugation

• bacterial form of mating (sexual reproduction)

• two cells form conjugation bridge: facilitates transfer of genetic material

  • formed from sex pili found on donor male

  • to form pilus, bacteria must have plasmids known as sex factors that contain necessary genes, like F (fertility) factor in E. Coli

  • Hfr (high frequency of recombination): sex factor plasmid being integrated into host genome → able to transfer entire host genome into other bacteria

• unidirectional transfer: donor male (+) to recipient female (-)

Bacterial Transduction

• only genetic recombination process requiring vector: virus carries genetic material from one bacterium to another

• Viruses are obligate intracellular pathogens, cannot reproduce outside host cell

• bacteriophages: viruses that infect bacteria

  • may accidentally incorporate segment of host DNA during assembly

  • when infecting other bacteria may put in that segment of host DNA

Bacterial Transposons

• genetic elements capable of inserting and removing themselves from genome

• both in prokaryotes and eukaryotes

• may disrupt gene if inserted in coding region of gene

Bcterial Growth

• Lag phase: bacteria first adapt to new local conditions

• Exponential/log phase: bacteria exponentially increase once they adapt to local conditions

• Stationary phase: when reduction of resources slows reproduction

• Death phase: bacteria exceeded ability of environment to support number of bacteria

Viral Structure

• Capsid: protein coat of virus

• Envelope: surrounding capsid, composed of phospholipids and virus-specific proteins

  • sensitive to heat, detergents and desiccation, so enveloped viruses easier to kill

• Virions: viral progeny released to infect additional cells

Viral structure - Bacteriophages

• tail sheath: syringe, injecting genetic material into bacterium

• tail fibers: help bacteriophage recognize and connect to correct host cel

Viral genomes - Single Stranded RNA viruses

• Positive sense single stranded RNA viruses: genome directly translated to functional proteins by ribosomes of host cell (like mRNA)

• Negative-sense single-stranded RNA viruses: template for synthesis to complementary strand → template for protein synthesis

  • Must have RNA replicase to ensure complementary strand is synthesized

• Retroviruses: enveloped, single-stranded RNA viruses

  • Two identical RNA molecules

  • carry reverse transcriptase: synthesized DNA from single-strand DNA → integrate into host cell genome and replicate and transcribed like host cell’s own DNA

Viral Life Cycle

1. infection: virus binds to correct receptor on cell and enveloped viruses fuse with cell membrane

2. translation of viral genetic material: translocation of genetic material to correct location in cell → translate into protein with host ribosomes, tRNA, amino acids and enzymes with capsids

3. progeny release: cell death → spill viral progeny, or host cell lyse, or virus fuse with plasma membrane (extrusion) to keep host cell alive and allow continued use by virus (productive cycle)

Viral Life Cycle - Bacteriophages

1. Lytic cycle: cell lyses, virulent: viruses in lytic state

2. Lysogenic cycle: virus does not lyse bacterium, but integrate into host genome as provirus or prophage, replicated as bacterium reproduces but environmental factors may cause provirus to leave genome and revert to lytic cycle

3. superinfection: simultaneous infection with other phages; infection with one strain of phage makes bacterium less susceptible to this

Prions

• infectious proteins, nonliving things

• trigger misfoldings of other protieins (a helical to b pleated sheet)

• protein aggregates form → interfere with cell function

Viroids

• small pathogens with very short circular single-stranded RNA that infect plants

• bind to large number of RNA sequences and silent genes on plant genome

• also present for humans too

Diploid vs Haploid Cells

diploid (2n): 2 copies of each chromosome, autosomal cells = 46

haploid (n): 1 copy of each chromosome, germ cells = 23

Cell Cycle

cell cycle: specific series of phases during which cell grows, synthesizes DNA and divides

interphase: G1, S, G2, longest part of cell cycle

• chromosomes not visible with light microscopy because in less condensed form known as chromatin

G0 stage: cell simply living nand carriying out its functions without preparation ofr division

Cell Cycle: G1 Stage: Presynthetic Gap

• cells make organelles for energy and protein production while also increasing their size

• passage into next stage governed by restriction point: need to contain proper complement of DNA

  • main protein in control is p53

Cell Cycle: S Stage: Synthesis of DNA

• cell replicates genetic material so daughter cell have identical copies

• after replication, two identical chromatids bound at specialized region known as centromere

• still only have 46 chromosomes, even though 92 chromatids are present

  • double content, but content stay same

Cell Cycle: G2 Stage: Postsynthetic Gap

• cell passes through another quality control checkpoint:

  • cell checks to ensure enough organelles and cytoplasm for two daughter cells

  • make sure that DNA replication proceeded correctly

  • main protein in control is p53

Cell Cycle: M Stage: Mitosis

• mitosis + cytokinesis

• occur in somatic cells (not involved in sexual reproduction)

Molecules responsible for cell cycle

Cyclin-dependent kinases (CDK): CDK require presence of right cyclins

During cell cycle, concentrations of specific cyclins increase and decrease during specific stages

cyclin binds to CDK → CDK-cyclin complex → phosphorylate transcription factor → promote transcription of genes required for next stage of cell cycle

Cancer

• cell cycle control becomes deranged and damaged cells are able to undergo mitosis

• mutation of gene producing p53 → not able to stop and repair damaged DNA

• tumors: cancer cells that undergo rapid cell division

• metastasis: distant spread of cancerous cells through bloodstream or lymphatic systems

Mitosis: Prophase

• condensation of chromatin into chromosomes

• centrosome: region outside of nucleus where paired cylindrical organelles help divide DNA → form spindle fibers made up of microtubules

• asters: anchor centrioles to cell membrane

• Kinetochores: appear at centrosome on centromeres that attach specific fibers of spindle apparatus (kinetochore fibers)

Mitosis: Metaphase

• centriole pairs at opposite ends of cell

• kinetichore fibers and fibers of spindle apparatus interact to align chromosomes at metaphase plate (equatorial plate)

Mitosis: Anaphase

• centromeres split so chromatid has own distinct centromere

• pulled towards opposite poles of cell by shortening of kinetochore fibers

Mitosis: Telophase and Cytokinesis

Telophase:

• reverse of prophase

• spindle apparatus disappears

• nuclear membrane reform around each set of chromosomes

• nucleoli reappear

• chromosomes uncoil

Cytokinesis:

• separation of cytoplasm and organelles

Mitosis

• occur in gametocytes (germ cells) and results in up to four nonidentical sex cells (gametes)

• Meiosis 1: homologous chromosomes separated, generating haploid daughter cells (reductional division)

• Meiosis 2: separation of sister chromatids without change in ploidy (equational division)

Meiosis: Prophase I

• chromatin condense into chromosomes

• spindle apparatus form

• nucleoli and nuclear membrane disappear

• homologous chromosomes come together and intertwine in process called synapsis

  • each chromosome has two sister chromatids, so synaptic pair has tetrad (four chromosomes)

  • all held together by synaptonemal complex

  • recombination: chromatids of homologous chromosomes may break at point of contact (chiasma/chiasmata) and exchange equivalent pieces of DNA via crossing over

  • explains Mendel’s second law of independent assortment: inheritance of one allele has no effect on likelihood of inheriting certain alleles for other genes

Meiosis: Metaphase I

• homologous pairs (tetrads) align at metaphase plate

• held by one spindle fiber

Meiosis: Anaphase I

• homologous pairs separate (father vs mother) and pulled to opposite poles of cell (disjunction)

• accounts for Mendel’s first law of segregation: during disjunction, each chromosome of paternal origin separate from homologue of maternal origin, and either chromosome can end of up either daughter cell → distribution of homologous chromosomes is random with respect to parental origin (segregation)

Meiosis: Telophase I

nuclear membrane forms around new nucleus → cells are now haploid (n=23)

may have interkinesis: short rest period during which chromosomes partially uncoil

Meiosis: Prophase II

• nuclear envelope dissolves

• nucleoli disappear

• centrioles migrate to opposite poles

• spindle apparatus begins to form

Meiosis: Metaphase II

• chromosomes line up on metaphase plate

Meiosis: Anaphase II

• centromeres divide, separating chromosomes into sister chromatids

• chromatids pulled to opposite poles by spindle fibers

Meiosis: Telophase II

• nuclear membrane forms around each new nucleus

• two daughter cells formed

• four haploid daughter cells produced by gametocyte

Chromosomal Sex - X Chromosome

• X chromosome: sizeable amount of genetic information

  • mutations in these genes → sex-linked (X-linked) disorders

  • hemizygous: males, many of genes on X chromosome because only have one copy so will necessarily express that allele of disease-causing allele on unpaired part of X chromosome

  • most x-linked disorders are recessively inherited

  • carriers: females with diseased allele on X chromosome but not exhibiting disease

Chromosomal Sex - Y Chromosome

• Y chromosome: very little genetic information

  • SRY (sex-determining region Y): codes for transcription factor initiating testis differentiation

Male Reproductive Anatomy - Testes

• primitive gonads → testes

  • seminiferous tubules: sperm produced, nourished by sertoli cells

  • interstitial cells of leydig: secrete testosterone and other male sex hormones (androgens)

• testes located in scrotum, external pouch below penis (maintain 2 to 4 degrees celsius lower than body via muscle, ductus deferens)

Male Reproductive Anatomy - Pathway of Sperm

• sperm → epididymis until ejaculation → vas deferens → ejaculatory duct → urethra

Male Reproductive Anatomy - Semen

• semen: sperm mixed with seminal fluid (produced by seminal vesicles, prostate gland and bulbourethral gland)

  • seminal vesicles: fructose to nourish sperm

  • seminal vesicle and prostate gland: fluid mildly alkaline properties so sperm survive in acidity of female reproductive tract

  • bulbourethral (cowper’s glands): clear viscous fluid that cleans out remnants of urine and lubricates urethra during sexual arousal

Spermatogenesis

formation of haploid sperm via meiosis

• spermatogonia: diploid stem cells

• diploid primary spermatocytes: after replicating genetic material (S stage)

• haploid secondary spermatocytes: after first meiotic division

• spermatids: after second meiotic division

• spermatozoa: mature spermatids

Structure of Sperm

• midpiece: ATP from fructose, filled with mitorchondria, generate energy for swimming through female reproductive tract to reach ovum in fallopian tubes

• Acrosome: covers sperm head, derived from Golgi apparatus and necessary to penetrate ovum

• head: contain genetic material

• flagella: motility

Female Reproductive Anatomy:

• gonads → ovaries: produce estrogen and progesterone

  • thousands of follicles: multilayers sacs that contain, nourish and protect immature ova (eggs)

  • one egg per month ovulated into peritoneal sac: lines abdominal cavity

• egg → fallopian tube/oviduct lined with cilia to push egg forward → uterus

• sperm go from vaginal canal → cervix → uterus

• vulva: external parts of female genital organs

Oogenesis

• production of female gametes

• primary oocytes (2n): arrested in prophase I

• after menarche (first menstrual cycle):

  • complete meiosis I → secondary oocyte (ample cytoplasm to one cell and polar body pretty empty) → wait until fertilization for remainder of meiosis II to complete

Oocyte Structure

• Zona pellucida surround oocyte itself and acellular mixture to protect oocyte and contain compound necessary for sperm binding

• corona radiata: lies outside zona pellucida and layer of cells that adheres to oocyte during ovulation

• Sperm acrosomal enzymes → penetrates the two layers → trigger completion of meiosis II → forms another mature ovum and polar body

• Ovum: contributes nearly everything to zygote (half of DNA, all of cytoplasm, organelles, mitochondria and RNA for early cellular processes)

• Completion of meiosis II: haploid pronuclei of sperm and ovum join → diploid zygote

Male Sexual Development

Fetal period:

• Y chromosome → production of androgens

Puberty:

• Testosterone by testes rises a lot during puberty

  • sperm production:

    • FSH stimulate Sertoli cell and trigger sperm maturation

    • LH causes interstitial cells to produce testosterone

  • development of secondary sexual characteristics: facial and axillary hair, deepening of voice, increased muscle and bone mass

  • negative feedback to anterior pituitary gland: synthesize and release follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and hypothalamus: restrict production of gonadotropin-releasing hormone (GnRH)

Female Sexual Development - Estrogen

• secreted in response to FSH

• development and maintenance of female reproductive system and female secondary sexual characteristics: breast growth, widening of hips, changes in fat distribution

• in embryo: estrogen stimulate development of reproductive tract

• in adult: estrogen → thickening of lining of uterus (endometrium) each month to prepare for implantation of zygote

Female Sexual Development - Progesterone

• secreted by Corpus luteum: remains of ovarian follicle following ovulation in response to LH

• development and maintenance of endometrium but not initial thickening of endometrium

• by end of first trimester, progesterone is supplied by placenta, and corpus luteum atrophies and ceases to function

Menstrual cycle - Follicular Phase

begin when menstrual flow (shed uterine lining) begins

GnRH secretion from hypothalamus increased and decreased estrogen and progesterone toward end of cycle

increased secretion of FSH and LH → develop several ovarian follicle → produce estrogen → negative feedback effects → stimulate regrowth of endometrial lining, stimulating vascularization and glandurization of decidua

Menstrual cycle - Ovulation

estrogen later parodoxically result in positive feedback effects of GnRH, LH, and FSH → ovulation, release of ovum from ovary into abdominal/peritoneal cavity

Menstrual cycle - Luteal Phase

• ruptured follicle forms corpus luteum → secretes progresterone → progesterone levels rise and estrogen levels remain high to maintain uterine lining for implantation → progesterone cause negative feedback on GnRH, FSH, and LH → prevent ovulatinon of multiple eggs

Menstrual cycle - Menstruation

• no implantation → corpus luteum loses stimulation from LG, progesterone levels decline, uterine lining sloughed off → loss of estrogen and progesterone removes block on GnRH so next cycle can begin

Menstrual cycle - Pregnancy

• fertilization → zygote become blastocyst that will implant on uterine lining and secrete human chorionic gonadotropin (hCG) analog of LH → maintain corpus luteum → critical during first trimester development to keep uterine lining in place → second trimester placenta make own progesterone and estrogen by itself so no more corpus luteum → negative feedback on GnRH secretion

Menstrual cycle - Menopause

• ovaries less sensitive to FSH and LH → ovarian atrophy

• estrogen and progesterone levels drop → endometrium also atrophies and menstruation stops

• blood levels of FSH and LH rise because no more negative feedback

• symptoms: flushing, hot flashes, bloating and headaches between 45-55

Fertilization

• Secondary oocyte ovulated from follicle on day 14 of menstrual cycle in ampulla (widest part of fallopian tube)

• Acrosomal apparatus (tube-like structure) after acrosomal reaction → cortical reaction (release of calcium ions) → fertilization membrane (now depolarized and impenetrable membrane)

Twins

Dizygotic (fraternal) twins: form from fertilization of two different eggs released during one ovulatory cycle by two different sperm

Monozygotic (identical) twins: single zygote splits into two

• incomplete division → conjoined twins

• monochorionic/monoamniotic: share same amnion and chorion

• monochorionic/diamniotic: own amnion but share same chorion

• dichorionic/diamniotic twin: have their own amnions and chorions

Cleavage

• move to uterus for implantation → rapid mitotic cell divisions called cleavage → first cleavage makes embryo

• Indeterminate cleavage: cells that can still develop into complete organisms

• Determinate cleavage: result in cells with fates taht are already determined → committed to differentiating into a certain type of cell

Blastulation

Zygote → morula (solid mass of cells) → blastula

• blastulation: forms blastula/blastocyst

  • fluid-filled inner cavity known as blastocoel

  • inner cell mass (protrudes into blastocoel): gives rise to organism

  • trophoblast: hollow surrounding of blastocoel, giving rise to chorion (extraembroynic membrane that develops into placenta) and placenta

Implantation

Blastula moves → uterus → burrow into endometrium

• chorion → chorionic villi which are finger-like projections that penetrate endometrium → maternal-fetal gas exchange

• umbilical cord - connects embryo to placenta via two arteries and one vein

• yolk sac: supports embryo until placenta is functional, site of early blood cell development

• allantois: extraembryonic membrane involved in early fluid exchange between embryo and yolk sac

• amnion: surrounds allantois, thin, tough membrane filled with amniotic fluid that shock absorber during pregnancy

Gastrulation

• generation of three distinct cell layers: endoderm, ectoderm, mesoderm

• archenteron: membrane invagination into blastocoel → gut

• blastopore: opening of archenteron → anus for deuterostomes, mouth for protosomes

Differentiation of cells

• via selective transcription of genome (some genes transcribed)

• induction: ability of one group of cells to influence fate of nearby cells, mediated by inducers (chemical substances) which diffuse from organizing cells to responsive cells

Neurulation

• development of nervous system from ectoderm

• notochord: rod of mesodermal cells, forms along long axis of organism like primitive spine (intervertebral discs)

• overlying ectodermal cells slide inward to form neural folds → surround neural groove → grow toward each other until fuse into neural tube → CNS

• neural crest cells: on tip of each neural fold → migrate outward to form PNS and other cell types in other tissues

• ectoderm then covers all of this

Problems in Early Environment

• Teratogens: substances that interfere with embryo development, causing defects or even death

• maternal health also affects embryo development

Cell Specification/Determination

• Specification: initial stage of cell specialization, where cell is reversibly designated as specific cell type

• Determination: commitment of cell to particular function in the future

  • Due to mRNA and protein in parent cell being split differently to daughter cells

  • Morphogens: secretion of specific molecules form nearby cells that cause them to follow specific developmental pathway