Ch3: Embryogenesis and Development

Fertilization 1: Ovulation

Secondary oocyte ovulated on day 14 into fallopian tube

Available for fertilization for 24h in ampulla

Fertilization 2: Sperm Meet Oocyte

Release acrosomal enzymes to penetrate corona radiata and zona pellucida

Fertilization 3: Acrosomal Apparatus

Tube penetrate oocyte membrane for pronucleus injection

Fertilization 4: Cortical Reaction

Ca2+ release depolarize ovum membrane → Fertilization membrane

Prevent multiple sperm fertilizing and increase zygote metabolic rate

Twins: Dizygotic

Fraternal

2 diff eggs released and fertilized by 2 diff sperm

Zygotes develop with own placenta, chorion, amnion

Twins: Monozygotic

Identical

1 egg released and fertilized by 1 sperm

1 zygote split into 2

Monozygotic: Conjoined Twins

Incomplete zygote division

Monozygotic: Monochorionic/Monoamniotic Twins

Share amnion and chorion

Monozygotic: Monochorionic/Diamniotic Twins

Share chorion, separate amnions

Monozygotic: Dichorionic/Diamniotic Twins

Separate amnions and chorions

Cleavage

Rapid mitotic cell divisions in zygote while moving for uterine implantation

Create embryo (multicellular)

Increase area for gas/nutrient exchange relative to volume

Indeterminate Cleavage

Cells able to develop into any cells in organism

Identical monozygotic twin genomes from cleaving same embryo

Determinate Cleavage

Cells committed to differentiating into specific cell type

Morula

Solid cell mass from embryotic divisions

Blastulation

Form blastula (blastocyst)

Blastula

Hollow cell ball with fluid-filled inner cavity (blastocoel)

Blastocyst: Trophoblast Cells

Surround blastocoel

Become chorion then placenta

Blastocyst: Inner Cell Mass

Protrude into blastocoel

Become organism

Implantation

Blastocyst move from fallopian tube to uterus and implant in endometrium

Form placenta

Trophoblast Cells: Chorion

Form chorion to connect maternal blood supply and embryo

Form chorionic villi penetrating endometrium → Develop into placenta to support gas exchange

Umbilical Cord

Connect embryo and placenta

Contain 2 arteries and 1 vein for O2 and nutrient exchange

Yolk Sac

Support embryo before placenta

Early blood cell development

Extraembryonic Membranes: Allantois

Early fluid exchange between embryo and yolk sac (form umbilical cord)

Surrounded by amnion

Extraembryonic Membranes: Amnion

Thin membrane producing amniotic fluid

Absorb shock

Inside chorion

Gastrulation

Form 3 distinct cell layers

Blastula cells invaginate to eliminate blastocoel and form gastrula

Gastrula: Archenteron

Membrane invagination into bastocoel

Develop into gut

Gastrula: Blastopore

Archenteron opening

Develop into anus (deuterostomes/humans) or mouth (protostomes)

Gastrulation: Primary Germ Layers

Cells migrating into remains of blastocoel

Establish 3 cell layers

Primary Germ Layers: Ectoderm

Outermost layer

Form integument (epidermis, hair, nails, epithelia of nose, mouth and lower anal canal), lens, nervous system (**adrenal medulla**), inner ear

“Attracto”derm: Things attracting us to others (cosmetic features and smarts)

Primary Germ Layers: Mesoderm

Middle layer

Form musculoskeletal system, circulatory system, gonads, muscular and connective tissue layers, **adrenal cortex**

“Means”derm: Means for organism to get around (bones and muscles), means of getting around body (circulatory), means of getting around (gonads)

Primary Germ Layers: Endoderm

Innermost layer

Form digestive and respiratory epithelial lining, pancreas, thyroid, bladder, distal urinary tracts, liver parts

"Endernal” organ lining: Digestive, respiratory, and accessory

Neurulation

Nervous system development

Neurulation 1: Notochord

Mesodermal cell rod form along organism

Neurulation 2: Neural Folds

Notochord induce overlying ectodermal cells to form neural folds surrounding neural groove

Neural crest cells at periphery differentiate into PNS and other tissues

Neurulation 3: Neural Tube

Neural folds fuse

Form CNS

Neurulation 4: Ectodermal Cell Migration

Cover neural tube and crest

Development Problems: Teratogens

Substances interfering with embryonic development

Cause defects or death

Variable effects

Ex: Alcohol, drugs, viruses, bacteria

Development Problems: Maternal Health

Physiological changes over/underexpose embryo to chemicals

Variable effects

Maternal Health: Diabetes

Increase fetal size

Hypoglycemia

Maternal Health: Folic Acid Deficiency

Neural tube defects

Cell Specialization Stages

Specification, determination, differentiation

Specialization 1: Specification

Cell reversibly designated type

Specialization 2: Determination

Cell irreversibly commitment to specific lineage

Determination Methods

Uneven material segregation during cleavage

Morphogen secretions from nearby cells

Specialization 3: Differentiation

Cell structure, function, and biochem changes to match cell type

Selective transcription

Differentiation: Selective Transcription

Only transcribe genes needed for cell type

Stem Cells

Non-differentiated or produce cells that will differentiate

Potency determine differentiation ability

In embryo and adults

Stem Cells: Totipotent

Greatest potency

Embryonic stem cells

Self-renewal

Differentiate into any cell type (germ cell layers and placental structures)

Stem Cells: Pluripotent

Self-renewal

Differentiate into any cells except placental

Stem Cells: Multipotent

Self-renewal

Differentiate into cell types within group

Ex: Hematopoietic cells → any blood cells

Cell-Cell Communication: Induction

Cells secrete inducer signals to influence nearby cells

Inducers diffuse from organizing cells to responsive cells

Ensure proximity of diff cell types working together in organ

Induction: Inducer Signal

Growth factors promoting differentiation and mitosis

Induction: Responder

Induced/responsive cell

Competent: Able to respond to inducing signal

Cell-Cell Communication: Autocrine Signals

Act on same cell secreting signal

Cell-Cell Communication: Paracrine Signals

Act on local cells

Cell-Cell Communication: Juxtacrine Signals

Directly stimulate receptors on adjacent cell

Cell-Cell Communication: Endocrine Signals

Act on distant cells

Hormones travel through bloodstream

Reciprocal Development

Two-way induction pathway

Signalling Method: Gradients

Morphogens diffusion cause cells closer to origin to be exposed to higher conc and undergo determination

Common Morphogens

Transform growth factor beta, sonic hedgehog, and epidermal growth factor

Cell Migration

Disconnect from adjacent structures and move to correct location after differentiation

Cell Death: Apoptosis

Programmed cell death during development

Via apoptotic signals or preprogramming

Prevent release of harmful substances into ECM

Apoptosis 1: Apoptotic Blebs

Self-contained protrusions dividing from cell

Apoptosis 2: Apoptotic Bodies

From breaking apoptotic blebs

Digested by other cells to recycle material

Cell Death: Necrosis

Cell death from injury

Internal substances leak into ECM to cause irritation and immune response

Regenerative Capacity

Organism ability to regrow body parts from stem cells

Complete Regeneration

Lost/damaged tissues replaced with identical tissues

Incomplete Regeneration

Lost/damaged tissues replaced with non-identical tissues

Differ in structure or function

Regenerative Capacity in Humans

High: Liver

Moderate: Kidneys

Low: Heart

Senescence

Biological aging

Cause cell division failure after 50 divisions

From shortened telomeres

Senescence: Telomeres

Chromosome ends knotted with high guanine and cytosine conc

Reduce genetic info loss and prevent DNA unraveling

Shorten with each division

Senescence: Telomerase

Reverse transcriptase synthesizing telomeres

Prevent senescence and allow for indefinite division

In germ, fetal, tumour cells

Placenta: Material Exchange

Oxygenation occurs

Exchange nutrient, gas, waste between mother and fetus

Separate maternal and fetal blood

Placental Exchange: Diffusion

Material exchange with gradient

Maternal Blood: Higher O2 partial pressure

Placental Exchange: Fetal Hemoglobin (HbF)

Higher O2 affinity

Increase O2 transfer and retention in fetal circulatory system

Placenta: Immunity

Antibodies cross placental membrane from mother to fetus

Placenta: Endocrine Organ

Produce progesterone, estrogen, hCG

Maintain pregnancy

Umbilical Arteries

Carry deoxygenated blood away from fetus to placenta

Umbilical Vein

Carry oxygenated blood toward fetus from placenta

Fetal Lungs and Liver

Non-functional, underdeveloped, sensitive to high bp

3 shunts carry blood away (2 lungs, 1 liver)

Foramen Ovale Shunt

Connect right and left atrium

Bypass lungs

Fetus: Higher right heart pressure push blood through opening

Adult: Pressure reverse, close opening

Ductus Arteriosus Shunt

Connect pulmonary artery to aorta

Bypass lungs

Higher pressure in right heart push blood through opening

Ductus Venosus Shunt

Connect placenta umbilical vein to inferior vena cava

Bypass liver

Liver receive blood from small hepatic arteries

Human Gestation Length

280 days

3 trimesters

First Trimester

Organogenesis

8 weeks: Most organs formed, brain developed, embryo → fetus

Second Trimester

Fetal growth and movement in amniotic fluid

Face formation

Third Trimester

7 and 8 months: Rapid growth and brain development

9 months: Antibody transport, slowed activity and growth (restricted room)

Parturition

Birth

Parturition Phase 1

Cervix thins

Amniotic sac ruptures

Parturition Phase 2

Strong rhythmic uterine contractions stimulated by prostaglandins and oxytocin birth fetus

Parturition Phase 3

Expel placenta and umbilical cord (afterbirth)