Lecture Final: Chapter 28

Question 1

Mechanisms of early embryonic development (4 ; 4sub1)

Answer 1

conserved among nearly all animals, especially early stages

1. Fertilization
2. Cleavage
3. Gastrulation: forming of three germ layers (endo, meso, ecto)
4. Neurulation (formation of a neural tube) and organ development
   - - Usually a result of interactions / release of chemical signals between the germ layers → Mesoderm produce neural cords to the ectoderm, which insulates it, creating the neural tube

Question 2

Fertilization of mammals (6)
2sub3
6sub3

Answer 2

Of 300 million sperm ejaculated, only about 200 reach the oviduct (location of fertilization – high rate of atrition)

1. Sperm undergo capacitation in female reproduce tract
   - - Stick to the side of the uterus where they undergo maturation (surface membrane changes so that they are more responsive to the egg)
2. Sperm must breach three barriers
   - - Corona radiata
   - - Zona pellucida (jelly capsule) – fated to become fertilization envelope in response to cortical reaction
   - - Plasma membrane
3. Sperm penetrates matrix btwn follicle cells (corona radiata) via membrane bound hyluronidase
4. Receptors on sperm PM bind to glycoprotein (ZP3) on zona pellucida, triggering acrosomal reaction
   - - Often takes multiple sperm to core its way through → first sperm may not be the fertilizing one
5. Fusion of sperm–egg PM results in entire sperm entering egg; flagellum and mitochondria of sperm disintergrate
6. Sperm delivers phosphilase C, which via second messenger IP3, realeses Ca2+ from ER into cytoplasm ER triggering the following
   - - Cortical reaction: slow block to polyspermy
   - - Completion of meiosis 2 by human egg
   - - Egg activation: use mRNA to make proteins – process cell division

Question 3

Cleavage (3 ; 2sub3 ; 3sub2)

Answer 3

Defined as the repeated mitotic cell division of a zygote with little or no growth; cell cycle during cleavage is Synthesis > Mitosis > S > M … → With each division, cells (called blastomeres) will become smaller

Cleavage patterns reflect the amount of yolk stored in the zygote: three levels of yolk deposition

1. Microlecithal: slight amount → sea urchins, humans (primarily get nutrients from placenta)
2. Meso: moderate
3. Macro: enormous → reptiles

YOLK impedes cytokinesis

• • When the yolk is sparse, mitotic furrows pass successfully through entire zygote → HOLOBLASTIC CLEAVAGE (centered blastocoel)
• • When yolk is plentiful, mitotic furrow is slowed, only a portion of the cytoplasm is cleaved → MEROBLASTIC CLEAVAGE (off center blastocoel)

Question 4

Sea Urchin + Frog Example

Answer 4

Sea Urchin example: microlecithal yolk, holoblastic cleavage → blastocoel (fluid filled cavity) is centered inside the blastula

Frog example: also holoblastic BUT mesolecithal yolk

• • First two divisions along meridian between animal and vegetal pole; cleavage delayed in vegetal pole by mesolecithal yolk.
• • During third and subsequent divisions, yolk displaces towards cleavage furrow towards animal pole, resulting in more division and smaller blastomeres near animal pole → blastocoel forms entirely in animal hemisphere

Question 5

Gastrulation in frogs (3 ; all sub 1)

Answer 5

1. Gastrulation begins on the dorsal side (grey crescent; opposite where the sperm enters the egg) as cells involute over the dorsal lip into the interior, forming the blastopore
   - - Involusion: inward movement of an expanding outer layer of cells
2. Blastopore extends around embryo as more surface cells invaginate.
   - - When the ends meet, blastopore forms a circle that shrinks as ectoderm spreads downward. Continued involution expands endoderm and mesoderm inward as archenteron forms.
3. Circular blastopore (fated to become anus) surrounds a yolk-plug (remaining patch of endodermal cells on the vegetal side, created during the formation of the dorsal lip)
   - - Cells remaining on surface are ectoderm, endoderm is innermost, mesoderm is intermediate.

Question 6

Gastrulation in the chick (5)

Answer 6

1. Due to extensive yolk deposits in eggs of reptiles, birds, and monotremes, DISCOIDAL CLEAVAGE is restricted to a cap of dividing cells at an animal pole
2. Flattened blastula is two layered: upper EPIBLAST (future embryo) and lower HYPOBLAST (future yolk sac and stalk that connects yolk to embryo), with a blastocoel in between
3. Primitive streak is functionally equivalent to blastopore; will be grey in color
4. Surface cells of epiblast migrate towards primitive streak and involute inside, forming endoderm (displaces hypoblast) and mesoderm
5. Cells remaining on surface become ectoderm

Question 7

Early embryonic development of a human (3)

Answer 7

Completely devoid of yolk yet cleavage is discoidal
– Gastrulation occurs via primitive streak (similar to birds)
At the end of cleavage, human blastocyst with inner cell mass (ICM) at one end of blastocoel

Implantation initiated by trophoblast (Extraembryonic)

• • Enzymes breakdown endometrium of uterus
• • ICM forms a flat disk with an inner EPIBLAST and an outer HYPOBLAST (similar to bird but now inner and outer instead of upper and lower)

Cleavage begins.
– Extraembryonic membranes of trophoblast will combine with endometrium to begin formation of placenta

Question 8

Extraembryonic membranes in amniotes (2)

+ functional parts (4)

Answer 8

Arise from embyronic germ layers and grow to surround developing embryo

Adaptations for a terrestrial existence in a dry world → portable pond insid
– Absent for dieshes and amphibians bc don’t need to worry about egg desiccation since alrdy developing in water

FUNCTIONAL PARTS:
– Chorion: exchange respiratory gases; will interact with (something) in placenta formation
– Allantoid: sequester waste products
– Yolk sac: transport nutrients
source of embryonic blood cells + future germ cells (ie in ovaries and testes) in placental mammals
– Amnion: grows around the embryo; create a tiny aquatic environment that allows embryo to float almost weightless; ruptures when you break water prior to delivery

Question 9

Organogenesis and neurulation in frog embryos (4)

+ different structures formed

Answer 9

1. Cells from 2 or 3 germ layers interact in organ formation via induction (molecular signals that make receiving cells differentiate into a specific cell type)
2. Mesodermal notochord induces ectodermal cells thicken and flatten to form a neural plate
   Notochord: flexible, gelatinous rod that acted as a rudimentary spinal cord for muscle contraction / movement BUT had an inability to telescope
3. As neural plate sinks, will roll into neural tube, forming the basis of the central nervous system
4. Neural crest (vertebrates only) break lost from neural folds and ultimately migrate throughout the embryo to differentiate into different structures:
   a. ganglia of PNS
   b. hormone producing cells in the medulla
   c. schwann cells in the CNS
   d. most of the cartilage and bone of lower jaw
   e. odontoblasts (embryonic / immature cells that create teeth)
   f. pigment cells
   g. connective tissue and smooth muscle of heart → ultimately derived from ectoderm

Question 10

Cell development

• determination, defined
• differentiation, defined

Answer 10

Cell developmental fate not through gene loss or gain but via selective gene expression (activation of certain genes and not others)

Determination: process by which cell becomes committed to a particular fate
– Eg. Selecting your major

Differentiation: resulting in specialization in structure and function via gene expression
– Eg. Taking the classes for your major

Question 11

Mechanisms of determination (2)

Answer 11

Cytoplasmic determinants:
– Fertilization yields cleavage and molecules within cytoplasm may not be equally split → ie more of molecule A in one daughter cell than in daughter B

Induction: two tissues interact via chemical signals

• • Recipient cells will influence gene expression
• • Ie induction of neural tube by mesodermal notochord

Question 12

Levels of Differentiation (3)

Answer 12

Totipotent: very early stages of the embryo; incredibly plastic; can form any stage of the adult

Pluripotent: still plastic / flexible; inner cell mass; can form any stage of the adult minus the embryonic cell layers
– By end of gastrulation, cell fate is fixed

Multipotent: still have some options but limited to the organ in which they function
– Ie stem cells in your body → bone marrow produces red blood cells and white blood cells

Question 13

Homeotic genes (4)

Answer 13

Encode transcription factors that regulate gene expression and specific identity of body segments in the fruit fly

“Master switch” / regulatory genes control identity of body structures along anterior/posterior acis in all animals except sponges

• • Control expression of other genes in order to build body parts
• • Can determine identity of the segment

Not only are nucleotide sequences highly conserved among vertebrates and invertebrates, so is their CO LINEAR ARRANGEMENT along chromosomes → when erroneously expressed, can lead to homeotic mutants
– Eg antennapedia: legs grow where antenna are supposed to

Order of genes mimic placement of structures on an animal – Arrangement cause from cranial to caudal
– Ie head genes are first; torso is second; tail is last