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UCALGARY / Biology / BIOL 305 / What is the 32 cell stage?

What is the 32 cell stage?

What is the 32 cell stage?


Lectures 7-10

What is the 32 cell stage?

6 -24 Hours After Intercourse

• Several hundred haploid sperm reach the upper third oviduct • Acrosome (head of sperm) releases digestive enzymes that penetrate  the egg

◦ Sperm proteins lock with egg membrane receptors

◦ Triggers the completion of meiosis II in the secondary oocyte  leading to the creation of a haploid oocyte

• Fertilization: one sperm's nucleus fuses with the ovum nucleus, forming a diploid zygote  

Four Days After Fertilization

• Morula forms: ball of 32 identical cells

◦ Occurs through the process of cleavage: division into identical  cells

◦ Occurs within the oviduct

• Growth of cells occur

◦ Single cell at fertilization > trillion cells at birth

How is the bilaminar embryonic disc formed?

◦ Increase in size of cells

6-7 Days After Fertilization: Embedding into Endometrium • Formation of Blastocyst: hollow ball with inner cell mass and water  tight cavity (blastocoel)

• Differentiation: cells begin to take up different identities (Eg. Heart,  brain)

• Morphogenesis: Changes in shape and form to fit a "human" shape END OF PRE-EMBRYONIC PERIOD

9 Days Post-Fertilization

• Embryonic disc develops  

15 Days Post-Fertilization

• Gastrulation: formation of trilaminar disc from bilaminar disk ◦ Bilaminar germ disc is made up of epiblast and hypoblast layers:  the entire embryo comes from the epiblast We also discuss several other topics like What are the basic operations performed by the algorithm that you would count towards its run time?

◦ Endoderm + Mesoderm form from gastrulated (travelled to  primitive streak) cells  

What is chronic obstructive bronchitis?

◦ Ectoderm forms from the cells that do not ingress

Tissues and Organs Derive from the Three Germ Layers

• Endoderm: outermost layer

◦ Becomes epidermis, nervous system (including brain), hair, nails, tooth enamel, parts of eye

• Mesoderm: middle layer

◦ Becomes heart, muscle, bone (including vertebrae), kidneys,  uterus, testes, ovaries, somites If you want to learn more check out What are the ideal characteristics of a victim?

• Ectoderm: innermost layer

◦ Becomes digestive tract, liver, pancreas, lungs, bladder, urethra,  vagina, prostate

Primitive Streak

• Tells the anterior and posterior ends of the embryo

• Node --- signals --- nervous system (brain & spinal cord) to form at  dorsal side of embryo

◦ Node also determines left-right symmetry

• As development proceeds the primitive streak gets shorter (towards  the posterior) as the nervous system begins to form


• Reverse left-right asymmetry (50% of the time) with chronic bronchitis  + male sterility

• At approx. 3 weeks: moving cilia on primitive streak causes flow of fluid to the left side which activates genes

◦ Mutation in cilia --- cilia does not move and organs end up on  opposite side

• Can result in  

◦ situs inversus: when every single organ is switched

• 25% of people with this have kartangener syndrome (with  chronic bronchitis)

◦ Random L/R symmetry (not all organs are switched) Don't forget about the age old question of What is scm (supply chain management)?

• Can be fatal

19 or 20 Days Post-fertilization

• Somites begin to form

◦ Type of tissue that is forms on each side of the neural tube ◦ Formed from the mesoderm

◦ Establishes the sequence of the body (head/anterior to  tail/posterior) Don't forget about the age old question of The sun is composed of what?
If you want to learn more check out What is dry matter?

◦ Formation finishes by day 30

◦ Somites divide into different population to make different parts of the body

• Three major populations

▪ Sclerotome: vertebrae and ribs (cartilage then bone)


• Migrate around neural tube to form the vertebrae and the  ribs

▪ Myotome: skeletal muscles (excluding some muscles of the back)


• Myotome formed

• Myoblasts (parts of myotome) migrate out to different  

locations of the body to form skeletal cells

• Myoblasts differentiate into muscle cells as they travel to  their final destination

▪ Dermatome: dermis of the back

◦ 37 somites in total

• 4 are used to make the occipital bone (part of the skull) • 33 become the 33 vertebrae

22 Days Post-Fertilization (week four)

• Embryo folds into a 3D structure

• Most organs begin to form (differentiation)

• Heart looping: the heart begins to fold into from a tubular structure  into a 3D shape containing the four major chambers Don't forget about the age old question of What are the four stages for forming a caldera?

◦ two atria on top and two ventricles below

• Relocation of atria from posterior to anterior

◦ Looping process finished by day 23, heart formation finished by  day 28

◦ Bilateral origin : cells that form the heart come from both sides of the primitive streak


• the original neural tube is one cell layer thick and is called germinal  neuroepithelium composed of neural stem cells

• The neocortex is responsible for all conscious thought, self-awareness,  communication, sensory perception, recognition and motor commands • Brain development in humans continues after birth, unlike chimps

28 Days Post Fertilization

• Neural tube finishes forming from the neural plate

• Neural plate forms -- section of the tube that "pinches off" ◦ Will become the brain and spinal cord

Developmental Defects of the Day: Neural Tube Defects • Anencephaly: caused by the failure of the anterior neuropore to close  (fatal)

• Craniorachischisis: failure of the entire neural tube to close (FATAL) • Spina bifida: failure of the posterior neuropore to close; symptoms  range from severe to mild to none

• Preventable by the consumption of folic acid before pregnancy EMBRYO CONSIDERED A FETUS AFTER WEEK 8

Human characteristics begin to appear

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