Embryology

Animal Development

• Early Views of development:
• Preformation. Belief that animals formed from a progressively enlarging preformed infant.
• Successive generations were progressively contained in each generation.
• Epigenesis.  Idea that animals develop from a simple structure an a series of stages.  Early microscopy supported this, however ignored sperm & egg fusion.

Sperm & Egg

• Acrosomal Reaction
• Describes the process of fertilization of egg by sperm for aquatic animals.
• Sperm has special proteins on its tip which attach to special receptor sites on egg.
• Specificity prevents fertilization by another species
• Once attached, enzymes are released from the tip of the sperm cell called an acrosome
• These enzymes function to penetrate an outer jelly coat and membrane to allow sperm nucleus to enter the ovum.
• Cortical reaction
• Functions to prevent polyspermy (multiple fertilization)
• When ovum is fertilized, calcium ions are released into cytoplasm
• Stimulate corticle granules (calcium filled vesicles) to fuse forming a fertilization membrane preventing further fertilizations

Acrosomal & Cortical Reactions

• Activation of egg
• Once fertilized, egg metabolism is stimulated (probably through internal pH changes)


• Mammal Fertilization
• Some important differences exist
• Sperm must pass through a series of cells surrounding/protecting egg called follicle cells
• Sperm must penetrate a tough protein matrix called the zona pellucida.
• Zona pellucida then is stimulated to become impenetrable once fertilization occurs.

Mammal Fertilization

Cleavage

• Refers to a rapid succession of divisions following fertilization
• Two types of cleavage;
• Spiral - Cells divide in a diagonal/corkscrew orientation
• Radial - Cells divide on 2 planes perpendicular to each other

• Often fertilized zygote omits G₁ & G₂ phases of mitosis
• Results in numerous cells called blastomeres
• Original zygote develops into two regions or poles separated by a region called the grey crescent.
• First is called the animal pole
• Will develop into organism's tissue
• Second is called vegetal pole
• Will develop into yolk

Early stages of frog zygote cleavage

• Division contiues until embryo develops into a ball of cells termed a Morula.
• Consists of 16-64 cells

morula

• Cleavage continues until embryo enlarges an develops a hollow center.
• Known as a Blastula
• Contains a hollow portion called a blastocoel.

Blastula

GASTRULATION

• As cleavage continues, a portion of the blastula invaginates from the vegetal pole forming a Gastrula
• New cavity is formed called an Archenteron. This is the organism's primitive gut
• New opening formed is called a blastopore
• Will determine what general classification the developing animal will become
• In protostomes the blastopore will become the mouth
• In deuterostomes the blastopore will become the anus
• Results in an embryo with 3 distinct layers of cells which will develop into specific tissues
• Ectoderm - Will form skin, nervous tissue etc…
• Endoderm - Digestive tract and many other organs
• Mesoderm - Blood and other organs…

Organogenesis

• Refers to rudimentary organ formation from germ layers
• Earliest for chordates in the formation of a notochord.
• Forms from ectoderm above archenterons
• Tissue on the ectoderm invaginates inwards forming a neural plate
• Ectoderm layers connect above forming a neural crest, and isolating a neural tube
• Also form structures called somites running along its axis
• Will eventually form ribs, vertebra and associated muscles and structures

            

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Early Human Development

• Human cleavage is relatively slow in comparison to other animals ~30 hours for first division ~60 for the second.
  • Probably due to the fact that fertilization occurs in fallopian tubes of the female and zygote will not receive nutrition until it reaches the uterine wall.
• During transit cell undergoes compaction.
  • Tightly adhere to each other.
• When cell reaches ~100 cells it is called a blastocyst (not blastula), which contains an Inner Cell Mass. This is surrounded by a layer of cells called the trophoblast.
• The embryo implants itself in the uterine lining with the help of enzymes secreted by the trophoblast.
• Trophoblast will develop into the fetal portion of the placenta.
• Inner cell mass will form embryo
•  

 

Human Development:

Weeks 1-6:

• Embryo develops rudimentary organs from the various dermal layers
• Placenta begins development.

Weeks 6-8:

• Embryo develops a recognizably human appearance
• Fully functioning placenta has developed.
  • Placenta is the only disposable human organ.
  • Allows nutrients, gasses, and wastes to be transported from mother to fetus, or vice versa.
  • Circulatory systems of mother and fetus pass parallel to each other.  Materials are exchanged by diffusion.  Blood systems do not connect.

 

Childbirth:

• Signaled by the hypothalamus of the fetus.  Starts a chemical chain reaction which stimulates contractions of the mother’s uterus.
• Pressure on the uterus stimulates the cervix to release oxytocin, which stimulates further uterine contractions.
• This results in a classic positive feedback control, in which each contraction stimulates further contractions of increasing strength.
• The cervix gradually dilates to the extent that the fetus’ body can pass through, and through the vaginal canal.
• Childbirth completes with the delivery of the placenta

Stages of Birth