Chapter 47 Animal Development p. 998

Embryology

Epigenesis: theory that the form of an embryo gradually develops from an egg

Preformation- the belief that the egg or sperm contains a miniature embryo was once the favored theory.

Read pages 1002-1008

 

Embryonic Development

            3 Key processes of Embryonic Development

1.      Cell Division- Production of large number of cells

2.      Differentiation- formation of specialized cells that make up tissues and organs.

3.      Morphogenesis- the movement of cells and tissues to produce body shape and form

 

 

Fertilization is a genetic and developmental event.

·        Combines the haploid sets of chromosomes and activates the egg by initiating metabolic reactions that trigger embryonic development.

 

The Acrosomal Reaction

·        P. 1000 Figure 47.2

·        The Acrosome is located at the tip of the sperm and discharges hydrolytic enzymes when it comes in contact with the jelly coat of an egg

 

·        Fertilization within the same species is assured when a protein on the surface of the acrosomal process attaches to specific receptor molecules on the vitelline layer of the egg

 

·        Fusion of the egg and sperm open Na⁺ ion channels, flooding the egg.

·        Depolarization of the membrane prevents other sperm cells from fusing with the egg, called fast block to polyspermy

 

Cortical Reaction

·        Membrane fusion also starts the Cortical Reaction

·        Ca+ ions flood the egg causing vitelline layer (outer most layer of the egg) harden; prevents additional sperm from entering; slow block to polyspermy

 

Protein synthesis and development in early embryos is directed by maternal mRNA

• After the sperm nucleus fuses with the egg nucleus, DNA replication begins in preparation of Cleavage division (begins development of the embryo)

           

Development of the embryo begins

 

Cleavage

• Figure 47.6  p. 1003
• succession of rapid cell divisions during which the embryo becomes partitioned into many cells called Blastomeres

 

• converts the embryo from a single large cell to many smaller cells that can operate independently

 

• The axis of the egg is defined by the vegetal pole (where the stored nutrients in yolk are most concentrated
• The opposite end is called the animal pole.

 

• cell does not grow in size; forms a solid “ball” of cells called the Morula ( a solid ball of 16 blastomeres) p. 1004 Figure 47.8 a-d

 

• Cells of the Morula arrange into a hollow ball by pressing against each other. This forms the Blastula (blastocoel- the fluid filled center)

 

• The large amount of yolk in the egg of birds’ results in Meroblastic cleavage restricted to the small disc of the cytoplasm in top of the yolk.

 

• Holoblastic cleavage is the complete division of eggs with little amounts of yolk

 

 

 

 

 

 

• Embryos differ in rates of division --depends upon
• Species
• Amount of yolk in egg
• Mammals divide to form a Blastocyst (name of embryo at the time of implantation)
• Cleavage in mammals occurs slowly, each division taking 12 to 24 hrs

 

• The Blastocyst contains 2 types of cells
• Trophoblast cells attach the embryo to the uterine wall and become part of the placenta

 

• The remaining cells become the embryo

 

Gastrulation (Germ Layer formation)

• Zygotes begins to change its shape.
• Blastula (or Blastocyst) invaginates forming a 2-layered cup. Inside of the cup is called the Archenteron (future gut)

 

• Cells migrate into the blastocoel and differentiate to form 3 Germ Layers (the movement and specialization of cells to form functioning organs)

 

• Cells move along Primitive streak (through a groove in the streak) eventually creating the germ layers.

 

 

3 Germ Layers

1.      Ectoderm: produces epidermis (skin), brain and spinal cord (nervous system) and eyes; hair and nails; secretory cells of sweat gland; enamel of teeth

2.      Endoderm: form the inner linings of digestive and respiratory tracts. Gives rise to accessory organs, pancreas, gall bladder and liver ( all are offshoots of digestive tract); lining of urinary bladder; thyroid and parathyroid glands.

3.      Mesoderm: everything else (bones, muscles, excretory, circulatory and reproductive systems

4.      Organogenesis- process of developing organs.

 

 Neurulation (Neurula)

• Begins with the formation of Notochord and neural tube
• The notochord will become the brain the neural tube will become the spinal cord….formation of the nervous system.
• The notochord is a supportive chord running from head to tail. The “organizer” for further embryonic development.

 

• Earliest organ formed (neural tube); arises from the ectoderm. Optic vesicles also rise from the neural tube.

 

• Above notochord, ectoderm rearranges to form the nervous system
• Cells along central line form Neural Plate
• The Neural plate gives rise to the Neural Tube; precursor of the Central Nervous System (CNS is the brain and spinal cord)

 

• Cells on the surface become skin; those that connected the neural tube to the surface (Neural Crest Cells) migrate away giving rise to other types of cells.

 

• Neurulation- the process of forming the neural tube and neural crest cells.

 

 

Extraembryonic membranes

In addition to primary germ layers some Animals have extraembryonic membranes.

• Internal Development (mother supplies nutrients)
• External Development (offspring develop outside the mother)

 

• Amnion- encases embryo in a fluid filled sac
•  prevents dehydration and cushions shock.

 

• Allantois- Stores metabolic wastes, gas exchange;
• precursor to urinary bladder and umbilical cord (connects embryo to placenta)… Allantois is a receptacle for uric acid wastes (nitrogenous waste)

 

• Chorion- works with Allantois in gas exchange;
• Produces HCG (human choronic gonadotropin) which initiates production of hormones that support pregnancy
• Estrogen and Progesterone (eventually placenta becomes responsible for hormone production)

 

• Yolk Sac- expands over yolk; provides stored nutrients for embryo
• Produces blood cells which then migrate into embryo
• Found in mammals birds and lizards; placental mammals contain least amount. In mammals the major function of the yolk sac is to produce blood cells.         

 

Stages of Development in order:

Zygoteŕ CleavageŕBlastulaŕGastrulaŕNeurulaŕEmbryoŕFetus

 

Programmed cell death contributes to normal development

• Development requires extensive “programmed cell death” called APOPTOSIS

 

·        Cells die quickly, are engulfed by phagocytes, and are digested without a trace

 

·        In vertebrates, programmed cell death provides for normal development of the nervous system, immune system, and body parts such as fingers and toes.

Primitive Streak and Determination p. 1009

 

Gestation (270 days in humans)

• Fallopian tube- location of fertilization
• Relatively early in development, each organ appears as a recognizable rudiment (initial stage) from which the final form will develop.
• In human, all organs are in place by the end of nine weeks, and the embryo is called a fetus.
• First Trimester
• 5^th Day-
• Blastula stage begins
• formation of placenta begins
• chorion, in birds, begins formation
• 7^th Day-
• Implantation- embryo buries itself into the inner lining of uterus.
• Formation of extra-embryonic membranes
• All major systems formed ( may not be operational)
• 2^nd week; mesoderm forms and vertebrae develop begins
• 2^nd month; human appearance (fetus)
• 9^th week: rudiments of all major organs are present; only the heart is functional; legally a fetus at this time.
• 9 to 12 weeks: the fetus doubles in length, external genitalia appear and start to develop, and kidney excrete their first urine.
• Second Trimester
• 12 to 16 weeks ovaries are differentiated and primary follicles contain oogonia
• 17 to 20 weeks: mother feels the first fetal movements
• the fetus is about 10 inches long.
• 22 weeks: Lungs, intestines and kidneys are sufficiently developed (could live outside of womb with intensive medical care)
• 24 weeks: Lungs begin to secrete surfactant, which allows them to fill with air.
• Third Trimester
• 26 weeks: Prematurely born fetus is likely to survive because the respiratory system is developed and functional
• 26 to 36 weeks: Increases body mass and size

 

 

 

 

 

A.     ectoderm

B.     omit

C.     endoderm

D.     Archenteron (primitive gut)

E.      Blastopore (anus)

F.      Archenteron

G.     Ectoderm

H.     Mesoderm

I.        Endoderm

J.       Dorsal lip

K.    Yolk plug (same as Blastopore; becomes anus)

 

 

 

A.     neural tube (CNS

B.     Neural crest (peripheral nervous system; skin; muscles)

C.     Somite (segmented vertebrae and muscles associated with vertebra)

D.     Archenteron

E.      Coelom (body cavity)

F.      Notochord (brain)

 

 

 

A.     Cleavage

B.     Blastula

C.     Animal pole

D.     Blastocoel

E.      Vegetal pole

 

 

A. Aminion