Topic 2 Human Reproductive Anatomy: Embryonic Development

Question 1

• Animal embryos follow four stages in growth and development: gametogenesis (sperm/egg formation), embryonic development (fertilization of egg until birth), reproductive maturity (puberty), and aging process to death. In mammals, development occurs in two stages: embryonic development followed by fetal development.
• A fetus is an embryo that resembles the human infant form, and in humans, an embryo is called a fetus at about 8 weeks.

Answer 1

Note

Question 2

1. Fertilization: sperm penetrates plasma membrane of secondary oocyte
   a. Recognition: before penetrating, the sperm secretes proteins that bind with receptors that reside on a glycoprotein layer surrounding the plasma membrane of the oocyte. In non-mammals, this layer is called the vitelline membrane. In mammals, this layer is the zona pellucida. In both organisms, the layer ensures same species fertilization.

Answer 2

1. Stages of embryonic development (sea urchin, echinoderm)

Question 3

• External glycoprotein membrane surrounding the plasma membrane (jelly coat) of an oocyte. This first appears in unilaminar oocytes, and is secreted by both the oocyte and follicular cells. At puberty, FSH stimulates growth of granulosa cells around the primary oocyte that secrete the viscous zona pellucida.
  i. When the zona pellucida binds sperm, the acrosome reaction is initiated.
  ii. The sperm releases the contents of its acrosome as it approaches the egg, and contributes to a charge- based ‘fast block’ of polyspermy.
  iii. 5 days after fertilization, the blastocyst undergoes zona hatching (zona pellucida degenerates and is replaced by the underlying later of trophoblastic cells so it can implant in the uterus)

Answer 3

2. Zona Pellucida

Question 4

• Fertilization cannot occur until capacitation and acrosomal reaction have taken place. In capacitation, secretions from the uterus’ wall and uterine tube destabilize the plasma membrane surrounding the head of the sperm (acrosome), making the head more fluid, which helps prepare it for fertilization and makes the sperm hyperactive (faster and wiggle more).
• The capacitated sperm moves through the corona radiata (dense layer of granulosa cells surrounding the oocyte) and comes into contact with the zona pellucida. The zona pellucida expresses specific receptor proteins called ZP3, which bind to proteins expressed in the head of the sperm. The binding of ZP3 triggers the acrosome reaction, during which the enzymatic contents of the acrosome are released.
• These enzymes help digest a path through the zona pellucida, allowing the sperm to enter the perivitelline space (space between the plasma membrane of the secondary oocyte and the zona pellucida), which then fuses with the oocyte’s plasma membrane.
• To ensure only one sperm penetrates the zona pellucida and fuses with the oocyte membrane, this fusion activates a fast block and a slow block to polyspermy. First, during the fast block, which takes place after fusion, the oocyte membrane depolarizes, preventing other sperm from fusing with it. Slow block to polyspermy is then stimulated by this depolarization — during slow block to polyspermy, a wave of intracellular calcium is released, causing small cortical granules beneath the oocyte membrane to release their contents outward, rendering ZP3 in the zona pellucida inactivate and making it impermeable. Note that intracellular Ca2+ triggers cortical granule release, but the granules themselves don’t contain Ca2+.
• In non-mammals, the zona pellucida is called vitelline membrane, and it plays an important role in preventing cross-breeding of different species, especially in species where fertilization occurs outside of the body. The vitelline membrane is also commonly used to control wildlife population via immunocontraception. When the vitelline membrane of one animal species is injected into the bloodstream of another, sterility of the second animal occurs due to an immune response. Fertilization cannot occur because antibodies have already bound to the vitelline membrane, thus preventing sperm from binding.
• Fertilization can be external in water (lots of eggs laid since the chance of fertilization is lower - frogs/amphibians) or internal (terrestrial vertebrates). The # of eggs laid is affected by the following factors: internal vs external fertilization, early development, and amount of parental care (less care = more eggs)

Answer 4

Note

Question 5

• Plasma membrane of sperm and oocyte fuse, and the sperm nucleus enters the oocyte

Answer 5

3. Penetration

Question 6

• The vitelline layer forms a fertilization membrane that blocks additional sperm (remember, this is due to the cortical reaction, which is the exocytosis of enzymes produced by cortical granules in egg cytoplasm during fertilization → called slow block in mammals)

Answer 6

4. Formation of fertilization membrane

Question 7

• Sperm penetration triggers meiosis II to complete. The oocyte was previously arrested in metaphase in humans. The result is an ovum and a second polar body, which is discharged through the plasma membrane

Answer 7

5. Completion of meiosis II in secondary oocyte

Question 8

• Sperm and ovum nuclei fuse → diploid zygote forms. This is associated with a sharp increase in protein synthesis and metabolic activity.

Answer 8

6. Fusion of nuclei and replication of DNA

Question 9

• Fertilization takes place in the oviduct (fallopian tubes); cleavage while swept; and the embryo is at the blastula stage by the time it reaches the uterus for implantation

Answer 9

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