Lecture Guide

• Developmental biology, traditionally known as embryology, studies the growth and development of organisms from zygote formation to the formation of a full embryo
• Ontogeny refers to the development of the individual, while phylogeny relates to the historical development of the species or evolutionary biology
• Phases of ontogenetic development include gametogenesis, fertilization, cleavage, gastrulation, organogenesis, growth, differentiation, morphogenesis, birth or hatching, and maturation/maturity
• Descriptive embryology involves direct observation and description of embryological development, while comparative embryology looks at similarities and differences in the development of animals
• Experimental embryology examines the cellular control of development, chemical embryology studies developmental stages using biochemical and physiological techniques, and teratology focuses on the study of malformations
• Reproductive biology deals with problems of conception and contraception
• Theories of development include the preformation theory, which posited that reproductive cells contained preformed adult individuals, and the epigenesis theory, which suggests the successive generation of new structures from pre-existing ones
• Gametogenesis involves the production of sperm in the testes (spermatogenesis) and egg formation in the ovaries
• The sperm consists of a head with a haploid nucleus and acrosome, a neck/midpiece with mitochondria and a tail for propulsion
• The ovum stores materials for growth and development, conserves and acquires more cytoplasm as it matures, and synthesizes and stores proteins like yolk for the developing embryo
• Spermatogenesis is the process by which sperm cells are produced in the testes, while oogenesis is the process by which egg cells are produced in the ovaries

  
• Spermatogenesis:

  • Occurs in the testes
  • Begins between ages 11 to 15 and continues until death
  • About 100 to 200 million sperm are made a day
  • Meiosis occurs, resulting in four non-identical cells, each with 23 chromosomes that will develop into sperm

  
• Steps in Sperm Formation:

  1. Spermatocytogenesis: spermatogenic cells form spermatids

  2. Spermiogenesis: spermatids differentiate into specialized cells known as sperm

  • Nuclear condensation, acrosome formation, flagellum formation, cytoplasmic reduction

  
• Oogenesis:

  • Occurs in the ovary
  • Starts in the fetus at around 7 weeks' gestation
  • Only about 400 to 500 oocytes will be released during a woman's reproductive years

  
• Phases of Oogenesis:

  • Primary oocytes have already entered meiosis I but are arrested
  • Meiosis resumes in one oocyte at a time, starting with the first menstrual cycle
  • Completion of meiosis results in the formation of a secondary oocyte and polar bodies

  
• Fertilization is the union of the sperm cell and egg cell, restoring the diploid number for a zygote

  
• Hazards of External Fertilization:

  • Eggs and sperm may not meet
  • Eggs may be eaten by predators
  • Environmental conditions may change

  
• Conditions for Fertilization:

  • Sperm and eggs have a short lifespan and must be released within a short period of time
  • There must be a fluid medium for sperm to swim to the egg

  
• Adaptations to Stimulate Mating Process:

  • Mating calls, special feathers or color patches, pheromones

  
• Internal Fertilization:

  • Occurs inside the body, specifically in the ampulla of the fallopian tube or oviduct

  
• Ectopic pregnancy is a pregnancy that occurs outside of the uterus, with the most common type being ampullary, which occurs in the fallopian tube

  
• Fertilization can occur in other parts of the uterine tubes if the oocyte is not fertilized in the ampullary region
• For fertilization to occur, coitus must happen no more than two days before ovulation or 24 hours after ovulation
• Steps in fertilization:

  1. Penetration of corona radiata

  2. Attachment to and penetration of zona pellucida
• Acrosome reaction: sperm bind to ZP proteins in the zona pellucida, releasing enzymes that allow the sperm to burrow through and fertilize the egg
• Fertilization facts:

  • Completed within 24 hours of ovulation
  • Approximately 400 to 600 million sperm are deposited at the cervical opening during ejaculation
  • Only about 200 sperm reach the fertilization site, with most degenerating and being absorbed by the female genital tract
• Cleavage is a series of mitotic divisions that transform the fertilized egg (zygote) into a multicellular embryo
• Classification of eggs based on the amount of yolk:

  • Alecithal: negligible amount of yolk or no yolk (e.g., mammals)
  • Microlecithal: small amount of yolk (e.g., amphioxus)
  • Mesolecithal: moderate amount of yolk (e.g., amphibians)
  • Megalecithal or polylecithal: large amount of yolk (e.g., birds and reptiles)
• Spermatogenesis is the process by which sperm cells are produced in the testes

  
• Oogenesis is the process by which egg cells are produced in the ovaries

  
• Table comparing spermatogenesis and oogenesis

  
• Ectopic pregnancy is a pregnancy that occurs outside of the uterus, with types including ampullary, isthmic, cornual, abdominal, fimbrial, ovarian, and cervical

  
• The acrosome reaction is depicted in the image

  
• The process of fertilization is illustrated in the image

  
• The image shows 4 different types of eggs based on the distribution of the yolk

  
• Centrolecithal cleavage is a type of superficial cleavage

  
• The image illustrates the process of twinning

  
• Position of testis in various species of animals

  1. Elephant - just behind the kidney
  2. Dolphins and whales - posterior end of the abdominal cavity
  3. Hedgehog - ventral abdominal wall
  4. Mole, shrew - at the base of the tail
  5. Pigs, rats, horses, rabbit, cats and dogs (non – pendulous) - scrotum underneath the anus
  6. Sheep, cattle and other ruminants - pendulous scrotum
• Parts of the sperm

   1. Head

  •       Haploid nucleus

  •       Little cytoplasm

  •       Acrosome

  2. Neck/Midpiece

  •       Mitochondria

  •       Centriole

  3. Tail (or propulsion system)

  •       Some species - ameboid motion

  •       Most sperm are propelled by flagella

  •       Formed by microtubles