Animal Development Lecture Notes

43.1 Reproduction Methods and Development

Learning Objectives

• Describe advantages and disadvantages of asexual and sexual reproduction
• Discuss sexual and asexual reproduction methods
• Explain the general process of development in animals

Asexual Reproduction

• Some animals usually reproduce asexually but may practice sexual reproduction occasionally.
• Asexual reproduction produces genetically identical offspring, advantageous in stable, uniform environments.

Sexual Reproduction

• Egg of one parent fertilized by sperm of another.
• Produces genetically variable offspring, which is advantageous in patchy, variable environments.
• Fertilization can be external (e.g., fish, frogs) or internal (e.g., salamanders, reptiles, birds, mammals).
• Animals usually produce gametes in specialized organs called gonads.
  • Testes produce sperm.
  • Ovaries produce eggs.
• Most animals are dioecious, meaning they have separate sexes.
• Some animals are monoecious (hermaphroditic), containing both male and female sex organs in a single body.
  • The majority practice cross-fertilization with other individuals.

Development

• Development includes all genetically-controlled changes in an individual organism from fertilization until death.
• Sexually reproducing animals start from a fertilized egg (zygote).
• Tissues and organs develop from the zygote based on genetic instructions in the zygote DNA.
• Cell determination begins when certain genes are activated or inactivated as cells become gradually committed to a certain pathway.
• Cell differentiation results from cell determination, as gene regulation increasingly commits cells to their biochemical and structural forms.
• Organisms of different species end up with different morphology and physiology because of genetically-programmed differences in development.
• Different individuals also end up with different morphology and physiology due to genetically-programmed differences in development.
• The zygote DNA has all the genetic information to control the differentiation of cells during development.
• All the somatic cells in the body retain all this genetic information.
• Cells and tissues become differentiated because they express different genes at different times.

43.2 and 43.6 Fertilization and Early Embryonic Development

Learning Objectives

• Describe sperm morphology
• Discuss how fertilization occurs
• Discuss the role of cleavage and gastrulation in animal development

Sperm Morphology

• Acrosome: organelle that contains enzymes that help penetrate the egg.
• Nucleus: contains a random subset of the father’s nuclear DNA.
• Mitochondrion: provides energy for the sperm (but the sperm mitochondrion does not enter the egg).
• Tail: for movement; an extension of the plasma membrane and cytoplasm.

Fertilization

• Sperm and egg fuse to form a zygote.
• Restores the diploid chromosome number.
• Produces a new mixture of genetic information from both parents.
• In many species, determines the sex of the offspring based on genetic information on sex chromosomes.
• Initiates the process of development.
• Egg membrane contains recognition proteins so the egg will be recognized by sperm from the same species. This is particularly necessary in animals with external fertilization.
• The egg (or ovum) is often surrounded by outer layers that protect the egg and help ensure fertilization by only one sperm of the proper species.
• Acrosomal enzymes in the sperm digest these layers.

Development Stages

• The embryo passes through several stages:
  • Cleavage
  • Blastula
  • Gastrula
  • Neurula
  • Organogenesis
• The details of animal development vary in different animal groups.
• The amount and distribution of yolk impact the processes of development.
  • Birds and reptiles have telolecithal eggs with a large amount of yolk taking up most of the egg.
  • Amphibians have mesolecithal eggs with a moderate amount of yolk.
  • Mammals and some invertebrates (such as sea urchins and starfish) have microlecithal eggs with minimal yolk.

Development in Echinoderms

• During cleavage, the zygote rapidly divides by mitosis into multiple cells without increasing in size.
• After several cell divisions, the embryo becomes a solid sphere of cells called a morula.
• The cells of the morula rearrange themselves to form a hollow blastula, with a fluid-filled cavity called the blastocoel.
• The blastula goes through the process of gastrulation to form the gastrula.
• The blastopore will become the first opening to the digestive tract (becomes the anus in vertebrates and echinoderms; the mouth in other invertebrates).
• Note that the blastopore is found in the gastrula, not in the blastula.
• Gastrulation is the beginning of the digestive tract (the primitive gut or archenteron).
• It is also the beginning of the three germ layers (ectoderm, mesoderm, and endoderm) that will develop into specific tissues and organs in the adult animal.

Germ Layers

• Specific organs develop from each layer.
• The cleavage seen in organisms with small amounts of evenly-distributed yolk is equal and holoblastic.
  • Equal = the cells are approximately the same size.
  • Holoblastic = the cells are completely separated from one another.

Cleavage in Amphibians

• Amphibians have mesolecithal eggs with holoblastic but unequal cleavage.
• The smaller cells of the “animal pole” have little yolk.
• The larger cells of the “vegetal pole” have more yolk.

Cleavage in Reptiles and Birds

• The nucleus and cytoplasm are restricted to the germinal disc at the animal pole.
• The yolk never divides—cell division occurs in the germinal disc.
• Telolecithal eggs of reptiles and birds have large amounts of yolk at the vegetal pole and a small amount of cytoplasm at the animal pole.
• Reptiles and birds have meroblastic and unequal cleavage.
• Cleavage occurs only in the germinal disc.

Gastrulation in Amphibians

• In amphibians, large yolk-filled cells obstruct inward movement at the vegetal pole.
• Cells from the animal pole migrate inward from the dorsal lip to form the endoderm and mesoderm.

Gastrulation in Birds

• In birds, development is restricted to the germinal disc.
• Cells of the epiblast migrate to the midline to form the primitive streak, the equivalent of the blastopore.
• Cells then move inward to form the mesoderm and endoderm.

43.7 Organogenesis and Vertebrate Embryos

Learning Objectives

• Describe the processes of neurulation and organogenesis
• Describe the structure and function of the extraembryonic membranes of amniote vertebrates (see section 29.4 in textbook)

Neurulation

• The central region of the ectoderm forms the neural tube, which gives rise to the brain and the spinal cord.
• An embryo of this stage is called a neurula.

Pattern Formation and Morphogenesis

• During development, differentiated cells become progressively organized into a multicellular animal.
• Morphogenesis proceeds through the process of pattern formation.
• Morphogenesis: shape/forming.
• Pattern formation locates cells in different regions with different signals (differentiation).
• Cells become progressively organized into different cell types, tissues, organs, etc.

Organogenesis

• Organogenesis is the development of organs and structures from the three original germ layers.

Germ Layers

• Ectoderm:
  • Nervous system
  • Sense organs
  • Outer skin layer (epidermis)
  • Nails, feathers, hair
• Mesoderm:
  • Skeleton
  • Muscles
  • Circulatory system
  • Blood
  • Excretory system
  • Reproductive system
  • Inner skin layer (dermis)
  • Muscular portion of the digestive tract
• Endoderm:
  • Inner lining of the digestive tract
  • Inner lining of the respiratory system
  • Liver and thyroid

Extraembryonic Membranes

• Amniotes (reptiles, birds, and mammals) produce an amniote egg that keeps the embryo moist on dry land.
• The amniote egg contains extraembryonic membranes that surround and support the embryo.
• Some of the extraembryonic membranes in mammals are modified to form the placenta, which connects the embryo to the mother and facilitates necessary transfer of oxygen, carbon dioxide, food, waste, and other substances between them.
• Four membranes: chorion, amnion, allantois, and yolk sac:
  • Chorion (outermost membrane): encloses the entire embryo and other membranes; major organ of gas exchange.
  • Amnion: encloses the embryo; secretes protective amniotic fluid that fills the amniotic cavity between the embryo and the amnion; also acts as a shock absorber (amniotic fluid can be analyzed for biochemical or chromosomal abnormalities).
  • Allantois: In reptiles and birds, it stores nitrogenous wastes; in mammals, its blood vessels contribute to the formation of umbilical vessels joining the embryo to the placenta.
  • Yolk sac: Encloses the yolk, slowly digests it, and makes it available to the embryo.