Biology Chapter 23 Notes Prof. Sen
Biology Chapter 23 Notes Prof. Sen Biol 112
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This 4 page Class Notes was uploaded by Anne Notetaker on Wednesday March 16, 2016. The Class Notes belongs to Biol 112 at College of Charleston taught by Dr. Senn in Spring 2016. Since its upload, it has received 29 views. For similar materials see General Biology: Evolution, Form, and Function in Biology at College of Charleston.
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Date Created: 03/16/16
Chapter 23: An Introduction to Animal Development -Gametes are haploid reproductive cells -male gametes are called sperm female gametes are called eggs -Development proceeds in ordered phases through an animal’s life cycle ~Fertilization ~Cleavage ~Gastrulation ~Organogenesis Steps of Fertilization Fertilization occurs when a haploid sperm and egg fuse, forming a diploid zygote (a fertilized egg) 1. Sperm are drawn to the egg 2. The head of the sperm binds to the jelly layer of the egg 3. Triggers acrosome reaction, which releases sperm contents 4. Flagella activity ramps up 5. Sperm penetrates egg coat 6. Membrane fusion of sperm and egg 7. Sperm nucleus enters egg Gametes from the Same Species Recognize Each Other -Sea urchins are a model system for studying fertilization: they produce large numbers of gametes and undergo external fertilization ~Bindin is a protein on the head of sea urchin sperm that binds to the surface of sea urchin eggs ~Bindin acts as a “key”-sperm binds only to eggs of the same species ~During sea urchin fertilization, species-specific bindin molecules on sperm interact with species-specific receptors on the surface of the egg Why Does Only One Sperm Enter the Egg? -Animals employ different mechanisms to avoid polyspermy- fertilization by more than one sperm -In sea urchins, fertilization stimulates the creation of a physical barrier: ~After fertilization, a Ca2+ based signal is rapidly induced and propogated throughout the egg. ~A fertilization envelope forms, which keeps away additional sperm Cleavage Cleavage is a set of rapid cell divisions that take place in animal zygotes immediately after fertilization -Cleavage is the first step in embryogenesis, the process that makes a single-celled zygote into a multicellular embryo -Cleavage partitions the egg cytoplasm without any additional growth of the zygote -The cells created by cleavage divisions are called blastomeres -When cleavage is complete, the embryo consists of a mass of blastomere cells called a blastula What Role do Cytoplasmic Determinants Play? -Cytoplasmic determinants are found in specific locations within the egg cytoplasm. ~they end up in specific populations of blastomeres -Cleavage initiates the step-by-step process that results in the differentiation of cells Cleavage in Mammals -Cleavage occurs in the mammalian oviduct: this connects the ovary, where the egg matures, to the uterus, where the embryo develops -Cleavage results in a blastocyst, a specialized blastula consisting of two populations of cells ~the external, thin-walled, and hollow trophoblast surrounds the inner cell mass (ICM) Fate of the Trophoblast and ICM -After the blastocyst embeds in the uterine wall, a mixture of trophoblast and maternal cells form the placenta ~placenta provides nourishment and waste removal for the developing embryo -The ICM contains the cells that under gastrulation and develop into the embryo. Gastrulation -Gastrulation results in the formation of the embryonic tissue layers ~a tissue is an integrated set of cells that function as a unit -Most early embryos have 3 primary tissue layers: ectoderm, mesoderm, and endoderm -These embryonic tissues are called germ layers because they give rise to adult tissues and organs Gastrulation in Frog Embryos -The frog blastula contains a fluid-filled space called the blastocel -Gastrulation begins with the formation of an opening called a blastopore -Cells from the periphery move inward through the blastopore, forming a tube-like structure that will become the gut Germ Layers -Ectoderm: forms the outer covering of the adult body and nervous system -Mesoderm: gives rise to muscle, most internal organs, and connective tissues such as bone and cartilage -Endoderm: produces the lining of the digestive tract or gut, along with some of the associated organs The Completion of Gastrulation -Another outcome of gastrulation is that the major body axes become visible -At the end of gastrulation: ~the 3 embryonic tissues are arranged in layers ~the gut has formed ~the major body axes have become visible Organogenesis -Organogenesis is the process of tissue and organ formation -During organogenesis, cells proliferate and become differentiated, meaning they become a specialized cell type. ~Differentiated cells have a distinctive structure and function because they express a distinctive suite of genes Organizing the Mesoderm into Somites -Early in organogenesis, the rod-like notochord appears in the dorsal mesoderm (muscle, internal organs, bone and cartilage) -This structure is unique to the animal group called the chorodates, which includes humans and all other vertebrates -The notochord functions as a key organizing element during organogenesis ~in many chordates, as organogenesis continues, the notochord cells undergo apoptosis Neural Tube Formation -Signals from the notochord trigger reorganization of the dorsal ectodermal cells, leading to neural tube formation ~the neural tube is a precursor to the brain and spinal cord Somite Formation -Once the neural tube forms, the mesodermal cells become organized into blocks of tissues called somites ~they form on both sides of the neural tube down the length of the body The Process of Determination -In the process of determination, somite cells differentiate in response to signals from nearby tissues -These signals diffuse away from cells in the notochord, the neural tube, and nearby ectoderm and mesoderm to act on specific populations of target cells in the somite.
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