Vertebrate Embryology

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Vertebrate Embryology

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The study of the development of a unicellular zygote into a complete multicellular organism. In 9 months, a unicellular human zygote undergoes cell division, cellular differentiation, and morphogenesis in preparation for life outside the uterus.

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Fertilization

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An egg can be fertilized within 12-24 hours following ovulation. Fertilization occurs in the lateral, widest portion of the oviduct when sperm traveling from the vagina encounter an egg. If more than one egg is fertilized, fraternal twins may be conceived.

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Cleavage

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A series of rapid mitotic divisions. These divisions lead to an increase in cell number without a corresponding growth in cell protoplasm. Thus, cleavage results in progressively smaller cells, with an increasing ratio of nuclear-to-cytoplasmic material. It also increases the surface-to-volume ratio of each cell, thereby improving gas and nutrient exchange. An indeterminate cleavage is one that results in cells that maintain the ability to develop into a complete organism. Identical twins are the result of an indeterminate cleavage. A determinate cleavage results in cells whose future differentiation pathways are determined at an early developmental stage. Differentiation is the specialization of cells that occurs during development. The first complete cleavage of the zygote occurs around 32 hours after fertilization. The second cleavage occurs after 60 hours, and the third around 72 hours, at which point the 8-celled embryo reaches the uterus. As cell division continues, a solid ball of embryonic cells,

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Gastrulation

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Once implanted in the uterus, cell migrations transform the single cell layer of the blastula into a three-layered structure called a gastrula. These three primary germ layers are responsible for the differential development of the tissues, organs, and systems of the body at later stages of growth. Ectoderm: integument (including the epidermis, hair, nails, and epithelium of the nose, mouth, and anal canal), the lens of the eye, the retina, and the nervous system. Endoderm: epithelial linings of the digestive and respiratory tracts (including the lungs), and parts of the liver, pancreas, thyroid, and bladder lining. Mesoderm: musculoskeletal system, circulatory system, excretory system, gonads, connective tissue throughout the body, and portions of digestive and respiratory organs.

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External Development

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Fish and amphibians lay eggs that are fertilized externally in the water. The embryo develops within the egg, feeding on nutrients stored in the yolk. Reptiles, birds, and some mammals develop eternally on land. Fertilization occurs internally and the fertilized egg is then laid. Eggs provide protection for the developing embryo. The eggs also include the following embryonic membranes: Chorion: lines the inside of the shell and is a moist membrane that permits gas exchange. Allantois: A sac-like structure that is involved in respiration and excretion and contains numerous blood vessels to transport O2, CO2, water, salt, and nitrogenous wastes. Amnion: A membrane that encloses the amniotic fluid. Amniotic fluid provides an aqueous environment that protects the developing embryo from shock. Yolk Sac: The yolk sac encloses the yolk. Blood vessels in the yolk sac transfer food to the developing embryo.

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Non-Placental Internal Development

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Marsupials and some tropical fish, develop in the mother without a placenta. Without a placenta, exchange of food and oxygen between the young and the mother is limited. The young may be born very young.

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Placental Internal Development

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The placenta and the umbilical cord are outgrowths of the four extra-embryonic membranes formed during development: the amnion, chorion, allantois, and yolk sac. The amnion is a thin, tough membrane containing a watery fluid called amniotic fluid. This acts as a shock absorber of external and localized pressure from uterine contractions during labor. Placenta formation begins with the chorion, a membrane that completely surrounds the amnion. A third membrane, the allantois, develops as an outpocketing of the gut. The blood vessels of the allantoic wall enlarge and become the umbilical vessels, while will connect the fetus to the developing placenta. The yolk sac, the site of early development of blood vessels, becomes associated with the umbilical vessels.

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Birth and Maturation

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Childbirth is accomplished by labor, a series of strong uterine contractions. Labor can be divided into three distinct stages. In the first stage, the cervix thins out and dilates, and the amniotic sac ruptures, releasing its fluids. During this time contractions are relatively mild. The second stage is characterized by rapid contractions, resulting in the birth of the baby, following by the cutting of the umbilical cord. During the final stage, the uterus contracts, expelling the placenta and the umbilical cord. The embryo develops into the adult through the process of maturation that involves cell division, growth, and differentiation. In some animals, maturation is suspended in a temporary state.