Biology: Vertebrate Embryology Flashcards
Embryology
Study of the development of a unicelluler zygote into a complete, multicellular organism.
Fertilization
In mammals, an egg can be fertilized within 12-24 hours after 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.
Cleavage
Early embryonic development is characterized by a series of rapid mitotic divisions known as cleavage. These divisions lead to an increase in cell number w/o a corresponding growth in cell protoplasm (i.e., the total volume of cytoplasm remains constant). Thus, cleavage results in progressively smaller cells, with an increasing ratio of nuclear-to-cytoplasmic material. Cleavage also increases the surface-to-volume ratio of each cell, thereby improving gas and nutrient exchange. This early developmental process consists of a series of very rapid, synchronous mitotic divisions that converts the zygote’s single large cell into a solid ball of cells, known as the merula, then into the blastula.
The first complete cleavage of the zygote occurs approximately 32 hours after fertilization. The second cleavage after 60 hours. Third after 72, at which point the eight-celled embryo reaches the uterus. As cell division continues, a solid ball of embryonic cells, known as the morula, is formed. Blastulation begins when the morula develops a fluid-filled cavity called the blastocoel, which by the fourth day becomes a hollow sphere of cells called the blastula. The blastula is the stage of the embryo that implants in the uterus.
Gastrulation
Once implanted in the uterus, cell migrations transform the single-cell layer of the blastula into a three-layered stucture 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.
Nerulation
By the end of gastrulation, regions of the germ layers begin to develop into a rudimentary nervous system; this process is known as nerulation. A rod of mesodermal cells, called the notochord, develops along the longitudinal axis just under the dorsal layer of ectoderm.
The notochord has an inductive effect on the overlying ectoderm, causing it to bend inward and form a groove along the dorsal surface of the embryo. The dorsal ectoderm folds on either side of the groove, these neural folds grow upward and finally fuse, forming a closed tube. This is the neural tube, which gives rise to the brain and spinal cord (CNS).
Once the neural tube is formed, it detaches from the surface ectoderm. The cells at the tip of each neural fold are called the neural crest cells. These cells migrate laterally and give rise to many components of the PNS, including the sensory ganglia, autonomic ganglia, adrenal medulla, and Schwann cells.
Internal Development
Follows the nerulation process.
Includes organogenesis, growth, and gametogenesis:
1) Organogenesis: the body organs begin to form. In this process, the cells interact, differentiate, change physical shape, proliferate, and migrate.
2) Growth: The organs increase in size, which is a continual process from infancy to childhood to adulthood.
3) Gametogenesis: Eggs develop in women and sperm in men, which results in reproduction possibilities.
The fertilization cycle continues, and then cleavage begins.
Placental Internal Development
The growing fetus receives oxygen directly from its mother through a specialized circulatory system. This system not only supplies oxygen and nutrients to the fetus but removes carbon dioxide and metabolic wastes as well. The two components of this system are the placenta and the umbilical cord, which both develop in the first few weeks after fertilization.
The placenta and the umbilical cord are outgrowths of the four extra-embryonic membranes formed during development: the amnion, chorion, allantois, and yolk sac.
Amnion
The amnion is a thin, tough membrane containing a watery fluid called amniotic fluid. Amniotic fluid acts as a shocker absorber of external pressure during gestation and localized pressure from uterine contractions during labor.
Chorion
Placenta formation beings with the chorion, a membrane that completely surrounds the amnion.
Allantois
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, which will connect the fetus to the developing placenta.
Yolk Sac
The yolk sac, the site of early development of blood vessels, becomes associated with the umbilical vessels.
Birth Stages
Childbirth is done by labor, a series of strong uterine contractions. Labor can be divided into 3 stages.
First stage, the cervix thins out and dilates, and the amniotic sac ruptures, releasing its fluids. During this time, contractions relatively mild.
Second stage is characterized by rapid contractions, resulting in birth of baby, followed by cutting the umbilical cord.
During the final stage, the uterus contracts, expelling the placenta and the umbilical cord.
