Mammalian Embryonic Development Quiz

Mammalian Embryonic Development

• Fertilization and embryo development are internal in mammals
• Eggs of higher mammals (Eutheria) are alecytic (oligolecitic), lacking significant yolk
• Embryonic development stages (cleavage, blastula, gastrula, organogenesis) are common to all multicellular organisms

Cleavage

• Cleavage begins hours after fertilization
• Cleavage transforms a single-celled zygote into a multicellular embryo through mitotic divisions without cell growth
• The nucleus-to-cytoplasm ratio changes during cleavage, with the nucleus size remaining relatively constant by maintaining nucleoplasm quantity
• Cleavage is a key process for initial embryonic cell division
• Cleavage morphology in mammals is total-equal (holoblastic), where all blastomeres are similar in size; this distinguishes it from partial (meroblastic) cleavage observed in some other organisms
• During cleavage, DNA synthesis is intensive, increasing the total amount of embryonic DNA along with the number of cells
• A morula forms when the embryo reaches approximately 16 cells

Blastula

• The morula is a short-lived stage transitioning to the blastocyst
• Cavitation results in a fluid-filled blastocoel (blastocyst cavity), defining the blastula stage
• The blastocyst is the blastula stage in mammals
• The blastocyst consists of two distinct cell types: an outer trophoblast layer and an inner cell mass (embryoblast), demonstrating cell specialization
• The inner cell mass will develop into the embryo itself
• The trophoblast will form the placenta in further embryogenesis

Gastrulation

• Gastrulation is the stage where germ layers form from the undifferentiated blastoderm
• Processes of formation such as invagination, migration, delamination, and epiboly shape the gastrula
• The gastrula, particularly in oligolecitic eggs, undergoes invagination
• Primary germ layers (ectoderm and endoderm) are established during the first gastrulation phase
• Mesoderm forms in the second gastrulation stage, a secondary germ layer
• Gastrulation in mammals involves the formation of a primitive streak for cell migration
• Cell migration is focused in the region of Henzen's node, determining head development
• Three germ layers become visibly distinct in gastrulation cross-sections

Germ Layer Differentiation

• The three germ layers (ectoderm, mesoderm, and endoderm) give rise to various body tissues and organs
• The diagrams demonstrate the specialization and derivation of tissues from each layer

Extraembryonic Membranes

• Yolk sac, amnion, chorion, and allantois are extraembryonic membranes that support the developing embryo
• The yolk sac, derived from the endoderm, doesn't have a significant nutritional role in mammals but is crucial in the early development of blood stem cells.
• The amnion, formed early in embryogenesis, is filled with amniotic fluid providing a protective environment for the growing embryo
• The chorion plays a role in forming the chorionic villi, which connect with the maternal blood vessels
• The allantois develops as an outgrowth of the hindgut and helps with waste removal and gas exchange, though its role in mammals is partially reduced
• In humans, the allantois contributes to the umbilical cord's formation

Chorion

• The chorion is the outermost extraembryonic membrane
• The chorion structure, specifically the chorionic villi, plays a critical role in connecting the mammalian embryo to the maternal tissues in the uterus
• The chorion exhibits various arrangements like cotyledonary, diffuse, discoidal, and zonal
• Chorionic villi penetrate the uterine wall (endometrium), forming the placenta during placental development

Placenta

• The placenta is a fetomaternal organ
• The placenta is composed of fetal (chorionic villi) and maternal tissue
• The placenta acts as a transport system for substances between the fetus and the mother
• Blood from the placenta reaches the fetus via the umbilical vein and returns via the umbilical arteries
• The functions of the placenta include nutrition, gas exchange for the embryo, removal of waste, and secretion of hormones such as estrogen, progesterone, and human chorionic gonadotropin (hCG)