Introduction to Embryology

Embryology Egg Meets Sperm: Semen is a basic fluid holding sperm cells with a pH of about 7.4 (a little basic) Luteinizing hormone (LH) triggers the ovaries to release the egg and estrogen causes glycogen release is broken down to lactate (making the acidity around the egg very low, around a pH of 3.8) Copper Intrauterine Devices (IUDs) release copper to take advantage of the sperm’s properties, decreasing its motility and even causing its destruction Egg, Sperm, & Fertilization: In the middle portion of the sperm cell are a whole bunch of mitochondria, which allow for propulsion of the flagella (giving the sperm cell increased motility) The genetic material is found in the head of the sperm cell The acrosome is also found in the tip of the head There isn’t really anything cool about the sperm cell, its primary function is to simply get to the egg (move and transfer genetic material) The egg cell is massive, about 10x more massive than a sperm cell (isn’t meant for movement like the sperm cells is and is therefore round in shape) The zona pellucida is a thick layer of glycoproteins on the exterior surface of the egg cell (protective layer for the sperm cell to get through) Once the sperm cells makes it past the zona pellucida and to the plasma membrane, fertilization has occurred Steps of fertilization: 1. Sperm binds to the zona pellucida 2. Acrosome reaction: the acrosomal proteins are released from the sperm cell and start to digest away the zona pellucida, which allows the sperm head to dive deeper through the zona pellucida and to the plasma membrane 3. Cortical reaction: when the sperm touches the plasma membrane of the egg cell, cortical enzymes are released from the egg and begin digesting the zona pellucida further, which makes it harder for other sperm cells to bind (blocks polyspermy) 4. Genetic transfer is then allowed into the egg Human Embryogenesis: Embryogenesis occurs during the first eight weeks of development following fertilization A zygote is simply one cell that is formed immediately following fertilization (before it begins to divide) During the first 12-24 hours following fertilization, zygotes are found in the cleavage phase (rapid cell division without growth) The cells do not have time to grow, however, and remain around the same size as the original zygote, even until the 32 cell stage (known as a morula) During blastulation, the mass of cells begins to form a hollow ball Following this, the cells begin to differentiate and form small cavities Two layers begin to develop: the trophoblast (a hollow outer shell that differentiates to different structures to help the embryo implant into the mother’s uterus) and the inner cell mass/embryoblast (which is pushed off to one side of the sphere) During gastrulation, three germ layers form (Week 3) Early Embryogenesis Cleavage, Blastulation, Gastrulation, & Neurulation: Eventually, the embryoblasts begin to clump together at one end, leaving a cavity (blastocoel) at the other side (still surrounded by trophoblasts) → zona pellucida is being disintegrated to allow room for growth Then, a cavity is developed inside of the embryoblasts known as the amniotic cavity and a layer on the bottom of the inner cell mass is known as the hypoblasts (the layer above is the epiblast) Primitive streak marks the beginning of the next stage, known as gastrulation → formation of the germ layer (cells migrate out from the primitive streak forming the germ layer) Germ Layers: 1. Ectoderm 2. Mesoderm 3. Endoderm Neurulation begins to occur and in the mesoderm through differentiation forming a structure known as the notochord (causes the thickening of the ectoderm above it known as the neural plate) The notochord starts to form a ring/tube structure known as the neural tube and cells begin breaking off into the mesoderm and are known as the neural crest cells Implantation: Eventually the blastocyst comes to find a crest in the endometrium This contact is known as apposition Trophoblasts rapidly replicate in order to invade the endometrium in order to anchor the blastocyst Syncytiotrophoblasts begin to fuse into the endometrium (become multinucleated and grow in size) Cytotrophoblasts are the cells that have yet to begun fusion The placenta forms from all of this shit Gestation: Organs have formed by week 10 Full term is around 37 - 40 weeks Counting in terms of weeks is the best for scientific understanding Germ Layer Derivatives: Endoderm primarily goes on to form the GI tract, lungs, liver, and pancreas (throat stuff as well) Mesoderm primarily goes on to form some inner layers of skin, muscles (including cardiac muscle), bones, kidneys, bladder, and ovaries/testes Ectoderm primarily goes on to form the outer layer of the skin, skin related items such as sweat glands, hair, etc. and even the nervous system Extra Notes: Neural crest cells in the ectoderm give rise to the nervous system