Embryogenesis

Question 1

Development

Answer 1

• Fertilization: Occurs in Ampulla (widest part of Fallopian tube). Sperm meets and binds to Secondary Oocyte and penetrates its corona radiata and zona pellucida with the help of acrosomal enzymes. Sperm pronucleus can freely enter oocyte after completion of Meiosis II and bind with ovum pronucleus to form diploid zygote. Depolarized and impenetrable Fertilization Membrane forms around zygote.
• Cleavage: Unicellular Zygote undergoes Cleavage (rapid mitotic cell divisions) to produce multicellular Embryo. Cleavage increases nuclear-to-cytoplasmic (N:C) ratio and surface area-to-volume ratio for maximizing gas and nutrient exchange.
• Blastulation: After several divisions, Embryo becomes Morula (solid mass of cells), which undergoes Blastulation to form Blastula/Blastocyst (hollow ball of cells) with Blastocoel (fluid-filled inner cavity), Inner Cell Mass (gives rise to organism itself), and Trophoblast Cells (gives rise to chorion and placenta).
• Implantation: Blastocyst moves to uterus for implantation in endometrium. Blastocyst supported by Yolk Sac until placenta becomes functional. Trophoblastic cells give rise to Chorion and then Placenta. Embryo connected to placenta by Umbilical Cord consisting of two umbilical arteries carrying deoxygenated blood/wastes to placenta (away from embryo heart) and one umbilical vein carrying oxygenated blood to embryo (away from placenta).
• Gastrulation: Begins with invagination of blastula to form Gastrula with Archenteron (invagination that eventually becomes gut) and Blastopore (opening to archenteron) and three Primary Germ Layers (ectoderm, mesoderm, endoderm). In Deuterostomes, blastopore develops into anus and merging forms mouth at other opening. In Protostomes, blastopore develops into mouth and merging forms anus at other opening.
• Neurulation: Development of nervous system begins after formation of primary germ layers in which mesodermal Notochord (primitive spine) induce overlying ectodermal cells to fold inward to form Neural Tube (which gives rise to CNS) and Neural Crest Cells (which give rise to PNS). Ectodermal cells migrate over neural tube and crest to cover rudimentary nervous system.

Question 2

Primary Germ Layers

Answer 2

• Ectoderm: Outermost layer of gastrula, gives rise to integument and nervous system, along with epithelia of mouth and anus. Attract-oderm (attracts us to others via cosmetic features and intelligence).
• Mesoderm: Middle layer of gastrula, gives rise musculoskeletal, circulatory, and urogenital systems, along with muscular and connective tissue of digestive and respiratory systems. Means-oderm (means of getting around physically, internally, and sexually).
• Endoderm: Innermost layer of gastrula, gives rise to epithelial lining of digestive and respiratory tracts (except mouth and anus), along with pancreas, thyroid, bladder, urethra, and liver. Enter-oderm (linings of internal organs and their accessories).
• Dual Origin of Adrenal Glands: Adrenal Medulla (as part of nervous system) is derived from Ectoderm, and Adrenal Cortex is derived from Mesoderm.

Question 3

Cell Specialization and Stem Cells

Answer 3

• Selective Transcription: Only genes needed for that particular cell type are transcribed.
• Induction: Ability of one group of cells to influence the fate (specialization) of nearby cells via Inducers/Morphogens. To be induced, responder must be competent (able to respond to inducing signal).
• Cell-cell communication can be Autocrine (signals act on same cell that secreted them), Paracrine (signals act on nearby cells), Juxtacrine (cell directly stimulates receptors of adjacent cell without diffusion of signals), and Endocrine (secreted hormones travel through bloodstream to distant target tissues).
• Stem Cells: Undifferentiated cells that give rise to other cells that will differentiate. Totipotent cells have the highest Potency, are able to differentiate into any cell type, and are found only during the first few postfertilization cell divisions. Pluripotent cells have intermediate Potency, are able to differentiate into any non-placental cell type, and are found in blastula (inner cell mass) and gastrula (primary germ layer cells) and beyond. Multipotent cells have the lowest Potency, can differentiate into multiple types of cells within a particular group (such as hematopoietic stem cell) and are found in adults.

Question 4

Senescence

Answer 4

• Biological aging due to shortening of Telomeres (ends of chromosomes); results in failure of cells to divide.
• Telomeres reduce the loss of genetic information from the ends of chromosomes and help prevent the DNA from unraveling as their high concentration of guanine and cytosine (high G-C content) enables telomeres to “knot off” the end of the chromosome. Eventually, the telomeres become too short, and the cell is no longer able to replicate.
• Telomerase can prevent senescence by lengthening telomeres through its reverse transcriptase activity.

Question 5

Fetal Circulation

Answer 5

• Placenta: Organ where nutrient, gas, and water exchange occurs between mother and fetus through diffusion.
• Oxygen is transferred to fetal blood from maternal blood due to presence of higher partial pressure of oxygen in maternal blood and greater oxygen affinity of fetal hemoglobin (HbF) in comparison to maternal/adult hemoglobin (HbA).
• Umbilical Arteries carry deoxygenated blood/wastes away from fetus (to placenta). Umbilical Vein carries oxygenated blood to fetus (away from placenta).
• Foramen Ovale shunts blood from right atrium to left atrium, bypassing lungs to be directly pumped through aorta into systemic circulation. Ductus Arteriosus shunts leftover blood from pulmonary artery to aorta, bypassing lungs again. Ductus Venosus shunts blood from umbilical vein to inferior vena cava, bypassing liver.