Development

  • Structure of sperm 

  • 4 parts 

  • Acrosome (cap) - breaks through zona pellucida to connect with egg and prevents cross-species fertilization 

  • Head – containing the nucleus 

  • Neck – containing many mitochondria to power the tail 

  • Flagellum – mobility 

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  • Zygote vs cleavage stage vs morula 

  • Zygote: The zygote is the single, fertilized cell formed immediately after the sperm and egg fuse, marking the start of embryonic development. 

  • Cleavage Stage: Following zygote formation, the cleavage stage involves rapid mitotic cell divisions without an increase in overall size, producing smaller cells called blastomeres. 

  • Morula: After several rounds of cleavage, the embryo reaches the morula stage (around 16–32 cells), forming a compact, solid ball of cells with a shape resembling a mulberry. 

  • Notes:  

  • Cleavage forms a 16-cell morula (solid ball of totipotent cells that enter uterus and floats around dividing 

  • Eventually forms a hollow ball stage (blastula) that surrounds a cavity called the blastocoel (hollow cavity in blastocyst) 

  • Morula enters uterus 

  • What is a blastula? 

  • Hollow ball stage of cleavage that surrounds a cavity called the blastocoel (hollow cavity in blastocyst) 

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  • Trophoblast vs embryoblast 

  • Trophoblast: The outer cell layer of the blastocyst that contributes to forming the placenta and supporting tissues needed for embryo implantation and nutrient exchange with the mother. Becomes placenta 

  • Invades endometrium, grows root into endometrium, damaging it 

  • Endometrium responds by growing around blastocyst, resulting in a successful implantation (about 7 days in humans). Next, embryonic stage begins. 

  • Embryoblast: The inner cell mass of the blastocyst that will develop into the embryo itself, giving rise to all the tissues and organs of the body. Becomes embryo/baby 

  • How implantation occurs? 

  • Troph goes in endo, endo grows around troph 

  • Role of primitive streak 

  • Embryoblast thickens and invaginates to form a primitive streak. These migrating cells give rise to the mesoderm and endoderm where we get the differentiation from totipotent to pluripotent cells.  

  • What is gastrulation 

  • After embryoblast thickens and forms primitive streak, gastrulation begins. Embryo folds in on itself, forming the 3 tissue layers (endo/meso/ecto-derm) with the resulting opening called the blastopore (future anus). Hollow space formed is the archenteron (primitive gut). 

  • How neurulation occurs 

  • Forms alongside gastrulation 

  • Neural plate forms with ectoderm of dorsal portion of embryo begins to thicken. 

  • Mesoderm simultaneously forms outpockets. Forms the coelom of the embryo and gives rise to somites. Also results in formation of notochord.  

  • Triggers neural plate to invaginate and close off to form neural tube (future spinal cord). 

  • Organs/tissues by tissue layer 

  • Endoderm: lining of the gut and organs that outcrop from the gut. Lungs, liver, bladder, pancreas, and other endocrine organs.  

  • Ectoderm: outside stuff.  

  • Outer ectoderm forms epidermis and its derivatives: glands, hair, eye lens, inner ear, part of pituitary 

  • Inner ectoderm forms CNS and derivatives: motor nerves, eye retina, part of pituitary 

  • Mesoderm: Urogenital organs and gonads, dermis of skin, bone, and muscles (including smooth muscle of resp. and dig. tracts) 

  • 4 embryonic membranes and roles 

  • Yolk sac: provides nutrients to the embryo, forms from endoderm, draws nutrients from trophoblast. 

  • Amnion: provides aquatic environment and cushioning  

  • Allantois: Provides storage for nitrogenous waste 

  • Chorion: provides gas exchange 

  • What structures make up the placenta? 

  • Chorion and allantois 

  • What happens in the embryo stage? 

  • Structures form and blood bypasses lungs and liver as they develop, occurs between 2-8 weeks 

  • ~ 4 weeks: head, jaw, heart, blood, and limbs form 

  • ~ 5-8 weeks: growth and development of features 

  • ~ 8 weeks: Most major organs formed 

  • What happens in the fetal stage? 

  • Additional development of organs and features occur 

  • All body systems mature and become functional 

  • Bones grow, brain develops, lungs grow, etc. 

  • Pregnancy concludes with birth of offspring 

  • Hormones affect parturition (birth) 

  • At 40 weeks through a positive feedback mechanism 

  • PG  (prostoglandins): inc stretch 

  • Oxytocin: causes more and stronger contractions 

  • Oxytocin from pituitary gland triggers uterine muscles to contract 

  • Moves fetus downward, stretching/dilating oxytocin, which triggers the release of more oxytocin and triggers the amniotic sac to rupture (which lubricates cervix and vagina). 

  • Embryonic blood flow and its purpose 

  • Ductus venosus bypasses liver, shunting blood from placenta directly to the heart. After birth, it becomes a suspensory ligament of the liver. If this fails to close after birth, a partosystemic shunt (PSS) forms. 

  • Foramen ovale connects right and left atria to bypass the lungs by shunting blood directly between atria. It snaps shut at birth due to pressure of 1st breath. If it fails to shut, a patent foramen ovale forms (heart murmur). 

  • Ductus arteriosus connects pulmonary artery and aorta. It shunts blood away from lungs to the systemic circuit. It snaps shut at birth due to pressure of 1st breath. If this fails ot shut, a patent ductus arteriosus (PDA).