HHP3115 Unit 1 Notes Chapter 3 PDF

# Chapter 3 - Basic Embryology Embryology: study of the origin and development of single individuals ## Prenatal period includes... * **Embryonic period:** first 8 weeks in utero * **Fetal period:** remaining ~30 weeks in utero Note: there is not a direct connection between oviduct and ovary ##...

# Chapter 3 - Basic Embryology Embryology: study of the origin and development of single individuals ## Prenatal period includes... * **Embryonic period:** first 8 weeks in utero * **Fetal period:** remaining ~30 weeks in utero Note: there is not a direct connection between oviduct and ovary ## Embryonic period: first 8 weeks post-fertilization **Major events:** * Organs form 3 primary germ layers * The basic body plan emerges ## Week 1 events: * **Conception:** occurs in the lateral third of uterine tube * **Zygote:** fertilized oocyte (one cell) * Moves towards the uterus * **Blastomeres:** daughter cells formed from zygote * Formed via cleavage: cell division without growth * No growth because it occurs so fast, and it would become an ectopic pregnancy if it grew too big that it had to implant in Fallopian tube * **Morula:** cluster of 12-16 blastomeres * **Blastocyst:** fluid-filled structure containing ~60 cells * **Outer layer of cells (trophoblast):** becomes the extraembryonic membranes * **Inner clump of cells (inner cell mass):** becomes the person * **Implantation** into endometrium of the uterus occurs on **DAY 6** ### Stone baby Resulted from sperm fertilizing the egg in gap between ovary and tube (resulting in ectopic pregnancy, but this time it was viable) * It found a blood supply and grew to be 32 weeks, but there was obviously no way for it to be delivered * Died after it couldn't be delivered * Since the genetic material was different than mom, the body calcified around it (to isolate it), and it turned into a 4 pound calcified tumor. ## Week 2 events: * **Bilaminar embryonic disk:** the inner cell mass is divided into 2 sheets * **Epiblast:** top layer * **Hypoblast:** bottom layer * **Amniotic sac:** formed by an extension of epiblast * Outer membrane forms **amnion** * Inner membrane forms **amniotic sac cavity** * Filled with amniotic fluid * **Yolk sac:** formed by an extension of hypoblast * Small part of it will eventually be incorporated into our digestive tube. In other animals, it's used as a source of nutrition. But placental mammals have a placenta to nourish us, so we don't need it. ## Week 3 events: * **Primitive streak:** epiblast cells migrate inward via a process called **gastrulation**. * Forms the **gastrula** * The side of the epiblast that forms the primitive streak becomes the **tail end** * Gastrulation causes the 3 germ layers to be formed: * **Endoderm:** formed by first migrating cells (replaces the hypoblast) * **Mesoderm:** second group of migrating cells (lands between epiblast and endoderm) * **Ectoderm:** cells left at the surface, which don't migrate inward (formed by epiblast cells) * **Notochord:** develops from mesoderm on the cranial side * This is a straight line that basically spans from end of the primitive streak to the opposite side of the gastrula (dark blue park. * Induces neurulation in overlying ectoderm * **Neurulation:** ectoderm starts forming the brain and spinal cord * **Neural plate:** ectoderm thickens * **Neural groove:** ectoderm folds inward * **Neural tube:** a hollow tube forms * Cranial part of the neural tube becomes the brain * **Neural crest cells:** ectoderm that breaks off during neural tube formation * Forms sensory nerve cells, ganglia, and melanocytes * **Somites:** bulging patterns of mesoderm next to the notochord ("paraxial mesoderm") * Our first body segments * Become vertebrae, ribs, and axial muscles * **Intermediate mesoderm:** just next to the paraxial mesoderm * Becomes the urogenital system ## Week 4 events: * **Lateral folding** occurs, giving us the future gut (digestive tube) * **Yolk sac** pinched off ## Major derivatives of embryonic germ layers: | Layer | Example | |---|---| | **Ectoderm** | Epidermis, hair, nails, glands of skin, brain and spinal cord, neural crest and derivatives, pigment cells, bones and blood vessels of the head | | **Mesoderm** | Notochord, somite, intermediate mesoderm, lateral plate mesoderm (somatic & splanchnic) | | **Endoderm** | Nucleus pulposus of intervertebral discs, sclerotome, dermatome, myotome, kidneys, gonads, parietal serosa, dermis of ventral body region, connective tissues of limbs (bones, joints, and ligaments), wall of digestive and respiratory tracts (except epithelial lining), visceral serosa, heart, blood vessels | | **Epiblast** | Epithelial lining and glands of digestive and respiratory tracts | ## Week 5 events: * Head is 50% of body * Upper appendages, called **paddles**, begin to form * By end of 2nd month (8 weeks), all organs have appeared * The placenta is also fully functioning ## Fetal period: week 9 post-fertilization to birth **Major events:** organs grow in size and complexity * **Beginning of 3rd month:** * **Head growth begins to slow, and the body increases in length.** * **Ossification centers appear in bones** * **Sex will soon be able to be determined** * **3rd and 4th month:** * Fingers, eyes, ears, etc. form * Finally resembles a human * **5th-7th months:** * Mother begins to feel fetal movement * Really begins to look like a human (fingernails, eyebrows, etc. are being formed) - Still not viable to survive outside the womb, because the lungs lack **surfactant** (a detergent that breaks up water tension in alveoli, allowing them to remain inflated) * At around 22-24 (6 months) weeks the fetus could survive outside the womb * **8th and 9th month:** * **Fetus usually rotates so that head is pointed down towards the cervix (most inferior part of the uterus)** * **Around 38-40 weeks is full term (around 9-10 months)**