Chapter 3: Embryogenesis and Development

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Fertilization - occurs in wide part of fallopian tube/ampulla. Sperm binds to oocyte and releases acrosomal enzymes that allow head of sperm to penetrate. Oncer sperm comes to direct contact with secondary oocyte's cell membrane, it forms tubelike structure/ acrosomal apparatus, which extends to and penetrates cell membrane. Its pronucleus may freely enter oocyte once meiosis 2 comes to completion. After penetration, cortical reaction = release of calcium ions occurs. Calcium ions depolarize membrane of ovum, which prevents fertilization of ovum by multiple sperm cells and increases metabolic rate of newly formed zygote (diploid).

Twins - 2 mechanisms diygotic/fraternal twins or monozygotic/identical twins

dizygotic = form from 2 different eggs released during 1 ovulatory cycle by 2 different sperm. Not genetically same

monozygotic = form when a single zygote splits into 2. Genetically similar

Cleavage - after fertilization. While zygote moves to uterus for implantation, it undergoes rapid mitotic cell divisions in process called cleavage

- first cleavage creates an embryo. By dividing into progressively smaller cells, the cells increase N:C ration (nuclear to cytoplasmic) and surface area to volume ration. Allows for increased area for gas and nutrient exchange

Indeterminate cleavage - 1 type of cleavage. Results in cells that can still develop into complete organisms.

Determinate cleavage - results in cells with fate that are already determined/ these cells are committed to differentiating into certain cell type

Blastulation - several divisions later embryo becomes solid mass of cells/morula. Once formed, undergoes blastulation which forms the blastula (hollow ball of cells with fluid filled inner cavity).

Mammalian blastocyst made of 2 cell groups 1. trophoblast cells -- give rise to chorion and later placenta. 2. inner cell mass -- gives rise to organism itself

Implantation - blastula moves to the uterus and burrows into endometrium. Trophoblast cells give rise to chorion (extra embryonic membrane that develops into placenta). Trophoblasts form chorionic villi, support maternal-fetal gas exchange. Embryo connected to placenta by umbilical cord (2 arteries/deoxygenated and 1 vein/oxygenated blood). Yolk sac= site of early blood cell development, and supports embryo until placenta is functional. Allantois = involved in early fluid exchange between embryo and yolk sac. Amnion= surrounds allantois, thin tough membrane filled with amniotic fluid (shock absorber)

Gastrulation - once cell mass implants, begins further development aka gastrulation. Generation of 3 distinct cell layers. Archenteron= membrane invagination into blastocoel, which later develops into the gut. Blastophore= opening of archenteron. Humans=deuterostomes, the blastopore develops into the anus. Protostomes=it develops into the mouth

The primary germ layers (3) = some cells migrate into remains of the blastocoel, this establishes 3 layers

Ectoderm - "attracto" derm. Outermost layer. Gives rise to integument (hair, nails, mouth, lower anal canal, lens of eye, nervous system etc)

Mesoderm - "means" oderm. Middle layer. Means of getting around, such as bones, circulatory system, gonads/excretory system

Endoderm - linings of "endermal" organs. Innermost layer. Forms epithelial lining of digestive and respiratory tracts. Pancreas, thyroid, bladder, distal urinary tracts derived

Differentiation - cells of same genes are able to develop into distinct cell types w/specialized functions mainly by selective transcription of the genome (only genes needed for that particular cell type are transcribed)

Dual embryonic origin of adrenal gland!! Adrenal cortex= derived from mesoderm. Adrenal medulla is derived from ectoderm (b/c it contains some nervous tissue)

Neurulation