Chapter 47: Animal Development Flashcards
Stages of fertilization
Usually occurs in the widest part of the fallopian tube called the ampulla
Comprised of three stages:
- Acrosomal reaction
- Cortical reaction
- Egg activation
Acrosomal reaction
Is triggered the moment the sperm head contacts the egg surface
The acrosome at the tip of the sperm releases hydrolytic enzymes that allow it to penetrate the jelly coat
A structure called the acrosomal process forms, elongates, and penetrates the coat
Plasma membranes fuse and sperm nucleus enters the egg causing rapid influx of Ca2+ that acts as a fast block to prevent polyspermy
Cortical reaction
Seconds after the sperm binds to the egg cortical granule vesicles just beneath the egg plasma membrane release their contents and form a fertilization envelope
Fertilization envelope acts as the slow block to polyspermy
The cortical reaction requires a high concentration of Ca2+ ions in the egg
Egg activation
The rise in Ca2+ in the cytosol increases the rates of cellular respiration and protein synthesis by the egg cell
These rapid changes in metabolism mark activation
The proteins and mRNAs needed for activation are already present in the egg
The sperm nucleus merges with the egg nucleus and cell division begins
Fertilization in mammals
A sperm must travel through a layer of follicle cells surrounding the egg before it reaches the zona pellucida, or extracellular matrix of the egg
The process of fertilization is relatively slow in mammals; first cell division occurs 12–36 hours after sperm binding
Cleavage
Fertilization is followed by cleavage, a period of rapid mitotic cell division without growth
Division into progressively smaller cells increases the nuclear-to-cytoplasmic ratio and the surface area-to-volume ratio
Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres
Blastulation
Initial rounds of division form a solid mass of cells known as a morula
Subsequent divisions produce a hollow ball of cells called the blastula that surround a fluid-filled cavity called a blastocel
Mammilian blastula is known as a blastocyst and contains two main cell groups
- The trophoblast cells surround the blastocell and give rise to the chorion and placenta
- The inner cell mass protrudes into the blastocel and gives rise to the organsim itself
Implantation
Blastula migrates and imbeds itself in the endometrium
Trophoblastic cells interface between maternal blood supply and developing embryo
Give rise to an extraembryonic membrane that develops into the placenta called the chorion
Trophoblasts form microscopic finger-like projections that penetrate the endometrium called chorionic villi that support gas exchange
Extraembryonic membranes
- Yolk sac- supports the embryo until the placenta is functional and is the site of early blood cell development
- Allantois- involved in early fluid exchange between the embryo and yolk sac and later forms the umbilical cord
- Amnion- tough membrane that surrounds the allantois and is filled with amniotic fluid
- Chorion- nourishes the embryo and forms the placenta; also forms the outermost membrane around the amnion
Gastrulation
Rearranges the cells of a blastula into a three-layered embryo called a gastrula
Involves the inward movement from the epiblast
The three layers produced by gastrulation are called embryonic germ layers
- The ectoderm forms the outer layer; develops into primarily exteranal structures such as skin and hair as well as the nervous system; “attracto”derm- cosmetic features
- The mesoderm partly fills the space between the endoderm and ectoderm; develops into the musculoskeletal, circulatory, and excretory systems; “means”oderm- means of getting around (movement, circulatory, reproductive)
- The endoderm forms the inner layer; develops into cells of the digestive and respiratory tract as well as other organs; “endernal”- internal organs
Neurulation
Begins as cells from the dorsal mesoderm form the notochord, a rod extending along the dorsal side of the embryo
Signaling molecules secreted by the notochord induce other mesodermal cells that cause the ectoderm above to form the neural plate
The neural plate soon curves inward forming the neural tube which will become the central nervous system
The neural crest cells develop along the neural tube of vertebrates and migrate in the body forming the PNS and other specialized cells
Morphogenesis
Movements of parts of a cell can bring about cell shape changes or can enable a cell to migrate to a new location
The microtubules and microfilaments of the cytoskeleton are essential to these events
Transmembrane glycoproteins called cell adhesion molecules play a key role in migration
Convergent extension
A morphogenetic movement in which a sheet of cells undergoes rearrangement to form a longer and narrower shape
Cells elongate and wedge between each other to form fewer columns of cells
Cell fate specification
Specification- cell is reversibly designated as a specific cell type
Determination- process by which a cell or group of cells becomes irreversibly committed to a particular fate
Differentiation- refers to the resulting specialization in structure and function after its fate has been determine
Potency
The variety of tissues to which a stem cell can differentiate into
Totipotent- the greatest potency and include embryonic stem cells which can differenctiate into any cell type
Pluripotent- cells can differentiate into any cell type except for those found in placental structures
Multipotent- cells can differentiate into multiple types of cells within a particular group
Fetal shunts
Two different shunts redirect blood from the lungs:
- Foramen ovale- one-way valve that connects the right atrium to the left atrium; right atrium is at a higher pressure in the developing fetus
- Ductus arteriosus- shunts leftover blood from the pulmonary artery to the aorta
The liver is bypassed by the ductus venosus which shunts blood returning from the placenta via the umbilical vein directly into the vena cava
