AP BIOLOGY

SHANNON BRADY2010

Chapter 47: Animal Development

A Body Building Plan for Animals

Cytoplasmic determinants: maternal substances that cause early embryonic cells to become different because of their uneven distribution within the unfertilized egg.

Cell Differentiation: The Specialization of cells in their structure and function.

Fertilization; Uniting the sperm & egg

The main function of fertilization is to combine haploid sets of chromosomes from two individuals

into a single diploid cell, the zygote.

There are two main processes that occur during fertilization; the Acrosomal Reaction and the

Cortical Reaction.

The Acrosomal Reaction

The Acrosomal Reaction Cont.

1. Contact. The sperm cell contacts the egg’s jelly coat, triggering exocytosis from the sperm’s acrosome.

2. Acrosomal Reaction. Hydrolytic enzymes released from the acrosome make a hole in the jelly coat, while growing actin filaments from the acrosomal process. This structure protrudes from the sperm head and penetrates the jelly coat, binding receptors in the egg cell membrane that extend through the vitelline layer.

3. Contact & Fusion of sperm & egg membranes. A hole is made in the vitelline layer, allowing contact and fusion of the gamete plasma membranes. The fused membrane becomes depolarized, resulting in the fast block to polyspermy ( prevents additional sperm cells from fertilizing the egg)

4. Entry of Sperm Nucleus.

The Cortical Reaction

The Cortical Reaction Cont.

1. Fusion of the gamete membranes triggers the release of Ca2+ from the ER into the

egg’s cytosol, causing cortical granules in the egg to fuse with the plasma membrane and discharge their contents. This leads to

swelling of the perivitelline space, hardening of the vitelline layer, and clipping of sperm- binding receptors. The resulting

fertilization envelope is the slow block to ployspermy.

Activation of the Egg

Another result of the increase in Ca2+ is substantial increase in the rates of cellular respiration and protein synthesis by the egg cell. This artificial activation switches on the metabolic

responses of the egg and causes it to begin developing by parthenogenesis.

Fertilization in Mammals

1. The sperm migrates through the coat of follicle cells and binds to receptor molecules in the zona pellucida (extracellular matrix) of the egg.

2. This binding induces the acrosomal reaction, in which sperm releases hydrolytic enzymes into the zona pellucida.

3. Breakdown of the zona pellucida by these enzymes allows the sperm to reach the plasma membrane of the egg. Membrane proteins of the sperm bind receptors on the egg membrane, and the two membranes fuse.

4. The nucleus and other components of the sperm cell enter the egg.

5. Enzymes released during the cortical reaction harden the zona pellucida, which now functions as a block to polyspermy.

Cleavage

During Cleavage a cell undergoes DNA synthesis and Mitosis but not protein synthesis.

Cleavage partitions the cytoplasm of one large cell, the zygote, into many smaller cells called blastomeres, each with it’s own nucleus. The first 5-7 divisions form a cluster known as the

morula (lobed surface embryo). A fluid filled cavity called the blastocoel begins to

form within the morula and is fully formed into the blastula ( a hollow ball of cells.)

Body Axes Establishment in Amphibians

The eggs and zygotes of sea urchins and other animals have a definite polarity.

yolk is most concentrated toward the vegetal pole; but decreases significantly toward the opposite animal pole.

Following fusion of the egg and sperm, rearrangement of the amphibian egg cytoplasm establishes One of the body axes. 1) The polarity of the egg determines the anterior posterior axis in fertilization. 2) At fertilization the pigmented cortex slides over the underlying cytoplasm toward the point of sperm entry. This rotation exposes a region of light colored cytoplasm, the gray crescent, which marks the dorsal side. 3) The first cleave division bisects the gray crescent. Once the anterior posterior, and dorsal-ventral axes are defined, so if the left-right axis.

Cleavage in a Chick Embryo

Most of the zygote is yolk with a small disk of cytoplasm located at the animal pole. The cell is surrounded by egg whites, but cleavage is restricted to the disk of cytoplasm.

Meroblastic Cleavage: The incomplete division of yolk-rich egg (chick embryo).

Holoblastic Cleavage: The complete division of eggs having little yolk (sea urchins) or some yolk (frogs).

Early cleavage division in a bird embryo produce a cape of cells

called a blastoderm, which rests undivided on the yolk. The blastoderm sorts into upper and lower layers; the epiblast and hypoblast. The cavity between the two is the avian version of the blastocoel.

Gastrulation

What is it?-A dramatic rearrangement of the cells of the blastula to

form a three layered embryo with a primitive gut.-Changes in motility, cell shape, and cellular adhesion to

other cells and to molecules of extra cellular matrix.

The result. A three layered embryo called the gastrula. The three layers form the embryonic germ layers; the ectoderm forms

the outer layer; the endoderm lines the embryonic digestive tract; and the mesoderm partly fills the space between.

These germ layers eventually turn into adult tissue and organs.

Gastrulation Diagram

Organogenesis

Organogenesis involves more localized morphogenic changes in tissue and cell shape. The first evidence of this process is the appearance of folds, splits, and dense clustering of cells.

Developmental Adaptations of Amniotes

All vertebrate embryos require an aqueous environment for development.

Only two such structures exist today that allow vertebrates to reproduce in dry environments.

1) the shelled egg of birds and other reptiles.2) the uterus of placental mammals. (embryos are surrounded by fluid within a sac by a membrane called the amnion)

Reptiles including birds are therefore called amniotes.

In some shelled eggs “extra life support” is needed in the form of embryonic membranes.

Mammalian Development

FOUR STAGES1) Fertilization2) Cleavage.

3) Gastrulation.4) Organogenesis.

Morphogenesis

The movement of cells through:- reorganization of the skeleton- cell migration- convergent extension.( depicted to the right )

Roles of the Extracellular Matrix & Adhesion Molecules

Several methods of guiding cells in the right direction during morphogenesis include:

ECM, (extracellular matrix) which is a mixture of secreted glycoproteins.

CAM’s ( Cell Adhesion Molecules ) Cadherins, an important type of cell to cell adhesion

molecules.

The Development Fate of Cells

Developmental fate depends on history and inductive signals, two general principles describe where differentiation occurs in cells:

1) During Cleavage divisions, embryonic cells must somehow become different from one another.2) Once initial cells asymmetries are set up, subsequent interactions among the embryonic cells influence their fate, usually by causing changes in gene. (Induction)

Establishing Cellular Asymmetries

The Axis of the body plan must be established.Bilaterally symmetrical animals have an anterior-posterior axis,

ventral-dorsal axis, and left and right sides.

The “Organizer” of Spemann & Mangold

Cell fate by Inductive Signals.In a 1920’s experiment Hans Spemann & Hilde

Mangold concluded that the dorsal lip of the balstopore in early gastrula functions as an

organizer of the embryo by initiating a chain of induction that result in the formation of the notochord, neural tube, etc. A growth factor

BMP-4 is suspected to cause induction.

Formation of the vertebrate Limb

Positional information controls pattern formation that forms limbs in the body.

The two major limb bud organizers are the: 1) apical ectodermal ridge (AER) 2) zone of polarizing activity (ZPA)

These two organizers send signals to the cells, which determine whether a limb will be a fore limb or a hind limb based on developmental histories.