1. Introduction 2. Germ Cells 3. The Roux-Weismana Thory 4. Experimental Embryology

Embryology means "study of embryos". It is the descriptive and experimental study of changes in morphogenesis (form or shape) of animals during their embryonic phase.

Each multicellulor organisms begin life as single cell, the fertilized egg, which is almost similar to all other cells but differs mainly by its potential to divide and produce all the cells that is going to make an adult organism. The cells of the embryo diverge from one another structurally and functionally during the process of development and eventually organized into an adult organism.

1. Germ Cells: Bridging the Generation Gap.

Aristotle originated the idea that form of an embryo gradually emerges during development. Nearly 2000 years later in the 17^th anf 18^th century many embryologists rejected this idea. They proposed that egg contains a miniature fully formed embryo. Bonnet (1745) formalized this concept in his theory of "emboitment".

Casper Wolff (1759) gave an alternative theory of development called "epigenesis", which means that adult gradually develops from a formless egg as originally proposed by Aristotle.

Nature and significance of Germ Cells remain unclear for almost a century until Schwann (1839) recognized that egg is a cell.

Schweigger-Seidel and St. George (1876) determined that sperm is a cell

Hartwig (1876) stated that fertilization results from the union of sperm and egg. He also observed that germ cell nuclei are the vehicles of inheretance.

Van Beneden (1883) gave the first detailed description of the behavior of chromosome in fertilized egg.

2. The Roux-Weisman Theory.

Wilhem Roux (1883) stated his idea that hereditary material represents different characteristics of of the organism and fertlized egg receives all of these substances which during cell division are linearly arranged on the chromosomes. These substances are then distributed unequally to daughter cells during cell division. According to his idea this "qualitative division" fixes the fate of cells and their descendents.

First Experiment in Embryology conducted by Roux (1888): Father of Experimental Embryology.

Roux (1888) confirmed his theory by conducting an experiment on Frog egg. He used a hot needle to destroy one of the two cell of a 2-cell frog embryo. The uninjured cell develop to form half embryo, lacking complete structures. Roux used this expt. To support his theory that the undamaged half frog embryo developed into half embryo because it lack the informations that was destroyed by killing the other half embryo.

Hans Driesch (1892) used sea urchin 2-cell stage embryo to disprove Roux's theory. He separated the 2-cell embryo by mechanical shaking. These half embryo developed normally but become dwarf. Later he separated the 2 and 4 cell embryo in calcium free sea water and found that both types of embryo develop into normal adult.

3. Mendelian Era (Nature of Inhretanace)

Mendel (1865) proposed that characteristics of organism are determined by factors that retain their characteristics through generations of breeding.

Thodor Boveri (1902) established precisely the role of chromosome in development.

The final proof of the "Chromosome theory of inheretance" came from the work of T. H. Mogan (1906-1940) on fruit fly .

Inheritance is the transmission of genes from parents to offspring. 2. Genes are located on chromosome 3. Each gene occupy a specific site on the chromosome 4. Genes can exists in several alternative forms 5. Allelic genes in a zygot donot blend or contaminate each other but segregate independently of each other. 6. Chromosomes and genes on the chromosome assort independently. 7. Genes are duplicated during cell division

Spermatogenesis: 1. Spermatogenesis 2. Sperm structure 3.Germ Cell-Somatic Cell Interaction 4.Hormonal regulation of spermatogenesis 5.Spermiogenesis 6.Spermiation 7.Mammalian Sperm maturation

GONAD A gamete-producing organ; an ovary or testis.

Spermatogenesis: Process that leads to the formation of sperms. Initial cell in gern cell line are the "primordial germ cells". Close association between germ cells and somatic cells in gonads.

Mammalian testes contain numerous seminiferous tubule. Spermatogenesis occurs in the seminiferous tubule. Somatic cell surrounding germ cell are called "sertoli cell". Premeiotic proliferative germ cells are called spermatogonia. Spermatogonia remain at the bases of sertoli cell.

Spermatogonia enters Ist meiotic division as a primary spermatocytes. This division produces two secondary spermatocytes, each of which will produce two spermatids.

Spermiogenesis- The process of differentiation of spermatid into mature sperm .

Spermiation: At the completion of spermiogenesis, individual sperm are released .

Sperm Structure: Primary component of most sperm are a nucleus, an acrosome, and a flagellum. Nucleus contains a highly condensed mass of chromatin, acrosoma has a variety of morphology and assis in penetration of egg accessory layers. Flagella are not universal, heps in locomotion.

Mammalian Sperm: two major region; head and tail. Head has acorosomal cap and post acrosomal region. Tail has 4 segments; neck, middle piece, principal piece and end piece.

acrosome- Astructure contained in the head of the sperm; it contains the acrosomal granule

acrosomal enzymes- The enzymes contained in the acrosomal granule of the sperm.

crosmal filament- A filamentous rod covered with acrosomal membrane that extends from the head of the sperm.

acrosomal process- A structure consisting of the acrosomal filament and the acrosomal membrane.

acrosomal reaction- A reaction when the sperm enters the jelly layer surrounding the egg; two events are involved: the acrosomal membrane fusing with sperm plasmalemma and exocytosis occurs; and actin undergoes rapid polymerization to form the acrosomal filament.

Axoneme: Motor apparatus of sperm tail that consists of 2 central microtubules which are surrounded by an array of nine doublet microtubule. The arms associated with outer microtubule doublets composed of a protein -Dynein-1 with ATPase activity and responsible for converting chemical energy into mechanical energy of motion of the sperm tail.

Immotile Cilia Syndrome: The sperm tail of a sterile male that is devoid of Dynein arm and so are incapable of mechanical movement.

Androgen: The principal promoters of germ cell differentiation in male vertebrates. Cells of Leydig synthesizeandrogen

Luteinizing hormone : is also known as Interstitial Cell Stimulating Hormone (ICSH).

Classical Feedback Inhibition: When LH production increases, androgen production is elevated. This, in turn, leads to a reduction in LH level.

Seminiferous Growth factor: the substance produce by mammalian sertoli cells that function as a growth factor which stimulates somatic cell proliferation and blood vessel production during fetal and prenatal development

Hormonal Regulation of Spermatogenesis: Principal regulators of germ cell differentiation in male vertebrates are Androgen, synthesized by interstitial cells (cells of leydig). Androgen production is regulated by leuteinizing hormone (LH); also known as ICSH. Follicle stimulating Hormone (FSH) is also involved in regulating spermiogenesis by acting directly on sertoli cells. Production of LH and FSH is regulated by gonadotropin releasing hormone (GnRH) produced by hypothalamus.

WEEK 3&4 (CH 3)

1. Oogenesis 2. Oocyte-Accessory Cell Interactions During Oogenesis 3. Organization and Differentiation of Oocyte 4. Hormonal Contol of Oogenesis 5. Oocyte maturation and ovulation 6. Egg Envelopes

Oogenesis: :

Process that leads to the formation of "ovum" or egg. Initial cell in gern cell line are the "primordial germ cells". Close association between germ cells and somatic cells in gonads. Somatic cell surrounding germ cell are called "Follicle cells". Premeiotic proliferative germ cells are called oogonia. Spermatogonia enters Ist meiotic division as a primary oocyte. This division produces one secondary oocytes and a small polar body. The secondary oocyte divide by a second meiotic division to produce a second polar body and a haploid "ovum", which is the only functional sex cell from the meiotic division.

Follice cells:Mammalian egg develop in conjunction with follicle cell

In animals several follicle matures while in human only one follicle matures per menstrual cycle

Gamete Differentiation: In Male it occurs after meiosis, In Female Oocyte is fully formed before completion of meiosis

Species With Yolky eggs (Vitellogenesis-yolk deposition): Most oocyte growth in these spp.during vitellogenesis

Oocyte-Accessory Cell Interaction During Oogenesis: Two types of accessory cells ; (1). Follicle cells-derived from somatic cells that forms a layer surrounding the oocyte and (2) Nurse cells-derived from germ cell line and remain associated with the oocyte via cytoplasmic bridges.

Mammalian Follicle: Expanding follicle are called Granulosa cells. Space between granulosa cells and oocytes are filled with sulfated glycoprotein secreted by follicle which forms a contnuous coat- known as ZONA PELLUCIDA

Organization of Egg : Egg polarity-Animal pole and Vegetal pole-distinct in amphibian egg, where ribosome, mitochondria and pigment granules are prevalent in animal pole and gradually decreases toward vegetal pole. Yolk platelets are prevalent on the vegetal pole.

Germinal Vesicle: Enlarged nucleus of the oocyte.

Mitochondria and Germ Plasm: Mitochondria stock piled during oogenesis. The ratio of nuclear DNA to mitochondrial DNA in somatic xenopus is 100:1, during oogenesis this ratio changes to 1:1 to 1:100. This is mitochondrial amplification.

Yolk-Amount and Distribution: Oligolecithal, isolecithal or homolecithal-Eggs with small amount of evenly distributed yolk (Sea Urchin, lower chordates).

Telolecithal-Segregation of cytoplas and yolk with cytoplasm restricted to a thin layer covering the yolk, Cytoplamic layer thickens at the animal pole to form a cytoplasmic cap which contains the nucleus (reptile, bony fishes,some mollusks).

Mesolecithal or moderately telolecithal: Amphibian eggs

Oviparous species where egg develops outside mothers body such as amphibian eggs have seasonal oogenesis cycle. Ovarian growth triggered by environmental cues.

Oogenesis is regulated by modulations in the concentrations of circulating Hormones. Hypothalamus release Gonadotropin-releasing hormone (GnRH), which regulate the secretion of gonadotropins (LH, FSH) from the pituitory. These hormones stimulate follicle cells in the ovary to synthesize steroid hormes (estrogen and progesterone), and the somatic cells in the testis to synthesize testosterone.

In Drosophila Gonadotropic hormone is the Juvenile Hormone(JH) secreted by the corpus allatum . JH stimulates ovary to produce vitellogenin (ylk precursor), ovary produces Ecdysone hormone which stimulates Fat body to produce Vitellogenin. Dual origin of vitellogenin.

In amphibian oocyte growth and differentiation is regulated by modulations in the concentrations of circulating Hormones. Hypothalamus release Gonadotropin-releasing hormone (GnRH), which regulate the secretion of gonadotropins (LH, FSH) from the pituitory. Estrogen stimualtes live to synthesize vitellogenin. Gonadotropin promotes its uptake in the ovary. In response to gonadotrophin stimulation follicle cells produce progesterone that induces oocyte maturation in Xenopus

Menstrual Cycle: Ovarian cycle of primates which involves cyclic production and ovulation of ripe eggs.

FSH production by pituitary notably increases at menstruation

After ovulation LH promotes follicle to become an endocrine structure called Corpus luteum

Inhibin decreases FSH levels by classical feedback inhibition

If eggs are not fertilized Corpus Luteum is degenerated

There is a inverse relationship between FSH levels and Inhibin; meaning if one increases the other decreases.

If egg is fertilized production of LH continues, corpus luteum expands and produces high level of progesterone until 4 months

Placenta becomes the principal source of steroid hormone production during the latter half of the pregnancy.

Contraceptives are synthetic progesterone that inhibit LH surge during the mid-cycle of menstruation which prevent ovulation.

In most mammals stage at which egg remains until fertilization is Prophase I

Conspicuous events of Oocyte maturation are; germinal vesicle breakdown, formation of spindle, production of polar body etc.,

To induce oocyte maturation progesterone binds to oocyte surface receptor like a protein hormone bind to a target cells plasmamembrane

Maturation promotion factor (MPF) is a nuclear factor that mediates hormonal effect on oocyte

Injection of MPF triggers production of more MPF in an autocatalytic reaction

M-phase promoting (MFP) factor may be universal cell cycle regulator

1. Egg and Sperm Structure 2. Events Before Sperm-Egg Fusion 3. Regulation of Human Fertility-Immunocontraception and In Vitro Fertilization. 4. Sperm and Egg Fusion 5. Events following sperm-egg fusion 6. Initiation of Development 7. Parthenogenesis

Sperm and Egg structure: Sperm often travel long distance to fertilize egg and so the sperm is composed of a locomotory tail that is surmounted by a head containing nucleus and acrosome.

Egg Structure: Sea Urchin- Egg contain glycocalyx, an extracellulor coat consisting of 2 acellulor layers; the jelly coat-the outer layer consisting of several small peptides and large acidic polysaccharides. The inner layer/vitelline envelope is composed of glycoprotein fibers.

Mammalian Egg(Mouse Egg): Outer layer is Corona Radiata-single layer follicle cells, a formidable barrier to sperm penetration. Inner layer is Zona Pellucida is composed of glycoprotein molecules.

Birds: Additional Layers;Chorion and Egg Shells. Sperm usually cannot penetrate chorions around the egg of some organisms and must gain access through a microphyle

Sperm Egg Fusion:

acrosome- Astructure contained in the head of the sperm; it contains the acrosomal granule.

acrosomal enzymes- The enzymes contained in the acrosomal granule of the sperm.

acrosmal filament- A filamentous rod covered with acrosomal membrane that extends from the head of the sperm.

acrosomal process- A structure consisting of the acrosomal filament and the acrosomal membrane.

acrosomal reaction- A reaction when the sperm enters the jelly layer surrounding the egg; two events are involved: the acrosomal membrane fusing with sperm plasmalemma and exocytosis occurs; and actin undergoes rapid polymerization to form the acrosomal filament.

Additional extracellulor layers (chorions and egg shells) are deposited around the eggs of some organisms only by the somatic cells of the reproductive tract

When the sea urchin sperm enter seawater which has a pH of about 8.0, a coupled exchange of internal H+ for external Na+ is initiated which quickly raises the internal pH to 7.6 and the sperm begin to move

Dynein (an ATPase) that is bound to the surface of the microtubule in the sperm flagellar axoneme activity is very sensitive to pH.

The fertilization potential temporarily changes the voltage across the egg plasma membrane from -70mV to about + 10mV.

In contrast to sea urchins there is no electrical block to polyspermy in Mammals

Block to polyspermy in Mammals involves structural changes in Zona pellucida

The fertilization wave was described by E E Just

Whereas the fast block is an electrical event, the slow block to polyspermy is a mechanical process

In sea urchin egg as the as the cortical granules undergo exocytosis, they fill the perivitteline space with hydrated proteins and mucopolysaccharides causing an elevation of vitelline envelope above egg surface. The elevated vitelline envelope is called fertilization envelope.

During sperm-egg fusion Ca++ is released from Cortical ER causing a major rise in cytoplasmic Ca++ level

The majorr Ca++ rise originate from inside the egg while the minor Ca++ rise comes from outside the egg

Write short notes on the following:

In mammals, the sperm activator and the receptors is a single molecule: the glycoprotein ZP-3

In sea urchin sperm penetration of the vitelline envelope is accomplished by the release ofa specific chymotrypsin like protease from the acrosome

Fertilization cone is the cortical layer in the mammalian egg

Fast block to polyspermy is mediated by fertlization potential

Parthenogenogenesis-Eggs develop without paternal genetic contribution.

1. Modified Cell Cycles in Cleaving Embryos 2. Mechanism of Cleavage 3. Pattern of Cleavage 4. Cleavage patterns in Different Animals

Unfertilized eggs are arrested in DNA synthesis and cell division. Zygot after fertilization begins a series of rapid and highly synchronous cell divisions which cleave it into multiple cells, or blastomeres. Blastomeres are group of cells formed through mitosis during cleavage; collectively, they form the blastula and surround the blastocoel

Blastocoel-An internal cavity that forms at the blastula stage; in frog gastrulae, it allows cells to involute over the blastopore lips, where they move into the interior to form the the archenteron and mesoderm; it is later squeezed out of existence

Mesenchyme blastula- In sea urchins, the stage during which primary mesenchyme cells are ingressing into the blastocoel; this occurs during the blastula stage.

Blastulation- The process during which the blastula and blastoceol form; it results form a series of rapid mitotic divisions.

MORULA -- (mulberry) A solid ball of cells in an early mammalian embryo.

Fertilization triggers zygot to re-enter the cell cycle. A somatic cell cycle consists of 4 well defined phases:

In amphibians, there is an abrupt change in the duration and synchrony of the cell cycle, which is referred to as the mid-blastula transition (MBT).

Cell division consists of karyokinesis, the division of the nucleus, and cytokinesis , the division of the cytoplasm.

Karyokinesis is controlled by the mitotis apparatus. The mitotic apparatus is a cage-like structure consisting of spindle fibers with attached chromosomes, paired centrioles at either pole, and asters, which emanate from the centrioles toward the periphery of the cell. During mitosis, the chromosomes attach to spindle microtubules, become aligned at the metaphase plate, and are withdrawn into the daughter cells. The spindle fibers and asters are composed of microtubules. The presence of microtubules in the mitotic apparatus of sea urchin embryos can be demonstrated by staining with tubulin antibodies, or by treating cleavinig embryos with colcemid, which depolymerizes microtubules. Tubulin antibodies stain both the spindle fibers and the asters. Colcemid treatment causes the mitotic apparatus to disappear, leaving the chromosomes stranded at the metaphase plate.\

The cleavage furrow is a constriction of the egg surface that splits the egg or blastomere into two parts during the telophase. The cleavage furrow is formed by the activity of a thickened region of the egg cortex, the contractile ring which is present only during cytokinesis. The contractile ring pinches the zygote into two parts by a mechanism similar to muscle contraction. The presence of actin and myosin in the contractile ring has been demonstrated by several different experiments. First, eggs treated with cytochlasin B do not form a contractile ring or cleavage furrow. Second, actin and myosin antibodies are injected into one of the blastomeres of a two cell starfish embryo, the injected blastomere does not cleave, although its nucleus continues to multiply. The results of these experiments indicate that the cleavage furrow contains an actin-myosin contractile system.

The quantity of yolk and its distribution in the egg vary markedly between different animals. Some eggs, such as those of most mammals, have little or no yolk. These are known as alecithal eggs. Many echinoderm, mollusks, annelids and tunicates have modest quantities of evenly distributed yolk and are called isolecithal eggss. Arthropods, Cepahalopods, Amphibians, Reptiles and Birds have enormous quantities of yolk and are either called telolecithal or centrolecithal depending on the distribution.

Based on further geometric considerations, cleavage is divided into five categories: radial; bilateral; rotational

cleavage furrows- Spaces that separate the blastomeres during cleavage; in holoblastic cleavage, they pass through the entire egg, whereas in meroblastic cleavage, they do not extend all the way to the periphery of teh cytoplasm; cleavage furrows pass through teh vegetal hemispere much more slowly than through the animal hemispere.

Partial or Meroblastic cleavage- In chick embryos, a type of cleavage that is restructed to the circular disc of the cytoplasm; with such cleavage, the cleavage furrows neither extend all the way to the periphery of the cytoplasm nor cut entirely through its thickness.

Total or holoblastic cleavage- A type of cleavage in which the cleavage furrows pass through the entire egg; for example in sea urchin embryos, the entire egg undergoes cleavage, not just the animal pole, as in some organisms.

Blastodisc: The presence of large quantities of yolk in some meroblastic eggs restricts the mitotic apparatus and cleavage furrows to a small yolk-free zone at the animal pole, the blastodisc

Discoidal cleavage- In chick embryos, a type of cleavage that is resticted to the circular disc of the cytoplasm.

Embryo proper is formed from the from the blastodisc while the remainder of the zygote forming the yolk sac.

During the interphase during the 3rd and the 4th cleavage mammalian blastomere undergo a process called compaction in which they become tightly packed into a compact morula.

In mammals by 32 cell stage, the blastomere secrete fluid into spaces within the embryo to create a blastocyst cavity. this embryonic stage is called blastocyst

The cells of the outside of the blastocyst form a flattened epithelial layer, the trophectoderm

Superficial cleavage occurs in centro-lecithal eggs (Drosophila).

Energid- A small island of cytoplasm within the central yolkmass in insect eggs where the Synkaryon (Zygote nucleus) is located.

1. Embryonic Polarization 2. Gastrulation 3. Cell movement during Gastrulation 4. The Primary Organizer

Polarization of Embryo: Amphibian and Insects extensively studied, most others unknown. In Xenopus laevis polarization initiated at fertilization, and polarity is defined by differences in pigmentation along animal-vegetal axis. Animal hemisphere contains pigment granules which lacks in vegetal side.Pigment granules aggregate at sperm entry on animal pole forming a dark spots remianing throughout development. Between fertlization and first cleavage, egg cortex begins to rotate with respect to internal cytoplasm. Cortical rotation shifts the egg cortex toeard the site of sperm entry revealing gray crescent on the opposite site of sperm entry. Gray crescent is caused by the exposure lightly pigmented area of cyroplasm in the marginal zone as the darkly pigmented cortex roates towards the sperm entry point. According to fate mapping dorso-ventral axis is determined by the site of sperm entry and the gray crescent. Thus cortical rotation is vital for polarization. Agents that inhibit cortical rotation produced Ventralized Embryo: Embryos lacking dorsal Structure. Xenopus eggs treated with high hydrostatic pressure, low temperature, and ultraviolet radiation of the vegetal hemisphere develop into ventralized embryo.

Heavy water (DH2O) caused excessive polymerization of microtubles in amphibian eggs and fertlilized egg trated with heavy water develop dorsalized embryo with duplicated dorsal structure.

Animal hemisphere- The half of the egg that contains the animal pole; in frog embryos, it contains the heavily pigmented cortex; it consists of a layer of four or five cells thick; it is composed of blastomeres that contain very little yolk.

Animal pole- The uppermost part of the pigmented region of the outer portion of the egg(cortex); it corresponds to the cranial end of teh future embryo; in the amphibian egg, the blastocoel is displaced toward the animal pole; the first meiotic division occurs in the animal pole of the primary oocyte.

Animal-vegetal axis- The axis defined by the animal and vegetal poles.

Gartrulation: Process by which cells of the blastoderm are translocated to new position in the embryo producing three primary germ layers.

The three primary germ layers are ectoderm, mesoderm and endoderm. Ectoderm give rise to endoderm give rise to epidermis and nervous tissues mesoderm give rise to the muscles, bones, and connective tissues of most organs and the endoderm gives rise to the organs of the gut and all the accessory glands.

Embryos of most animal species develop bilateral symmetry and become polarized along three axes: the anteroposterior, the dorsoventral and left-right axis. The body axes are determined by the embryonic polarization, the process in which bilateral symmetry is superimposed on the egg's radial symmetry.

Bilateral symmetry- the characteristic of having right and left sides.

Radial Symmetry: The chracteristics of an object where any division of the body which is constructed along a central axis results in two similar half.

In amphibians, the primary organizer is the dorsal lip of the blastopore

Dorsal blastoporal lip- A liplike structure that forms just above the blastopore during the gastrula stage; it constitutes the organizer of teh frog embryo.

Ventral blastoporal lip- In frog embryos, the blastoporal lip that forms after the dorsal and lateral blastoporal lips form during gastrula stages.

Lateral blastoporal lips- In frog embryos, the blastoporal lips that form after the dorsal lip forms.

Blastopore- In frog embryos, a depression that forms below the gray cresent as cells initiate involution; it represents the future caudal end; throughout gastrulation, it is occupied by the yolk plug; in sea urchin and frog embryos, it is the first opening; it forms the anus.

Blastocyst- In mouse embryos, the stage following the morula in which the conceptus consits of a trophoblast, inner cells mass, and blastocoel.

Blastoderm- The structure in chick embryos that is formed when the blastodisc initiates cleavage (that is, subdivision by mitosis into cells called blatomeres.

Blastodisc- The structure in chick embryos consisting of a circular disc appoximately 35mm in diameter and containing the germinal vesicle; the blastodisc is a cytoplasmic cap that forms at one pole of teh egg, which later cleaves into the form the multicellular blastoderm.

The basic types of cell movement that occur during gastrulation: Epiboly, Intercalation, Convergent Extension, Invagination, Ingression and Involution.

The following types cell movements are responsible for the displacement of cell into the interior of the embryo during gastrulation; Invagination, Ingression and Involution.

Epiblast is a layer of cells above the blastocoel in the discoidal embryo

The presumptive mesoderm and endoderm cells of the chick epiblast appear to reach their internal locations by ingression, which results in the formation of a long cleft in the blastodisc which is called primitive disc

Invertebrates typically gastrulate by a combination of epiboly and ingression and or/invagination.

Insects embryo gastrulate by invagination

Ingression is the primary mechanism of gastrulation in birds

The embryo proper in insects such as Drosophila is derived from the ventral blastoderm

The sea urchin blastula is a single layered epithelium surrounding a blastocoel

In relatively non-yolky embryos, such as those of many invertebrates, gastrulation is initiated near the vegetal pole

The source of ectoderm, mesoderm and endoderm in the chick embryo is epiblast

Mammalian gastrulation is similar to that of Chick

The epiblast is the source of the ectoderm, mesoderm, and endoderm in the chick embryo

In Xenopus laevis all surface cells of blastula will remain as surface cells through gastrulation

The blastula of Amphioxus consists of a single-layered epithelium surrounding a blastocoel. Amphioxus gastrulates by invagination

Primitive streak forms by convergent extension.

In Drosophila Invagination of of presumptive mesoderm cells leads to ventral furrow formation

1. Neurulation: Establishment of the Body Axis and Separation of Ectoderm into subpopulations 2. Mechanism of Neurulation 3. Development of Central Nervous System 4. Development of Peripheral Nervous System 5. Growth Cone Guidance 5. Ectodermal Placodes 6. Epidermis and Cutaneous Appendages.

Body plan- The structure of the body of the embryo; the tube-within-a-tube body plan is characteristic of a vertebrate embryos; it consists of an outer ectodermal tube, forming the skin, and an inner endodermal tube, forming the gut.

Postgastrula and post neurula-stage embryos of various vertebrates are structurally similar which was stated by K.E. von Baer in 1828 and later by Haeckel in 1868

Process of neurulation in Vertebrate embryo produces 3 germ layers

In amphibian gastrulation results in a sphere consisting of 3 layers

Gastrulation in amphibians results in a sphere consisting of 3 layers: an enveloping layer of ectoderm, an inner rchenteron lined with endoderm and sheet of mesoderm in between. This type of body plan that is laid down during gastrulation has been referred to as a "tube within a tube" body plan.

In the chick embryo the 3 germ layers are arranged in concentric rings, with ectoderm covering the embryo, the endoderm forming a closed tube in the center, and mesoderm forming an intermediate layer.

Because the chick develops on a large yolk mass, the three germ layers are arranged initially as stacked sheets and although the body walls flex ventrally, the germ layers never assume the same configuration as in the early amphibian

Gastrulation doesn't produce a completely closed archenteron cavity in the chick

The partitioning of ectoderm after gastrulation is accomplished by the formation and inward displacement of the neural tube in a process known as neurulation

Soon after gastrulation this germ layer ectoderm will become partitioned into 3 cell population;epidermis, the neural ectoderm and the neural crest.

The initial indication of neurulation is the flattening and thickening of the dorsal ectoderm to form the neural plate

The neural tube is the rudiment of the central nervous system

Developing nervous system dominates the external morphology of early vertebrate embryo

Initially amphibian neural plate constitutes an oval disc encompassing the entire dorsal hemisphere of the gastrula.

In amphibian neurulation the anterior neural plate is considerably broader and will eventually form thebrain

In amphibian neurulation the neural plate elongates rapidly and folds up almost simultaneously along its length to form the neural tube.

The basic process of neurulation in chick and the amphibians are significantly similar.

In chick, all mammals and reptiles the neurulation and accompanying process of body plan organization such as formation od somites from mesoderm, begin at the anterior end while the posterior end is still undergoing gastrulation.

Two important cell shape changes accompany neurulation in chick; the first change is the development of neural plate because neural ectoderm change from a cuboidal to a columnar shape and the second is rolling of neural plate to form neural tube.

The glial cell that produce myelin membranes in the central nervous system are the oligodendrocytes

The early brain is composed of 3 distinct swellings known from anterior to posterior as prosencephalon, mesenchepalon,and rhombencephalon.

Telencephalon gives rise to olfactory lobes of the brain and cerebral hemisphere (Association-intelligence, memory)

Mesencephalon gives rise to crura cerebri and optic tectum (visual reflexes)

The mantle layer house neuron cell body and is devoid of axon and darker appearance which is known as the gray matter.

The marginal layer, glial cells from mantle layer covers axon in the marginal layer and produce myelin membranes giving the axon a glistening white appearance . Therefore the marginal layer is known as white matter.

In chick head neural crest cells give rise to portions of cranial nerves and ganglion, pigment cells and parts of the connective tissue and skeleton of the head.

25. In the trunk the neural crest cells give rise to sensory ganglia, sympathetic ganglia the medulla of adrenal gland, pigment cell of the skin, and the enteric ganglia of the gut.

Higher vertebrate's axon display precise path finding

Horseradish peroxide techniques used by Landmesser in 1978 to trace the neurons from the location of their cell bodies in the spinal cord to the particular muscle mass that they innervated.

An important source of neurons in addition to neural tube and the neural crest cells is the thickening of the ectoderm known as placodes.

At the completion of gastrulation, the mesoderm is a loosely associated layer of cells between the ectoderm and endoderm.

In amphibians mesoderm is a sheet on the dorsal surface of the endoderm.

In amniote the anterior end of endoderm is in close contact with an inpocketing of the ectoderm known as the stomodeum,these two tissues fuse to form oral plate

In chick the notochord is laid down from cells from Hansen's node

In Chick embryo the mesoderm adjacent to the notochord forms a thick band known as the paraxial mesoderm.

Intermediate mesoderm will eventually give rise to components of urogenital system.

7. Lateral plate mesoderm eventually splits to form two layers; splanchnic mesoderm and paraxial

Soon after neurulation the paraxial mesoderm begins to condense into somites.

Recent morphological studies in chick early embryogenesis have shown that the pattern of prospective somites is laid down during late gastrulation.

The paraxial mesoderm is believed to be organized into somitomeres by Hensen's node and regressing primitive streak in chick embryo.

Somitomeres: Circular swirls of the mesodermal cells in the segmental plate reflecting the pattern of future somites in the trunk.

Somitomeres always appear in paraxial mesoderm just behind regressing node.

Somites in amniotes are an epithelial ball with few loose cells filling the cavity.

In amniote the ventromedial wall of the somite loses its epithelial organization and becomes a loose association of single cells known as a sclerotome which are precursors of vertebrae and ribs..

The mesenchymal portion of the somite is known as the sclerotome

The pronephric kidney is functional only in larva of amphibians and the adults of the evolutionary lowest vertebrates.

The mesonephric kidney is the functional kidney in the embryos of higher vertebrates, adult amphibians and fishes

Mesoderm gives rise to the linings of the pericardial cavity and also major components of the heart. (Hint:germ layer)

Dermatome: Epithelial portion of the somite that become the dermis of the skin

Myotome develop into the musculature of the back, abdominal wall and limbs

Once the sclerotome cells are in place they begin to differentiate into cartilage, the tough but pliant connective tissue common in the embryo.

The intermediate mesoderm is present in the trunk only as a stalk of cells connecting the somites with the lateral plate mesoderm.

Anteriorly, the lateral plate mesoderm forms the heart and the pericardial cavity in chick.

In amphibians, the gut is already tubular at the termination of gastrulation, and the mesodern is sandwiched between the ectoderm and endoderm.

In amniotes, the gut is still a flat endodermal sheet overlying the yolk at the end of gartrulation.

The first evidence of heart development in amphibians is the proliferation of cells from the tip of the mesodermal mantles which eventually hollows out to form an endothelial tube-the endocardium.

The heart in amniotes develops initially as paired primordia that then come together and fuse as body folds swing toward the midline and fuse.

By 30 hours of incubation of chick embryo has beating tubular heart.

The adult heart in the chich is formed by five days of incubation.

In higher vertebrates the first pharyngeal pouches become the eustachian tube and tympanic cavities of the ear and second pharyngeal pouches produces the tonsils.

In the human, the earliest evidence of the liver is an endodermal outgrowth from the gut in the region that will become the duodenum. This diverticulum is known as the hepatic diverticulum.

In egg laying reptiles and birds, the extraembryonic membrane accomplish the vital function of Respiration and waste removal.

The 4 extraembryonic membranes are chorion, allantois, amnion, yolksac.

The chorion becomes a portion of the placenta in mammals.

F The embryo and the extraembryonic membranes in chick develop at the same time, where as in mammals . extraembryonic membranes develop well ahead of the embryo.

The human embryo gives rise to blastocyst by day 4

Gastrulation in human begins around day 16

The placenta is composed of 2 sources of tissue;chorion and a portion of the lining of the uterus known asdecidua.

In human embryo the neural tube undergoes flexture at day 24 as it does in chick development and the amniotic cavity expand

The portion of the uterine decidua that is unperturbed by the invading

embryo is known as the parietal decidua.

allantois- One of the extraembryonic membranes that originates from the splanchnopleure; it is formed by expansion of the endo dermal allantois rudiment and surrounding splanchnic mesoderm; in 72-hour chick embryos, it is a saclike structure somewhat encircled by the tail; in mouse embryos, it extends from the caudal end of the embryo into the extraembryonic part of the egg cylinder; in 10-mm pig embryos, it is the an endoderm-lined cavity that emerges from the umbilical cord to join the the urogenital sinus ventrally.

amnion- An extraembryonic membrane formed in chick embryos by the elevation and fusion of the amniotic folds; it all vertebrates containing an amnion, it originates from somatopleure (somatic mesodermand ectoderm); contraction of the amnion causes the early movement of the embryo.

boundary of the amnotic folds- In 48-hour chick embryos, the distinct curvature between the covered and uncovered parts of the cranial half of the embryo; in the 72-hour chick embryos, an oval or circular boundary over the caudal trunk region.

chorioallantoic membrane- In chick embryos, an extraembryonic membrane that is formed when the fully-developed allantois fuses with the chorion; fusion occurs whereever the splanchnic mesoderm of the allantois contacts teh somatic mesoderm of teh chorion.

chorion- In chick embryos, an outer extraembryonic membrane formed simultaneously with the amnion by the elevation and fusion of amniotic folds; in chick, mouse, and pig embryos, the chorion is an extraembryonic membrane formed from somatopleure.

chorionic cavity- In mouse embryos, the cavity contained within the chorion of the gastrulating/neurulating embryo, within the extraembryonic part of the egg cylinder; in chick, and pig embryos, the chorionic cavity is called the extraembryonic coelom.

coelom- See also the specific divisions of; a space that forms within the the lateral plate mesoderm, between teh somatic and splanchnic mesodermal layers.

extraembryonic coelom- Prior to body folding, it is the space contained within the extraembryonic lateral plate mesoderm, and it is continuos medially with the intraembryonic coelom; it becomes separated from the intraembryonic coelom due to the action of the somatopleural component of teh lateral body folds; with the initiation of formation of teh amnion, it extends into the somatic mesoderm-lined cavity within each amniotic fold; after fusion of the amniotic folds, it is teh space contained within the chorion, into which the allantois expand to form the chorioallantoic membrane; in 10-mm pig embryos, the intestinal loop herniates into the extraembryonic coelom of the umbilical cord, forming the temporary umbilical hernia.

compaction- The process that occurs during the morula stage as it begins its transformation into the blastocyst; during compaction, blastomeres become tightly apposed to one another.

diencephalon- A level of the neural tube formed by the prosencephalon; it is the region of the brain from which the optic vesicles evaginate.

ectoderm- one of the three germ layers; in frog and chick embryos, it is the outermost germ layer; in mouse embryos, it is the innermost germ layer, lining the amniotic cavity of the egg cylinder.

neural or ectodermal cells, neural ectoderm- The ectoderm composing the nueral plate, neural groove, and nueral tube.

inner or deep ectodermal layer- In frog embryos, one of two layers that constitute the ectoderm; it is far less pigmented than the outer ectodermal layer.

endocardial cushion- A midline gas of lightly stained tissue in the heart; it separates the right and leftatriaventricular canals.

endocardial tubes- The two primitive heart tubes; they are organized when cells emerge medially fron each cardiac primordium; thes tubes fuse ventral to the foregut in craniocaudal seqyence, due to the action of the splanchnopleural component of the lateral body folds.

endocardium- The inner layer of the heart; it is derived from the splanchnic mesoderm.

endoderm- One of the three germ layers; in frog and chick embryos, it is the innermost germ layer; in mouse embryos, it is the outermost germ layer, which forms the external surface of the embryonic part of the egg cylinder.

extraembryonic endoderm- The endoderm that contributes to the extraembryonic membranes (namely, the allantois and yolk sac).

epiblast- In chick embryos, the surface layer of the area pellucida; in mouse embryos, the layer of the blastoderm that lines the amniotic cavity during the gastrula stage.

exogastrulation- Aprocess of abnormal gastrulation; in sea urchin embryos, it causes the archenteron to evaginate rather than invaginate; in frog embryos, surface cells move, but fail to involute over the blastoporal lips-thus normal morphogenetic movements are inhibited and the formation of the archenteron is prevented.