Embryonic Embryonic DevelopmentDevelopment

Submitted by:Ananya Choudhary

BSc (Hons) Medical Biotech

Sem-6

BMB/08/101

Embryonic DevelopmentEmbryonic Development

EmbryogenesisEmbryogenesis  is the process by which is the process by which the embryo is formed and develops, until the embryo is formed and develops, until it develops into a fetus. It starts with it develops into a fetus. It starts with the fertilization of the ovum (or egg) by the fertilization of the ovum (or egg) by sperm. The fertilized ovum is referred to sperm. The fertilized ovum is referred to as a as a zygotezygote. The zygote undergoes . The zygote undergoes rapid mitotic divisions with no significant rapid mitotic divisions with no significant growth (a process known as growth (a process known as cleavagecleavage) ) and cellular differentiation, leading to and cellular differentiation, leading to

development of an embryodevelopment of an embryo

FertilizationFertilization

Functions of fertilizationFunctions of fertilization Transmission of genes Transmission of genes Restoration of the diploid Restoration of the diploid

number of chromosomes number of chromosomes reduced during meiosisreduced during meiosis

Initiation of development Initiation of development in offspring in offspring

Steps Of Fertilization

Chemotaxis Sperm activation/acrosomal reaction activation/acrosomal reaction Sperm/egg adhesion. adhesion. Cleavage: cell division creates a : cell division creates a

hollow ball of cells called a blastulahollow ball of cells called a blastula Gastrulation: cells are rearranged into : cells are rearranged into

a three-layered gastrulaa three-layered gastrula Organogenesis: the three layers : the three layers

interact and move to give rise to interact and move to give rise to organsorgans

FertilizationFertilization activates the egg, initiating metabolic activates the egg, initiating metabolic

processesprocesses

acrosomal reactionacrosomal reaction sperm are activated sperm are activated acrosomal processacrosomal process sperm and egg membranes fusesperm and egg membranes fuse ion channels open, allowing Naion channels open, allowing Na++ to flow in to flow in fast block to polyspermyfast block to polyspermy

cortical reactioncortical reaction egg’s ER releases Caegg’s ER releases Ca2+2+ into the cytosol at site of into the cytosol at site of

sperm entrysperm entry

FertilizationFertilization slow block to polyspermyslow block to polyspermy

CaCa2+2+ causes cortical granules underneath causes cortical granules underneath the plasma membrane to fusethe plasma membrane to fuse

Mucopolysaccharides draw water into the Mucopolysaccharides draw water into the space, swelling itspace, swelling it

Vitelline layer becomes the fertilization Vitelline layer becomes the fertilization membranemembrane

CleavageCleavage rapid divisions following fertilizationrapid divisions following fertilization

often skip G1 and G2 phasesoften skip G1 and G2 phases blastomeres resultblastomeres result

most animal eggs have polaritymost animal eggs have polarity substances are heterogeneously distributed substances are heterogeneously distributed

in cytoplasmin cytoplasm vegetal polevegetal pole animal poleanimal pole

Animal pole

Vegetal pole

Process of Cleavage

Mammalian DevelopmentMammalian Development

Primitive streakPrimitive streak:: primitive streak is the structure that will

establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation. To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the first embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation 

Henson’s Node : In birds it is known as "Hensen's node", and is named

after its discoverer Victor Hensen. In amphibians, it is known as "Spemann's organizer",

and is named after Hans Spemann

GastrulationGastrulation series of cell migrations to positions where series of cell migrations to positions where

they will form the three primary cell layersthey will form the three primary cell layers inward movement of cells through the inward movement of cells through the

primitive streakprimitive streak

ImplantationImplantation ICM forms flat disk with 2 layers (epiblast and ICM forms flat disk with 2 layers (epiblast and

hypoblast)hypoblast) embryo develops from epiblast cells, hypoblast embryo develops from epiblast cells, hypoblast

forms yolk sacforms yolk sac

Human Development

GastrulationGastrulation takes place after cleavage and the formation of the blastula and primitive streak.

Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers. Each layer gives rise to specific tissues and organs in the developing embryo.

Embryonic Germ LayersEmbryonic Germ Layers

EctodermEctoderm Forms skin, hair, sweat glands, nervous Forms skin, hair, sweat glands, nervous

system, epitheliumsystem, epithelium

MesodermMesoderm Forms muscles, connective tissuesForms muscles, connective tissues

EndodermEndoderm Forms digestive and respiratory organsForms digestive and respiratory organs

Mammalian DevelopmentMammalian Development

Allantois- outpocketing of embryo’s gut, - outpocketing of embryo’s gut, incorporated into umbilical cord, forms incorporated into umbilical cord, forms blood vessels of umbilical cordblood vessels of umbilical cord

This sac-like structure is primarily This sac-like structure is primarily involved in involved in nutrition and excretionnutrition and excretion, and , and is webbed with blood vessels.is webbed with blood vessels.

The function of the allantois is to The function of the allantois is to collect collect liquid waste from the embryoliquid waste from the embryo, as well as , as well as to exchange gases used by the embryo.to exchange gases used by the embryo.

Human Embryo Development

Notochord formed from dorsal

mesoderm

Neural tube plate of dorsal ectoderm

that rolls itself (CNS), process called neuralation

Development Of Nervous System

OrganogenesisOrganogenesis SomitesSomites

serially arranged strips of lateral mesodermserially arranged strips of lateral mesoderm become vertebrae and muscles of axial become vertebrae and muscles of axial

skeletonskeleton

Neural crestNeural crest created when neural tube pinches from created when neural tube pinches from

ectodermectoderm cells migrate and form teeth, bones, PNS, cells migrate and form teeth, bones, PNS,

medulla of adrenal glandmedulla of adrenal gland

Human GestationHuman Gestation

1st trimester1st trimester fertilization resulting in zygote formationfertilization resulting in zygote formation cleavage (about 24 hours after fertilization)cleavage (about 24 hours after fertilization) blastocystblastocyst implantationimplantation placental formation (exchange structure)placental formation (exchange structure) organogenesisorganogenesis fetus- all major structures are present fetus- all major structures are present human chorionic gonadotropin (hCG)- produced by human chorionic gonadotropin (hCG)- produced by

embryo, maintains corpus luteumembryo, maintains corpus luteum mucous plug formation in cervixmucous plug formation in cervix negative feedback- cessation of ovulation and menstrual negative feedback- cessation of ovulation and menstrual

cyclescycles

Human GestationHuman Gestation

2nd trimester2nd trimester increased movement of fetusincreased movement of fetus hCG levels decline, leading to hCG levels decline, leading to

deterioration of corpus luteumdeterioration of corpus luteum placenta secretes progesteroneplacenta secretes progesterone uterus increases to visible sizeuterus increases to visible size

Human GestationHuman Gestation

3rd trimester3rd trimester rapid growth of fetusrapid growth of fetus estrogens and oxytocin initiate laborestrogens and oxytocin initiate labor positive feedback- oxytocin stimulates positive feedback- oxytocin stimulates

prostaglandin secretion by placenta, prostaglandin secretion by placenta, leading to increased contractionsleading to increased contractions