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PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor

Embryology Lecture 5

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Development involves:1) Cell division and differentiation

Differentiation is the creation of different types of cells2)  Changes in anatomical structures

Anatomical changes include gradual modification of physical and physiological characteristics

Development begins at fertilization

Differentiation and development

Development can be divided into: Prenatal and postnatal development

Prenatal development begins at fertilization and ends with birth Prenatal development includes:

Embryological development Changes occurring during the first two months after

fertilization Fetal development

Begins at the start of the ninth week and continues until birth

Postnatal development Commences at birth and continues to maturity

Stages of development

Fertilization is fusion of two haploid gametes (egg and sperm) each with 23 chromosome to produce a zygote that contains 46 chromosomes

Fertilization occurs in the uterine tubes Within a day of ovulation

Spermatozoa cannot fertilize an ovum until after capacitation

Fertilization (conception)

Fertilization Steps

• Step 1 Ovulation• At ovulation the oocyte is in metaphase

of meiosis II• Both the occyte and the polar body is

surrounded by corona radiate• Step 2 Fertilization

• Oocyte is surrounded by the sperms• Acrosomal enzyme from several sperms

creates gaps in corona radiata• One sperm makes contact with the

oocyte membrane• Sperm and oocyte fused • The process of meiosis will

complete

Fertilization

The first trimester is the period of embryological and early fetal development

Four processes occur during the first trimeter1) Cleavage2) Implantation 3) Placentation 4) Embryogenesis

The First Trimester

Cleavage - cleavage is the first cell division The first cleavage occurs at 32 hours Second cleavage occurs at 60 hour Third cleavage occurs at 72 hours Several divisions later the embryo becomes a solid mass of cells

known as morula Molurla becomes a blastocyst (also known as blastula) Blastocyte includes:

Trophoblast – outer layer of cells that give rise to the chorion and placenta

Inner cell mass – cluster of cells at one end of blastocyst Blastocoel – hallow fluid-filled inner cavity

The First Trimester

Cleavage

Cleavage and Blastocyst Formation

Implantation Attachment of blastocyst into the uterine

endometrium The uterus is prepared for implantation by the hormone

progesterone (see reproductive and endocrine lectures)Occurs about 7 days after fertilization

Placentation Blood vessels form around blastocyst and

placenta developsThe placenta is a complex organ that permits

exchange between the maternal and embryonic circulatory systems

The First Trimester

Implantation and Placentation

GastrulationAfter implantation, cell migration transforms

blastula (single cell layer) to gastrula (3 layered structure)

Gastrulation is the generation of 3 distinct germ (cell) layers

EndodermMesodermEctoderm

The First Trimester

The Inner Cell Mass and Gastrulation

Gastrulation By day 12 surface cells move toward the primitive streak A third germ layer forms

The three germ layers are: Ectoderm – superficial cells that did not migrate

Will become integument (epidermis, hair, nail, epithelium, nose, mouth and anal canal), lense of eye, ear and nervous system

Endoderm – cells facing the blastocoele Will become liver, pancreas, thyroid, bladder, lungs, epithelium of

digestive and respiratory systems. Mesoderm – migrating cells between ectoderm and endoderm

Will become musculskeletal system, circulatory system, excretory system, gonads, connective tissue of digestive and respiratory systems, smooth and cardiac muscle.

Germ layers

Germ layers

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Germ layers

Organs and glands are consist of or include more than one germ layerAdrenal cortex is derived from mesodermAdrenal medulla is derived from ectoderm

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NeurulationBy the end of gastrulation, regions of the germ

layers begin to develop into a rudimentary nervous system

Neurulation is the process of rudimentary nervous system formation Notochord – a rod of mesodermal cells that

develop along the axis. Notochord cause ectoderm to bend inward and

form a groove Ectoderms grow on either side of the groove and then

fuse together to form a neural tube Neural tube becomes central nervous system (Brain and

spinal cord)

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Fetal Respiration Growing fetus receives oxygen and eliminates carbon

dioxide through a specialized circulatory system The two components of this system are:

Placenta Umbilical cord

Placenta and umbilical cord are outgrowths of the 4 extra-embryonic membrane (Amnion, Chorion, Allantois and yolk sac) that are formed during the development Chorion extends villi into uterine wall and these villi become

closely associated with endometrial cells Chorionic villi forms intricate branching network for maternal

blood Umbilical cord connects fetus to placenta

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Four extraembryonic membranes:Yolk sac AmnionAllantoisChorion

Extraembryonic Membranes

Extraembryonic Membranes and Placenta Formation

Extraembryonic Membranes and Placenta Formation

Extraembryonic Membranes and Placenta Formation

Yolk sac Important site of blood cell formation

Amnion Encloses fluid that surrounds and cushions developing

embryo Allantois

Eventually becomes bladder Chorion

Placenta formation begins for chorion Chorion is a membrane that completely surrounds

amnion

Embryo Anatomy

A Three-Dimensional View of Placental Structure

Trophoblast secretes hormones to maintain pregnancy Human chorionic gonadotropin (hCG) – Prevents disintegration of corpus

luteum At the ovaries corpus luteum produce progesterone

Estrogens – in addition to having role in secondary sexual characteristics it is involved in the thickening of the endometrium

Progesterone - enriches the uterus with a thick lining of blood vessels and capillaries so that it can sustain the growing fetus

Human Placental Lactogen (hPL) - decreases maternal insulin sensitivity, and therefore raises maternal blood glucose levels, whilst decreasing maternal glucose utilization, which helps ensure adequate fetal nutrition

Placental prolactin- stimulates the mammary glands to produce milk Increased serum concentrations of prolactin during pregnancy cause enlargement of

the mammary glands of the breasts and increases the production of milk. However, the high levels of progesterone during pregnancy stop ejection of milk.

After childbirth progesterone decreases and milk ejection initiates. Relaxin

Hormones of the placenta

Second trimesterOrgan systems increase in complexity

Third trimesterMany organ systems become fully functionalFetus undergoes largest weight changeAt end of gestation fetus and uterus push

maternal organs out of position

Second and Third Trimesters

The Second and Third Trimesters

Growth of the Uterus and Fetus

Growth of the Uterus and Fetus

Developing fetus totally dependent on maternal organsMaternal adaptations include increased

Respiratory rateTidal volumeBlood volumeNutrient and vitamin uptakeGlomerular filtration rate

Structural and Functional Changes in the UterusProgesterone inhibits uterine muscle

contraction Opposed by estrogens, oxytocin and

prostaglandinsMultiple factors interact to produce labor

contractions in uterine wall

Factors Involved in the Initiation of Labor and Delivery

Labor and Delivery

Stages of laborDilation

The cervix dilates and fetus moves toward cervical canal

Expulsion The cervix completes dilation and fetus emerges

Placental Ejection of the placenta

Goal of labor is parturition