1

EMBRYOLOGY SPRING 2012

2

REPRODUCTIVE ANATOMY – FEMALES Ovaries –

Female gonad (2) Produces egg cells & reproductive hormones Surface of ovary is covered in follicles 400,000

found in most women; formed before birth Follicle –

A cluster of cells that surround and nourish a developing egg cell

Secretes estrogen (hormone) ~ 1 follicle matures and releases an egg cell in

women every 28 days from puberty menopause This releasing of the egg cell is called ovulation

3

REPRODUCTIVE ANATOMY – FEMALES After ovulation, the remaining follicle tissue

continues to grow w/in the ovary Forms a solid mass called the corpus luteum The CL secretes:

Progesterone (hormone) that maintains the uterine lining during pregnancy

More Estrogen (hormone)

If egg is NOT fertilized CL disintegrates & a new follicle matures during next cycle

More later on if the egg is fertilized…

4

REPRODUCTIVE ANATOMY – FEMALES Oviduct / Fallopian Tube –

“Tube” in which egg cell travels after ovulation Cilia push the egg cell toward the uterus Fertilization usually occurs in the oviduct

Uterus / womb – Site of pregnancy Lined with a rich supply of blood vessels –

endometrium Embryo implants in the endometrium linning

Embryo is considered a fetus at the ninth week of development

5

REPRODUCTIVE ANATOMY

6

REPRODUCTIVE ANATOMY – MALES Testes –

Male gonad (2) Produces sperm (in the seminiferous tubules w\

in) and reproductive hormones “Housed” outside the body in the scrotum

sperm cannot develop at human core temperature, therefore the scrotum placement allows sperm forming cells to function normally

Vas Deferens – Moves sperm away from the testes

7

FORMATION OF SPERM CELLS Spermatogenesis –

Formation of sperm cells Takes 65-75 days in humans Cells differentiate into primary spermatocytes (diploid)

Undergo Meiosis Two secondary spermatocytes are produced (haploid)

Undergo Meiosis II Four sperm cells are produced (haploid)

8

SPERM CELLS Acrosome –

Contains enzymes that help sperm penetrate the egg

Surrounds the head/nucleus

Tail – Flagellum

Neck & Middle piece – Contain mitochondria Mitochondria provide ATP for movement of the

tail

9

OOGENESIS Oogenesis –

Formation of egg cells Majority of the process occurs in the ovaries, prior to birth

10

FORMATION OF EGG CELLS At birth, each follicle contains a

dormant primary oocyte (diploid)

“Frozen” in Prophase I

At puberty, a hormonal change (FSH) triggers the primary oocyte to complete Meiosis I every 28 days

Haploid

Follicle enlarges primary oocyte completes Meiosis I and begins M2

Meiosis then STOPS again at Metaphase 2 (haploid)

11

FORMATION OF EGG CELLS (CON’D) Larger haploid cell is now the Secondary oocyte

Smaller haploid disintegrates The secondary oocyte is the stage that is released by the ovary (ovulation) If a sperm cell enters the ova, the secondary oocyte completes meiosis 2 Meiosis 2 creates another polar body and the actual egg cell Egg + Sperm = zygote

12

HORMONES FSH –

Follicle-Stimulating Hormone Increases sperm production (males) Stimulates the growth of the follicle (females)

LH – Luteinizing Hormone In males, stimulates sperm production In females:

Stimulates growth of follicle Production of secondary oocyte Promotes ovulation

13

EMBRYONIC DEVELOPMENT If fertilization occurs:

The sperm and egg fuse zygote Remember: Chromosome number never changes after

this point!!!

First two major stages of development:1) Cleavage 2) Gastrulation

14

CLEAVAGE Cleavage is the rapid chain of cell divisions

(differentiation) that create a multi-cellular embryo (blastula) from the single- celled zygote Zygote does not grow in size during cleavage

As cells divide, a solid ball of cells is formed morula

Morula

15

CLEAVAGE Cell divisions split the zygote into many smaller cells

Divisions act as an organizer dividing the embryo into developmental regions (more on this…)

As cleavage continues, a fluid-filled cavity forms in the center called the blastocoel

The blastocoel is surrounded by 1+ layers of cells

This new “hollow ball” is called the blastula

AKA - blastocyst in humans

The blastula now enters the gastrulation stage

16

GASTRULATION The 2nd major phase of embryonic

development Adds more cells to the embryo Cell division continues (now called a gastula) Cells are sorted into three distinct layers of

embryonic tissue:1) Ectoderm (outer most layer)2) Endoderm (inner most layer)3) Mesoderm (middle layer)

These three cell layers eventually develop into the various parts of our human body

17

GASTRULATION A small groove (the Blastopore), forms at

one side of the gastula

Cells that will form the ectoderm spread across the surface of embryo; mesoderm cells spread inside the embryo

Endoderm cells create a digestive cavity called the archenteron

18

GASTRULATION Gastrulation is complete when the embryo is

3-layered

• The endoderm & archenteron fill the blastocoel space

• The entire surface (except the yolk plug) is covered by the ectoderm

- Yolk plug will be future anus

19

GASTRULA LAYERS Ectoderm –

Epidermis, nervous system, spinal cord, eyes, tooth enamel, hair

Endoderm – Lining of digestive tract, digestive organs, pancreas,

liver, respiratory system, lungs, tonsils, urinary tract, reproductive system, tongue

Mesoderm – Most organs (kidneys, heart), skin, bones, muscles,

blood, circulatory system, excretory system

20

ORGAN FORMATION Once the 3-layers form, cells in each layer

differentiate into tissues and organs begin to form!

Notochord develops (from mesoderm) between the digestive tract and nerve cord

Present only in embryos replaced by backbone

Neural tube also forms (from ectoderm) Gives rise to the brain and spinal cord

21

EMBRYONIC DEVELOPMENT SEQUENCE

1. Zygote forms2. Cleavage of cells

3. Blastula forms4. Gastrulation

5. Organs form!

22

GESTATION Another term for “pregnancy” Average of 40 weeks for humans Cleavage begins ~ 24 hours after fertilization By 7th day embryo blastocyst

Blastocyst implants in uterine wall Outer layer called the trophoblast

Aids in implantation and forms the placenta

Implantation occurs after ~ one week Gastrulation begins after implantation

23

GESTATION Blastocyst & trophoblast give rise to the

extraembryonic membranes. This forms during 1st trimester and consists of: Amnion – encloses the amniotic sac containing embryo

and fluid; protects embryo; breaks just prior to birth (water breaks)

Chorion – forms part of placenta; secretes HCG hormone HCG VERY IMPT! W/O this menstration would occur emrbyo

would abort

Yolk Sac – produces embryos 1st blood cells

Allantois – forms part of umbilical cord (connects embryo to placenta via the “belly button”

24

25

PLACENTA Provides nourishment and oxygen to the embryo; stores

embryonic waste Chorionic villi – contain embryonic blood vessels; villi

absorb nutrients & oxygen from mom’s blood and pass on to embryo NOTE: Mom’s blood & embryo blood NEVER touch

C.V. also carry waste away from embryo Placenta allows protective antibodies to pass from mom to fetusHOWEVER, placenta cannot protect against certain viruses (HIV) or drugs/alcohol serious injury to fetus

26

3 TRIMESTERS 1ST Conception birth is divided into 3 trimesters First trimester (1-3 mo)

Multiple tissue layers and organs have developed

All organs and major body parts are present Fingers and toes are developing

9 weeks

27

Second trimester (4-6 mo) Increases size and refines structures Placenta takes over progesterone secretion Has eyebrows, eyelashes, fingernails, toenails,

body hair Very active Eyes open and teeth form

3 TRIMESTERS 2ND

28

Third trimester (7-9 mo) Gains weight and strength Respiratory and circulatory systems prepare for

air breathing Bones harden and muscles thicken Loses body hair

3 TRIMESTERS 3RD

29

CHILDBIRTH Labor – rhythmic contractions of the uterus

Oxytocin – hormone that stimulates contractions of the uterus and mammary glands

Post-birth, babies lungs expand and are used for the first time!

30

REPRODUCTIVE OPTIONS

Technology can help infertile couples In Vitro Fertilization (IVF)

Most common type of assisted reproductive technology

Woman’s eggs (secondary oocyte) are mixed with sperm in a culture dish and incubated to aid fertilization

After an embryo has been developed (8 cells), it is inserted into the woman’s uterus

Higher risk of birth defect/low birth rate with IVF babies Approx. $10,000 a try

31

REPRODUCTIVE OPTIONS Fertility drugs are also commonly used Only available for women Drugs stimulate follicle development increase the number of eggs released Unfortunately, this is very risky 2+ fetusesincreases the risk of birth defect, low birth rate, mental retardation, etc.

32

STEM CELLS

Unspecialized cells that can differentiate and create new, specialized cells These then become tissues/organs

Many types (amniotic, umbilical cord, etc.) Two we will focus on:

1) Embryonic 2) Adult

33

STEM CELLS Embryonic –

Come from blastocysts (human embryos) Can differentiate into all types of specialized

cells (aka – can become the cell of any body tissue)

Adult – Come from three sources:

1) Bone marrow2) Adipose tissue (fat/lipid cells)3) Blood

Can only differentiate into some cell types

34

STEM CELL POSSIBILITIES Under the right conditions (grown in a

culture), embryonic stem cells can reproduce indefinitely

Using Bob’s cells, the nucleus is taken out and inserted into the ES cell Since these stem cells are now genetically

identical to Bob’s own cells, his body will not reject these new cells!

Embryonic stem cells could cure/help many diseases: Diabetes, heart disease, Parkinson’s,

Alzheimer's, Lou Gehrig’s, arthritis, and/or organ failure

35

STEM CELL CONTROVERSY Ethical issues –

ES cells can only come from discarded embryos (donated by infertile patients) Many fear that embryos will be created just to be

destroyed