Chapter 47 Animal Development p. 998

Embryology

Epigenesis: theory that the form of an embryo gradually develops from an egg

Preformation- the belief that the egg or sperm contains a miniature embryo was once the favored theory.

Read pages 1002-1008

 

 

3 Key processes of Embryonic Development 1.   Cell Division- Production of large number of cells 2.   Differentiation- formation of specialized cells that make up tissues and organs. 3.   Morphogenesis- the movement of cells and tissues to produce body shape and form

 

 

Fertilization is a genetic and developmental event.

·       Combines the haploid sets of chromosomes and activates the egg by initiating metabolic reactions that trigger embryonic development.

 

·       P. 1000 Figure 47.2

·       The Acrosome is located at the tip of the sperm and discharges hydrolytic enzymes when it comes in contact with the jelly coat of an egg

 

·       Fertilization within the same species is assured when a protein on the surface of the acrosomal process attaches to specific receptor molecules on the vitelline layer (outer most layer) of the egg

 

·       Fusion of the egg and sperm open Na⁺ ion channels, flooding the egg.

·       Depolarization of the membrane prevents other sperm cells from fusing with the egg, called fast block to polyspermy

 

·       Membrane fusion also starts the Cortical Reaction

·       Ca+ ions flood the egg causing vitelline layer (outer most layer of the egg) harden; prevents additional sperm from entering; slow block to polyspermy

 

·       Protein synthesis and development in early embryos is directed by maternal mRNA

·       After the sperm nucleus fuses with the egg nucleus, DNA replication begins in preparation of Cleavage division (begins development of the embryo)

           

Development of the embryo begins

  

 

·       Cleavage

·       Sea Urchin 1-4 Cell Cleavage

·       Figure 47.6  p. 1003

·       succession of rapid cell divisions during which the embryo becomes partitioned into many cells called Blastomeres

 

·       converts the embryo from a single large cell to many smaller cells that can operate independently

 

·       The axis of the egg is defined by the vegetal pole (where the stored nutrients in yolk are most concentrated

o     The opposite end is called the animal pole.

 

·       cell does not grow in size; forms a solid “ball” of cells called the Morula ( a solid ball of 16 blastomeres) p. 1004 Figure 47.8 a-d Mammalian Cleavage and Morula

 

·       Cells of the Morula arrange into a hollow ball by pressing against each other. This forms the Blastula (blastocoel- the fluid filled center) Blastula

 

·       The large amount of yolk in the egg of birds’ results in Meroblastic cleavage restricted to the small disc of the cytoplasm in top of the yolk.

 

·       Holoblastic cleavage is the complete division of eggs with little amounts of yolk

·       Amphibian Stages of Development

 

 

Embryos differ in rates of division --depends upon

o     Species

o     Amount of yolk in egg

o     Mammals divide to form a Blastocyst (name of embryo at the time of implantation)

§       Cleavage in mammals occurs slowly, each division taking 12 to 24 hrs

§       The Blastocyst contains 2 types of cells

·       Trophoblast cells attach the embryo to the uterine wall and become part of the placenta

·       The remaining cells become the embryo

 

 (Germ Layer formation)

·       Gastrulation

·       Zygote begins to change its shape.

o     Blastula (or Blastocyst) invaginates forming a 2-layered cup. Inside of the cup is called the Archenteron (future gut)

 

·       Cells migrate into the blastocoel and differentiate to form 3 Germ Layers (the movement and specialization of cells to form functioning organs)

 

o     Cells move along Primitive streak (through a groove in the streak) eventually creating the germ layers.

 

 

 

3 germ layers (see table 47.1 p. 1007 for more info)

1. Ectoderm (outer layer)                         becomes epidermis of skin, lining of mouth and rectum, eyes, various glands brain and spinal cord (nervous system) and eyes; hair and nails; secretory cells of sweat gland; enamel of teeth

2. Mesoderm (middle layer)                         becomes notochord, and the following systems: skeletal, muscular, circulatory, lymphatic and excretory.

3. Endoderm (inner layer)                         becomes liver, pancreas, urinary bladder, innermost lining of digestive tract and associated organs, and reproductive organs Gives rise to accessory organs, pancreas, gall bladder and liver ( all are offshoots of digestive tract);thyroid and parathyroid glands.

 

 

 

(Neurula)

o     Neurulation- the process of forming the neural tube and neural crest cells.

·       Begins with the formation of Notochord and neural tube

o     The notochord will become the brain

1.   The notochord is a supportive chord running from head to tail. The “organizer” for further embryonic development.

2.   Above notochord, ectoderm rearranges to form the nervous system

3.   Cells on the surface become skin; the other cells (Neural Crest Cells) migrate away giving rise to other types of cells

 

o     the neural tube will become the spinal cord….formation of the nervous system.

1.   Earliest organ formed (neural tube); arises from the ectoderm. Optic vesicles also rise from the neural tube.

2.   Cells along central line form Neural Plate

3.   The Neural plate gives rise to the Neural Tube; precursor of the Central Nervous System (CNS is the brain and spinal cord)

 

 

 

In addition to primary germ layers some Animals have extraembryonic membranes.

·       Internal Development (mother supplies nutrients)

·       External Development (offspring develop outside the mother)

 

o     Amnion- encases embryo in a fluid filled sac §        prevents dehydration and cushions shock.

o     Allantois- Stores metabolic wastes, gas exchange; §       precursor to urinary bladder and umbilical cord (connects embryo to placenta)… Allantois is a receptacle for uric acid wastes (nitrogenous waste)

o     Chorion- works with Allantois in gas exchange; §       Produces HCG (human choronic gonadotropin) which initiates production of hormones that support pregnancy ·       Estrogen and Progesterone (eventually placenta becomes responsible for hormone production)

o     Yolk Sac- expands over yolk; provides stored nutrients for embryo §       Produces blood cells which then migrate into embryo §       Found in mammals birds and lizards; placental mammals contain least amount. In mammals the major function of the yolk sac is to produce blood cells.

 

 

Zygoteà CleavageàBlastulaàGastrulaàNeurulaàEmbryoàFetus

 

Human Development Animation

Weeks 3-8

 

 

Development requires extensive “programmed cell death” called APOPTOSIS

·       Cells die quickly, are engulfed by phagocytes, and are digested without a trace

·       In vertebrates, programmed cell death provides for normal development of the nervous system, immune system, and body parts such as fingers and toes.

Primitive Streak and Determination p. 1009

 

Gestation (270 days in humans)

·       Fallopian tube- location of fertilization

·       Relatively early in development, each organ appears as a recognizable rudiment (initial stage) from which the final form will develop.

·       In human, all organs are in place by the end of nine weeks, and the embryo is called a fetus.

 

·       5^th Day-

o     Blastula stage begins

o     formation of placenta begins

o     chorion, in birds, begins formation

·       7^th Day-

o     Implantation- embryo buries itself into the inner lining of uterus.

o     Formation of extra-embryonic membranes

o     All major systems formed ( may not be operational)

o     2^nd week; mesoderm forms and vertebrae develop begins

o     2^nd month; human appearance (fetus)

o     9^th week: rudiments of all major organs are present; only the heart is functional; legally a fetus at this time.

o     9 to 12 weeks: the fetus doubles in length, external genitalia appear and start to develop, and kidney excrete their first urine.

 

o     12 to 16 weeks ovaries are differentiated and primary follicles contain oogonia

o     17 to 20 weeks: mother feels the first fetal movements

§       the fetus is about 10 inches long.

o     22 weeks: Lungs, intestines and kidneys are sufficiently developed (could live outside of womb with intensive medical care)

o     24 weeks: Lungs begin to secrete surfactant, which allows them to fill with air.

 

o     26 weeks: Prematurely born fetus is likely to survive because the respiratory system is developed and functional

o     26 to 36 weeks: Increases body mass and size

 

 

 

 

Primary sex characteristics- the structures directly involved in reproduction: uterus and ovaries (female); testes (males)

Secondary sex characteristics- body hair, muscle and fat, breast, voice...indicate sexual maturity.

 

 

 

Meiotic cell division that produce eggs (oogenesis) and sperm (spermatogenesis).

 

1. Spermatogensis

2.      Oogenesis begins during embryonic development

1.      Primary oocytes are produced and remain in prophase I until puberty. At this time one primary oocyte will mature each month.

2.      The maturing oocyte will do so within the follicle (a group of cells encircling the oocyte)

3.      Upon maturity the oocyte will undergo an unequal division of the cytoplasm creating a daughter cell that contains most of the cytoplasm (secondary oocyte),

4.      the other daughter cell is called the polar body. Polar bodies will disintegrate

 

 

 

 

·       Adrenal cortex (sits on top of right kidney) release Cortical Sex hormones

·       responsible for secondary sex characteristics

·       Testosterone is responsible for development of the sex organ and stimulates testes (male reproductive organ) to manufacture sperm cells…does so by causing cells in the testes to undergo meiosis.

·       Cortical sex hormones and testosterone both cause

·       Increases in body hair, muscle growth and facial hair

 

 

·       testis (plural testes)

·      highly coiled tubes (seminiferous tubes) where sperm forms.

·       produce hormones and gametes. The hormones are produced by interstitial cells.

 

·   ·       scrotum and penis

·       sperm production cannot occur at normal body temperatures

·       gonads must be held outside abdominal cavity

·       scrotum highly folded, temp. about 2^o C lower

·       Epididymis

·       sperm will pass into the epididymis where they mature, gaining motility and the ability to fertilize.

·       during ejaculation sperm is propelled from the epididymis

·       Vas deferens

·       run from scrotum, behind urinary bladder into urethra…transfers sperm to the uterus…sperm exits the body through the uterus

Sperm

Sperm head- contains the haploid nucleus (23 chromosomes)

At the tip of the sperm head is the acrosome

Midpiece- contains mitochondria that supplies energy to the flagella.

·       Semen contains

·       mucus

·       amino acids

·       large amounts of fructose (energy source for sperm)

 

Accessory organs which add semen

1.               A pair of seminal vesicles ·       secretes thick, clear fluid ·       also secretes ·        prostaglandins which stimulate contractions of the uterine muscles helping semen up the uterus ·       proteins in the semen “corral” sperm together to make movement to uterus easier.

2. Prostate Gland ·       largest semen secreting gland ·       secretes directly into the urethra through several small ducts. ·       Prostatic fluid ·       thin and milky and contains several enzymes ·       high alkalinity ·       neutralizes any residual urine in the urethra and the natural acidity of the vagina ·       helps activate sperm   ·       Prostate cancer common medical problem for men over 40 ·       Benign (non cancerous) enlargement occurs in more than 50% of all men.

 

 

 

·       Ovum-

·       female gamete

·       haploid

·       nonmotile egg

·       unfertilized egg is usually large in size.

 

1. Ovary-
1. the organ that produces the ova (ovum) or egg. Each female has 2 ovaries. 

 

Oviduct or fallopian tube

Eggs move from the ovary to the uterus through the oviduct.

Uterus

A fertilized ovum implants on the inside wall, or endometrium, of the uterus. Development of the embryo occurs here until birth

Vagina

At birth, the fetus passes through the cervix, the opening to the uterus, through the vagina, and out of the body

·       Ovulation-

·       the release of an egg from ovaries. In humans, ovarian follicle releases an egg during each menstrual cycle.

·       Follicle- microscopic structure in the ovary that contains the developing ovum and secretes estrogen.

 

 

·       Ovarian cycle

·       Cyclic recurrence of the follicular phase, regulated by hormones.

 

 

1.   release hormones ·       estrogen and progesterone( both involved in the menstrual cycle). ·       considered an Endocrine gland ·       Endocrine system- release hormones and contains the endocrine glands. Endocrine glands release hormones directly into the bloodstream without the use of ducts.

2.               manufactures female gamete called  the Ova

 

 

 

Ovarian Cycle- cyclic recurrence of the follicular phase, regulated by hormones

 

 

1.   Follicular Phase

2.   Luteal Phase

3.   Menstruation