Fertilization ,implantation, Pregnancy, Growth and Development

Site Does not occur in the uterus Chemical signals from oocyte attract the gamete specific proteins on the surface of the sperms The fertilization process takes about 24 hours

Stages of Development Pre-Embryonic Development Fertilization Cleavage Gastrulation Organogenesis Fetal Development

Conception A secondary oocyte can be fertilized for about 24 hours after ovulation Sperm remain viable for up to 48 hours within the female reproductive tract This gives a three day “window” for intercourse to result in fertilization: two days before to one day after ovulation

Fertilization usually takes place in the outer one- third of the uterine tube, but can take place in the abdominal cavity Sperm swim up the female reproductive tract, aided by muscular contractions of the uterus stimulated by prostaglandins in the semen. The oocyte may also secrete a chemical that attracts sperm

Sperm undergo a functional change in the female tract – called capacitation During this process the membrane around the acrosome becomes fragile, and its enzymes are released. It requires the combined action of many sperm to allow one sperm to penetrate the oocyte.

When the first sperm enters the egg, the cell depolarizes causing the release of calcium ions inside the cell. This stimulates the release of granules that cause changes in the zona pellucida to prevent entry of other sperm. Secondary oocyte completes division, and nuclei of ovum and sperm unite to form a zygote.

Phases of Fertilization Passage of sperm through corona radiata Penetration of zona pellucida Fusion of the plasma membrane of the oocyte and sperm Completion of second meiotic division & formation of female pronucleus Formation of male pronucleus

Passage of Sperm Through Corona Radiata Dispersal of the follicular cells of the corona radiata surrounding the oocyte and zona pellucida This results mainly from the action of hyaluronidase enzyme released from the acrosome Tubal mucosal enzymes also appear to assist in dispersal

Penetration of Zona Pellucida Zona reaction occurs once the sperm penetrates the zona pellucida It makes zona pellucida impermeable to other sperms The composition of this extracellular glycoprotein coat changes after fertilization

Fusion of the Plasma Membrane of the Oocyte and Sperm The plasma membranes of the oocyte and sperm fuse and break down at the area of fusion The head and tail of the sperm enter the cytoplasm of oocyte Sperm’s plasma membrane remains behind

Completion of Second Meiotic Division Penetration of the oocyte by a sperm activates the oocyte into completing the second meiotic division Mature oocyte and a second polar body are formed Nucleus of the mature oocyte becomes a female pronucleus

Formation of Male Pronucleus Cytoplasm of oocyte and nucleus of the sperm enlarges to form the male pronucleus Tail of the sperm degenerates Morphologically the male and female pronuclei are indistinguishable The oocyte containing two haploid pronuclei is called an ootid

Formation of a Zygote As the pronuclei fuse into a single diploid aggregation of chromosomes, the ootid becomes a zygote

Results of Fertilization Stimulates the penetrated oocyte to complete 2nd meiotic division Restores the normal diploid number of chromosomes Determines chromosomal sex of embryo Initiates cleavage (cell division) of zygote

Twins Dizygotic or fraternal twins occur when two separate eggs are ovulated. May be of different sexes. Monozygotic or identical twins occur when a single egg is fertilized but dividing cells break into two groups and develop into two individuals. Genetically identical (clones)

Zygote undergoes rapid mitotic cell division, but these do not increase the size of the zygote – called cleavage divisions Cleavage produces a solid sphere of cells, still surrounded by zona pellucida – now called a morula. At 4.5 to 5 days, cells have developed into a hollow ball of cells – blastocyst. It is at this stage that it enters the uterus.

Cleavage the first series of cell divisions by mitosis after fertilization Cell division is rapid, new cells do not take time for the growth phase G1 cell growth does not occur so cells decrease in size with each cleavage division

Cleavage of Zygote Consists of repeated mitotic divisions of zygote Rapid increase in the number of cells These smaller embryonic cells are called Blastomeres Normally occurs in the uterine tube

Cleavage of Zygote Zygote divides first into 2 then 4 and 8 Zygote is within the thick zona pellucida during cleavage Zona pellucida is translucent under light microscope Begins about 30 hrs after fertilization

Compaction Blastomere change their shape after 9 cell stage Tightly align themselves against each other Form a compact ball of cells Probably mediated by cell surface adhesion glycoprotein Permits greater cell to cell interaction

Morula When there are 12-32 blastomeres the developing human is called MORULA Enters the uterus at this stage Spherical morula forms about 3 days after fertilization Has outer and inner cell layer Resembles mulberry or blackberry

Formation of Blastocyst Morula enters the uterus 4 days after fertilization A fluid-filled space called the blastocystic cavity appears inside the morula The fluid passes from the uterine cavity through the zona pellucida to form this space

Formation of Blastocyst Blastomere is separated into two parts as the fluid increases in the blastocystic cavity A thin outer cell layer called trophoblast Inner cell mass that gives rise to embryo called embryoblast

Blastocyst has an outer layer of cells called the trophoblast, an inner cell mass, and a fluid filled cavity called the blastocele. The trophoblast and part of the inner cell mass will form the membranes of the fetal portion of the placenta, the rest of the inner mass forms the embryo.

Implantation The blastocyst remains free in the uterus a short time, during which the zona pellucida disintegrates. Blastocyst nourished by glycogen from glands of the endometrium. At about 6 days after ovulation blastocyst implants – orients cell mass toward endometrium, and secretes enzymes which allow it to penetrate (digest) the endometrial wall. This nourishes the blastocyst for about a week after implantation.

Implantation can also occur in uterine tube, cervix, or the abdominal cavity. Implantation anywhere outside the uterus is called an ectopic pregnancy. It is possible for fetus to grow in the abdominal cavity, but growth inside the uterine tube causes the tube to rupture, resulting in severe bleeding.

As early as 8 -12 days after fertilization, the blastocyst begins to secrete human chorionic gonadotropin or hCG. hCG keeps the corpus luteum active until the placenta can produce estrogens and progesterone. The presence of hCG is the basis for pregnancy tests.

Early Pregnancy Factor Is an immunosuppressant protein Secreted by trophoblast Appears in maternal serum within 24-48 hrs It forms the basis of a pregnancy test during the first 10 days of development

Inner cell mass forms two cavities: The yolk sac Amniotic cavity In humans the yolk sac produces blood cells and future sex cells The amniotic cavity becomes the cavity in which the embryo floats. Fluid is produced from fetal urine, and secretions from the skin, respiratory tract, and amniotic membranes.

Embryo: The beginning developmental processes are always the same in all animals: 1) cleavage 2) growth 3) differentiation

Primary germ layers Endoderm Mesoderm Ectoderm In between the yolk sac and the amniotic cavity is the embryonic disc, which gives rise to the primary germ layers: Endoderm Mesoderm Ectoderm

Gestation period Divided into three trimesters. During first trimester individual starts out as a zygote, then morula, blastocyst, and after implantation, is called an embryo. Embryonic phase of development lasts from fertilization until the 8th week of gestation, when it becomes a fetus. By day 35 the heart is beating, and eye and limb buds are present.

Germ Layers Ectoderm Outer layer Nervous system including brain, spinal cord and nerves Lining of the mouth, nostrils, and anus Epidermis of skin, sweat glands, hair, nails

Germ Layers Mesoderm Middle Layer Bones and muscles Blood and blood vessels Reproductive and excretory systems Inner layer (dermis) of skin

Germ Layers Endoderm Inner Layer Lining of digestive tract Lining of trachea, bronchi, and lungs Liver, pancreas Thyroid, parathyroid, thymus, urinary bladder

By month four, the rudiments of all organ systems are formed and functioning, and from then on, fetal development is primarily a matter of growth. By the end of the third month the placenta is functioning.

The placenta The chorion develops into the fetal part of the placenta. The chorionic villi connect the fetal circulation to the placenta Composed of both fetal and maternal tissues

Placenta Organ made up of both mother’s and embryo’s blood vessels Site of nutrient, oxygen, and waste exchange Does NOT allow mother and embryo blood to mix rather, the nutrients and wastes DIFFUSE between the blood vessels Acts a filter for SOME substances (alcohol, caffeine, some viruses CAN pass through) Umbilical cord – attaches the embryo to the placenta. Is cut away after birth and the scar becomes the naval.

Functions of the placenta: 1 Transfer gasses 2 Transport nutrients 3 Excretion of wastes 4 Hormone production – temporary endocrine organ – estrogen and progesterone 5 Formation of a barrier – incomplete, nonselective – alcohol, steroids, narcotics, anesthetics, some antibiotics and some organisms can cross

First Trimester Up to week 12 Organogenesis – development of body organs Heart beats around week 4 By week 8 all major structures of the adult are present in rudimentary form At end, fetus is only 5cm long

Second Trimester Months 4-6 Fetus grows about 30cm and is very active

Third Trimester Fetal activity decreases as it fills available space Fetus is ~50 cm long Lungs complete development

Quickening The first movement of the fetus felt by the mother, usually occurring during the fourth or fifth month of pregnancy By month seven the fetus is quite active During the last month the fetus becomes less active (usually due to space considerations.)

At the end of pregnancy both the mother and the uterus become “irritable” The uterus undergoes Braxton-Hicks contractions: intermittent, painless contractions which can come 10 to 20 minutes apart. Become more frequent as gestation progresses, and can be mistaken for onset of labor Cervix begins to thin and dilate

Labor (parturition) Stage one – the period from the onset of true labor contractions until the cervix is completely dilated at 10 cm. The uterine contractions cause the cervix to dilate, and the amniotic sac may rupture. Usually lasts 6 – 24 hours depending on the number of previous deliveries.

Stage 2 Period from maximal cervical dilation until the birth of the baby Lasts minutes to an hour Contractions become more intense and frequent.

Stage 3 The expulsion of the placenta Usually occurs within 15 minutes after the birth of the baby, but can range from 5 to 60 minutes.

The End !! That’s it ! You’ve made it ! Study well ! Good luck !