Fertilization and Development Pregnancy: events that occur from - - PowerPoint PPT Presentation

Fertilization and Development

• Pregnancy: events that occur from fertilization

until infant is born

• Conceptus
• Gestation period

– time from last menstrual period until birth – fertilization approx. 14 d into gestation – ~280 days

• Embryo: fertilization through week 8
• Fetus: week 9 through birth

Diagram shows the approximate size of a human conceptus from fertilization to the early fetal stage.

Fertilization 1-week conceptus 3-week embryo (3 mm) 5-week embryo (10 mm) Embryo 8-week embryo (22 mm) 12-week fetus (90 mm)

Fertilization

• Sperm must reach secondary oocyte

– Oocyte viable for 12 to 24 hours – Sperm viable 24 to 48 hours after ejaculation

• Only a few thousand out of millions of

ejaculated sperm make the trip through the uterine tubes

– 12-cm (5-inch) journey – Most are lost

• Fertilization: sperm’s chromosomes combine

with those of secondary oocyte to form fertilized egg, called a zygote

Sperm Capacitation

• Secretions of female tract help to weaken and thin
• ut acrosome membrane

– Motility enhanced – Sperm cell membranes begin releasing hydrolytic enzymes (delaying until needed at corona radiata) – Occurs after several hours exposed to female reproductive secretions – Incapacitated sperm can’t fertilize

• Sperm have olfactory receptors that can follow

chemical trail released by egg or surrounding cells

Acrosomal Reaction and Sperm Penetration

• Oocyte protective layers must be digested
• Corona radiata and zona pellucida slowly degraded

by the action of hundreds of sperm

• Cells displaced by sperm enzymes degrading cell

to cell connections

• Zona degraded by releases of many acrosomes
• The single sperm reaching the membrane binds

acrosomal markers to oocyte membrane receptors

• Sperm is bound and contents ‘emptied’ into oocyte

Sperm, delivered to the vagina and capacitated in the female reproductive tract, stream toward a secondary

• ocyte.

Sperm Zona pellucida Oocyte nucleus arrested in meiotic metaphase II Polar body Granulosa cells of corona radiata Sperm Extracellular space

• Approach. Aided by enzymes
• n its surface, a sperm cell weaves

its way past granulosa cells of the corona radiata. 1 Zona pellucida Extracellular space Oocyte plasma membrane

2 3 4 5 Zona pellucida sperm-binding receptors Oocyte sperm-binding membrane receptors Cortical granules Sperm nucleus Mitochondria Microtubules from sperm flagellum Acrosomal reaction. Binding of the sperm to receptors in the zona pellucida causes Ca2+ levels within the sperm to rise, triggering the acrosomal

• reaction. Acrosomal enzymes from many

sperm digest holes through the zona pellucida, clearing a path to the

• ocyte membrane.
• Binding. The

sperm’s membrane binds to the oocyte’s sperm-binding receptors.

• Fusion. Sperm

and oocyte plasma membranes fuse. Sperm contents enter the oocyte. Blocks to polyspermy. Oocyte sperm-binding membrane receptors are shed. Ca2+ levels in the oocyte’s cytoplasm rise, triggering the cortical reaction (exocytosis of cortical granules). As a result, the zona pellucida hardens and the zona pellucida’s sperm-binding receptors are clipped off.

Blocks to Polyspermy

• Polyspermy prevented in humans

– One-sperm-per-oocyte condition

• Two mechanisms ensure monospermy

– first encounter between sperm and oocyte membrane receptors causes destruction of remaining receptors – Zygote destroys zona receptors shedding remaining sperm from the area

2 1 3 4

Extracellular space Corona radiata Zona pellucida Second meiotic division of oocyte Second meiotic division of first polar body Male pro- nucleus Female pro- nucleus (swollen

• vum nucleus)

Polar bodies Male pronucleus Mitotic spindle Centriole Female pronucleus Zygote Male and female pronuclei Polar bodies Sperm nucleus After the sperm penetrates the secondary oocyte, the oocyte completes meiosis II, forming the ovum and second polar body. Sperm and ovum nuclei swell, forming pronuclei. The DNA in each pronucleus replicates. The pronuclei approach each other and a mitotic spindle forms between them. Chromosomes of the pronuclei intermix. Fertilization is accomplished and the cell, now called a zygote, is ready for the first cleavage division.

Completion of Meiosis II and Fertilization

Cleavage

• Zygote is slowly moving toward uterus
• Rapid mitotic divisions

– No cell growth: Cell size optimized for exchange – First occurs after ~36 hours

• Two daughter cells called blastomeres
• After 72 hours, cluster of cells contains 16 or

more cells and is referred to as a morula

Zygote to Blastocyst Implantation

Figure 28.3 Cleavage: From zygote to blastocyst. Zygote (fertilized egg) 4-cell stage 2 days Morula (a solid ball

• f blastomeres)

3 days Early blastocyst (Morula hollows out, fills with fluid, and “hatches” from the zona pellucida) 4 days Implanting blastocyst (Consists of a sphere

• f trophoblast cells and

an eccentric cell cluster called the inner cell mass) 7 days Degenerating zona pellucida Blastocyst cavity Zona pellucida Sperm Uterine tube Fertilization (sperm meets and enters egg) Oocyte (egg) Ovary Ovulation Uterus Endometrium Cavity of uterus Trophoblast Blastocyst cavity Inner cell mass

Blastocyst Formation

• Around day 4 or 5 blastocyst reaches uterus

– Embryo now consists of ~100 cells

• Trophoblast cells

– Participate in placenta formation

• Inner cell mass: cluster of 20–30 rounded cells

– Becomes embryonic disc, which will form embryo and three or four extraembryonic membranes

• Fourth extraembryonic membrane (chorion) is formed

by trophoblast

– Now called embryoblast

Implantation and Placentation

Implantation

• Blastocyst floats for about 2–3 days

– Nourished by uterine secretions

• Begins 6–7 days after ovulation

– Trophoblast cells grow favorable connection to endometrium penetrating it – Blastocyst incorporated under endometrial epithelium – Endometrium still being supported by ovarian hormones and hCG

• Complete by day 12

Failed implantation may be as high as 2 out of every three

Figure 28.4a Implantation of the blastocyst.

Endometrium Uterine endometrial epithelium Inner cell mass Trophoblast Blastocyst cavity or blastocoele Lumen of uterus

Figure 28.4b Implantation of the blastocyst.

Endometrial stroma with blood vessels and glands Syncytiotrophoblast Cytotrophoblast Blastocyst cavity Lumen of uterus

Human Chorionic Gonadotropin

• Embryo dependent on endometrium at this time
• Secreted by trophoblast cells and later the chorion
• Prompts corpus luteum to continue secretion of

progesterone and estrogen

• Promotes placental development
• hCG levels begin decline (2 months) as placenta

begins to secrete progesterone and estrogen

• Low values occur at 4 months and continue for rest
• f pregnancy

Figure 28.5 Hormonal changes during pregnancy.

Human chorionic gonadotropin Estrogens Progesterone Relative blood levels Gestation (weeks) Ovulation and fertilization Birth 4 8 12 16 20 24 28 32 36

Figure 28.6a Events of placentation, early embryonic development, and extraembryonic membrane formation.

Maternal blood vessels Cytotrophoblast Amniotic cavity Proliferating syncytiotrophoblast Bilayered embryonic disc

• Epiblast
• Hypoblast

Implanting 7½-day blastocyst. The syncytio- trophoblast is eroding the endometrium. Cells of the embryonic disc are now separated from the amnion by a fluid-filled space. Endometrial epithelium

Figure 28.6b Events of placentation, early embryonic development, and extraembryonic membrane formation.

Endometrium Maternal blood vessels Proliferating syncytiotrophoblast Cytotrophoblast Amniotic cavity Bilayered embryonic disc

• Epiblast
• Hypoblast

Amnion Yolk sac Extraembryonic mesoderm Chorion being formed Lumen of uterus 12-day blastocyst. Implantation is complete. Extraembryonic mesoderm is forming a discrete layer beneath the cytotrophoblast.

Figure 28.6c Events of placentation, early embryonic development, and extraembryonic membrane formation.

Lacuna (intervillous space) containing maternal blood Chorionic villus Chorion Amnion Yolk sac Extraembryonic mesoderm Lumen of uterus Extraembryonic coelom Allantois Forming umbilical cord

• Endoderm
• Mesoderm
• Ectoderm

Amniotic cavity Primary germ layers 16-day embryo. Cytotrophoblast and associated mesoderm have become the chorion, and chorionic villi are elaborating. The embryo exhibits all three germ layers, a yolk sac, and an allantois, which forms the basis of the umbilical cord.

Figure 28.9 Folding of the embryonic body, lateral views. Tail Head Amnion Yolk sac Ectoderm Mesoderm Endoderm Trilaminar embryonic disc Future gut (digestive tube) Lateral fold Somites (seen through ectoderm) Neural tube Notochord Primitive gut Foregut Hindgut Yolk sac Yolk sac Tail fold Head fold

Figure 28.6d Events of placentation, early embryonic development, and extraembryonic membrane formation.

Decidua basalis Maternal blood Chorionic villus Umbilical blood vessels in umbilical cord Amnion Amniotic cavity Yolk sac Extraembryonic coelom Chorion Decidua capsularis Lumen

• f uterus

4½-week embryo. The decidua capsularis, decidua basalis, amnion, and yolk sac are well formed. The chorionic villi lie in blood-filled intervillous spaces within the endometrium. The embryo is nourished via the umbilical vessels that connect it (through the umbilical cord) to the placenta.

Placentation

• Placenta is a temporary organ that originates

from both embryonic and maternal tissues

• Provides

– Nutrition – Gas exchange – Waste removal and fluid balance – Transport for hormones – Pathway for substances in mother’s blood to fetal blood

• Fully functional by third month

Placentation

• Trophoblast cells proliferate and differentiate into

– Chorionic villi – Umbilical blood vessels – surrounded by blood-filled lacunae in the stratum functionalis of the endometrium

• Exchange occurs here

– Maternal and embryonic blood supplies normally do not intermix

• Together chorionic villi and layers of functional endometrium

make up placenta

• Layers of cells in the inner cell mass will differentiate into

three cell layers which form the origin of all of the cells in the body - called embryonic disc

Figure 28.6e Events of placentation, early embryonic development, and extraembryonic membrane formation.

Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic cavity Umbilical cord Uterus Lumen of uterus Decidua capsularis Extraembryonic coelom 13-week fetus.

Figure 28.7 Detailed anatomy of the vascular relationships in the mature decidua basalis. Decidua capsularis Chorion Amnion Amniotic fluid Lumen of uterus Mucous plug Yolk sac Placenta Umbilical cord Uterus Umbilical arteries Umbilical vein Amnion Umbilical cord Connection to yolk sac Fetal portion

• f placenta

(chorion) Maternal portion of placenta (decidua basalis) Stratum basalis of endometrium Myometrium Maternal veins Maternal arteries Fetal venule Fetal arteriole Maternal blood in lacuna (intervillous space) Chorionic villus containing fetal capillaries Chorionic villi Decidua basalis

Embryonic Development: Extraembryonic Membranes

Form during first 2–3 weeks of development and include:

• Amnion: transparent sac filled with amniotic fluid

that envelopes embryo

– Buoyant, constant homeostatic temperature, freedom of movement, prevents parts from fusing together

• Yolk sac
• Allantois
• Chorion - Encloses embryonic body and all other

membranes