Fertilization and Development

• Pregnancy involves fertilization and early embryonic development.

Ovum Structure

• The ovum contains follicle cells, corona radiata, nucleus, nucleolus, cytoplasm, zona pellucida, plasma membrane (vitelline membrane), and perivitelline space.

Sectional View of Ovary

• The Ovary contains:
• Primary follicle
• Secondary follicle
• Tertiary follicle
• Graafian follicle
• Blood Vessels
• Corpus Luteum
• Ovum

Sperm structure

• A Sperm contains:
• Head (with Acrosome and Nucleus)
• Acrosome – digestive enzymes
• Nucleus – 23 chromosomes
• Mid piece (with Mitochondria and Centriole)
• Collar (containing many mitochondria)
• Tail (with Flagellum)
• Flagellum – causes sperm to swim

Definition of Terms

• Growth is an increase in size through an increase in cell number by mitosis.
• Development includes growth and changes in life phases to accomplish physiologic changes.
• Fetus development can be divided into 4 stages.
• Fertilization takes 24 hours.
• Blastocytes form between the 2nd and 6th day.
• The embryonic stage is from the 7th day to the 4th week.
• The fetal stage is from the 4th week onward.

Zygote Development

• The stage of development is Zygote -> Embryo -> Fetus -> Baby.

Fertilization

• Fertilization, also known as conception, is the fusion of the genetic material of a spermatozoon with a mature ovum (secondary oocyte).
• The ovum's nucleus is surrounded by cytoplasm, the zona pellucida (jelly coat), and the corona radiata (follicular cells).
• Sperm includes structures like the acrosome, sperm nucleus, cortical granules, cytoplasm, zona pellucida, and follicle cells.
• During fertilization, the oocyte is moved by cilia and peristaltic contractions of the uterine tube.
• The cervix secretes alkaline mucus to attract spermatozoa.
• After deposition in the posterior fornix of the cervix, 2 million sperms reach loose cervical mucus and uterine tubes out of 300 million sperm -Some sperm are destroyed by the acidic medium in the vagina
• Sperm undergo capacitation, which involves physiological changes such as increased flagella action and removal of glycoproteins, enabling penetration and fertilization within the uterine tube.
• The zona pellucida of the oocyte produces chemicals that attract capacitated sperms.
• Hyaluronidase, an enzyme from the acrosome of sperm, disperses the corona radiata, allowing access to the zona pellucida.
• Only one spermatozoon penetrates the zona pellucida, after which the cell membrane becomes impenetrable to other sperms.
• Oocyte maturation occurs when the plasma membranes of sperm and egg fuse.
• The sperm contains either a Y or an X chromosome, while the ovum contains only an X chromosome
• XY sperm yields a male and XX sperm yields a female
• Fraternal (dizygotic) twins form from two ooctyes released independently. Monozygotic (identical) twins develop from a single zygote that divides into two during division/cleavage.
• Dizygotic twins have genetically distinct separate placentas
• Monozygotic twins have genetically identical shared placentas during zygote splits

Development of Zygote

• The development of a zygote includes three phases: pre-embryonic, embryonic, and fetal periods.
• Pre-embryonic period (2 weeks after fertilization)
• Embryonic period (2 to 8 weeks after fertilization)
• Fetal period (weeks 8 to birth)

Pre-embryonic Period

• After fertilization, the zygote travels along the uterine tube to the uterus, surrounded by the zona pellucida.
• The zygote is nourished by nutrients from goblet cells and secretory cells in the uterine tubes and uterus.
• The zygote undergoes mitotic division called cleavage, resulting in small cells called blastomeres.
• Cell division occurs:
• 2 cells on day 1
• 4 cells on day 2
• 8 cells on day 2.5
• 16 cells on day 3 forming a morula

Pre-Embryonic period: Cavitation & Blastocyst

• Cavitation occurs when the outermost cells secrete fluid into the morula, creating a fluid-filled cavity, forming a blastula through the process of blastulation.
• During cleavage, the size of the developing structure does not initially increase, but cells divide into smaller cells bounded by the zona pellucida.
• By day 4, the blastocyst enters the uterus.
• Endometrial glands secrete fluid that nourishes the blastomeres.
• The outer cell mass, or trophoblast, and the inner cell mass, or embryoblast, makeup a blastocyst.
• Day 1: Fertilization
• Day 2: Cleavage occurs
• Day 3: Compaction begins
• Day 4: Differentiation in cells are seen
• Day 5: Cavitation
• Day 6: Zona Hatching
• Day 7: Implantation
• Day 9: Cell mass differentiation
• Day 12: Bilaminar Disc Formation

Blastocyte

• The inner cell mass (embryoblast) within the blastocyst attaches to the wall of the uterus via the endometrium.
• A blastocyst contains the inner cell mass, trophoblast, and blastocyst cavity (blastocoele).
• The inner cell mass becomes the embryo, amnion, and umbilical cord, and the trophoblast becomes the placenta and chorion.
• Implantation of the trophoblast into the decidua (endometrium) begins with the help of proteolytic enzymes.
• The trophoblast invades the decidua and forms finger-like projections called villi.
• The spaces between the villi are called lacunae which fill with maternal blood.

Embryoblast Differentiation & Gastrulation

• Embryoblast cells differentiate into two types: epiblast (closest to the trophoblast) and hypoblast (closest to the blastocyst cavity).
• Epiblast and hypoblast then differentiate and fuse into 3 layers, called the tri-laminar embryonic disc, through a process called gastrulation.

Gastrulation and Germ Layers

• At day 15, there are 3 layers in the primitive streak
• Ectoderm: forms the epidermis layer of skin, hair, nails, and nervous system
• Mesoderm: forms muscles, skeleton, dermis of skin, urogenital glands, blood vessels, blood and lymph cells, and connective tissues
• Endoderm: forms the epithelial lining of digestive, respiratory and urinary systems, and glandular cells of organs (e.g., liver, pancreas)

Early Placenta Development

• Trophoblast produces hCG, which ensures the endometrium is ready for implantation.
• Decidualization of the endometrium occurs, with increased vascularity, and structural changes.
• Decidua basalis: between the embryo and stratum basalis of uterus at implantation site.
• Decidua capsularis: covers developing embryo.
• Decidua vera/parietalis: lines remainder of uterine cavity.
• During embryonic development, the uterine glands produce nutrients the embryo requires prior to full placenta development.
• Adaptation of the mother's immune system prevents rejection of the fetus.
• Implantation follows prelancunar and lacunar stages.
• Prelacunar is the stage before villi develop
• Lacunar stage refers to when the villi have formed, and spaces between them are evident (the spaces are called lacuna, which further develops to form intervillous space)

Placental Development

• During weeks 0 - 40, the placenta develops alongside the fetus, eventually forming the uterus wall, placental vili, cytotrophoblast, syncytiotrophoblast, and the maternal blood.

Placenta at Term

• The fully formed structure is a spongy disc that is circular, 20 cm in diameter, 2.5 cm thick, dark red.
• The placenta is fully formed by the 16th week of pregnancy, but starts developing around the 14th day after fertilisation.
• The placenta weighs around 500g, and has a fetal size to placenta ratio of 7:1.
• It is mainly composed of chorionic villi, also know as Chorion frondosum.
• The maternal surface of the placenta is the one attached to the uterine wall.
• The adjacent fetal surface lies to the fetus, covered by the shiny amnion.
• It is divided into 15-20 lobes separated by deep grooves and has an umbilical cord attached near its center.

Placenta Villi

• The developing chorion villi are of two kinds:
• The ones that deeply enter the basal deciduas (anchoring the placenta)
• The ones that hang loosely in the maternal blood, absorbing nutrients through the nutritive vili.
• The mature chorionic villi are tree like, and contain the foetal capillaries network arising from one stem.
• Each villum is covered with few layers of tissue, which make it impossible for the foetal and maternal blood to mix in the placenta.
• Foetal blood is pumped by the foetal heart through umbilical arteries to capillaries in the chorionic villi
• In the chorionic villi, the blood absorbs oxygen and nutrients from the maternal blood, and returns via umbilical vein to the foetus.

Placenta Function

• Respiratory: Oxygen from the mother's hemoglobin goes to the fetus; the fetus lets off carbon dioxide by simple diffusion.
• Nutrition: The fetus gets nourishment from the mother's diet.
• Excretory: Waste products and carbon dioxide move into maternal blood by diffusion.
• Storage: Stores glycogen, iron, and fat soluble vitamins.
• Protection. The placenta acts as barrier against illnesses, but some organisms like rubella and syphilis may pass through them.
• Endocrine: Releases Human Chorionic Gonadotrophin (HCG, for maintaining pregnancy), oestrogens, progesterone, and Human Placental Lactogen (HCL).
• Human placental lactogen stimulates somatic growth, breast tissue growth, food intake and weight gain.

Fetal & Maternal circulation

Placental structure, composition and function

• The basal and chorionic plates meet to form the membrane around the amniotic fluid
• The chorion-amnion membranes are composed of:
• the amnion
• the chorion

Amnion

• It is comprised of an inner membrane derived from the inner cell mass
• Its is a tough, smooth, translucent, and continuous with outer umbilical cord
• It contains amniotic fluid, it moves atop the aiding mucous
• It produces small amounts amniotic fluid and prostaglandin in small amounts, playing role in labour initiation

Chorion

• Its is the continuous outermost membrane touching the placenta
• It is a thick, rough, fibrous, and opaque membrane connected to the decidura vernix
• It also produced platelet activating factors, oxytocin, and prostaglandins
• It can easily rupture and be retained in the uterus after birth

The amniotic fluid

• Fluid is alkaline, transparent, and yellowish inside the amniotic sac.
• It's 99% water circulating constantly.
• The placenta and amnion emit it during pregnancy.
• Later, the fetus swallows and releases it through the kidneys.
• The outer chorion and inner amnion form the fetal sac.
• Its volume changes, as at 10 weeks it can be 2oml, and at term, between 500 - 1200ml
• if lower then 500ml it termed oligohydramniouS, and above 1500 volume it termed polyhydramnious

Amniotic fluid functions

• It enables fetal movement.
• It acts as a shock absorber.
• It equalizes the pressure from uterine contractions.
• It keeps a stable intrauterine temperature and protects against heat.
• It provides the fetus with few smaller nutrients

Amniotic Fluid Composition and Significance

• Amniotic fluid contains 99% water alongside solid, and fetal skin cells.
• Vernix caseosa and lanugo can also be found in Amniotic fluid
• Vernix caseosea is a whitish, cheese substance. Long fine hair called Lanugo cover newborns.
• Meconium in amniotic fluid indicates fetal distress.
• Amniocentesis can assess fluid for condition compromises.
• During labour, when membranes break, the liquor cleans birth canal.
• Aseptic and bactericidal fluid helps labour progress

Umbilical Cord (Funis)

• The Umbilical Cord, also known as Funis, extends from the foetus to the placenta.
• It is formed by 5 weeks of pregnancy
• It is roughly 50 centimetres in lengths, and is considered shorts if it measures below 40 Centimetres
• The function of the Umbilical cord os to transport nutrients and oxygen as, and remove wastes from the foetus
• It possesses no nerves, and therefore cutting does not cause it pain
• Structure of the Umbilical Cord (Funis)
• Two arteries carry blood from fetus to placenta. One umbilical vein transports blood to fetus
• Blood vessels are covered and protected by the Wharton's Jelly, and by a layer of Amnion

Problems of the Umbilical cord

• It features true and false knots (Wharton's Jelly)
• A long umbilical cord( above 100 cms ) may wrap around the body or knotted, preventing normal circulation.

Anatomical Variation of Placenta

• Succenturiate Lobe
• separated lobe joined by vessels, which is also referred to as the significant variations. It can cause hemorrhage and infections
• Circumvallate placenta
• A ring forms from form from opaque membranes. The placenta is nearer to centre and in 2 sections
• Bipartite Placenta
• Here 2 lobes from the placenta exists. There are 2 cords, adjoined to the placenta
• Tripartite placenta
• Here there are 3 sections of the placenta exists. It is similar to the Bipartite

Variations of Cord

• Velamentous
• Cord inserted in membranes
• Vasa praevia
• Vessels cross in the os of the uterus
• Battledore insertion
• In the Battledore the Cord exist on edges
• Malformation of structure can causes structural issues, issues with complete incomplete separate of the septum and malformation of tissues.