Growth and Development 1: Fertilisation and Implantation

Terms

Pregnancy: events that occur from fertilisation until the infant is born

Conceptus: the developing offspring, i.e. bound end and sperm regardless of the stage of pregnancy

Gestation period: from the last menstrual period until birth

Pre embryo: conceptus from fertilisation until 2 weeks of age in mother

Embryo: conceptus during weeks 3-8

Foetus: conceptus from 9 weeks until birth

What happens during fertilisation?

The oocyte is only viable for 12-48 hours. The sperm are viable for no longer than 24-72 hours. For fertilisation to occur, coitus (conception) must occur no more than 3 days before ovulation and 24 after ovulation. Fertilisation occurs when a spermatozoon fuses with a secondary oocyte to form a zygote. Two haploid gametes therefore are forming a zygote.

An ovulated oocyte, as mentioned in Female Reproductive System posts, is encapsulated by the corona radiata and zona pellucida, as well as the extracellular matrix. A sperm is able to break bonds between the corona radiata cells with help of enzymes it contains called hyaluronidase. The sperm then binds to the zona pellucida and enzymes are released near the secondary oocyte itself to digest this layer. The sperm therefore is said to 'digest' a path through the zona pellucida to the oocyte surface where it fuses with it.

Upon entry of the sperm, the secondary oocyte completes meiosis 2, released a second non-functional polar body and the ovum regenerates. The ovum and sperm nuclei swell and the two nuclei approach each other. Spindle fibres form and when fully swollen, the two nuclei are called pronuclei. 

Fertilisation is when the two pronuclei come together and fuse in a process called amphimixis.

Because only one sperm is able to penetrate the oocyte, two mechanisms are in play to ensure monospermy:

1. The membrane depolarises, preventing sperm from fusing
2. Inhibiting proteins destroy sperm receptors in the zona pellucida, causing sperm already bound to receptors to detach

Formation of the Pre-Embryo

Almost immediately, the 46 chromosomes line up along the metaphase plate and the cell prepares to divide (cleavage). The first cleavage produces 2 daughter cells called blastomeres, which continue to divide, producing more and more daughter cells.

Once 16 or more daughter cells have been produced, the morula forms. This is when the cells reach a stage where they are able to implant themselves on the uterine wall (72 hours). By the 5th or 6th day, the pre-embryo consists of 1000 or so cells (blastocyst). A blastocyst is a fluid-filled hollow sphere composed off a single layer of trophoblasts and an inner cell mass.

Trophoblasts take part in plasma formation, providing nutrients to the developing embryo and protecting and insulating the inner cell mass. The inner cell mass becomes the embryonic disc.

How does implantation occur?

Implantation occurs 6-7 days after ovulation. Trophoblasts adhere to the endometrium and the enzymes shed the zona pellucida. This triggers the trophoblasts to divide rapidly, forming two distinct layers; the cytotrophoblast (inner) and syncytiotrophoblast (outer). These cells invade the endometrium and erode a path through the uterine wall until they are no longer in contact with the uterine cavity.

Once all this has occurred (day 10), the inner cell mass detaches from the trophoblasts and forms a blastodisc (functional disc), which becomes the embryonic disc. An amniotic cavity forms on one side, with the yolk sac on the other.

Formation of the gastrula and germ layers

The embryonic disc (2 layered) becomes a three-layered embryo. By day 12, the blastocyst develops into a gastrula with three primary germ layers:

1. Ectoderm (closest to amniotic cavity)
2. Mesoderm (in between ectoderm and endoderm)
3. Endoderm (closest to yolk sac)

These layers form the different body system. Thus, the process of gastrulation, the formation of the gastrula with a 3 layered embryonic disc, sets the stage for organogenesis. 4 extraembyronic membranes are also formed. These surround the cavities, i.e. amniotic cavity, within the gastrula.

Extra embryonic membranes

Embryonic Membrane 1: Yolk Sac

• Gastrulation forms a sac on the ventral side of the embryo
• Primary nutrient source for early embryonic development
• Yolk sac → digestive tube and blood cells/vessels

Embryonic Membrane 2: Amnion

• Formed through inner cell mass separating from the trophoblast/new layers from ectoderm and mesoderm
• Fluid filled cavity from maternal blood and foetal urine
• Buoyant environment that regulates temperature

Embryonic Membrane 3: Allantois

• Small, outpocketing at the base of the yolk sac
• Structural base for umbilici cord
• Forms part of urinary bladder

Embryonic Membrane 4: Chorion

• Encloses the embryonic body and all other membranes 
• Forms part of the placenta
• Has villi to make contact with maternal capillaries

What is gestation?

Following fertilisation, prenatal development of the cell occurs to form an infant. It is divided into three trimesters:

First trimester (0-12 weeks)

• Embryological and early foetal development
• Rudiments of major organ systems occur
• If not properly fused, miscarriage occurs

Second trimester (13-27 weeks)

• Development of the foetus and its organs and systems

Third trimester (28-40 weeks)

• Rapid foetal growth and deposition of adipose tissue (energy stores as a back up)
• Organ systems become fully functional