The term 'embryo' has varied meanings across different authorities.
Some authorities define the embryo as the fertilized egg or the two-cell stage.
Others assert that an individual becomes an embryo when it is 16 days old and consists of three primary germ layers: ectoderm, mesoderm, and endoderm.
Preembryonic Stage:
This stage covers the first 16 days following fertilization.
Sperm Migration
Fertilization Window:
The egg must be fertilized within 12 to 24 hours of ovulation to survive.
Sperm must encounter the egg somewhere in the distal one-third of the uterine tube.
Challenges Faced by Sperm:
A vast majority of sperm do not reach the egg due to various factors:
Destroyed by vaginal acid or draining from the vagina.
Fail to penetrate the mucus of the cervical canal.
Destroyed by leukocytes in the uterus.
Half of the sperm travel up the wrong uterine tube.
Of the 300 million sperm ejaculated, approximately 200 spermatozoa successfully reach the vicinity of the egg.
Fertilization
Sperm Viability:
Sperm can remain viable for up to 6 - 7 days after ejaculation.
Fertilization is optimal if sperm are deposited a few days prior to ovulation or within 14 hours after.
Enzymatic Action:
When a sperm encounters an egg, it releases enzymes necessary for penetrating the egg, allowing it to pass through granulosa cells and then the zona pellucida.
Mechanism of Fertilization:
Fertilization combines the haploid (n) set of sperm chromosomes with the haploid set of egg chromosomes, producing a diploid (2n) set.
Polyspermy Prevention Mechanisms:
Fast Block:
The binding of a sperm to the egg opens sodium (Na+) channels in the egg membrane.
Slow Block:
Involves secretory vesicles and cortical granules located just below the membrane.
Meiosis II and Formation of Zygote
Secondary Oocyte:
The secondary oocyte begins meiosis II prior to ovulation but completes it only upon fertilization.
It forms the second polar body, discarding one chromatid from each chromosome.
Pronuclei Formation:
Both sperm and egg nuclei swell and become pronuclei.
Zygote Formation:
Each pronucleus ruptures, and the chromosomes from the two gametes mix into a single diploid set.
The fertilized egg is now called the zygote, which is prepared for its first mitotic division.
Major Stages of Prenatal Development
Pregnancy Timeline:
Pregnancy is divided into three-month intervals known as trimesters.
First Trimester (Weeks 1-12):
More than half of all embryos die from natural causes during this time.
The conceptus is notably vulnerable to stress, drugs, and nutritional deficiencies.
Second Trimester (Weeks 13-24):
Most organs complete their development.
The fetus begins to resemble a distinctly human form.
Chance of survival increases (with intensive care) if born near the end of this trimester.
Third Trimester (Weeks 25-Birth):
The fetus experiences rapid growth and achieves sufficient cellular differentiation to survive outside the womb.
By 35 weeks and weighing 5.5 lbs, the fetus is considered mature.
The Preembryonic Stage
Overview of Preembryonic Stage:
The preembryonic stage is the first two weeks of development, culminating in the formation of an embryo, described as involving three major processes:
Cleavage
Implantation
Embryogenesis
Cleavage and Early Development
Cleavage Process:
Cleavage refers to the mitotic divisions occurring in the first 3 days as the conceptus migrates down the uterine tube.
The first cleavage happens within 30 hours after fertilization, resulting in the zygote splitting into two daughter cells known as blastomeres.
By the time the conceptus reaches the uterus (around 72 hours post-ovulation), it becomes a morula—a solid ball of approximately 16 cells, resembling a mulberry.
The morula remains no larger than the zygote while producing increasingly smaller blastomeres.
Blastocyst Formation
Blastocyst Development:
The morula lies free in the uterine cavity for 4 to 5 days, further dividing into about 100 cells.
The zona pellucida disintegrates, releasing the conceptus, now referred to as a blastocyst.
Blastocyst Structure:
Outer layer: Trophoblast (destined to form the placenta and assist in nourishing the embryo).
Inner cell mass: Embryoblast (destined to develop into the embryo).
Implantation Process
Attachment to Uterine Wall:
The blastocyst attaches to the uterine wall approximately 6 days after ovulation, usually on the fundus or posterior wall.
Implantation refers to the process in which the blastocyst adheres to the endometrium.
Embryogenesis:
Refers to the arrangement of blastomeres into three primary germ layers within the embryoblast: ectoderm, mesoderm, and endoderm.
Hormonal Support During Early Development
Human Chorionic Gonadotropin (HCG):
The trophoblast secretes HCG which stimulates the corpus luteum to secrete estrogen and progesterone.
Progesterone plays a crucial role in suppressing menstruation.
HCG levels rise in the mother’s blood until the end of the second month.
Chorion Formation:
The trophoblast develops into a membrane called the chorion, which takes over the role of the corpus luteum, making HCG unnecessary.
After this point, ovaries become inactive for the remainder of the pregnancy, while both estrogen and progesterone levels rise from the chorion.
Embryonic and Fetal Stages
Embryonic Stage:
Begins when all three primary germ layers are present, typically on day 16 post-fertilization.
The placenta forms over the next 6 weeks, becoming the primary source of nutrition for the embryo.
Organogenesis:
This is the process by which the germ layers differentiate into organs and organ systems.
By 8 weeks, organs are present, although not fully functional, during which the embryo transitions to being termed a fetus.
Embryonic Folding and Organogenesis
Embryonic Folding:
During the embryonic stage, the flat embryonic disc transforms into a cylindrical form around week 4.
The embryo grows quickly, folding around a yolk sac, forming a C-shape with the head and tail nearly touching.
The lateral margins of the disc fold around the yolk sac to create the ventral surface of the embryo.
Tissue Formation:
Ectoderm now covers the entire surface, eventually forming the epidermis of the skin.
Mesoderm splits into two layers: one adheres to the ectoderm, and the other to the endoderm, resulting in the formation of the coelom—the body cavity between the two mesoderm layers.
Organ Formation Timeline
Presence of Organs:
At 8 weeks, all organs are present in the fetus, measuring approximately 3 cm long.
Key derivatives from the primary germ layers are as follows:
Ectoderm:
Epidermis, nervous system, lens and cornea, integumentary glands, internal ear.
Mesoderm:
Skeleton, muscle, cartilage, blood, dermis, lymphoid tissue, gonads and ducts, kidneys and ureters.
Endoderm:
Gut and respiratory epithelium and glands, bladder, and urethra.