Embryonic Development Stages

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Questions and Answers

What characterizes situs inversus in terms of organ positioning?

Only the heart is flipped while other organs remain unchanged.

Organs are positioned correctly but with altered functionality.

All organ systems are completely absent.

Thoracic and abdominal viscera are arranged as a mirror image. (correct)

During neurulation, which structure is responsible for inducing the formation of the neural plate?

Neural crest cells

Somites

Ectoderm

Notochord (correct)

What is the final outcome of the fusion of the neural groove during neurulation?

Formation of the heart

Development of the spinal cord

Formation of the neural tube (correct)

Detaching from the ectoderm

What primarily happens to the cranial end of the neural tube during development?

It expands to form the brain. Signup and view all the answers

What term describes the abnormal positioning of internal organs, which may include conditions like dextrocardia?

Laterality anomalies Signup and view all the answers

What is the main role of the trophoblast during implantation?

To differentiate and invade the endometrium Signup and view all the answers

Which term describes the initial direct contact between the blastocyst and the endometrium?

Apposition Signup and view all the answers

What forms the layers within the inner cell mass during early embryonic development?

Epiblast and hypoblast Signup and view all the answers

What is the purpose of the lacunae formed by maternal vessels during implantation?

To provide a space for oxygen exchange Signup and view all the answers

What initiates the series of complex signaling events between the endometrium and the trophoblast during implantation?

The blastocyst adhering to the endometrium Signup and view all the answers

What is the initial product of fertilisation?

Zygote Signup and view all the answers

What is the primary process that leads to the formation of a morula?

Cleavage Signup and view all the answers

How are blastomeres arranged within the morula?

Tightly compacted Signup and view all the answers

What role do the outer cells of the blastocyst play?

Become the placenta Signup and view all the answers

What do the hypoblast and epiblast together establish?

Dorsoventral axis Signup and view all the answers

What is the significance of chromosomal anomalies in zygotes or blastocysts?

They often result in spontaneous pregnancy loss. Signup and view all the answers

At what stage does implantation begin after fertilisation?

Day 6 Signup and view all the answers

What component can be removed from a blastocyst for genetic testing in IVF?

Blastomere Signup and view all the answers

What could be a reason for unsuccessful implantation?

Blastocyst does not reach the uterus Signup and view all the answers

Where can an embryo implant if not in the body of the uterus?

Cervix Signup and view all the answers

What process allows sperm to acquire the ability to fertilize an egg?

Capacitation Signup and view all the answers

What serious issue can arise from invasive placentation?

Severe haemorrhage Signup and view all the answers

Which layer is formed from the differentiation of the embryoblast during early development?

Bilaminar disc Signup and view all the answers

What significant process occurs during gastrulation in week 3?

Formation of a trilaminar disc Signup and view all the answers

What is the function of the primitive streak during development?

Serves as a site for cell migration Signup and view all the answers

What is the primary consequence of polyspermy during fertilization?

Impaired development of the embryo Signup and view all the answers

During which stage does the formation of the three germ layers occur?

Gastrulation Signup and view all the answers

What does the trilaminar disc develop into?

Embryonic tissues Signup and view all the answers

How does a complete mole occur?

Single sperm fertilizes an egg with no genetic material Signup and view all the answers

What term describes implantation that occurs outside the uterus?

Ectopic implantation Signup and view all the answers

Which germ layer is formed during the invagination at the primitive streak?

Mesoderm Signup and view all the answers

What primarily aids the movement of sperm into the fallopian tube?

Contractions of smooth muscle cells Signup and view all the answers

What is indicated by a molar pregnancy test?

Presence of abnormal trophoblast tissue Signup and view all the answers

What does the zona pellucida do during fertilization?

Becomes impermeable to prevent polyspermy Signup and view all the answers

Study Notes

Learning Objectives

Describe the stages of development from fertilization to week 4.
Understand how a blastocyst differentiates into an embryoblast and trophoblast.
Understand how an embryoblast develops into a bilaminar disc.
Understand gastrulation and the formation of three germ layers.
Understand neurulation.
Understand that congenital anomalies result from disrupted embryonic development.

Fertilization

Millions of sperm enter the female reproductive tract, but few reach the uterine tube.
Sperm are "conditioned" (capacitated) in the uterine tube before fertilization.
Fertilization is the fusion of gametes.

How Sperm Moves to the Tube?

Contractions from smooth muscle cells in the uterine wall move sperm upwards into the fallopian tube.

What Happens After Sperm Enters the Tube

Sperm survive in the fallopian tube for up to 5 days.
Sperm are conditioned (capacitated) before fertilization.

Capacitation

A period of time sperm need to reside in the female reproductive tract before acquiring the ability to fertilize oocytes.

Fertilization Process

Sperm reaches the ovum.
Chemicals are released, triggering the acrosome reaction.
The acrosome swells and fuses with the sperm cell membrane.
Digestive enzymes are released.
Enzymes digest follicle cells and the zona pellucida.
Sperm fuses with the ovum's membrane.
Nuclei of ovum and sperm fuse.
Enzymes thicken the ovum's outer layer, preventing other sperm entry.

Polyspermy Prevention

The zona pellucida becomes impermeable, which prevents multiple sperm from fertilizing the ovum.

Clinical Relevance: Molar Pregnancy

Two types:
- Complete mole: Single sperm fertilizes an egg lacking genetic material. Abnormal trophoblast develops, but no embryo.
- Partial mole: Normal egg fertilized by two sperms. Embryo develops but cannot survive.
Molar pregnancy gives a positive test result.

The Zygote

Fertilization produces a zygote that is genetically unique.

Cleavage

Zygote undergoes cell division immediately after fertilization.
No overall increase in size during early divisions.

Morula

Cleavage process produces a morula.

Blastomeres

Cells formed from cleavage of the fertilized ovum.

Blastocyst Arrangement (Morula)

Blastomeres are tightly compacted within the morula.

Blastocyst Structure

Surrounded by the zona pellucida (glycoprotein coat).
Cavity separates the cells into inner and outer groups.
Inner cell mass (ICM) develops into the embryo.
Outer cells (trophoblast) develop into the placenta.
ICM cells differentiate into hypoblast and epiblast.

Blastocyst Hatching

Blastocyst sheds its outer membrane and attaches to the uterine lining to prepare for implantation.

Implantation

Three Stages:
- Apposition: First physical contact between blastocyst and endometrium.
- Adhesion: Blastocyst adheres to endometrium.
- Invasion: Penetration of the endometrium by the trophoblast
Complex signaling is necessary between blastocyst and endometrium for a successful implantation.
Blastocyst must implant adequately but not too deeply.
Blastocyst makeup is different from maternal tissue. This prevents rejection by the mother.

Implantation: Differentiation of Trophoblast

Cytotrophoblast at the embryonic pole proliferates.
Cytotrophoblast invades endometrium.
Syncytiotrophoblast forms maternal blood filled spaces (lacunae).
Trophoblast contacts maternal vessels, enabling gas exchange between maternal and fetal circulation.

Implantation: Changes in the Inner Cell Mass

Inner cell mass flattens into two layers.
Layers are epiblast and hypoblast.
Blastocyst cavity is the primitive yolk sac.

Clinical Relevance: Implantation Problems

Implantation may be unsuccessful if the blastocyst is abnormal or the endometrium is not receptive.
Ectopic pregnancy occurs when implantation occurs outside the uterus (e.g., fallopian tubes, ovaries, abdomen, cervix).
Suboptimal implantation can cause placental problems later in pregnancy.

Clinical Relevance: Ectopic Pregnancy

Tubal (uterine tube) ectopic pregnancy.
Pregnancy occurs outside the uterus.

Clinical Relevance: Invasive Placentation

Placenta invades too far into the uterine muscle layers.
Placenta may attach to adjacent tissues.
Placenta does not detach easily after birth.
Can cause severe hemorrhage.

Gastrulation (Week 3)

Bilaminar disc develops into a trilaminar disc.
Establishes three germ layers for development.
Primitive streak: Marks the beginning of gastrulation.

Primitive Streak

Appears as a groove (caudal end) on the epiblast.
Cells migrate and invaginate through the epiblast.
Cells form mesoderm and other layers.

Neurulation (Week 4)

Process that forms the neural tube.
Neural plate formation (ectoderm thickening) is induced by notochord.
Neural plate bends to form a groove.
Cells at the top of the folds become neural crest cells.
Two sides of the groove fuse to form the neural tube.
Neural tube detaches from the ectoderm.
- Cranial end expands to form the brain.
- Caudal end remains tubular, forming the spinal cord.

Clinical Relevance: Laterality Anomalies

Congenital conditions involving abnormal "sidedness".
Situs inversus: Thoracic and abdominal viscera are flipped.
Dextrocardia: Heart is flipped.

Other Key Concepts

Embryonic development is critical.
Genetic abnormalities can cause spontaneous pregnancy loss.

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Description

This quiz covers the critical stages of embryonic development, from fertilization through the first four weeks. Understand the differentiation of the blastocyst and the formation of germ layers, as well as how congenital anomalies may arise from disrupted development. Test your knowledge on the processes that form the foundation of human development.