Embryonic Development in Human

A study of the development of an organism from fertilization to the formation of young one is known as embryogeny.

A series of rapid mitotic divisions of the zygote without an increase in size is called cleavage.

A solid, mulberry-like ball of cells formed after cleavage is called morula.

The process of formation of blastula from morula is called blastulation.

A hollow embryonic stage formed after morula in humans is called blastocyst.

An extra-embryonic sac involved in waste storage and placental development is called allantois.

A-J. Stages in the segmentation of fertilized ovum and formation of blastocyst

Relationship between the human embryo and extraembryonic membranes about five weeks ovulation

Cleavage is a series of rapid mitotic divisions of the zygote that produces smaller cells called blastomeres without increasing the overall size of the embryo.

Characteristics of Cleavage:

• Occurs as the zygote moves through the fallopian tube.
• Increases the number of cells, not the size of the embryo.
• Restores the normal nucleo-cytoplasmic ratio.
• Leads to the formation of a multicellular stage called the morula.

Cleavage Timeline:

Human Cleavage:

• Human cleavage is holoblastic.
• It is also indeterminate.
• This property explains why identical twins can arise from early embryonic stages.

Morula:

Morula is a solid ball of 16–32 blastomeres formed after repeated cleavage divisions.

Blastulation is the process by which the morula is transformed into a hollow, fluid-filled structure called the blastocyst.

Structure of Blastocyst:

The blastocyst has three major parts:

• Trophoblast – outer cell layer
• Inner Cell Mass (ICM) – inner group of cells
• Blastocoel – fluid-filled cavity

Functions of the Parts:

Implantation is the attachment and embedding of the blastocyst into the endometrium of the uterus.

Sequence of Implantation:

1. The blastocyst reaches the uterus.
2. The zona pellucida disappears.
3. The trophoblast comes in contact with the uterine endometrium.
4. Trophoblast cells invade the endometrial tissue.
5. The blastocyst becomes embedded in the uterine wall.

Trophoblast Differentiation:

The trophoblast differentiates into two layers:

Hormonal Significance:

• hCG (human chorionic gonadotropin) is secreted after implantation.
• hCG maintains the corpus luteum.
• The corpus luteum continues to secrete progesterone.
• Progesterone maintains the uterine lining and supports pregnancy.

Gastrulation is the process by which the bilaminar embryonic disc is converted into a trilaminar disc containing the three primary germ layers.

Germ Layers Formed

• Ectoderm
• Mesoderm
• Endoderm

Process Overview:

• A primitive streak appears in the embryonic disc.
• Cells migrate inward through this region.
• These movements produce the three germ layers.
• Gastrulation establishes the basic body plan of the embryo.

Germ Layer Derivatives:

Extra-embryonic membranes are structures formed around the embryo that protect it and support its development.

Main Extra-Embryonic Membranes:

Functional Summary:

• Chorion participates in placenta formation.
• Amnion protects the embryo from shock and desiccation.
• Allantois contributes to early circulation-related structures.
• Yolk sac plays a role in early embryonic nourishment and blood formation.

• Following fertilization, the single-celled zygote undergoes rapid mitotic divisions known as cleavage to form a solid, multicellular ball of cells termed the morula.
• The morula continues to divide and transforms into a hollow structure called the blastocyst, which consists of an outer trophoblast layer and an inner cell mass.
• By approximately the seventh day post-fertilization, the blastocyst embeds itself into the endometrial lining of the mother's uterus in a critical process called implantation.
• The inner cell mass subsequently undergoes gastrulation, a phase where it differentiates into three primary embryonic germ layers: the ectoderm, mesoderm, and endoderm.
• During organogenesis, these three germ layers systematically give rise to all specific tissues, internal organs, and organ systems, typically beginning with the formation of the heart.
• A temporary structural and functional organ known as the placenta develops from trophoblastic and maternal tissues to facilitate the essential exchange of nutrients, gases, and wastes.
• The complete sequence of embryonic and foetal development is sustained throughout a gestation period of approximately nine months, ultimately culminating in childbirth.