Developmental Biology Overview - Anatomy & Physiology

Introduction

Embryo, when applied to mammals, is the term given to the developing organism from fertilisation to birth. Developmental biology, or embryology, is the study of the embryo as it transforms from a unicellular zygote to a multicellular, mulitsystemed organism which in some cases is ready to function autonomously at birth. Developmental biology is of interest to vets in understanding why organs and systems are the way they are, but also in understanding genetic diseases and applying cell based therapies to treat loss or damage to tissues.

Perhaps quite remarkably, all animals follow the same developmental "plan" - until the latter stages of development, it is difficult to tell a human embryo from a chicken embryo. This suggests an evolutionary programme for cell development.

Embryogenesis is driven by three cellular process:

1. Cell division and growth

   Fertilisation produces a unicellular zygote which undergoes a series of mitotic divisions to eventually become a multicellular organism known as a blastocyst. Mitosis produces (i) growth by hyperplasia (ii) an increased number of cells for greater diversity of function, and thus more a complex organism. The first differentiation of cells creates trophoblasts that contain molecules that adhere to the endometrium during implantation. The extent of cellular growth and division and the timing of implantation varies between species.

2. Cell differentiation

   As embryogenesis progresses, cells become specialised in structure and function. Regulation of gene expression allows different proteins to be expressed in some cells, allowing tissue differentiation even though all cells contain the same DNA. At the blastomere stage (16 cells) cells are totipotent, meaning they have the capacity to form every adult cell type. As the embryo progresses and enters gastrulation cells become pluripotent, meaning they can form several but not all cell types. Gastrulation is the process of forming the three germ layers; ectoderm, mesoderm and endoderm. Eventually, most cells terminally differentiate and can no longer form any other cell type.

3. Morphogenesis

   During embryogenesis two cell types exist; mesenchymal and epithelial. Mesenchymal cells are single cells or loosely linked to other cells and irregularly shaped. Epithelial cells are tightly attached to each other or a membrane and have a regular shape (cuboidal or columnar).

The Development of Anatomical Structures

The nervous system develops from ectoderm in the anterior part of the embryo, beginning with the formation of the neural plate. Some of the ectoderm will also develop into the epidermis in response to signalling factors from elsewhere in the embryo; formation of the neural ectoderm is the default pathway. The neural plate develops into the neural tube, which is the precursor to the brain and spinal cord.

In the verterbrate embryo, the anterior of the embryo also begins to form into blocks of cells known as somites. Somites are transient structures that will give rise to the vertebrae and ribs, the dermis of the dorsum, and the skeletal muscle of the body wall, back and limbs; they originate from the subsection of mesoderm known as paraxial mesoderm.

The mesoderm also gives rise to the urinary system and some parts of the reproductive system; these develop from intermediate mesoderm. The development of the male or female reproductive system is mediated by the genetic sex of the embryo.

Lateral plate mesoderm differentiates into the heart, the vascular system, the lymphatic system, the limbs, and the tissue and smooth muscle surrounding the alimentary canal, or digestive system.

The formation of bone, or osteogenesis has several different origins - the skull develops at the junction of the neural plate and the epidermis, the limb skeleton develops from lateral plate mesoderm, and the axial skeleton develops from paraxial mesoderm. Both bones and cartilage continue to develop into several well differentiated types specific to their anatomical position and function.

The endoderm will form the lining of the alimentary canal and the glandular structures that develop within it. Endoderm also develops into other digestive organs such as the pancreas; it also gives rise to non digestive structures such as the thyroid gland which is formed from a downgrowth of the pharyngeal endoderm of the developing tongue. Initially, the alimentary canal is supended within two compartments; further development alters this arrangement so that a single peritoneal cavity is formed.

The Development of Specialised Structures

Alimentary

• Tooth Development

Reproductive

• Testicular Descent

Integumentary

• Hair Development
• Mammary Gland Development.

Endocrine

• Pituitary Gland Development
• Adrenal Gland Development

Lymphatic

• Bone Marrow Development
• Bursa of Farbricus Development
• Thymus Development

Test yourself with the embryology quiz

Embryology quiz

References

• Scott F. Gilbert Developmental Biology, 6th edition
• T.A.McGeady, P.J. Quinn, E.S.Fitzpatrick, M.T.Ryan Veterinary Embryology

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