Embryonic Period Weeks

I. INTRODUCTION. All major organ systems begin to develop during the embryonic period, causing a craniocaudal and lateral body folding of the embryo. By the end of the embryonic period (week 8), the embryo has a distinct human appearance. During the embryonic period, the basic segmentation of the human embryo in a craniocaudal direction is controlled by the Hox (homeobox) complex of genes. All Hox genes contain a 180-base pair sequence (homeobox) that encodes a 60 amino acid long region (homeodomain) that binds to DNA. All homeodomain proteins are gene regulatory proteins (i.e., they control transcription).

II. GASTRULATION (Figure 4-1) is a process that establishes the three primary germ layers of ectoderm, mesoderm, and endoderm, thereby forming a trilaminar embryonic disk. This process is first indicated by the formation of the primitive streak within the epiblast.

A. Ectoderm gives further rise to neuroectoderm and neural crest cells.

B. Endoderm remains intact.

C. Mesoderm gives further rise to paraxial mesoderm (somitomeres and 35 pairs of somites), intermediate mesoderm, and lateral mesoderm.

D. All adult cells and tissues can trace their embryologic origin back to the three primary germ layers (Table 4-1).

III. CLINICAL CORRELATIONS

A. Sacrococcygeal teratoma (Figure 4-2)

1. This tumor arises from remnants of the primitive streak, which normally degenerates and disappears.

2. A sacrococcygeal teratoma often contains various types of tissue (e.g., bone, nerve, hair) because it is derived from pluripotent cells of the primitive streak.

3. This tumor occurs more commonly in female infants.

4. The tumor usually becomes malignant during infancy and must be removed by 6 months of age.

B. Chordoma

1. This tumor arises from remnants of the notochord.

2. It may be found either intracranially or in the sacral region.

3. It occurs more commonly in men late in life (over age 50).

Level and view of sections A and B

Level and view of sections A and B

Cloacal membrane (future anus)

C Caudal
Primitive Streak Embryo

Figure 4-1. Gastrulation. The embryoblast in the upper left is provided for orientation. (A) Dorsal view of the epiblast. The primitive streak consists of the primitive groove, node, and pit. (B) Arrows show the migration of cells through the primitive streak. The notochord (i.e., mesoderm located between the primitive node and prochordal plate) induces the formation of the neural tube. The cardiogenic area is the future site of the heart. (C) Epiblast cells migrate to the primitive streak and insert themselves between the epiblast and hypoblast. Some epiblast cells displace the hypoblast to form endoderm; the remainder migrate cranially, laterally, and along the midline to form mesoderm. After gastrulation, the epiblast is called ectoderm.

Figure 4-1. Gastrulation. The embryoblast in the upper left is provided for orientation. (A) Dorsal view of the epiblast. The primitive streak consists of the primitive groove, node, and pit. (B) Arrows show the migration of cells through the primitive streak. The notochord (i.e., mesoderm located between the primitive node and prochordal plate) induces the formation of the neural tube. The cardiogenic area is the future site of the heart. (C) Epiblast cells migrate to the primitive streak and insert themselves between the epiblast and hypoblast. Some epiblast cells displace the hypoblast to form endoderm; the remainder migrate cranially, laterally, and along the midline to form mesoderm. After gastrulation, the epiblast is called ectoderm.

4. It may be either benign or malignant. C. Caudal dysplasia (sirenomelia)

1. The term "caudal dysplasia" refers to a constellation of syndromes ranging from minor lesions of the lower vertebrae to complete fusion of the lower limbs.

2. This disorder is caused by abnormal gastrulation, in which the migration of mesoderm is disturbed.

3. It can be associated with various cranial anomalies, such as:

Cranial end

Caudal end

Primitive node

Cranial end

Caudal end

Prochordal plate (future mouth)

Primitive node

Cloacal membrane (future anus)

Table 4-1

Summary of Germ Layer Derivatives

ECTODERM

Epidermis, hair, nails, sweat and sebaceous glands

Utricle, semicircular ducts, vestibular ganglion of CN VIII

Saccule, cochlear duct, spiral ganglion of

CN VIII Olfactory placodes, CN I Ameloblasts Adenohypophysis Lens of eye

Anterior epithelium of the cornea Acinar cells of parotid gland Acinar cells of mammary gland Epithelial lining of: Lower anal canal Distal part of male urethra External auditory meatus

Neuroectoderm

All neurons within brain and spinal cord (CNS) Retina, iris, ciliary body, optic nerve (CNS), optic chiasm, optic tract, dilator and sphincter pupillae muscles Astrocytes, oligodendrocytes, ependymocytes, tanycytes, choroid plexus cells Neurohypophysis Pineal gland

Neural crest:

Ganglia (dorsal root, cranial, autonomic)

Schwann cells

Odontoblasts

Pia and arachnoid

Chromaffin cells (adrenal medulla)

Parafollicular C cells of thyroid

Melanocytes

Aorticopulmonary septum Pharyngeal arch skeletal components Bones of neurocranium

MESODERM

Muscle (smooth, cardiac, skeletal) Extraocular muscles (preotic somites), ciliary muscle of eye, sclera, choroid, substantia propria of cornea, corneal endothelium Muscles of tongue (occipital somites) Pharyngeal arch muscles Laryngeal cartilages Connective tissue Dermis of skin Bone and cartilage Dura mater

Endothelium of blood and lymph vessels RBCs, WBCs, microglia, and Kupffer cells Spleen Kidney

Adrenal cortex Testes and ovary

ENDODERM

Hepatocytes

Principles and oxyphil of parathyroid

Thyroid follicular cells

Epithelial reticular cells of thymus

Acinar and islet cells of pancreas

Acinar cells of submandibular and sublingual glands Epithelial lining of: Gl tract

Trachea, bronchi, lungs Biliary apparatus

Urinary bladder, female urethra, most of male urethra Vagina

Auditory tube, middle ear cavity Crypts of palatine tonsils

CN = cranial nerve; CNS = central nervous system; Gl = gastrointestinal; RBCs = red blood cells; WBCs = white blood cells

(From Dudek RW, Fx JD: Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p. 36.)

a. VATER, an acronym for vertebral defects, anal atresia, tracheoesophageal fistula, and renal defects b. VACTERL, which is similar to VATER but also includes cardiovascular defects and upper limb defects

D. Missed menstrual period

1. A missed period is usually the first indication of pregnancy. Week 3 of embryonic

Infantile Sacral Chordoma
Figure 4-2. Infant with a sacrococcygeal teratoma. (From Sadler TW: Langman's Medical Embryology, 7th ed. Baltimore, Williams «Si Wilkins, 1995, p 62.)

development coincides with the first missed menstrual period. (Note that at this time the embryo has already undergone 2 weeks of development.)

2. It is crucial that the woman becomes aware of a pregnancy as soon as possible because the embryonic period is a period of high susceptibility to teratogens.

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Responses

  • kari
    What are the 3 distinct layers during the embryonic period?
    5 years ago

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