Gastrulation and Reproductive Anatomy

Introduction to Female Reproductive Anatomy

• The reproductive anatomy in females consists of ovaries, uterine tubes (Fallopian tubes), and uterus.
• Ovaries produce oocytes (female haploid gametes) and hormones such as progesterone and estrogens.
• Uterine tubes receive oocytes from ovaries and are the site of fertilization.
• The uterus is where the embryo develops and the placenta and membranes form.

Basics of Ovulation and Fertilization

• Remember meiosis?
• Progesterone helps prepare the uterus for implantation.

Gastrulation

• Gastrulation is the process of forming three embryonic germ layers: ectoderm, endoderm, and mesoderm.
• Ectoderm is typically found on the exterior of the organism, endoderm is typically found on the interior, and mesoderm is found between the two.
• Endoderm and mesoderm can also be extra-embryonic.

Week 2 and 3 Embryology

• Coelom is a fluid-filled cavity.
• Cephalad means towards the head region, and caudad means towards the tail region.
• The epiblast and hypoblast differentiate into two distinct layers during gastrulation.

Implantation

• Implantation of the embryo is complete around day 10.
• The embryo is surrounded by syncytiotrophoblast cells.
• Decidual cells accumulate glycogen and lipids throughout the uterus.

Embryonic Folding

• Folding begins at the end of the 3rd week and is easy to see in the 4th week.
• Cranial folding incorporates part of the endoderm of the umbilical vesicle into the embryo as the foregut.
• Lateral folding is caused by the rapidly growing spinal cord and somites.
• Tail folding incorporates part of the endoderm germ layer into the embryo as the hindgut.

Germ Layer Derivatives

• The three germ layers formed during gastrulation give rise to the primordia of all tissues and organs.
• Ectoderm derivatives include the CNS, PNS, sensory epithelia, and epidermis.
• Mesoderm derivatives include connective tissue, cartilage, bone, and muscles.
• Endoderm derivatives include the epithelial lining of the digestive and respiratory tracts, and organs such as the liver, pancreas, and thyroid.

Development of the Nervous System

• The neural plate forms at day 19 and eventually gives rise to the CNS and PNS.
• Neural crest cells are important for the development of the nervous system.

Pharyngeal Arches

• The pharyngeal arches give rise to various bone and cartilage derivatives.
• Each pharyngeal arch has a specific nerve and muscle derivative.

Thyroid Development

• The thyroid gland is the first endocrine gland to develop in the embryo.
• It forms from a median endodermal thickening in the floor of the primordial pharynx.
• The developing thyroid gland descends in the neck and eventually reaches its final site.

Week 1-2: Embryonic Development

• Day 6-7: Syncytiotrophoblast invades the stroma, and the inner cell mass (embryoblast) differentiates into two distinct layers:
  • Epiblast: will become the embryo proper
  • Hypoblast: will line the blastocystic cavity (coelom) and form the primary yolk sac, also known as the primary endoderm

Week 2-3: Embryologic Terms

• Coelom: a fluid-filled cavity
• Gastrulation: the process of forming three embryonic germ layers:
  • Ectoderm: typically found on the exterior of the organism
  • Endoderm: typically found on the interior of the organism
  • Mesoderm (intraembryonic mesoderm): found between the ectoderm and endoderm
• Note: endoderm and mesoderm can also be extra-embryonic (not classified as gastrulation)

Week 2-3: Implantation and Folding

• Completion of implantation: day 10
  • The embryo is completely embedded within the endometrium
  • Surrounded by syncytiotrophoblast cells
• Decidualization: stromal cells undergo changes, accumulating glycogen and lipids, and eventually undergoing apoptosis, releasing stored nutrients for embryonic growth
• Epiblast and hypoblast:
  • Epiblast enlarges and gives rise to amnioblasts, which surround the developing amniotic cavity
  • Hypoblast extends around the entire interior surface of the blastocoel

Week 3-4: Neural Development

• Longitudinal neural groove develops in the neural plate, flanked by neural folds
• Fusion of neural folds forms the neural tube, the primordium of the CNS
• Neuroectodermal cells form a neural crest between the surface ectoderm and neural tube
• Mesoderm on each side of the notochord condenses to form longitudinal columns of paraxial mesoderm, which give rise to somites

Week 3-4: Coelom and Blood Vessels

• Coelom (cavity) arises as isolated spaces in the lateral mesoderm and cardiogenic mesoderm
• Coelomic vesicles coalesce to form a single, horseshoe-shaped cavity that gives rise to the body cavities
• Blood vessels first appear in the wall of the umbilical vesicle, allantois, and chorion
• Fetal and adult erythrocytes develop from different hematopoietic precursors

Week 3-4: Embryonic Folding

• Embryonic folding: the process by which the embryonic disk becomes more cylindrical in shape
• Folding occurs in two general planes:
  • Median plane: anterior and posterior ends of the embryo move ventrally (cranial-caudal folding)
  • Horizontal plane: lateral edges of the embryonic disk move ventrally (lateral folding)

Week 4: Cranial and Caudal Folding

• Cranial folding:
  • Part of the endoderm of the umbilical vesicle is incorporated into the embryo as the foregut
  • Foregut lies between the brain and heart
  • Oropharyngeal membrane separates the foregut from the stomodeum (primordium of the mouth)
• Caudal folding:
  • Part of the endodermal germ layer is incorporated into the embryo as the hindgut
  • Connecting stalk is attached to the ventral surface of the embryo, and the allantois is partially incorporated into the embryo

Germ Layer Derivatives

• Ectoderm: CNS, PNS, sensory epithelia, epidermis, and its appendages
• Mesoderm: connective tissue, cartilage, bone, heart, blood, and lymphatic vessels
• Endoderm: epithelial lining of the digestive and respiratory tracts, parenchyma of the liver, pancreas, and thyroid glands

Development of the Nervous System

• Neural plate can be seen at day 19
• Neural plate forms the neural tube, which gives rise to the CNS
• Pharyngeal arches: the main formative elements of the face, nasal cavities, mouth, larynx, pharynx, and neck

Development of the Pharyngeal Apparatus

• Pharyngeal arches begin to develop early in the fourth week
• Pharyngeal arches are separated from each other by pharyngeal grooves
• Pharyngeal arches give rise to:
  • First arch: maxilla, mandible, and parts of the vomer
  • Second arch: parts of the face, nasal cavities, and mouth
  • Third arch: parts of the pharynx, larynx, and neck

Development of the Thyroid

• Thyroid gland begins to form 24 days after fertilization from a median endodermal thickening in the floor of the primordial pharynx
• Thyroid gland descends in the neck, passing ventral to the developing hyoid bone and laryngeal cartilages
• Thyroid gland is connected to the tongue by a narrow tube, the thyroglossal duct, which usually degenerates by 7 weeks