2011 Lab 3 - Week 4

Week 4

Axes - Human Embryo (Carnegie stage 11), 24 days, 13 somite pairs

Key Events of Human Development during the fourth week (week 4) following fertilization or Clinical week 6 (LMP).

These notes cover the fourth week of embryonic development, which is the beginning of organogenesis, (specific tissues and systems are beginning to differentiate) from the trilaminar embryo. With many parallel processes, descriptions begin to get complicated! Many of the described processes begin and extend over a broader range of time. Some developmental processes will be discussed later in the practical to simplify matters.

Ectoderm on the embryo surface undergoes segmentation: The central portion of the embryonic disc forms the neural plate, the edge of this plate forms neural crest and the remainder outside this forms the epitheium of the skin and other structures.

Neurogenesis

Central Nervous System (CNS) - the neural plate undergoes morphological changes to form the primitive central nervous system (brain, spinal cord). An epithelial layer of cells which contributes all neural (brain, spinal cord, peripheral nervous system) and the external epithelium (surface layer of the skin) of the embryo. Neurogenesis begins towards the end of week 3, when the neural tissues separate from this germ cell layer.

Peripheral Nervous System (PNS) - the neural crest cells in the body region migrate and spread to different regions of the embryo forming the PNS (dorsal root ganglia, sympathetic ganglia, enteric nervous system) and many other embryonic tissues. Neural crest cells in the head region form skeletal and other structures.

Pharyngeal Arches

In the head region, a series of ventral folds form under the brain in a rostral to caudal sequence, these are the pharyngeal arches.

Placodes

In the head region, ectoderm small patches form pairs of specialised placodes that eventually contribute to specific sensory components, cranial ganglia and the anterior pituitary (adenohypophysis).

Limb Buds

In the body region, limb buds form initially as ectoderm and mesoderm (somitic and somatic) components and are the "paddle-like" projections from the trunk which will form all the upper and lower limb components. (An overview of limb development will be covered in week 8).

Cardiogenesis

Within the embryo mesoderm, the heart tube and vascular development continues. Cardiogenesis will be covered in week 5, when septation begins.

Stage 10
Stage 11
Stage 12
Stage 13

Neurogenesis

Neural Groove (stage 10)

Caudal Neural Pore (stage 12)

Developmental sequence: neural plate -> (day 18-19) neural groove -> neural tube -> Central Nervous System (brain and spinal cord)

Central Nervous System (CNS) - the neural plate undergoes morphological changes to form the primitive central nervous system. An epithelial layer of cells which contributes all neural (brain, spinal cord, peripheral nervous system) and the external epithelium (surface layer of the skin) of the embryo. Neurogenesis begins towards the end of week 3, when the neural tissues separate from this germ cell layer.

Peripheral Nervous System (PNS) - the neural crest cells in the body region migrate and spread to different regions of the embryo forming the PNS and many other embryonic tissues. Neural crest cells in the head region form skeletal and other structures.

Human Neuralation - Early Stages

The stages below refer to specific Carneigie stages of development.

stage 8 (about 18 postovulatory days) neural groove and folds are first seen
stage 9 the three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open
stage 10 (two days later) neural folds begin to fuse near the junction between brain and spinal cord, when neural crest cells are arising mainly from the neural ectoderm
stage 11 (about 24 days) the cranial neuropore (rostral, cephalic or anterior) closes within a few hours; closure is bidirectional, it takes place from the dorsal and terminal lips and may occur in several areas simultaneously. The two lips, however, behave differently.
stage 12 (about 26 days) The caudal (posterior) neuropore takes a day to close
the level of final closure is approximately at future somitic pair 31
corresponds to the level of sacral vertebra 2
stage 13 (4 weeks) the neural tube is normally completely closed.

Three primary brain vesicles develop initially due to the neural plate being broader at the cranial (brain) end than the narrower caudal (spinal cord) end. When the plate fuses to form a tube, these 3 initial expansions (vesicles) result.

Pharyngeal Arches and Placodes

In the head region, two main components of head development form the pharyngeal arches and sensory placodes.

Pharyngeal arches form a series of ventral folds under the brain in a rostral to caudal sequence. These arches will form most of the head and neck structures of the embryo and contain all three germ layers (ectoderm, mesoderm and endoderm). The topic of head and sensory development is covered in detail in BGD cycle B.

Small patches of ectoderm form pairs of specialised placodes that eventually contribute to specific sensory components, cranial ganglia and the anterior pituitary (adenohypophysis).

Embryo Stage 13

Movies - Embryo Carnegie stage 13 - These are rotating animations based upon reconstruction of individual serial slice images of the stage 13 embryo.


Central Nervous System	Gastrointestinal	Cardiovascular

Week 4 Movies

Note that many of the movies start in week 4 and continue on through later embryonic development.

Ectoderm


Neural Plate	Neural Tube

**Neural crest migration in the head in chicken** (chicken neural crest movies overview)

Mesoderm


Mesoderm	Somite Structures	Vertebra

Endoderm