Lateral plate mesoderm

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Here is a comprehensive overview of the lateral plate mesoderm (LPM) based on authoritative embryology textbooks.

Lateral Plate Mesoderm

Origin and Position

The lateral plate mesoderm is one of three subdivisions of the intraembryonic mesoderm that differentiates by the end of the third week of development. It lies lateral to the intermediate mesoderm and extends toward the sides of the embryo. The three mesodermal subdivisions are:

Region	Derivatives
Paraxial	Somites → vertebrae, ribs, skeletal muscle, dermis of back
Intermediate	Urogenital system
Lateral plate	Body cavities, cardiovascular system, limb skeleton, body wall

BMP4 signaling ventralizes the mesoderm to form intermediate and lateral plate mesoderm; in its absence, BMP4 is antagonized by Chordin/Noggin/Follistatin (secreted by the notochord and neural plate) and mesoderm becomes paraxial.

Splitting: Parietal vs. Visceral Layers

Shortly after it forms as a solid sheet, clefts appear within the lateral plate mesoderm and coalesce to divide it into two layers, creating the intraembryonic (primitive) body cavity between them:

Parietal (somatic) layer — adjacent to surface ectoderm; continuous with extraembryonic parietal mesoderm of the amnion
- Together with overlying ectoderm → somatopleure
Visceral (splanchnic) layer — adjacent to endoderm of the gut tube; continuous with extraembryonic visceral mesoderm over the yolk sac
- Together with underlying endoderm → splanchnopleure

Cross sections of an embryo showing parietal and visceral mesoderm layers, intraembryonic cavity, body wall, gut, and serous membranes

FIGURE 6.13 — Cross sections through a 21-day embryo showing parietal and visceral mesoderm layers. The intraembryonic cavity communicates with the extraembryonic (chorionic) cavity. By the end of the 4th week, parietal mesoderm and overlying ectoderm form the ventral and lateral body wall, with the peritoneal (serous) membrane visible. — Langman's Medical Embryology

Derivatives of the Parietal Layer

Lateral body wall folds — parietal LPM + overlying ectoderm fold ventrally and fuse in the midline, closing the ventral body wall by the end of week 4
Dermis of skin in the body wall and limbs
Bones and connective tissue of the limbs
Sternum — develops independently in the parietal layer from two sternal bands, one on each side of the midline, which later fuse
Costal cartilages, limb muscles, and most body wall muscles — from sclerotome and muscle precursor cells that migrate across the lateral somitic frontier into the parietal LPM
Serous (mesothelial) membranes — lining the peritoneal, pleural, and pericardial cavities

Derivatives of the Visceral Layer

Wall of the gut tube — visceral LPM + embryonic endoderm
Smooth muscle of the dorsal aorta and large vessels (along with neural crest cells)
Thin serous membrane around each organ
Heart — the LPM is the source of both the primary and secondary heart fields
Hematopoietic cells and vasculature — blood islands appear in LPM at ~3 weeks, giving rise to hemangioblasts → endothelial cells + blood cells (vasculogenesis)

Intraembryonic Body Cavity

The space between the two LPM layers is the primitive body cavity, which initially is a single continuous cavity communicating with the extraembryonic chorionic cavity. During the fourth week it is progressively subdivided into:

Pericardial cavity
Pleural cavities
Abdominopelvic (peritoneal) cavity

The Lateral Somitic Frontier

A key boundary exists between each somite and the parietal LPM called the lateral somitic frontier. This frontier separates:

Primaxial domain — somite-derived tissues only
Abaxial domain — parietal LPM and somite-derived cells that have migrated across the frontier

This boundary also defines:

Dermis from dermatomes (back) vs. dermis from LPM (body wall and limbs)
The extent of rib development

Summary Table of Key Derivatives

Structure	Layer	Notes
Serous membranes (peritoneum, pleura, pericardium)	Parietal	Line body cavities
Limb skeleton	Parietal	Bones + connective tissue
Sternum	Parietal	Two sternal bands fuse
Dermis (body wall & limbs)	Parietal
Heart (primary & secondary fields)	Visceral
Blood vessels / vasculature	Visceral	Via vasculogenesis from hemangioblasts
Gut wall smooth muscle	Visceral	With endoderm
Dorsal aorta smooth muscle	Visceral	+ neural crest cells

— Langman's Medical Embryology; Creasy & Resnik's Maternal-Fetal Medicine

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